JP2007119884A - Method for producing high strength and high toughness steel material excellent in strength at intermediate temperature zone - Google Patents
Method for producing high strength and high toughness steel material excellent in strength at intermediate temperature zone Download PDFInfo
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Abstract
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本発明は、中温域において降伏強さが５５０ＭＰａ以上の鋼材の製造方法に関し、特に蒸気配管用高強度高靭性溶接鋼管用途に好適なものに関する。 The present invention relates to a method for producing a steel material having a yield strength of 550 MPa or more in an intermediate temperature range, and particularly relates to a material suitable for high strength and high toughness welded steel pipe use for steam piping.
油層からオイルサンドを回収する方法として、露天堀による方法と高温・高圧の蒸気を鋼管により挿入するスチームインジェクション法があるが、露天掘りが適用可能な地域は少なく、多くの地域ではスチームインジェクション法が適用されている。 There are two methods for recovering oil sand from the oil reservoir: the open-pit method and the steam injection method in which high-temperature and high-pressure steam is inserted through steel pipes, but there are only a few areas where open-pit digging is applicable, and the steam injection method is applied in many regions. Has been.
スチームインジェクション法では、油層内へ３００〜３５０℃の温度域（以下、中温域という）の蒸気を、１３ＭＰａ前後の高圧で送り込むが、従来、この中温・高圧の蒸気に耐えうるスチームインジェクション用の蒸気輸送鋼管として、特許文献１、特許文献２、特許文献３に示されるＡＰＩ Ｘ６５、７０グレード相当の継目無管が使用され、鋼管外径は最大で１６インチであった。
近年のエネルギー需要の増加に伴い、重質油の回収率の向上ならびに敷設コストの低減を目的として、使用される鋼管の大径化ならびに高強度化が要望されている。 With the recent increase in energy demand, there is a demand for increasing the diameter and increasing the strength of steel pipes used for the purpose of improving the recovery rate of heavy oil and reducing laying costs.
そこで、本発明は、ＡＰＩグレードＸ８０以上の蒸気輸送用高強度高靭性溶接鋼管に要求される、中温域においても降伏強さ５５０ＭＰａ以上（ＡＰＩグレードＸ８０以上）の鋼板を安価に提供することを目的とする。 Accordingly, the object of the present invention is to provide a steel sheet having a yield strength of 550 MPa or more (API grade X80 or more) at a low temperature, which is required for a high-strength, high-toughness welded steel pipe for steam transportation of API grade X80 or more. And
本発明者等は高強度大径溶接管における中温域での特性について鋭意検討し、適切なＮｂ量を固溶したＮｂ系鋼に、制御圧延後の加速冷却とその後の再加熱という製造プロセスにおいて、ベイナイト変態途中に再加熱を行うと、加速冷却時のベイナイト変態による強化に加え、再加熱時にベイナイトならびに未変態オーステナイトから析出する微細析出物による析出強化ならびに中温域での転位回復の抑制によって、中温域での強度低下の抑制が可能になるという知見を得た。 The present inventors diligently examined the characteristics in the middle temperature range of the high-strength large-diameter welded pipe, and in the manufacturing process of accelerated cooling after controlled rolling and subsequent reheating to Nb steel in which an appropriate amount of Nb was dissolved. When reheating during bainite transformation, in addition to strengthening due to bainite transformation during accelerated cooling, precipitation strengthening due to fine precipitates precipitated from bainite and untransformed austenite during reheating, and suppression of dislocation recovery in the intermediate temperature range, The inventor obtained that it was possible to suppress the strength decrease in the middle temperature range.
すなわち、ＴｉＮが存在する場合、Ｎｂが固溶し難くなり、Ｔｉを添加しない場合に比べて加速冷却後の再加熱時の微細なＮｂ炭化物の分散析出が低下し、中温域での強度低下の抑制が困難となるが、式（１）で求められるＮｂｅｆｆ．値が０．０２５％以上の場合にはＴｉ添加の場合においても再加熱時の微細なＮｂ炭化物の分散析出が十分に得られ、中温域での強度低下の抑制が可能になる。
Ｎｂｅｆｆ．（％）＝０．００２×（１−２５Ｔｉ）／（Ｃ＋０．８６Ｎ）・・・（１）
Ｔｉ，Ｃ，Ｎ：質量％
また、Ｎｂは鋼中で炭化物を形成する元素であり、ＮｂＣの析出により鋼を強化することは従来行われているが、本発明では加速冷却後、再加熱する際の加熱速度を増加して加熱時の析出物の成長を抑制することにより、Ｎｂを基本として含有する炭化物を鋼中に多量に微細析出させ、中温域での強度低下抑制効果を得ている。
That is, when TiN is present, it becomes difficult for Nb to dissolve, and compared with the case where Ti is not added, the dispersion and precipitation of fine Nb carbides at the time of reheating after accelerated cooling is reduced, and the strength is lowered in the intermediate temperature range. Although it becomes difficult to suppress, Nb eff. When the value is 0.025% or more, even when Ti is added, fine dispersion of fine Nb carbide at the time of reheating is sufficiently obtained, and it is possible to suppress a decrease in strength in the middle temperature range.
Nb eff. (%) = 0.002 × (1-25Ti) / (C + 0.86N) (1)
Ti, C, N: mass%
In addition, Nb is an element that forms carbides in steel, and strengthening the steel by precipitation of NbC has been conventionally performed, but in the present invention, the heating rate at the time of reheating is increased after accelerated cooling. By suppressing the growth of precipitates during heating, a large amount of carbides containing Nb as a basis are finely precipitated in the steel, and the effect of suppressing the strength decrease in the intermediate temperature range is obtained.
そのため、本発明では、従来の工業的に用いられている大気炉での加熱速度よりも高速で加熱することにより、Ｎｂを基本とする炭化物の成長を抑制させ、粒径が１０nm未満の極めて微細な析出物を多量に得ている。当該１０ｎｍ未満の析出物の個数は、たとえば降伏強度が５５０ＭＰａ以上の高強度鋼板とするためには、２×１０３個/μｍ３以上析出させることが好ましい。 For this reason, in the present invention, the growth of carbides based on Nb is suppressed by heating at a higher speed than the heating rate in a conventional industrially used atmospheric furnace, and the particle size is extremely fine with a particle size of less than 10 nm. A large amount of precipitates is obtained. For example, in order to obtain a high-strength steel plate having a yield strength of 550 MPa or more, the number of precipitates of less than 10 nm is preferably deposited 2 × 10 3 pieces / μm 3 or more.
更に、加速冷却後の加熱による微細炭化物の分散析出に先立ち、粒内組織中に多量の転位を導入するため900℃以下での累積圧下率と圧延仕上温度を規定し、圧延ならびに加速冷却の両工程にて粒内の転位を増加させる。 Furthermore, prior to the dispersion precipitation of fine carbides by heating after accelerated cooling, in order to introduce a large amount of dislocations in the intragranular structure, the cumulative reduction ratio and rolling finish temperature below 900 ° C are specified, and both rolling and accelerated cooling are performed. Increase dislocation within the grain in the process.
上述したように、本発明は圧延と加速冷却による転位の増加と、加速冷却後の加熱により分散析出する微細炭化物による中温域での転位の回復抑制により、中温域での高強度を確保するもので、すなわち、本発明は、
１． 質量％で、Ｃ：０．０４〜０．０８％、Ｓｉ：０．０５〜０．２％、Ｍｎ：１．５〜２％、Ｐ：０．０２％以下、Ｓ：０．００２％以下、Ｍｏ：０．０５〜０．３％、Ｎｂ：０．０３〜０．０７％、Ｔｉ：０．０２％以下、Ａｌ：０．０４％以下、ＲＥＭ：０．０１５％以下、Ｎ：０．００６％以下を含有し、残部がＦｅ及び不可避的不純物からなり、（１）式で示されるＮｂｅｆｆ．：０．０２５％以上の鋼を１１００〜１２００℃に加熱後、９００℃以下での累積圧下率が５０％以上、かつ圧延終了温度が８５０℃以下で熱間圧延後、５℃／秒以上の冷却速度にて４００〜５５０℃まで加速冷却した後、直ちに０．５℃/s以上の昇温速度で５５０〜７００℃まで再加熱を行うことを特徴とする、中温域での強度に優れた高強度高靭性鋼板の製造方法。
Ｎｂｅｆｆ．（％）＝０．００２×（１−２５Ｔｉ）／（Ｃ＋０．８６Ｎ）・・・（１）
Ｔｉ，Ｃ，Ｎ：質量％
２． 質量％で、Ｃ：０．０４〜０．０８％、Ｓｉ：０．０５〜０．２％、Ｍｎ：１．５〜２％、Ｐ：０．０２％以下、Ｓ：０．００２％以下、Ｍｏ：０．０５〜０．３％、Ｎｂ：０．０３〜０．０７％、Ｔｉ：０．０２％以下、Ａｌ：０．０４％以下、ＲＥＭ：０．０１５％以下、Ｎ：０．００６％以下、更に、Ｃｕ：０．５以下、Ｎｉ：０．５％以下、Ｃｒ：０．５％以下、Ｖ：０．０８％以下、Ｃａ：０．０００５〜０．００４％のうち１種または２種以上を含有し、残部がＦｅ及び不可避的不純物からなり、（１）式で示されるＮｂｅｆｆ．：０．０２５％以上の鋼を１１００〜１２００℃に加熱後、９００℃以下での累積圧下率が５０％以上、かつ圧延終了温度が８５０℃以下で熱間圧延後、５℃／秒以上の冷却速度にて４００〜５５０℃まで加速冷却した後、直ちに０．５℃/s以上の昇温速度で５５０〜７００℃まで再加熱を行うことを特徴とする、中温域での強度に優れた高強度高靭性鋼板の製造方法。
Ｎｂｅｆｆ．（％）＝０．００２×（１−２５Ｔｉ）／（Ｃ＋０．８６Ｎ）・・・（１）
Ｔｉ，Ｃ，Ｎ：質量％
３． １または２に記載の方法で得られた鋼板を管状に冷間成形し、その突合せ部を溶接することを特徴とする、中温域での強度に優れた高強度高靭性鋼管の製造方法。
As described above, the present invention secures high strength in the intermediate temperature range by increasing dislocations due to rolling and accelerated cooling, and suppressing recovery of dislocations in the intermediate temperature range due to fine carbides that are dispersed and precipitated by heating after accelerated cooling. That is, the present invention
1. In mass%, C: 0.04 to 0.08%, Si: 0.05 to 0.2%, Mn: 1.5 to 2%, P: 0.02% or less, S: 0.002% or less , Mo: 0.05 to 0.3%, Nb: 0.03 to 0.07%, Ti: 0.02% or less, Al: 0.04% or less, REM: 0.015% or less, N: 0 .006% or less, and the balance is Fe and inevitable impurities, and Nb eff. : After heating 0.025% or more of steel to 1100 to 1200 ° C, the cumulative rolling reduction at 900 ° C or less is 50% or more, and after hot rolling at a rolling end temperature of 850 ° C or less, 5 ° C / second or more After accelerating and cooling to 400 to 550 ° C. at a cooling rate, immediately reheating to 550 to 700 ° C. at a temperature rising rate of 0.5 ° C./s or more, excellent in strength in the middle temperature range Manufacturing method of high strength and tough steel sheet.
Nb eff. (%) = 0.002 × (1-25Ti) / (C + 0.86N) (1)
Ti, C, N: mass%
2. In mass%, C: 0.04 to 0.08%, Si: 0.05 to 0.2%, Mn: 1.5 to 2%, P: 0.02% or less, S: 0.002% or less , Mo: 0.05 to 0.3%, Nb: 0.03 to 0.07%, Ti: 0.02% or less, Al: 0.04% or less, REM: 0.015% or less, N: 0 0.006% or less, Cu: 0.5 or less, Ni: 0.5% or less, Cr: 0.5% or less, V: 0.08% or less, Ca: 0.0005 to 0.004% 1 type or 2 types or more, and the balance consists of Fe and inevitable impurities, and Nb eff. : After heating 0.025% or more of steel to 1100 to 1200 ° C, the cumulative rolling reduction at 900 ° C or less is 50% or more, and after hot rolling at a rolling end temperature of 850 ° C or less, 5 ° C / second or more After accelerating and cooling to 400 to 550 ° C. at a cooling rate, immediately reheating to 550 to 700 ° C. at a temperature rising rate of 0.5 ° C./s or more, excellent in strength in the middle temperature range Manufacturing method of high strength and tough steel sheet.
Nb eff. (%) = 0.002 × (1-25Ti) / (C + 0.86N) (1)
Ti, C, N: mass%
3. A method for producing a high-strength, high-toughness steel pipe excellent in strength in an intermediate temperature range, wherein the steel plate obtained by the method according to 1 or 2 is cold-formed into a tubular shape and the butt portion is welded.
本発明によれば、操業の効率化が達成できる、中温域において降伏強さが５５０ＭＰａ以上を有する大径の蒸気輸送用高強度高靭性溶接鋼管の製造可能な原板が安定して得られ、産業上極めて有益である。 According to the present invention, it is possible to stably obtain an original plate capable of producing a high-strength, high-toughness welded steel pipe for steam transport having a large diameter having a yield strength of 550 MPa or more in an intermediate temperature range, which can achieve operational efficiency. Very useful.
本発明に係る鋼板は、厚板ミルや熱延ミルにて製造され、ＵＯＥ成形、プレスベンド成形、ロール成形などにより冷間成形され、サブマージドアーク溶接などの溶接法により溶接接合されて、高温・高圧の蒸気を輸送するための鋼管として利用される。 The steel plate according to the present invention is manufactured by a thick plate mill or a hot rolling mill, is cold formed by UOE forming, press bend forming, roll forming, etc., and is welded and joined by a welding method such as submerged arc welding.・ Used as a steel pipe for transporting high-pressure steam.
以下に、成分組成の限定理由を示す。％は質量％とする。 Below, the reason for limitation of a component composition is shown. % Means mass%.
Ｃ：０．０４〜０．０８％
Ｃは固溶強化ならびに析出強化により鋼の強度を確保するために必要な元素であり、特に固溶Ｃ量の増加と析出物の形成は中温域での強度確保に重要である。０．０８％を超える過剰なＣの添加は靭性ならびに溶接性の劣化を招くため添加量の上限を０．０８％とした。一方、０．０４％未満では室温ならびに中温域において所定の強度を確保することが難しくなるため、Ｃの含有量は０．０４〜０．０８％とした。
C: 0.04 to 0.08%
C is an element necessary for ensuring the strength of the steel by solid solution strengthening and precipitation strengthening. In particular, the increase in the amount of solid solution C and the formation of precipitates are important for securing the strength in the intermediate temperature range. Addition of excess C exceeding 0.08% causes deterioration of toughness and weldability, so the upper limit of the addition amount was set to 0.08%. On the other hand, if it is less than 0.04%, it becomes difficult to ensure a predetermined strength at room temperature and in the middle temperature range, so the C content is set to 0.04 to 0.08%.
Ｓｉ：０．０５〜０．２％
Ｓｉは脱酸のために添加され、０．０５％未満では充分な脱酸効果が得られない。一方、０．２％を越えると靱性の劣化を招くため、Ｓｉの含有量は０．０５〜０．２％とした。
Si: 0.05 to 0.2%
Si is added for deoxidation, and if it is less than 0.05%, a sufficient deoxidation effect cannot be obtained. On the other hand, if it exceeds 0.2%, the toughness is deteriorated, so the Si content is set to 0.05 to 0.2%.
Ｍｎ：１．５〜２％
Ｍｎは鋼の強度および靱性の向上に有効な元素で、１．５％未満ではその効果が小さく、また２％を越えると靭性ならびに溶接性が著しく劣化するため、Ｍｎの含有量は１．５〜２％とした。
Mn: 1.5-2%
Mn is an element effective for improving the strength and toughness of steel. If the content is less than 1.5%, the effect is small, and if it exceeds 2%, the toughness and weldability are significantly deteriorated. ˜2%.
Ｐ：０．０２％以下
Ｐは不純物元素であり靱性を著しく劣化させるため、極力低減することが望ましいが、過度のＰ低減は製造コストの上昇を招くため、Ｐの含有量を０．０２％以下とした。
P: 0.02% or less P is an impurity element and significantly deteriorates toughness, so it is desirable to reduce it as much as possible. However, excessive P reduction causes an increase in manufacturing cost, so the content of P is 0.02%. It was as follows.
Ｓ：０．００２％以下
Ｓは不純物元素であり靭性を著しく劣化させるため、極力低減することが望ましい。また、Ｃａを添加してＭｎＳからＣａＳ系の介在物に形態制御を行ったとしても、Ｘ８０グレードの高強度材の場合には微細に分散したＣａＳ系介在物も靱性劣化の要因となり得るため、Ｓの含有量を０．００２％以下とした。
S: 0.002% or less Since S is an impurity element and significantly deteriorates toughness, it is desirable to reduce it as much as possible. In addition, even if Ca is added to control the morphology from MnS to CaS inclusions, in the case of X80 grade high strength material, finely dispersed CaS inclusions can also cause toughness deterioration. The S content was 0.002% or less.
Ｍｏ：０．０５〜０．３％
Ｍｏは固溶あるいは析出物の形成により室温ならびに中温域での強度上昇に大きく寄与するが、０．０５％未満ではその効果が小さく室温ならびに中温域で十分な強度が得られない。一方、０．３％を超えて添加すると靭性ならびに溶接性を劣化させるため、Ｍｏの添加量を０．０５〜０．３％とした。
Mo: 0.05-0.3%
Mo greatly contributes to an increase in strength at room temperature and medium temperature by solid solution or formation of precipitates, but if it is less than 0.05%, the effect is small and sufficient strength cannot be obtained at room temperature and medium temperature. On the other hand, if adding over 0.3%, the toughness and weldability are deteriorated, so the addition amount of Mo was made 0.05 to 0.3%.
Ｎｂ：０．０３〜０．０７％
Ｎｂは本発明において重要な元素であり、炭化物を形成し室温ならびに中温域での強度確保に必要な成分である。また、スラブ加熱時と圧延時の結晶粒の成長を抑制することによりミクロ組織を微細化し、充分な強度と靱性を付与するためにも必要である。その効果は０．０３％以上で顕著であり、０．０７％を超えるとその効果がほぼ飽和して靭性を劣化させるため、Ｎｂの含有量を０．００５〜０．０７％とした。
Nb: 0.03 to 0.07%
Nb is an important element in the present invention, and is a component that forms carbides and is required for securing strength at room temperature and in the middle temperature range. It is also necessary to refine the microstructure by suppressing the growth of crystal grains during slab heating and rolling, and to impart sufficient strength and toughness. The effect is remarkable at 0.03% or more, and when it exceeds 0.07%, the effect is almost saturated and the toughness is deteriorated. Therefore, the Nb content is set to 0.005 to 0.07%.
Ｔｉ：０．０２％以下
ＴｉはＴｉＮを形成してスラブ加熱時や溶接熱影響部の粒成長を抑制し、ミクロ組織の微細化をもたらして靱性を改善する効果があるが、０．０２％を越えて添加すると靱性の劣化を引き起こすため、Ｔｉの含有量を０．０２％以下とした。
Ti: 0.02% or less Ti forms TiN and suppresses grain growth at the time of slab heating or welding heat affected zone, brings about refinement of the microstructure and improves toughness, but 0.02% If added over the range, the toughness is deteriorated, so the Ti content is set to 0.02% or less.
Ａｌ：０．０４％以下
Ａｌは脱酸剤として添加されるが、０．０４％を超えると鋼の清浄性が低下し靱性の劣化を引き起こすため、Ａｌの含有量を０．０４％以下とした。
Al: 0.04% or less Al is added as a deoxidizer, but if it exceeds 0.04%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is 0.04% or less. did.
ＲＥＭ：０．０１５％以下
ＲＥＭは酸硫化物の形成により組織粗大化を抑制し靭性の向上に寄与するが、０．０１５％を超えると靭性が劣化するため、ＲＥＭ含有量は０．０１５％以下に規定する。好ましくは０．００１〜０．００７％である。
REM: 0.015% or less REM contributes to the improvement of toughness by suppressing the coarsening of the structure due to the formation of oxysulfides. However, if it exceeds 0.015%, the toughness deteriorates, so the REM content is 0.015%. It is defined below. Preferably it is 0.001 to 0.007%.
Ｎ：０．００６％以下
ＮはＴｉと共にＴｉＮを形成し、１３５０℃以上に達する溶接熱影響部の高温域において微細分散することにより、溶接熱影響部の旧オーステナイト粒を細粒化し溶接熱影響部の靭性向上に大きく寄与する。０．００６％を超えて添加すると、析出物の粗大化ならびに固溶Ｎの増加による母材靭性の劣化と、鋼管での溶接金属の靭性劣化を招くため、Ｎ含有量は０．００６％以下とした。
N: 0.006% or less N forms TiN together with Ti and finely disperses in the high temperature region of the weld heat affected zone reaching 1350 ° C. or higher, thereby refining the prior austenite grains in the weld heat affected zone and affecting the heat of welding. This greatly contributes to improving the toughness of the part. Addition exceeding 0.006% leads to deterioration of base metal toughness due to coarsening of precipitates and increase in solute N, and toughness deterioration of weld metal in steel pipes, so the N content is 0.006% or less. It was.
Ｎｂｅｆｆ．（％）：０．０２５以上
Ｎｂｅｆｆ．（％）＝０．００２×（１−２５Ｔｉ）／（Ｃ＋０．８６Ｎ）・・・（１）
但し、Ｔｉ，Ｃ，Ｎ：質量％
本パラメータ式は上記成分範囲で構成される鋼を中温域で優れた強度を有する鋼とするための重要な因子で、Ｎｂｅｆｆ．（％）が０．０２５％未満の場合には冷却後の再加熱時に析出する微細分散炭化物が少なく、強度、特に中温域での強度を確保することが困難であるため、Ｎｂｅｆｆ．（％）は０．０２５％以上とした。
Nb eff. (%): 0.025 or more Nb eff. (%) = 0.002 × (1-25Ti) / (C + 0.86N) (1)
However, Ti, C, N: mass%
This parameter equation is an important factor for making the steel having the above-mentioned composition range into a steel having excellent strength in the middle temperature range, and Nb eff. When (%) is less than 0.025%, the amount of finely dispersed carbide that precipitates during reheating after cooling is small, and it is difficult to ensure strength, particularly strength in the middle temperature range, so Nb eff. (%) Was 0.025% or more.
本発明は以上の成分組成で優れた特性が得られるが、更に特性を向上させる場合、Ｃｕ，Ｎｉ,Ｃｒ，Ｖ，Ｃａの一種または二種以上を添加する。 In the present invention, excellent properties can be obtained with the above component composition. However, when further improving the properties, one or more of Cu, Ni, Cr, V, and Ca are added.
Ｃｕ：０．５％以下
Ｃｕは靭性の改善と強度の上昇に有効な元素の１つであるが、０．５％を超えるＣｕの含有は溶接性を阻害するため、Ｃｕを添加する場合は０．５％以下とした。
Cu: 0.5% or less Cu is one of the elements effective for improving toughness and increasing strength. However, when Cu is added in excess of 0.5%, weldability is impaired. 0.5% or less.
Ｎｉ：０．５％以下
Ｎｉは靭性の改善と強度の上昇に有効な元素の１つであるが、０．５％を超えると効果が飽和し製造コストの上昇を招くため、Ｎｉを添加する場合は０．５％以下とした。
Ni: 0.5% or less Ni is one of elements effective in improving toughness and increasing strength. However, if it exceeds 0.5%, the effect is saturated and the manufacturing cost is increased, so Ni is added. In the case, it was 0.5% or less.
Ｃｒ：０．５％以下
Ｃｒは強度の上昇に有効な元素の一つであるが、０．５％を超えて添加すると溶接性に悪影響を与えるため、Ｃｒを添加する場合は０．５％以下とした。
Cr: 0.5% or less Cr is one of the elements effective in increasing the strength, but if added over 0.5%, the weldability is adversely affected. Therefore, when Cr is added, 0.5% It was as follows.
Ｖ：０.０８％以下
ＶはＴｉと共に複合析出物を形成し、強度上昇に寄与する。しかし、０.０８％を超えると溶接熱影響部の靭性が劣化するため、Ｖ含有量は０.０８％以下に規定する。
V: 0.08% or less V forms a composite precipitate with Ti and contributes to an increase in strength. However, if it exceeds 0.08%, the toughness of the weld heat affected zone deteriorates, so the V content is specified to be 0.08% or less.
Ｃａ：０．０００５〜０．００４％
Ｃａは硫化物系介在物の形態を制御し靱性を改善するが、０．０００５％以上でその効果が現われ、０．００４％を超えると効果が飽和し、逆に清浄度を低下させて靱性を劣化させるため、Ｃａを添加する場合は０．０００５〜０．００４％とした。
Ca: 0.0005 to 0.004%
Ca improves the toughness by controlling the form of sulfide inclusions, but its effect appears at 0.0005% or more, and when it exceeds 0.004%, the effect is saturated, and conversely, the cleanliness is lowered and the toughness is reduced. When Ca is added, the content is made 0.0005 to 0.004%.
次に、製造方法の限定理由について説明する。
加熱温度：１１００〜１２００℃
熱間圧延に際し、オーステナイト化ならびに炭化物の固溶を十分に進行させ、室温ならびに中温域での十分な強度を得るため、鋼片の加熱温度を１１００℃以上とする。
Next, the reason for limiting the manufacturing method will be described.
Heating temperature: 1100-1200 ° C
At the time of hot rolling, the heating temperature of the steel slab is set to 1100 ° C. or more in order to sufficiently advance austenitization and solid solution of carbides and obtain sufficient strength at room temperature and in the middle temperature range.
一方、加熱温度が１２００℃を超えると、オーステナイト粒の成長が著しく、母材靱性が劣化するため、加熱温度は１１００〜１２００℃とした。 On the other hand, when the heating temperature exceeds 1200 ° C., the austenite grains grow remarkably and the base material toughness deteriorates, so the heating temperature was set to 1100 to 1200 ° C.
９００℃以下での累積圧下率≧５０％、かつ圧延終了温度：８５０℃以下
本プロセスは本発明の重要な製造条件である。９００℃以下での温度域において累積にて圧延を行い、仕上温度を８５０℃以下とすることにより、オーステナイト粒が伸展し板厚、板幅方向で細粒となると共に、圧延により導入される粒内の転位密度が増加する。
Cumulative rolling reduction at 900 ° C. or less ≧ 50% and rolling end temperature: 850 ° C. or less This process is an important production condition of the present invention. Rolling is performed cumulatively in a temperature range of 900 ° C. or less, and by setting the finishing temperature to 850 ° C. or less, austenite grains are expanded and become fine grains in the plate thickness and width directions, and grains introduced by rolling The dislocation density inside increases.
９００℃以下での累積圧下率が５０％以上で圧延終了温度を８５０℃以下とすることにより、この効果が顕著に発揮され、強度、特に中温域での強度が上昇し靱性が著しく向上する。 When the cumulative rolling reduction at 900 ° C. or less is 50% or more and the rolling end temperature is 850 ° C. or less, this effect is remarkably exhibited, and the strength, particularly the strength in the middle temperature range, is increased and the toughness is remarkably improved.
９００℃以下での累積圧下率が５０％未満あるいは圧延終了温度が８５０℃を超える場合には、オーステナイト粒の細粒化が十分でなく、粒内の転位の増加量が小さいため、中温域での強度ならびに靭性が劣化するため、９００℃以下での累積圧下率は５０％以上、かつ圧延終了温度は８５０℃以下とする。 When the cumulative rolling reduction at 900 ° C. or less is less than 50% or the rolling end temperature exceeds 850 ° C., the austenite grains are not sufficiently refined, and the increase in dislocations in the grains is small. Since the strength and toughness of the steel deteriorate, the cumulative rolling reduction at 900 ° C. or less is 50% or more and the rolling end temperature is 850 ° C. or less.
加速冷却の冷却速度：５℃/s以上
鋼板強度は加速冷却での冷却速度の増加に伴い上昇する傾向を示す。加速冷却時の冷却速度が５℃/s未満の場合、変態組織が高温で変態し、冷却中に転位の回復も進行するため、室温ならびに中温域にて十分な強度を得ることができないため、加速冷却時の冷却速度を５℃/s以上とする。
Accelerated cooling rate: 5 ° C./s or more Steel plate strength tends to increase with increasing cooling rate in accelerated cooling. When the cooling rate during accelerated cooling is less than 5 ° C / s, the transformation structure is transformed at a high temperature, and the recovery of dislocation proceeds during cooling, so that sufficient strength cannot be obtained at room temperature and in the middle temperature range. The cooling rate during accelerated cooling is set to 5 ° C / s or more.
加速冷却の冷却停止温度：４００〜５５０℃
鋼板強度は加速冷却の冷却停止温度が低下するに従い上昇する傾向を示すが、加速冷却の冷却停止温度が５５０℃を超える場合、炭化物の成長を促進させ固溶炭素量が低減するため、十分な強度、特に中温域での十分な強度が得られない。
Cooling stop temperature for accelerated cooling: 400-550 ° C
The strength of the steel sheet tends to increase as the cooling stop temperature for accelerated cooling decreases. However, when the cooling stop temperature for accelerated cooling exceeds 550 ° C, the growth of carbides is promoted and the amount of solute carbon is reduced. The strength, particularly sufficient strength in the middle temperature range cannot be obtained.
一方、冷却停止温度が４００℃未満の場合には、低温変態生成物の析出が顕著になり母材靭性が劣化すると共に、中温域での低温変態生成物の分解により中温域での強度が著しく低下するため、加速冷却の冷却停止温度は４００〜５５０℃とする。 On the other hand, when the cooling stop temperature is less than 400 ° C., the precipitation of the low temperature transformation product becomes remarkable and the base material toughness deteriorates, and the strength in the middle temperature region is remarkably increased due to the decomposition of the low temperature transformation product in the middle temperature region. In order to reduce, the cooling stop temperature of accelerated cooling shall be 400-550 degreeC.
加速冷却後の昇温速度：速度０．５℃/s以上、ならびに再加熱温度：５５０〜７００℃本プロセスは本発明において重要で、室温ならびに中温域での強化に寄与する微細析出物を再加熱時に析出させる。微細析出物を得るためには、加速冷却後直ちに５５０〜７００℃の温度域まで再加熱する必要がある。 Temperature increase rate after accelerated cooling: speed 0.5 ° C./s or more, and reheating temperature: 550 to 700 ° C. This process is important in the present invention, and fine precipitates that contribute to strengthening at room temperature and medium temperature range are regenerated. Precipitate on heating. In order to obtain fine precipitates, it is necessary to reheat to a temperature range of 550 to 700 ° C. immediately after accelerated cooling.
再加熱は、冷却後の温度より少なくとも５０℃以上昇温することが望ましい。昇温速度が０．５℃/s未満では、目的の再加熱温度に達するまでに長時間を要するため製造効率が悪化し、また析出物が成長するため、微細析出物の分散析出が得られず十分な強度を得ることができない。 In reheating, it is desirable to raise the temperature by at least 50 ° C. from the temperature after cooling. If the rate of temperature increase is less than 0.5 ° C / s, it takes a long time to reach the target reheating temperature, so that the production efficiency deteriorates, and the precipitate grows, so that a dispersion precipitate of fine precipitates is obtained. Therefore, sufficient strength cannot be obtained.
再加熱温度が５５０℃未満ではＭｏとＮｂの析出温度域から外れるため十分な析出強化が図れず、一方、７００℃を超えると析出物が粗大化し室温ならびに中温域で十分な強度が得られないため、再加熱の温度域を５５０〜７００℃に規定する。 If the reheating temperature is less than 550 ° C., it will be out of the precipitation temperature range of Mo and Nb, so sufficient precipitation strengthening cannot be achieved. On the other hand, if it exceeds 700 ° C., the precipitate becomes coarse and sufficient strength cannot be obtained at room temperature and medium temperature range. Therefore, the temperature range of reheating is specified to 550 to 700 ° C.
再加熱処理において、特に温度保持時間を設定する必要はない。また、再加熱後の冷却過程でもベイナイト変態と共に析出が進行するため、再加熱後の冷却速度は基本的には空冷とする。 In the reheating process, it is not necessary to set the temperature holding time. In addition, since precipitation proceeds with bainite transformation even in the cooling process after reheating, the cooling rate after reheating is basically air cooling.
尚、本発明で規定する加速冷却後の昇温速度：速度０．５℃/s以上は、板厚によっては大気炉で達成することが難しく、加熱装置として、鋼板の急速加熱が可能であるガス燃焼炉や誘導加熱装置を用いる事が好ましく、加速冷却を行うための冷却設備の下流側で搬送ライン上に設置するとより好ましい。 Note that the rate of temperature increase after accelerated cooling specified in the present invention: a rate of 0.5 ° C./s or more is difficult to achieve in an atmospheric furnace depending on the plate thickness, and as a heating device, rapid heating of the steel plate is possible. It is preferable to use a gas combustion furnace or an induction heating device, and it is more preferable that the gas combustion furnace or the induction heating device is installed on the transport line on the downstream side of the cooling equipment for performing accelerated cooling.
誘導加熱装置は均熱炉等に比べて温度制御が容易でありコストも比較的低く、冷却後の鋼板を迅速に加熱できるので特に好ましい。また複数の誘導加熱装置を直列に連続して配置することにより、ライン速度や鋼板の種類・寸法が異なる場合にも、通電する誘導加熱装置の数や供給電力を任意に設定するだけで、昇温速度、再加熱温度を自在に操作することが可能である。 The induction heating device is particularly preferable because temperature control is easier than in a soaking furnace, the cost is relatively low, and the cooled steel sheet can be heated quickly. In addition, by arranging a plurality of induction heating devices in series, even if the line speed and the type and size of the steel sheet are different, the number of induction heating devices to be energized and the supply power can be set by arbitrarily setting them. It is possible to freely control the temperature rate and the reheating temperature.
なお、鋼の製鋼方法については特に限定しないが、経済性の観点から、転炉法による製鋼プロセスと、連続鋳造プロセスによる鋼片の鋳造を行うことが望ましい。 In addition, although it does not specifically limit about the steel making method of steel, From a viewpoint of economical efficiency, it is desirable to cast the steel piece by the steelmaking process by a converter method, and the continuous casting process.
鋼管の成型方法は、冷間にて成形することが好ましく、ＵＯＥ成形、プレスベンド成形、ロール成形などにより成形し、サブマージドアーク溶接等により溶接接合して、溶接鋼管を製造する。鋼管製造後の熱処理は所望する特性に応じて実施すれば良く、特に規定しない。 The method of forming the steel pipe is preferably formed in the cold, and is formed by UOE forming, press bend forming, roll forming, or the like, and welded and joined by submerged arc welding or the like to produce a welded steel pipe. The heat treatment after the production of the steel pipe may be performed according to the desired characteristics and is not particularly defined.
尚、本発明に係る製造方法で得られる鋼の常温強度は５５０ＭＰａ以上、７００ＭＰａ以下である。 In addition, the normal temperature strength of steel obtained by the manufacturing method according to the present invention is 550 MPa or more and 700 MPa or less.
表１に示す化学成分を有する鋼Ａ〜Ｏを用いて、種々の製造条件にて作製した鋼板（板厚１５〜２５ｍｍ）を冷間成形後、シーム溶接により、外径６１０ｍｍ×管厚１５〜２５ｍｍの鋼管を作製した。 Steel plates (plate thickness 15-25 mm) produced under various production conditions using steels A to O having chemical components shown in Table 1 were cold-formed and then seam welded to obtain an outer diameter of 610 mm × tube thickness of 15 to A 25 mm steel pipe was produced.
鋼板特性について、室温での引張試験はＩＳＯ ６８９２に準拠し、全厚のＡＰＩ矩形試験片を用いて実施した。３５０℃での引張試験はＩＳＯ ７８３に準拠し、直径８．７５ｍｍの丸棒試験片を用いて実施した。 Regarding the steel sheet characteristics, the tensile test at room temperature was performed in accordance with ISO 6892, using API rectangular test pieces of full thickness. The tensile test at 350 ° C. was performed according to ISO 783, using a round bar test piece having a diameter of 8.75 mm.
鋼管の強度は、円周方向に引張試験片を採取し、室温ならびに３５０℃での降伏強度を求めた。室温での引張試験はＩＳＯ ６８９２に準拠し、全厚のＡＰＩ矩形試験片を用いて実施した。３５０℃での引張試験はＩＳＯ ７８３に準拠し、直径８．７５ｍｍの丸棒試験片を用いて実施した。 For the strength of the steel pipe, tensile test specimens were collected in the circumferential direction, and the yield strength at room temperature and 350 ° C. was obtained. The tensile test at room temperature was carried out in accordance with ISO 6892, using a full thickness API rectangular test piece. The tensile test at 350 ° C. was performed according to ISO 783, using a round bar test piece having a diameter of 8.75 mm.
鋼管の溶接熱影響部シャルピー試験は、入熱約５０ｋＪ／ｃｍのＳＡＷにてシーム溶接を行った鋼管を用いて、管厚中央部を中心にノッチ全体が溶接熱影響部となるように溶接ボンド部近接側から円周方向に２ｍｍＶノッチのフルサイズ試験片を３本採取し、実施した。試験方法はＩＳＯ １４８に準拠し、試験温度は−２０℃ならびに３５０℃にて行った。なお、−２０℃での試験にはアルコールで、３５０℃での試験では加熱炉を用いて所定の温度に保持した。靭性値は所定の温度でのシャルピー吸収エネルギーの３本の平均値（単位Ｊ）で評価した。表２に鋼板の製造条件ならびに鋼板、鋼管の試験結果を併せて示す。 The welded heat-affected zone Charpy test for steel pipes uses a steel pipe that has been seam welded at a SAW with a heat input of about 50 kJ / cm. Three full-size test pieces of 2 mmV notch were collected in the circumferential direction from the side close to the head and carried out. The test method was based on ISO 148, and the test temperature was -20 ° C and 350 ° C. The test at −20 ° C. was alcohol, and the test at 350 ° C. was maintained at a predetermined temperature using a heating furnace. The toughness value was evaluated by the average value (unit J) of three Charpy absorbed energy at a predetermined temperature. Table 2 also shows the manufacturing conditions of the steel sheet and the test results of the steel sheet and the steel pipe.
室温ならびに３５０℃での降伏強度（単位ＭＰａ）が５５０ＭＰａ以上で、‐２０℃ならびに３５０℃でのシャルピー吸収エネルギーが１００J以上の場合を良好とし、本発明例とした。 The case where the yield strength (unit MPa) at room temperature and 350 ° C. was 550 MPa or more and the Charpy absorbed energy at −20 ° C. and 350 ° C. was 100 J or more was determined to be good, and the example of the present invention was obtained.
化学成分、鋼板製造条件とも本発明範囲内である本発明鋼（１〜１２）は鋼板、鋼管の室温ならびに３５０℃での降伏強度（単位ＭＰａ）が５５０ＭＰａ以上を有し、かつ良好な溶接熱影響部靱性が得られている。 The steels of the present invention (1-12) that are within the scope of the present invention in terms of chemical composition and steel plate production conditions have a yield strength (unit MPa) of 550 MPa or more at room temperature and 350 ° C. of the steel plate and steel pipe, and good welding heat. Affected zone toughness is obtained.
一方、化学成分あるいは鋼板製造条件が本発明範囲外である比較鋼（１３〜１７、１９〜２５）は、室温あるいは３５０℃での強度および／または溶接熱影響部靱性が本発明鋼に対して劣っていた。 On the other hand, the comparative steels (13-17, 19-25) whose chemical composition or steel plate production conditions are outside the scope of the present invention have strength and / or weld heat affected zone toughness at room temperature or 350 ° C. It was inferior.
また、製管後、熱処理（Ｑ−Ｔ処理）を施した比較鋼１８は、３５０℃での強度が本発明鋼に対して劣っていた。 In addition, the comparative steel 18 subjected to heat treatment (QT treatment) after pipe making was inferior in strength at 350 ° C. to the steel of the present invention.
Claims (3)
Ｎｂｅｆｆ．（％）＝０．００２×（１−２５Ｔｉ）／（Ｃ＋０．８６Ｎ）・・・（１）
Ｔｉ，Ｃ，Ｎ：質量％ In mass%, C: 0.04 to 0.08%, Si: 0.05 to 0.2%, Mn: 1.5 to 2%, P: 0.02% or less, S: 0.002% or less , Mo: 0.05 to 0.3%, Nb: 0.03 to 0.07%, Ti: 0.02% or less, Al: 0.04% or less, REM: 0.015% or less, N: 0 .006% or less, and the balance is Fe and inevitable impurities, and Nb eff. : After heating 0.025% or more of steel to 1100 to 1200 ° C, the cumulative rolling reduction at 900 ° C or less is 50% or more, and after hot rolling at a rolling end temperature of 850 ° C or less, 5 ° C / second or more After accelerating and cooling to 400 to 550 ° C. at a cooling rate, immediately reheating to 550 to 700 ° C. at a temperature rising rate of 0.5 ° C./s or more, excellent in strength in the middle temperature range Manufacturing method of high strength and tough steel sheet.
Nb eff. (%) = 0.002 × (1-25Ti) / (C + 0.86N) (1)
Ti, C, N: mass%
Ｎｂｅｆｆ．（％）＝０．００２×（１−２５Ｔｉ）／（Ｃ＋０．８６Ｎ）・・・（１）
Ｔｉ，Ｃ，Ｎ：質量％ In mass%, C: 0.04 to 0.08%, Si: 0.05 to 0.2%, Mn: 1.5 to 2%, P: 0.02% or less, S: 0.002% or less , Mo: 0.05 to 0.3%, Nb: 0.03 to 0.07%, Ti: 0.02% or less, Al: 0.04% or less, REM: 0.015% or less, N: 0 0.006% or less, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less, V: 0.08% or less, Ca: 0.0005 to 0.004% Nb eff. Containing one or more of them, the balance being Fe and inevitable impurities, and represented by the formula (1) . : After heating 0.025% or more of steel to 1100 to 1200 ° C, the cumulative rolling reduction at 900 ° C or less is 50% or more, and after hot rolling at a rolling end temperature of 850 ° C or less, 5 ° C / second or more After accelerating and cooling to 400 to 550 ° C. at a cooling rate, immediately reheating to 550 to 700 ° C. at a temperature rising rate of 0.5 ° C./s or more, excellent in strength in the middle temperature range Manufacturing method of high strength and tough steel sheet.
Nb eff. (%) = 0.002 × (1-25Ti) / (C + 0.86N) (1)
Ti, C, N: mass%
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