JP5163196B2 - Austenitic stainless steel pipe for high frequency bending and bending method using the steel pipe - Google Patents
Austenitic stainless steel pipe for high frequency bending and bending method using the steel pipe Download PDFInfo
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Description
本発明は、高周波誘導加熱により高温に加熱した直管に曲げ加工を施すことにより、原子力発電用の軽水炉等で用いられるステンレス鋼製曲げ管を製造する際に用いられる高周波曲げ加工用オーステナイト系ステンレス鋼管、およびそのステンレス鋼管を素管として用いる曲げ加工方法に関する。 The present invention relates to an austenitic stainless steel for high-frequency bending used in manufacturing a stainless steel bending tube used in a light water reactor for nuclear power generation, etc., by bending a straight pipe heated to a high temperature by high-frequency induction heating. The present invention relates to a steel pipe and a bending method using the stainless steel pipe as a raw pipe.
原子力発電用の軽水炉等における高温純水環境下では、通常、ステンレス鋼製の配管が使用されている。配管を構成する際には、ステンレス鋼管を溶接により接続すると溶接時の熱影響により溶接部に応力腐食割れが発生する可能性があるため、溶接継手に代えてエルボ等や、曲げ部の両端に曲げ部と一体となった直管部を有する曲げ管が使用されている。 In a high-temperature pure water environment in a light water reactor for nuclear power generation, stainless steel piping is usually used. When constructing piping, if stainless steel pipes are connected by welding, stress corrosion cracking may occur in the weld due to the thermal effect during welding. A bending pipe having a straight pipe portion integrated with the bending portion is used.
しかし、ステンレス鋼管に曲げ加工を施してこのような曲げ部と直管部が一体に組み込まれた曲げ管とする場合、例えば、SUS304ステンレス鋼管を高周波誘導加熱により高温に加熱して熱間曲げ加工(この曲げ加工を、以下、「高周波曲げ加工」という)する際に、配管のためのスペース削減などの必要性から、外径の1.5倍程度の小さな曲げ半径で曲げ加工すると、曲げ外側の内部結晶粒界に微小な割れ(マイクロクラック)が発生することが知られている(特許文献1)。 However, when bending a stainless steel pipe into a bending pipe in which such a bending portion and a straight pipe portion are integrated, for example, a SUS304 stainless steel pipe is heated to a high temperature by high frequency induction heating to perform a hot bending process. (This bending process is hereinafter referred to as “high-frequency bending process”.) Because of the need for space reduction for piping, when bending with a small bending radius of about 1.5 times the outer diameter, It is known that minute cracks (microcracks) occur in the internal crystal grain boundaries (Patent Document 1).
そのため、特許文献1では、SUS304ステンレス鋼管に圧縮力を付与しながら曲げ外側を1000〜1050℃の加工温度範囲で加熱すると共に加工速度を0.5mm/sec以上とし且つ曲げ外側の伸び率が20%以下となるように高周波曲げ加工するステンレス鋼管の曲げ加工法が提案されている。すなわち、この方法では、管全体に軸方向の圧縮力をかける装置を使用して曲げ加工時の引張応力をできるだけ少なくし、伸びを抑えて、マイクロクラックの発生を防止している。 Therefore, in Patent Document 1, the outer side of the bend is heated in a processing temperature range of 1000 to 1050 ° C. while applying a compressive force to the SUS304 stainless steel pipe, the processing speed is set to 0.5 mm / sec or more, and the elongation rate on the outer side of the bending is 20 A method of bending a stainless steel pipe that is subjected to high-frequency bending so as to be less than or equal to% has been proposed. That is, in this method, a device that applies an axial compressive force to the entire tube is used to reduce the tensile stress during bending as much as possible, suppress elongation, and prevent the occurrence of microcracks.
このように、曲げ部と直管部が一体となった曲げ管やエルボ等は、管の材質が従来多用されているSUS304、SUS316等に相当するステンレス鋼であれば、前掲の特許文献1に記載される曲げ加工方法により製造することが可能である。 In this way, a bent pipe or elbow in which a bent portion and a straight pipe portion are integrated is a stainless steel equivalent to SUS304, SUS316, etc., in which the material of the tube is conventionally widely used. It can be produced by the bending method described.
しかし、マイクロクラックの発生を防止するため、曲げ加工温度、加工速度および曲げ外側の伸び率を20%以下と加工条件を厳しく制限する必要があると共に、曲げ加工時に管に圧縮力を付与する装置を設ける必要がある。さらに、より高い耐食性を必要とする環境下で使用されるCrやNiの含有量が高いステンレス鋼は、従来のSUS304やSUS316Lのステンレス鋼と比べて加工性が劣るため、曲率半径が小さい曲げ加工を行った場合、微小割れの発生が懸念される。そのため、高Cr、高Niステンレス鋼の高周波曲げ加工に適した材料設計および曲げ加工方法が求められている。 However, in order to prevent the occurrence of microcracks, it is necessary to severely limit the processing conditions such as the bending temperature, the processing speed, and the elongation rate outside the bending to 20% or less, and a device that applies a compressive force to the pipe during bending It is necessary to provide. Furthermore, stainless steel with high Cr and Ni content used in environments that require higher corrosion resistance is inferior in workability compared to conventional SUS304 and SUS316L stainless steel, so bending with a small curvature radius If this is done, the occurrence of microcracking is a concern. Therefore, a material design and a bending method suitable for high-frequency bending of high Cr and high Ni stainless steel are required.
また、曲げ加工後の偏肉を防止する技術として、特許文献2には、Cr:20.37%、Ni:8.83%、Si:1.21%を含有するステンレス鋼の直管を素管とし、これを管外径の2倍の曲げ半径で高周波曲げ加工して曲げ部と直管部が一体に組み込まれた曲げ管を製作するに当たり、予め曲げ管の背側となる側は曲げ加工による減肉分だけ増肉し、曲げ管の腹側となる側は曲げ加工による増肉分だけ減肉する偏肉加工を施して曲げ加工する方法が開示されており、これによって、曲げ部においてもほぼ均一な肉厚の曲げ管を得ることができるとしている。 As a technique for preventing uneven thickness after bending, Patent Document 2 discloses a straight stainless steel pipe containing Cr: 20.37%, Ni: 8.83%, Si: 1.21%. When producing a bent tube with a bending part and a straight pipe part integrated together by high-frequency bending with a bending radius twice the outer diameter of the pipe, the side that is the back side of the bending pipe is bent in advance. Disclosed is a method of bending by bending the thickness of the bending tube by increasing the thickness of the bent tube and reducing the thickness of the bent tube by increasing the thickness of the bent tube. It is said that a bent tube having a substantially uniform wall thickness can be obtained.
曲げ加工前に予め偏肉加工を施しておく方法は、特許文献3にも記載されている。すなわち、大径厚肉のエルボのような曲管を曲げ加工により製造する方法で、曲げ加工すべき素管を、機械加工により予め曲げ外側に当たる側を曲げ内側に当たる側より厚肉肉厚に形成しておく方法である。 A method of performing uneven thickness machining in advance before bending is also described in Patent Document 3. In other words, by bending a large-diameter, thick-walled elbow-like bent pipe, the pipe to be bent is formed thicker and thicker than the side that hits the inner side of the bending by machining. It is a method to keep.
本発明は、原子力発電用の軽水炉等における高温純水環境下で使用されるCrやNiの含有量が高いステンレス鋼製の曲げ管を、直管に高周波曲げ加工を施すことにより製造する際に、微小割れが発生しない高周波曲げ加工用オーステナイト系ステンレス鋼管、およびそのステンレス鋼管を素管として用いるオーステナイト系ステンレス鋼管の曲げ加工方法を提供することを目的としている。 The present invention provides a stainless steel bending pipe having a high Cr and Ni content used in a high-temperature pure water environment in a light water reactor for nuclear power generation, etc., by subjecting the straight pipe to high-frequency bending. An object of the present invention is to provide an austenitic stainless steel pipe for high-frequency bending that does not cause microcracking, and a method for bending an austenitic stainless steel pipe using the stainless steel pipe as a raw pipe.
前述のように、曲げ部と直管部が一体となった曲げ管を製造するに際しては、例えば、ステンレス鋼管に圧縮力を加えて曲げ加工時の背側(曲げ外側)の伸びを抑制しつつ高周波曲げ加工を行い、曲げ管の背側における微小割れの発生を防止する方法が知られている(前掲の特許文献1)。 As described above, when manufacturing a bending pipe in which a bending portion and a straight pipe portion are integrated, for example, a compressive force is applied to a stainless steel pipe while suppressing the elongation on the back side (bending outside) during bending. A method is known in which high-frequency bending is performed to prevent the occurrence of microcracks on the back side of the bending tube (Patent Document 1).
しかし、曲げ管の背側の伸びを抑える方法は、従来多用されているSUS304、SUS316などのステンレス鋼管に対してはそれだけで有効であるが、CrやNiの含有量がより高いステンレス鋼管では、後述する実施例で示すように、曲げ加工時に割れが発生する。 However, the method of suppressing the elongation on the back side of the bent pipe is effective only for stainless steel pipes such as SUS304 and SUS316 that have been widely used in the past, but in stainless steel pipes with higher contents of Cr and Ni, As shown in the examples described later, cracks occur during bending.
そこで、本発明者らは、曲げ加工時の背側の伸びを抑制するために曲げ加工される部分に偏肉加工を施すことに加え、管自体の熱間加工性の改善を試みた。熱間加工性を向上させることができれば、割れが生じにくくなると考えられるからで、具体的には、粒界に偏析して熱間加工性を低下させる鋼中のSを、Caを添加することによりCaSとして固定して熱間加工性を改善する方法である。 Therefore, the present inventors tried to improve the hot workability of the tube itself, in addition to performing uneven thickness processing on the portion to be bent in order to suppress the elongation on the back side during bending. If it is possible to improve the hot workability, it is considered that cracking is unlikely to occur. Specifically, S is added to the steel in the steel that segregates at the grain boundaries and decreases the hot workability. This is a method for improving hot workability by fixing as CaS.
その結果、供試材として高Cr、高Niのオーステナイト系ステンレス鋼からなる素管を高周波曲げ加工するに際し、素管の曲げ加工される部分に偏肉加工を施し、さらに、鋼中にCaを所定量含有させることにより、微小割れの発生を防止できることが判明した。 As a result, when subjecting a raw tube made of a high Cr, high Ni austenitic stainless steel as a test material to high frequency bending, the portion of the raw tube to be bent is subjected to uneven thickness processing, and Ca is further contained in the steel. It has been found that the inclusion of a predetermined amount can prevent the occurrence of microcracks.
本発明はこのような知見に基づきなされたもので、その要旨は、下記(1)の高周波曲げ加工用オーステナイト系ステンレス鋼管、およびそのステンレス鋼管を素管として用いる(2)のオーステナイト系ステンレス鋼管の曲げ加工方法にある。 The present invention has been made on the basis of such knowledge. The gist of the present invention is the following (1) austenitic stainless steel pipe for high-frequency bending, and (2) austenitic stainless steel pipe using the stainless steel pipe as a base pipe. It is in the bending method.
(1)質量%で、C:0.08%以下、Si:1.5%以下、Mn:0.1〜2.0%、Cr:22〜28%、Ni:18〜25%およびCa:0.0003〜0.01%を含有し、且つCa含有量と不純物中のSおよびO含有量とが下記(i)式を満足し、残部がFeおよび不純物からなり、不純物中のPが0.045%以下、Sが0.030%以下およびOが0.010%以下であるオーステナイト系ステンレス鋼管であって、曲げ加工される部分に、予め、曲げ管の背側になる部分の肉厚/曲げ管の腹側になる部分の肉厚の比が1.2以上となるように偏肉加工が施されていることを特徴とする高周波曲げ加工用オーステナイト系ステンレス鋼管。 (1) By mass%, C: 0.08% or less, Si: 1.5% or less, Mn : 0.1-2.0%, Cr: 22-28%, Ni: 18-25% and Ca: 0.0003 to 0.01% is contained, and the Ca content and the S and O contents in the impurity satisfy the following formula (i) , the balance is Fe and impurities, and P in the impurities is 0 0.045% or less, S is 0.030% or less, and O is 0.010% or less, an austenitic stainless steel pipe, and the thickness of the portion to be bent in advance on the back side of the bending tube / An austenitic stainless steel pipe for high-frequency bending, which is subjected to uneven thickness processing so that the thickness ratio of the portion on the ventral side of the bending pipe is 1.2 or more.
Ca−1.25×S−2.5×O≧−0.01 ・・・(i)
ただし、(i)式中の元素記号は、それぞれの元素の質量%での含有量を表す。
Ca-1.25 × S-2.5 × O ≧ −0.01 (i)
However, the element symbol in (i) Formula represents content in the mass% of each element.
前記の「曲げ管の背側になる部分の肉厚」とは、直管を曲げ加工したときに背側、つまり曲げ部の外側になる部分の肉厚を指し、「曲げ管の腹側になる部分の肉厚」とは、曲げ加工したときに曲げ部の内側になる部分の肉厚をいう。 The “thickness of the portion that becomes the back side of the bending tube” refers to the thickness of the portion that becomes the back side when the straight pipe is bent, that is, the outside of the bending portion. The “thickness of the part” means the thickness of the part that becomes the inside of the bent part when bent.
図1は、外径D、肉厚tの直管に、曲げ半径をR、曲げ角度を90°として曲げ加工を施して得られた曲げ管の外観を示す図(部分図)で、(a)は平面図、(b)は(a)に例示した曲げ管の端部の正面図である。曲げ管は、曲げ加工により形成された曲げ部の両端に直管部を有している。図示するように、曲げ管の背側になる部分とは、曲げ部の外側にあたる部分であり(図中に「背側」と表示)、曲げ管の腹側になる部分とは、曲げ部の内側にあたる部分である(図中に「腹側」と表示)。 FIG. 1 is a diagram (partial view) showing an external appearance of a bending pipe obtained by bending a straight pipe having an outer diameter D and a wall thickness t with a bending radius R and a bending angle 90 °. ) Is a plan view, and (b) is a front view of the end of the bent pipe illustrated in (a). The bending pipe has straight pipe portions at both ends of a bending portion formed by bending. As shown in the figure, the portion on the back side of the bending tube is the portion on the outside of the bending portion (indicated as “back side” in the figure), and the portion on the ventral side of the bending tube is the portion of the bending portion. It is a part corresponding to the inner side (displayed as “ventral side” in the figure).
実際に「曲げ管の腹側になる部分の肉厚」に対する「曲げ管の背側になる部分の肉厚」の比を求めるに際しては、曲げ管の背側になる部分のうちで外側に最も張り出す部位(図1において、管の中心軸(図中に一点鎖線で表示)を含む平面が曲げ部の外側の管壁と交わる部位)に相当する素管の肉厚を「曲げ管の背側になる部分の肉厚」とし、曲げ管の腹側になる部分のうちで最も内側に位置する部位(図1において、管の中心軸を含む平面が曲げ部の内側の管壁と交わる部位)に相当する素管の肉厚を「曲げ管の腹側になる部分の肉厚」として、それらの比を求めればよい。 When calculating the ratio of the “thickness of the bent tube's dorsal side” to the “thickness of the bent tube's dorsal side”, The thickness of the raw tube corresponding to the overhanging portion (the portion where the plane including the central axis of the tube (indicated by the alternate long and short dash line in the figure) intersects the outer tube wall of the bending portion) The portion located on the innermost side among the portions on the ventral side of the bent tube (in FIG. 1, the portion where the plane including the central axis of the tube intersects the tube wall inside the bent portion) The thickness of the raw tube corresponding to) is defined as “the thickness of the portion on the ventral side of the bent tube” and the ratio thereof may be obtained.
前記(1)に記載のオーステナイト系ステンレス鋼管が、質量%で、C:0.08%以下、Si:1.5%以下、Mn:0.1〜2.0%、Cr:22〜28%、Ni:18〜25%およびCa:0.0003〜0.01%を含有し、且つCa含有量と不純物中のSおよびO含有量とが前記の(i)式を満足し、残部がFeおよび不純物からなり、不純物中のPが0.045%以下、Sが0.030%以下およびOが0.010%以下の組成をもつものであれば、熱間加工性さらには耐食性が良好に維持される。 The austenitic stainless steel pipe described in the above (1) is in mass%, C: 0.08% or less, Si: 1.5% or less, Mn: 0.1-2.0%, Cr: 22-28% , Ni: 18 to 25% and Ca: 0.0003 to 0.01%, and the Ca content and the S and O contents in the impurities satisfy the above formula (i), with the balance being Fe And hot impurities, and corrosion resistance is good if it has a composition in which P is 0.045% or less, S is 0.030% or less, and O is 0.010% or less. It is maintained Ru.
(2)質量%で、C:0.08%以下、Si:1.5%以下、Mn:0.1〜2.0%、Cr:22〜28%、Ni:18〜25%およびCa:0.0003〜0.01%を含有し、且つCa含有量と不純物中のSおよびO含有量とが下記(i)式を満足し、残部がFeおよび不純物からなり、不純物中のPが0.045%以下、Sが0.030%以下およびOが0.010%以下であり、曲げ加工される部分に、予め、曲げ管の背側になる部分の肉厚/曲げ管の腹側になる部分の肉厚の比が1.2以上となる偏肉加工が施されているオーステナイト系ステンレス鋼管を用い、曲げ半径を(1.5〜2.0)×(管の外径)として高周波曲げ加工することを特徴とするオーステナイト系ステンレス鋼管の曲げ加工方法。 (2) By mass%, C: 0.08% or less, Si: 1.5% or less, Mn : 0.1-2.0%, Cr: 22-28%, Ni: 18-25% and Ca: 0.0003 to 0.01% is contained, and the Ca content and the S and O contents in the impurity satisfy the following formula (i), the balance is Fe and impurities, and P in the impurities is 0 0.045% or less, S is 0.030% or less, and O is 0.010% or less. In the portion to be bent, the thickness of the portion that becomes the back side of the bending tube / the ventral side of the bending tube Using an austenitic stainless steel pipe that has been subjected to uneven thickness machining so that the thickness ratio of the portion becomes 1.2 or more, the bending radius is (1.5 to 2.0) × (outer diameter of the pipe), and high frequency A method for bending an austenitic stainless steel pipe, characterized by bending.
Ca−1.25×S−2.5×O≧−0.01 ・・・(i)
ただし、(i)式中の元素記号は、それぞれの元素の質量%での含有量を表す。
Ca-1.25 × S-2.5 × O ≧ −0.01 (i)
However, the element symbol in (i) Formula represents content in the mass% of each element.
前記(2)に記載のオーステナイト系ステンレス鋼管の曲げ加工方法において、オーステナイト系ステンレス鋼管が、質量%で、C:0.08%以下、Si:1.5%以下、Mn:0.1〜2.0%、Cr:22〜28%、Ni:18〜25%およびCa:0.0003〜0.01%を含有し、且つCa含有量と不純物中のSおよびO含有量とが前記の(i)式を満足し、残部がFeおよび不純物からなり、不純物中のPが0.045%以下、Sが0.030%以下およびOが0.010%以下の組成をもつものであれば、熱間加工性さらには耐食性が良好に維持されたステンレス鋼管が得られる。 In the method for bending an austenitic stainless steel pipe as described in (2) above, the austenitic stainless steel pipe is, by mass, C: 0.08% or less, Si: 1.5% or less, Mn: 0.1-2. 0.0%, Cr: 22-28%, Ni: 18-25% and Ca: 0.0003-0.01%, and the Ca content and the S and O contents in the impurities are i) If the formula is satisfied, the balance is Fe and impurities, P in the impurities is 0.045% or less, S is 0.030% or less, and O has a composition of 0.010% or less, hot workability further corrosion resistance Ru well maintained stainless steel tube is obtained.
本発明の高周波曲げ加工用オーステナイト系ステンレス鋼管を素管として使用すれば、微小割れを発生させることなく曲げ加工を行うことができる。本発明のオーステナイト系ステンレス鋼管の曲げ加工方法は、このオーステナイト系ステンレス鋼管を素管として用い高周波曲げ加工する方法で、曲率半径が小さいステンレス鋼製曲げ管を製造することができる。この曲げ管は、CrやNiの含有量が高く、耐食性に優れているので、原子力発電用の軽水炉等における高温純水環境下など、過酷な腐食環境下で用いられる曲げ管として好適である。 If the austenitic stainless steel pipe for high-frequency bending according to the present invention is used as a base pipe, bending can be performed without generating microcracks. The bending method of the austenitic stainless steel pipe of the present invention is a method of high-frequency bending using this austenitic stainless steel pipe as a raw pipe, and a stainless steel bent pipe having a small curvature radius can be manufactured. Since this bent pipe has a high Cr and Ni content and excellent corrosion resistance, it is suitable as a bent pipe used in a severe corrosive environment such as a high-temperature pure water environment in a light water reactor for nuclear power generation.
本発明の高周波曲げ加工用オーステナイト系ステンレス鋼管(前記(1)に記載のステンレス鋼管)は、曲げ部と直管部が一体となった曲げ管を製造する際に素管として使用されるステンレス鋼管で、質量%で、Cr:22〜28%、Ni:18〜25%およびCa:0.0003〜0.01%を含有し(以下、鋼の化学組成を表す「%」は「質量%」を意味する)、且つCa含有量と不純物中のSおよびO含有量とが下記(i)式を満足するオーステナイト系ステンレス鋼管であって、曲げ加工される部分に、予め、曲げ管の背側になる部分の肉厚/曲げ管の腹側になる部分の肉厚の比が1.2以上となるように偏肉加工が施されているステンレス鋼管である。 The austenitic stainless steel pipe for high-frequency bending of the present invention (stainless steel pipe described in (1) above) is a stainless steel pipe used as a raw pipe when producing a bent pipe in which a bent portion and a straight pipe portion are integrated. And containing Cr: 22-28%, Ni: 18-25% and Ca: 0.0003-0.01% (hereinafter, “%” representing the chemical composition of steel is “mass%”. And an austenitic stainless steel pipe in which the Ca content and the S and O contents in the impurities satisfy the following formula (i): This is a stainless steel pipe that has been subjected to uneven thickness processing so that the ratio of the thickness of the portion to become / the thickness of the portion on the ventral side of the bending tube is 1.2 or more.
Ca−1.25×S−2.5×O≧−0.01 ・・・(i)
ただし、(i)式中の元素記号は、それぞれの元素の含有量(%)を表す。
Ca-1.25 × S-2.5 × O ≧ −0.01 (i)
However, the element symbol in (i) Formula represents content (%) of each element.
本発明のステンレス鋼管が、Cr:22〜28%、Ni:18〜25%を含有する高Cr、高Niのオーステナイト系ステンレス鋼管であることとするのは、SUS304やSUS316Lのステンレス鋼製曲げ管よりも高い耐食性が要求される過酷な環境下で使用される曲げ管の素材(素管)を提供することが本発明の目的だからである。さらに、Ca:0.0003〜0.01%を含有し、且つCa含有量が前記の(i)式を満足することとするのは、管自体の熱間加工性を向上させるためである。なお、これらの合金成分の作用および含有量の限定理由については、後に詳述する。 The stainless steel pipe of the present invention is a high-Cr, high-Ni austenitic stainless steel pipe containing Cr: 22-28% and Ni: 18-25%. SUS304 or SUS316L stainless steel bent pipe This is because it is an object of the present invention to provide a material (element tube) of a bending tube used in a severe environment where higher corrosion resistance is required. Further, the reason why Ca is contained in the range of 0.0003 to 0.01% and the Ca content satisfies the above formula (i) is to improve the hot workability of the tube itself. In addition, the effect | action of these alloy components and the reason for limitation of content are explained in full detail behind.
また、本発明のステンレス鋼管は、曲げ加工される部分に、予め、曲げ管の背側になる部分の肉厚/曲げ管の腹側になる部分の肉厚の比が1.2以上となるように偏肉加工が施されたものとする。これは、曲げ加工の際に背側の伸びを抑制して微小割れの発生を抑えるためで、前記の比を1.2以上と規定するのは、この比が1.2未満では曲げ加工時に割れ発生のおそれがあるからである。この曲げ管の背側になる部分の肉厚/曲げ管の腹側になる部分の肉厚の比の上限は特に定めない。この比が過度に大きいと、得られる曲げ管の背側の肉厚が厚くなりすぎて寸法公差を満足することができない等の支障が生じるので、前記肉厚比の適正範囲は、曲げ管の素管として用いるステンレス鋼管の外径や肉厚、曲げ半径等に応じて自ずと定まるからである。 Further, in the stainless steel pipe of the present invention, the ratio of the thickness of the portion on the back side of the bending tube / the thickness of the portion on the ventral side of the bending tube is 1.2 or more in advance in the portion to be bent. As shown in FIG. This is to suppress the occurrence of microcracking by suppressing the elongation on the back side during bending, and the ratio is defined as 1.2 or more when the ratio is less than 1.2. This is because cracks may occur. There is no particular upper limit on the ratio of the thickness of the portion on the back side of the bending tube / the thickness of the portion on the ventral side of the bending tube. If this ratio is excessively large, the thickness on the back side of the obtained bending tube becomes too thick, which causes problems such as failure to satisfy the dimensional tolerance. This is because it is naturally determined according to the outer diameter, thickness, bending radius, etc. of the stainless steel pipe used as the raw pipe.
前記の偏肉加工時の加工形状は、特定の形状に限定されることはない。素管が曲げ加工されて曲げ管の背側となる部分を増肉させることにより偏肉させてもよいし、曲げ管の腹側となる部分を減肉させて、その肉厚に対する背側となる部分の肉厚の比を増大させてもよい。また、曲げ管の背側となる部分を増肉させると共に、腹側となる部分を減肉させてもよい。なお、曲げ加工時には背側の肉厚が減少するので、曲げ管の背側になる部分の肉厚を大きくすることにより偏肉させるのが望ましい。 The processing shape at the time of uneven thickness processing is not limited to a specific shape. The base tube may be bent to increase the thickness of the portion that becomes the back side of the bending tube, or the portion that becomes the ventral side of the bending tube may be thinned, and the back side with respect to the thickness You may increase the ratio of the thickness of the part which becomes. Moreover, while increasing the part which becomes the back | dorsal side of a bending pipe, you may reduce the part which becomes abdominal side. In addition, since the thickness on the back side decreases during bending, it is desirable to make the thickness uneven by increasing the thickness of the portion on the back side of the bending tube.
図2は、本発明のステンレス鋼管の曲げ加工される部分に予め施されている偏肉加工の形状を例示する縦断面図で、(a)と(b)は曲げ加工されて曲げ管の背側となる部分を増肉した場合、(c)は曲げ加工されて腹側となる部分を減肉した場合である。 FIG. 2 is a longitudinal cross-sectional view illustrating the shape of uneven thickness machining that is preliminarily applied to a portion to be bent of the stainless steel pipe of the present invention. FIGS. 2 (a) and (b) are bent and the back of the bent pipe is illustrated. When the part which becomes the side is increased in thickness, (c) is the case where the part which becomes the ventral side is thinned by bending.
図2(a)に示したステンレス鋼管では、曲げ加工したときに背側になる部分が管の外側方向に増肉され、曲げ管の背側になる部分の肉厚t1が曲げ管の腹側になる部分の肉厚t2よりも厚くなっている。また、同図(b)に示したステンレス鋼管では、曲げ加工したときに背側になる部分が管内方向に増肉されている。一方、同図(c)に示したステンレス鋼管では、曲げ加工したときに腹側になる部分が管内面側で減肉され、曲げ管の背側になる部分の肉厚t1が曲げ管の腹側になる部分の肉厚t2よりも厚くなっている。 In the stainless steel pipe shown in FIG. 2 (a), the portion on the back side when bent is increased in the outer direction of the pipe, and the thickness t 1 of the portion on the back side of the bent pipe is the belly of the bent pipe. It is thicker than the wall thickness t 2 of the side portion. Further, in the stainless steel pipe shown in FIG. 5B, the portion that becomes the back side when bent is increased in the pipe inner direction. On the other hand, in the stainless steel pipe shown in FIG. 5C, the portion that becomes the ventral side when bent is thinned on the inner surface side of the tube, and the thickness t 1 of the portion that becomes the back side of the bent tube is the thickness of the bent tube. It is thicker than the thickness t 2 of the portion on the ventral side.
偏肉加工は、機械加工により行うことができる。なお、その際、管周方向に肉厚差をつけた偏心加工により偏肉させるのが望ましい。すなわち、曲げ管の背側になる部分のうちで外側に最も張り出している部位における増肉厚さを最大とし、その部位から管周方向に進むと共に増肉厚さを徐々に減少させる。 The uneven thickness processing can be performed by machining. In this case, it is desirable to make the thickness uneven by an eccentric process with a thickness difference in the pipe circumferential direction. That is, the thickness of the thickening at the portion of the bent tube that protrudes most outward is maximized, and the thickness of the bent tube is gradually reduced as it proceeds from the portion in the circumferential direction of the tube.
このように素管に偏肉加工を施して、曲げ加工時に管の背側に生じる引張応力を小さくし、管の腹側から降伏しやすくすることにより、背側の伸びを抑えて微小割れの発生を抑制することができる。 In this way, the base pipe is subjected to uneven thickness processing, the tensile stress generated on the back side of the pipe during bending is reduced, and it is easy to yield from the ventral side of the pipe. Occurrence can be suppressed.
本発明のステンレス鋼管は、高周波曲げ加工用の素管で、曲げ加工時に、高周波誘導加熱により熱間曲げ加工されることを前提としている。高周波誘導加熱によれば限定された狭い範囲を高温加熱することが可能であり、従来から熱間曲げ加工による曲げ管の製造に常用されているからである。 The stainless steel pipe of the present invention is a base pipe for high-frequency bending, and is premised on being hot-bended by high-frequency induction heating during bending. This is because high-frequency induction heating makes it possible to heat a limited narrow range at a high temperature and has been conventionally used for manufacturing bent pipes by hot bending.
本発明のステンレス鋼管は、前述のように、Cr:22〜28%、Ni:18〜25%を含有する高Cr、高Niのオーステナイト系ステンレス鋼管である。 As described above, the stainless steel pipe of the present invention is a high Cr, high Ni austenitic stainless steel pipe containing Cr: 22 to 28% and Ni: 18 to 25%.
Crは耐食性を改善する元素であり、特に、高温純水環境下において、加工や熱歪により硬化した領域で発生する応力腐食割れを抑制するために重要な元素である。Cr含有量が22%未満ではこの効果が十分に得られない。一方、Cr含有量が28%を超えると熱間加工性や延性が低下する。したがって、本発明のステンレス鋼管が含有するCr量は、22〜28%と規定した。望ましくは、24〜26%である。 Cr is an element that improves corrosion resistance, and in particular, is an important element for suppressing stress corrosion cracking that occurs in a region hardened by processing or thermal strain in a high-temperature pure water environment. If the Cr content is less than 22%, this effect cannot be obtained sufficiently. On the other hand, when the Cr content exceeds 28%, hot workability and ductility are lowered. Therefore, the amount of Cr contained in the stainless steel pipe of the present invention is defined as 22 to 28%. Desirably, it is 24 to 26%.
Niはオーステナイト相を安定させ、良好な耐食性や耐応力腐食割れ性の確保に重要な元素である。Ni含有量が18%未満では、特に、高温純水環境下において、加工や熱歪により硬化した領域で発生する耐応力腐食割れを抑制するには不十分である。一方、Ni含有量が25%を超えても、前記の耐食性や耐応力腐食割れ性を良好ならしめる効果はそれほど変わらず、製造コストの上昇を招くだけである。したがって、Ni含有量は、18〜25%とした。望ましくは、19〜22%である。 Ni is an important element for stabilizing the austenite phase and ensuring good corrosion resistance and stress corrosion cracking resistance. When the Ni content is less than 18%, it is insufficient to suppress stress corrosion cracking that occurs in a region hardened by processing or thermal strain, particularly in a high-temperature pure water environment. On the other hand, even if the Ni content exceeds 25%, the effect of improving the corrosion resistance and the stress corrosion cracking resistance is not changed so much, and only the production cost is increased. Therefore, the Ni content is 18-25%. Desirably, it is 19 to 22%.
本発明のステンレス鋼管は、さらに、Ca:0.0003〜0.01%を含有し、且つCa含有量が前記の(i)式を満足するものである。 The stainless steel pipe of the present invention further contains Ca: 0.0003 to 0.01%, and the Ca content satisfies the above formula (i).
Caを含有させるのは、前述のように、高Cr、高Niオーステナイト系ステンレス鋼は従来多用されているSUS304やSUS316Lステンレス鋼に比べて変形抵抗が高く、小さい曲げ半径で高周波曲げ加工したときの引張応力により微小割れが発生する危険性があるので、管自体の熱間加工性を改善して割れを生じにくくするためである。 As described above, Ca is contained when high Cr, high Ni austenitic stainless steel has higher deformation resistance than SUS304 and SUS316L stainless steel, which are widely used in the past, and when high frequency bending is performed with a small bending radius. This is because there is a risk of microcracking due to tensile stress, so that the hot workability of the tube itself is improved and cracking is less likely to occur.
Caは、鋼中に不純物として含まれ、粒界に偏析して熱間加工性を低下させるSをCaSとして固定することにより熱間加工性を改善する作用効果を有している。しかし、同じく不純物として含まれ、Caとの結合力がSよりも強いO(酸素)をCaOとして固定するので、Sと結合する有効Caが消費される。 Ca is contained as an impurity in the steel, and has an effect of improving hot workability by fixing S, which is segregated at the grain boundary and decreases hot workability, as CaS. However, since O (oxygen), which is also contained as an impurity and has a stronger binding force with Ca than S, is fixed as CaO, effective Ca binding with S is consumed.
そこで、Ca−1.25×S−2.5×O(ただし、Ca、SおよびOは、それぞれの元素の含有量(%)を表す)を有効Caと定義し、有効Caが異なる鋼を溶製して試験を行い、高周波曲げ加工時における割れの発生に及ぼす有効Caの影響を調査した。なお、前記の定義式における第2項(−1.25×S)はSと結合するCa(%)に、第3項(−2.5×O)はOと結合するCa(%)に相当する。 Therefore, Ca-1.25 × S-2.5 × O (where Ca, S and O represent the content (%) of each element) is defined as effective Ca, and steels having different effective Ca are defined. A test was conducted by melting, and the effect of effective Ca on the occurrence of cracks during high-frequency bending was investigated. In the above definition, the second term (-1.25 × S) is Ca (%) combined with S, and the third term (−2.5 × O) is Ca (%) combined with O. Equivalent to.
調査においては、前記溶製した有効Caが異なる鋼をそれぞれ熱間加工した後、機械加工により直径10mmの丸棒試験片を作製し、この丸棒試験片に、サーモレスタ試験機を用いて、加熱温度1000℃で0.005s-1の歪速度で40%の伸びを与えた時の割れ発生によって高周波曲げ加工時の割れ発生を模擬し、サーモレスタ試験における割れ発生の有無により、有効Caが割れ発生に及ぼす影響を評価した。その結果を表1に示す。 In the investigation, after each hot-melted steel having different effective Ca was hot-worked, a round bar test piece having a diameter of 10 mm was produced by machining, and this round bar test piece was heated using a thermoresta testing machine. Cracks generated during high-frequency bending are simulated by the occurrence of cracks when 40% elongation is applied at a strain rate of 0.005 s -1 at a temperature of 1000 ° C. Effective Ca cracks are generated depending on the presence or absence of cracks in the thermorestor test. The effect on the environment was evaluated. The results are shown in Table 1.
表1に示した結果から、有効Ca(Ca−1.25×S−2.5×O)が−0.01以上であれば(材質9〜11)、割れが発生しないことが確認された。そこで、Ca含有量と、鋼中に不純物として含まれるSおよびOの含有量とが前記の(i)式(この式の左辺は、有効Caを表す)を満たすこととした。 From the results shown in Table 1, it was confirmed that if effective Ca (Ca-1.25 × S-2.5 × O) was −0.01 or more (materials 9 to 11), no cracking occurred. . Therefore, the Ca content and the contents of S and O contained as impurities in the steel satisfy the above formula (i) (the left side of this formula represents effective Ca).
なお、前記(i)式を満たすか否かにかかわらず、Ca含有量が0.0003%未満では割れ発生防止効果が得られない。また、Ca含有量が0.01%を超えると、JIS G0571に規定される10%しゅう酸エッチング試験で粒界に顕著な溝状腐食が認められ、亀裂が粒界に沿って進展する可能性がある。そのため、Ca含有量は、0.0003〜0.01%であることが必要である。望ましくは0.0005〜0.005%である。 Regardless of whether or not the above formula (i) is satisfied, if the Ca content is less than 0.0003%, the crack prevention effect cannot be obtained. If the Ca content exceeds 0.01%, significant groove-like corrosion is observed at the grain boundary in the 10% oxalic acid etching test specified in JIS G0571, and cracks may develop along the grain boundary. There is. Therefore, the Ca content needs to be 0.0003 to 0.01%. Desirably, it is 0.0005 to 0.005%.
したがって、本発明のステンレス鋼管は、Cr:22〜28%、Ni:18〜25%を含み、さらに、Ca:0.0003〜0.01%を含有し、且つCa含有量が前記の(i)式を満足することとした。 Therefore, the stainless steel pipe of the present invention contains Cr: 22 to 28%, Ni: 18 to 25%, further contains Ca: 0.0003 to 0.01%, and the Ca content is (i ) Was satisfied.
本発明の高周波曲げ加工用オーステナイト系ステンレス鋼管は、C:0.08%以下、Si:1.5%以下、Mn:0.1〜2.0%、Cr:22〜28%、Ni:18〜25%およびCa:0.0003〜0.01%を含有し、且つCa含有量と不純物中のSおよびO含有量とが前記の(i)式を満足し、残部がFeおよび不純物からなり、不純物中のPが0.045%以下、Sが0.030%以下およびOが0.010%以下の組成をもつものである。 The austenitic stainless steel pipe for high-frequency bending work of the present invention is C: 0.08% or less, Si: 1.5% or less, Mn: 0.1-2.0%, Cr: 22-28%, Ni: 18 -25% and Ca: 0.0003-0.01%, and the Ca content and the S and O contents in the impurities satisfy the above formula (i), with the balance being Fe and impurities. , P 0.045% of the impurities less, S is Ru der those having a composition of 0.030% or less and O 0.010% or less.
このステンレス鋼管において、Cr、NiおよびCa以外の成分の作用効果と含有量を上記のように限定した理由は、次のとおりである。 In this stainless steel pipe, the reason why the effects and contents of components other than Cr, Ni and Ca are limited as described above is as follows.
C:0.08%以下
Cはオーステナイト相の安定に寄与する。しかし、Cを過剰に含有させると、炭化物が形成されて鋼の耐食性が劣化するだけでなく、熱間加工性も低下する。したがって、C含有量の上限を0.08%とした。望ましくは、0.05%、より望ましくは、0.03%である。なお、C含有量の下限は精錬における技術的または経済的制約により自ずと定まるので、特に限定しない。
C: 0.08% or less C contributes to the stability of the austenite phase. However, when C is contained excessively, carbides are formed and the corrosion resistance of the steel is deteriorated, and hot workability is also lowered. Therefore, the upper limit of the C content is set to 0.08%. Desirably, it is 0.05%, more desirably 0.03%. The lower limit of the C content is not particularly limited because it is naturally determined by technical or economic restrictions in refining.
Si:1.5%以下
Siは脱酸剤として利用される。しかし、Siを過剰に含有させると金属間化合物が形成され、靭性や熱間加工性が低下する。したがって、Si含有量の上限を1.5%とした。熱間加工性の観点からは、上限を1.0%とするのが望ましい。脱酸剤としての作用効果を得るために、Si含有量の下限は0.1%とするのが望ましい。
Si: 1.5% or less Si is used as a deoxidizer. However, when Si is excessively contained, an intermetallic compound is formed, and toughness and hot workability are deteriorated. Therefore, the upper limit of the Si content is set to 1.5%. From the viewpoint of hot workability, the upper limit is preferably 1.0%. In order to obtain the effect as a deoxidizer, the lower limit of the Si content is preferably 0.1%.
Mn:0.1〜2.0%
Mnは脱酸剤として利用され、熱間加工性やオーステナイト相の安定にも寄与するので、含有量の下限を0.1%とした。しかし、Mnを過剰に含有させると鋼の清浄度が低下し、熱間加工性が低下する場合がある。したがって、Mn含有量の上限を2.0%とした。望ましくは、1.5%である。
Mn: 0.1 to 2.0%
Since Mn is used as a deoxidizer and contributes to hot workability and stability of the austenite phase, the lower limit of the content is set to 0.1%. However, when Mn is contained excessively, the cleanliness of the steel is lowered and the hot workability may be lowered. Therefore, the upper limit of the Mn content is set to 2.0%. Desirably, it is 1.5%.
このステンレス鋼管は、上述した成分以外、残部がFeと不純物からなるものである。不純物としては、P、SおよびOの上限を抑えることが必要である。 The stainless steel pipe is composed of Fe and impurities in the balance other than the components described above. As impurities, it is necessary to suppress the upper limit of P, S and O.
P:0.045%以下
Pは、多量に存在すると鋼の耐粒界腐食性を劣化させる。したがって、P含有量を0.045%以下とした。
P: 0.045% or less P, if present in a large amount, deteriorates the intergranular corrosion resistance of steel. Therefore, the P content is set to 0.045% or less.
S:0.030%以下
Sは硫化物を形成して鋼の耐食性を劣化させる。したがって、S含有量を0.030%以下とした。
S: 0.030% or less S forms sulfides and degrades the corrosion resistance of steel. Therefore, the S content is set to 0.030% or less.
O:0.010%以下
Oは酸化物を形成して鋼の熱間加工性を低下させる。したがって、O含有量を0.010%以下とした。
O: 0.010% or less O forms an oxide to reduce the hot workability of steel. Therefore, the O content is set to 0.010% or less.
本発明のオーステナイト系ステンレス鋼管が上述したような組成を有するものであれば、熱間加工性さらには耐食性が良好に維持されるので、望ましい。 If the austenitic stainless steel pipe of the present invention has a composition as described above, it is desirable because hot workability and corrosion resistance are maintained well.
以上述べたように、本発明の高周波曲げ加工用オーステナイト系ステンレス鋼管は、高Cr、高Niのステンレス鋼で、従来多用されているSUS304やSUS316Lステンレス鋼に比べて変形抵抗が高いが、曲げ加工される部分に偏肉加工が施され、さらに、熱間加工性を向上させるCaが含まれているので、例えば、曲げ半径が(1.5〜2.0)D(ここで、Dは管の外径である)のような、曲率の小さい曲げ半径で高周波曲げ加工を行っても微小割れが発生することがない。曲げ半径が2.0Dよりも大きい曲げ加工用の素管としても、もちろん使用することができる。 As described above, the austenitic stainless steel pipe for high-frequency bending according to the present invention is a high Cr, high Ni stainless steel, which has a higher deformation resistance than SUS304 and SUS316L stainless steel, which are widely used in the past. For example, the bending radius is (1.5 to 2.0) D (where D is a tube). Even if high-frequency bending is performed at a bending radius with a small curvature such as the outer diameter of Of course, it can also be used as an element pipe for bending with a bending radius larger than 2.0D.
本発明のオーステナイト系ステンレス鋼管の曲げ加工方法(前記(2)に記載の曲げ加工方法)は、前記のように、Cr:22〜28%、Ni:18〜25%およびCa:0.0003〜0.01%を含有し、且つCa含有量と不純物中のSおよびO含有量とが下記(i)式を満足し、曲げ加工される部分に、予め、曲げ管の背側になる部分の肉厚/曲げ管の腹側になる部分の肉厚の比が1.2以上となる偏肉加工が施されているオーステナイト系ステンレス鋼管を用い、曲げ半径を(1.5〜2.0)Dとして高周波曲げ加工する方法である。 As described above, the bending method of the austenitic stainless steel pipe of the present invention (the bending method described in (2)) is Cr: 22 to 28%, Ni: 18 to 25%, and Ca: 0.0003 to 0.01% is contained, and the Ca content and the S and O contents in the impurities satisfy the following formula (i), and the portion to be bent is previously formed in the portion to be bent on the back side of the bending tube. Using an austenitic stainless steel pipe that has been subjected to uneven wall thickness ratio that the thickness ratio of the thickness / bent pipe side becomes 1.2 or more, the bending radius is (1.5 to 2.0) D is a method of high-frequency bending.
Ca−1.25×S−2.5×O≧−0.01 ・・・(i)
ただし、(i)式中の元素記号は、それぞれの元素の含有量(%)を表す。
Ca-1.25 × S-2.5 × O ≧ −0.01 (i)
However, the element symbol in (i) Formula represents content (%) of each element.
曲げ加工に供する素管として、前述の本発明の高周波曲げ加工用オーステナイト系ステンレス鋼管を用いるので、(1.5〜2.0)Dの小さい曲げ半径で高周波曲げ加工しても、微小割れが発生することはない。 Since the austenitic stainless steel pipe for high frequency bending according to the present invention described above is used as an element pipe to be bent, even if high frequency bending is performed with a small bending radius of (1.5 to 2.0) D, microcracks are not generated. It does not occur.
本発明のステンレス鋼管の曲げ加工方法においては、C:0.08%以下、Si:1.5%以下、Mn:0.1〜2.0%、Cr:22〜28%、Ni:18〜25%およびCa:0.0003〜0.01%を含有し、且つCa含有量と不純物中のSおよびO含有量とが前記の(i)式を満足し、残部がFeおよび不純物からなり、不純物中のPが0.045%以下、Sが0.030%以下およびOが0.010%以下の組成をもつステンレス鋼管を素管として用いる。
In the bending method of the stainless steel pipe of the present invention, C: 0.08% or less, Si: 1.5% or less, Mn: 0.1-2.0%, Cr: 22-28%, Ni: 18- 25% and Ca: 0.0003 to 0.01%, and the Ca content and the S and O contents in the impurities satisfy the above formula (i), and the balance is Fe and impurities. P 0.045% of the impurities less, S is Ru with a stainless steel tube 0.030% or less and the O has the following composition 0.010% as base pipe.
この素管は、前述のように、高周波曲げ加工用として望ましい本発明のオーステナイト系ステンレス鋼管である。したがって、この素管に曲げ加工を施して得られるステンレス鋼管は、曲率の小さい曲げ半径で高周波曲げ加工を行っても微小割れの発生がなく、熱間加工性さらには耐食性が良好に維持されているので、望ましい。 As described above, this blank is an austenitic stainless steel pipe of the present invention that is desirable for high-frequency bending. Therefore, the stainless steel pipe obtained by bending this element pipe does not generate microcracks even when subjected to high-frequency bending with a bending radius with a small curvature, and maintains good hot workability and corrosion resistance. Because it is desirable.
表2に示す化学組成を有する鋼を溶製し、熱間加工により曲げ加工素管となる種々の寸法の継目無管を製造した。材質AはSUS304、材質BはSUS316Lであり、材質CとDが高Cr、高Niのステンレス鋼で、そのうちの材質Dが、Caが規定量含まれ、且つ、有効Caが−0.01以上で前記の(i)式を満足する、本発明の高周波曲げ加工用オーステナイト系ステンレス鋼管が有している材質である。これらの素管の一部については、切削により曲げ加工部の曲げの背側が腹側よりも厚肉となるように偏肉加工を施した。 Steels having the chemical composition shown in Table 2 were melted, and seamless pipes having various dimensions to be bent into raw pipes were manufactured by hot working. The material A is SUS304, the material B is SUS316L, the materials C and D are high Cr, high Ni stainless steel, and the material D includes a specified amount of Ca, and the effective Ca is -0.01 or more. The material which the austenitic stainless steel pipe for high frequency bending according to the present invention satisfies the above-mentioned formula (i). Some of these blanks were subjected to uneven thickness machining so that the bending back side of the bent portion was thicker than the ventral side by cutting.
その後、これらの素管に、曲げ半径を1.5Dとして高周波曲げ加工を施し、微小割れの発生の有無を管断面の顕微鏡観察により確認した。管寸法、肉厚比(曲げ管の背側になる部分の肉厚/曲げ管の腹側になる部分の肉厚の比)、曲げ加工による背側の伸び率および割れの観察結果を表3に示す。 Thereafter, these elementary tubes were subjected to high-frequency bending with a bending radius of 1.5D, and the presence or absence of microcracks was confirmed by microscopic observation of the tube cross section. Table 3 shows the observation results of tube dimensions, wall thickness ratio (ratio of wall thickness on the back side of the bending tube / wall thickness on the ventral side of the bending tube), elongation on the back side due to bending, and cracking. Shown in
表3に示したように、従来のSUS304(材質A)やSUS316L(材質B)からなる曲げ管では、偏肉加工を施さない素管を高周波曲げ加工した場合、割れが発生した(A−1、B−1)が、肉厚比1.2以上の偏肉加工した素管を用いることにより、割れの発生を防止できた(A−2,B−2)。 As shown in Table 3, in a conventional bending pipe made of SUS304 (material A) or SUS316L (material B), cracking occurred when a raw pipe not subjected to uneven thickness machining was subjected to high-frequency bending (A-1). , B-1) was able to prevent cracks from occurring by using an unprocessed tube having a thickness ratio of 1.2 or more (A-2, B-2).
しかし、高Cr−高Niオーステナイト系ステンレス鋼では、偏肉加工を施した素管を用いて高周波曲げ加工を行った場合でも、割れが発生した(C−1)。 However, in the high Cr-high Ni austenitic stainless steel, cracking occurred even when high-frequency bending was performed using a raw pipe subjected to uneven thickness machining (C-1).
これに対し、本発明で規定する範囲のCaを含有させた高Cr−高Niオーステナイト系ステンレス鋼を素管として高周波曲げ加工を行った場合、偏肉加工を施さない素管では、管寸法に関係なく割れが発生したが(D−1、D−2)、肉厚比1.2以上の偏肉加工を施した素管を用いることにより、管寸法に関係なく、割れの発生を防止できた(D−3、D−4)。 On the other hand, when high-frequency bending is performed using a high Cr-high Ni austenitic stainless steel containing Ca in the range specified in the present invention as a base pipe, the pipe size is not used for the base pipe not subjected to uneven thickness processing. Regardless of cracks (D-1, D-2), the use of uncoated pipes with a wall thickness ratio of 1.2 or more can prevent cracks regardless of pipe dimensions. (D-3, D-4).
本発明の高周波曲げ加工用オーステナイト系ステンレス鋼管は、曲げ加工される部分に偏肉加工が施されると共に、熱間加工性を向上させるCaが所定量含まれているので、Cr、Niの含有量が従来多用されているSUS304、SUS316Lより高いにもかかわらず、微小割れを発生させることなく曲げ加工することができる。 Since the austenitic stainless steel pipe for high-frequency bending according to the present invention is subjected to uneven thickness processing at the portion to be bent and contains a predetermined amount of Ca to improve hot workability, the content of Cr and Ni Even though the amount is higher than that of SUS304 and SUS316L, which are widely used in the past, bending can be performed without generating microcracks.
本発明のオーステナイト系ステンレス鋼管の曲げ加工方法は、このオーステナイト系ステンレス鋼管を素管として用いて高周波曲げ加工する方法で、曲率半径が小さいステンレス鋼製曲げ管を製造することができる。この曲げ管は、耐食性に優れているので、原子力発電用の軽水炉等における高温純水環境下など、過酷な腐食環境下で用いられる曲げ管として好適である。 The bending method of the austenitic stainless steel pipe of the present invention is a method of high-frequency bending using this austenitic stainless steel pipe as a raw pipe, and a stainless steel bent pipe having a small curvature radius can be manufactured. Since this bent tube is excellent in corrosion resistance, it is suitable as a bent tube used in a severe corrosive environment such as a high-temperature pure water environment in a light water reactor for nuclear power generation.
したがって、本発明の高周波曲げ加工用オーステナイト系ステンレス鋼管、およびこれを素管として用いる本発明の曲げ加工方法は、曲げ管の製造に有効に利用することができる。 Therefore, the austenitic stainless steel pipe for high-frequency bending according to the present invention and the bending method according to the present invention using this as an element pipe can be effectively used for manufacturing a bent pipe.
Claims (2)
曲げ加工される部分に、予め、曲げ管の背側になる部分の肉厚/曲げ管の腹側になる部分の肉厚の比が1.2以上となるように偏肉加工が施されていることを特徴とする高周波曲げ加工用オーステナイト系ステンレス鋼管。
Ca−1.25×S−2.5×O≧−0.01 ・・・(i)
ただし、(i)式中の元素記号は、それぞれの元素の質量%での含有量を表す。 C: 0.08% or less, Si: 1.5% or less, Mn : 0.1-2.0%, Cr: 22-28%, Ni: 18-25% and Ca: 0.0003 -0.01% and the Ca content and the S and O contents in the impurity satisfy the following formula (i) , the balance is Fe and impurities, and the P in the impurity is 0.045% Hereinafter, an austenitic stainless steel pipe in which S is 0.030% or less and O is 0.010% or less ,
The portion to be bent is previously subjected to uneven thickness processing so that the ratio of the thickness of the portion on the back side of the bending tube / the thickness of the portion on the ventral side of the bending tube is 1.2 or more. An austenitic stainless steel pipe for high-frequency bending.
Ca-1.25 × S-2.5 × O ≧ −0.01 (i)
However, the element symbol in (i) Formula represents content in the mass% of each element.
曲げ半径を(1.5〜2.0)×(管の外径)として高周波曲げ加工することを特徴とするオーステナイト系ステンレス鋼管の曲げ加工方法。
Ca−1.25×S−2.5×O≧−0.01 ・・・(i)
ただし、(i)式中の元素記号は、それぞれの元素の質量%での含有量を表す。 C: 0.08% or less, Si: 1.5% or less, Mn : 0.1-2.0%, Cr: 22-28%, Ni: 18-25% and Ca: 0.0003 -0.01% and the Ca content and the S and O contents in the impurity satisfy the following formula (i), the balance is Fe and impurities, and the P in the impurity is 0.045% In the following, S is 0.030% or less and O is 0.010% or less, and the thickness of the portion that is the back side of the bending tube / the portion that is the ventral side of the bending tube Using an austenitic stainless steel pipe that has been subjected to uneven wall thickness ratio of 1.2 or more,
A method for bending an austenitic stainless steel pipe, characterized by performing high-frequency bending with a bending radius of (1.5 to 2.0) × (outer diameter of the pipe).
Ca-1.25 × S-2.5 × O ≧ −0.01 (i)
However, the element symbol in (i) Formula represents content in the mass% of each element.
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