JP3817887B2 - High toughness high strength steel and method for producing the same - Google Patents

Description

【００７４】
【表２】

【００７５】
【表３】

【００７６】
表４は、加工熱処理（熱間圧延、冷却および焼戻し）の条件を示す一覧表である。前記したように、上記の鋼の未再結晶温度域は９５０℃以下である。また、Ａr_３点は、５００〜６００℃の範囲にある。
【００７７】
【表４】

【００７８】
また、表５はこれらの条件によって製造された厚鋼板の板厚中心部の金属組織をしめす。
【００７９】
【表５】

【００８０】
これらの鋼板の板厚中心部より試験片を採取し、つぎの試験に供した。
【００８１】
母材強度については、引張試験（試験片：JIS Z 2204 4号、試験法：JIS Z 2241）によりＹＳとＴＳを求めた。母材の靭性は、２ｍｍＶノッチシャルピー衝撃試験（試験片：JIS Z 2202 4号、試験法：JIS Z 2242）およびＤＷＴＴ試験により行った。
【００８２】
ＤＷＴＴ試験はラインパイプの業界ではきわめて一般的に知られているアレスト性評価試験である。ナイフエッジで原厚の試験片にプレスノッチを入れ、落錘または大型ハンマにより衝撃加重を加え、ノッチから脆性亀裂を発生させ破断後の破面を観察し、延性破面から脆性破面に破壊様式が遷移する温度のみで評価する。有効な試験においては、プレスノッチ底から脆性破面が発生するが、その後、延性破面（延性亀裂の伝播には多大のエネルギを要する）に変化し、延性破面が破面全体の８５％以上の比率の場合（８５％ＦＡＴＴ）に、アレスト性がその試験温度においてあると判断する。ノッチ底から脆性亀裂が発生していない場合は有効な試験とされない。このような場合にはノッチ底に浸炭処理等を施し、さらに脆化させノッチ底から脆性亀裂が発生するようにする。本実施例においては、いずれの鋼の場合もプレスノッチしたままのノッチ底から脆性破面が発生していた。
【００８３】
一方、２ｍｍＶノッチシャルピー衝撃試験は、発生特性を主に評価するがアレスト性も一部に含んだ靭性評価試験といわれている。母材の２ｍｍＶノッチシャルピー衝撃試験では、試験温度−４０℃における吸収エネルギーを求めた。
【００８４】
溶接継手部の靭性試験は、溶接熱サイクル再現試験機により最高加熱温度１３５０℃、入熱４万Ｊ／ｃｍ相当の冷却速度で８００〜５００℃を冷却した試験片から２ｍｍＶノッチシャルピー試験片を採取して試験に供し、−２０℃の吸収エネルギーによって評価した。上記のとおり、この評価は主に発生特性の評価である。
【００８５】
現地溶接性はｙ開先溶接割れ試験（JIS Z 3158）により評価した。これら溶接割れは化学組成によってほとんど決まり母材の金属組織は影響しないので、加熱温度１１５０℃、仕上げ温度９００℃にて板厚２５ｍｍの厚鋼板を表１〜２の鋼について製造した。これら厚鋼板から原厚のｙ開先溶接割れ試験片を採取して試験に供した。溶接材料は市販の１００キロハイテン用手溶接棒とし、１．５ｃｃ／１００ｇ程度の水素となるよう、温度２０℃、湿度７５％の大気中に２時間放置した後、入熱１．７ｋＪ／ｍｍの条件でビードを置き、室温に冷却した後、JIS Z 3158にしたがって割れの有無を調査した。
【００８６】
表６は、これらの試験結果を示す一覧表である。
【００８７】
【表６】

【００８８】
比較例である試験番号Ｘ１〜Ｘ１０は、それぞれ鋼の合金元素、Ｃ過剰（Ｘ１）、Ｓｉ過剰（Ｘ２）、Ｍｎ過剰（Ｘ３）、Ｃｕ過剰（Ｘ４）、Ｎｉ過少（Ｘ５）、Ｃｒ過剰（Ｘ６）、Ｍｏ過剰（Ｘ７）、Ｖ過剰（Ｘ８）、Ｔｉ過剰（Ｘ９）、Ａｌ過剰（Ｘ１０）であるため、Ｘ１０を除いて母材の靭性、とくにアレスト性が目標に届かなかった。Ｘ１０は、靭性は確かに目標値に達しているが、強度が９００ＭＰａを満足しなかった。
【００８９】
比較例のＸ１１およびＸ１２は、それぞれＣeqが過大および過小であるが、Ｘ１１は靭性が低くかつ溶接割れを生じ、Ｘ１２は焼入性の不足に起因して強度、靭性ともに低かった。
【００９０】
鋼の化学組成は本発明の範囲に入るが、熱間圧延または冷却条件が適切な常法の範囲を外れ、金属組織(c)の範囲を大きく外れる比較例Ｙ１、Ｙ２、Ｙ６およびＹ１０の母材靭性はきわめて不満足な結果となった。
【００９１】
これに対して、本発明例ではＴＳ９００ＭＰａ以上の強度と−４０℃でのシャルピー衝撃試験において２００Ｊ以上の吸収エネルギーが得られた。最大の関心であった、ＤＷＴＴにおいては８５％ＦＡＴＴが−４０℃以下となり、きわめて満足すべきアレスト性を示した。さらに、溶接部性能および現地溶接性も良好であった。
【発明の効果】
【００９２】
本発明によれば、引張り強さ９００ＭＰａ以上を具備し、靭性、とくにアレスト性の良好な高張力鋼を得ることができる。その結果、十分な安全性を確保したうえでパイプライン敷設時の施工能率および操業時の輸送効率を飛躍的に改善することが可能となった。BACKGROUND OF THE INVENTION
[0001]
  The present invention is a high-strength steel used for line pipes for transportation of natural gas and crude oil, various pressure vessels, etc., and particularly has a tensile strength (TS) of 900 MPa which is excellent in brittle crack propagation stop characteristics and weld joint characteristics. It relates to the above high-tensile steel.
[Prior art]
[0002]
  In pipelines that transport natural gas, crude oil, etc. over long distances, improvements in transportation efficiency due to increased operating pressure have been continually pursued. In order to withstand the increase in operating pressure, it is conceivable to increase the wall thickness of the conventional strength grade steel for pipes, but it reduces the welding efficiency at the site and lowers the laying efficiency by increasing the weight of the structure. There are problems that arise. On the other hand, the request | requirement which restrict | limits the increase in thickness by making steel materials for pipes high is growing. For this reason, for example, in recent years, X80 grade steel has been standardized and put into practical use at the American Petroleum Institute (API). Here, the symbol “X80” indicates that the yield strength (YS) is 80 ksi (about 551 MPa) or more.
[0003]
  Furthermore, it has been proposed that X100 or X120 grade high-strength steel can be produced based on this X80 grade steel production technology. Specifically, X100 to X120 grade steel using precipitation strengthening of Cu and a method for manufacturing the same (JP-A-8-104922, JP-A-8-209287, JP-A-8-209288), and Proposals have been made on steels with an increased Mn content and methods for producing the same (JP-A-8-209290, JP-A-8-209291).
[0004]
  The former steel material using the precipitation strengthening of Cu is surely provided with excellent on-site weldability and high strength of the base material due to reduced hardness of the welded portion. However, due to Cu precipitates dispersed in the matrix, it is insufficient to provide a brittle crack propagation stopping property (hereinafter referred to as “arrestability”) to a sufficient extent. This arrestability is a characteristic required for steel materials from the viewpoint of preventing a major accident that a welded steel structure collapses instantaneously due to brittle fracture.
[0005]
  Generally, in a welded steel structure, it is expected from the beginning that a certain amount of defects exist in the welded joint portion. Even if a brittle crack occurs due to a defect in the welded joint, a major accident will not occur if the base metal can stop the propagation of the brittle crack. For this reason, in an important welded steel structure, brittle fracture occurrence characteristics (hereinafter referred to as “generation characteristics”) are required for the welded joint, and arrestability is required for the base material. Of course, the occurrence characteristics of the base material may be a problem. The two characteristics of generation characteristics and arrestability are not completely independent and irrelevant characteristics. For example, when the precipitates are cohered and hardened, both properties deteriorate. However, although another factor, for example, refinement of the metal structure has a great effect of improving the generation characteristics, there is a difference that the degree of improvement in arrestability is small (not zero). When discussing these two properties, it should be noted that depending on the impact test method, there are ways to produce results that include both properties. The Charpy impact test produces results that include both properties, but is said to include more of the nature of the generated properties. In order to obtain only arrestability results, comparatively large test pieces in which a brittle crack generation part and a brittle crack stop part are separated, such as DWTT described in Examples described later, or a double tensile test, etc. Must be used. Historically, these two properties were called “toughness” because of the properties obtained by the Charpy test etc. without distinguishing between these two properties. Even now, it is usually called toughness including arrestability and developmental characteristics. In this specification, unless otherwise specified, toughness refers to both properties.
[0006]
  In the high Mn steel disclosed in JP-A-8-209290, etc., hardenability can be secured by containing a large amount of low-cost Mn, so that there is an advantage that expensive alloy elements such as Ni and Mo can be reduced. . However, when Mn is raised and Ni is lowered, the generation characteristics of the weld joint and the arrestability of the base material may not be ensured. Steel materials with low base metal arrestability cannot be used for important welded structures, and their applications are limited.
[0007]
  The term “welded joint property” includes toughness of the welded joint, particularly “generation characteristics” and “strength”. The term “welded joint” refers to both a weld heat affected zone (Heat Affected Zone: so-called “bond”; hereinafter referred to as “HAZ”) and weld metal. Unless otherwise indicated, only HAZ.
[0008]
  In the above-mentioned line pipe, high pressure operation is scheduled in the near future, and an X120 grade steel material having arrestability and the like is demanded. In the case of X120 grade steel, YS needs to be 850 MPa or more, but TS becomes 900 MPa or more at this time. Such a high-strength grade steel with sufficient arrestability has never been put to practical use.
[Problems to be solved by the invention]
[0009]
  An object of the present invention is to provide a high-strength steel of TS900 MPa or more that is excellent in arrestability and generation characteristics of a welded joint, and a method for producing the same. Specific target performance is shown below. The description of the DWTT (Drop Weight Tear Test) for evaluating the content of each test item and the nature of the test, particularly the arrestability, will be given in the examples.
[0010]
  1. Base material performance
  TS: 900 MPa or more (the upper limit is not particularly limited, but the upper limit is approximately 1050 MPa)
  Arrestability: 85% FATT (Fibrous Appearance Transition Temperature) is -30 ℃ or less in DWTT test
  Generation characteristics: 2EV notch Charpy impact test vE-40 ≧ 150J
  2. Welding performance
  Welded joint TS: 900MPa or more
  Occurrence characteristics: HAZ 2mmV notch Charpy impact test vE-20 ≧ 150J
  On-site weldability: Crack prevention temperature above room temperature in y-groove weld cracking test
[Means for Solving the Problems]
[0011]
  In order to obtain TS900 MPa or higher high-tensile steel having a weld joint part property of TS900 MPa or higher, excellent arrestability, and relatively high heat input (3 to 10 kJ / mm), various compositions, As a result of intensive studies on steel having a structure, the following matters were confirmed.
[0012]
  a) By setting Ni to a content exceeding 1.2% by weight, excellent arrestability and HAZ toughness can be ensured while being a high-strength steel of TS900 MPa or more.
[0013]
  b) As a chemical composition, the following restrictions are necessary.
[0014]
  In order to target a relatively thin steel material, an upper limit of the carbon equivalent is set depending on the presence or absence of B in order to avoid excessive firing, that is, an excessive volume ratio of martensite, for example, 100% martensite. Further, from the viewpoint of securing the strength, a lower limit of carbon equivalent is set according to the presence or absence of B.
[0015]
  c) In order to further improve the arrestability of the base material, it is desirable to have a mixed structure of a constant ratio of lower bainite and martensite. And in order to make this mixed structure fine, the dislocation density accumulated by processing is increased so that the nucleation density of the lower bainite is increased. Therefore, the aspect ratio of the prior austenite (hereinafter “austenite” may be referred to as “γ”) grains having a good correspondence with the dislocation density is set to 3 or more.
[0016]
  d) In order to reduce the center segregation and remarkably improve the toughness of the center, the following (2)Vs represented by the formula is set to 0.10 to 0.42%.
[0017]
  The gist of the present invention is a high-strength steel described below and a method for manufacturing the high-strength steel, which have been completed through a combination of the above-described matters and undergoing tests on the manufacturing site.
[0018]
  (1)mass%C: 0.02 to 0.1%, Si: 0.6% or less, Mn: 0.2 to 2.5%, Ni: more than 1.2%, 2.5% or less, Nb: 0 0.01 to 0.1%, Ti: 0.005 to 0.03%, N: 0.001 to 0.006%, Al: 0.1% or less, Cu: 0 to 0.6%, Cr: 0 -0.8%, Mo: 0-0.6%, V: 0-0.1%, Ca: 0-0.006%, balance Fe and inevitable impurities, the following conditions (a) or (b ) Among the inevitable impurities, P is 0.015% or less, and S is 0.003% or less.
[0019]
  (a) Furthermore, B is in the range of 0 to 0.0004% and(1) formulaHas a carbon equivalent Ceq of 0.53 to 0.7%.
[0020]
  (b) Furthermore, B: more than 0.0004%, 0.0025% or less,(1) formulaHas a carbon equivalent Ceq of 0.4 to 0.58%.
[0021]
      (1): Ceq = C + (Mn / 6) + {(Cu + Ni) / 15} + {(Cr + Mo + V) / 5}
  Here, the element symbol represents the content (% by weight) in the steel.
[0022]
  (2) The high-tensile steel according to (1), wherein the metal structure satisfies the following (c).
[0023]
  (c) The mixed structure of martensite and lower bainite is 90% by volume or more in the metal structure, of which the lower bainite is 2% by volume or more and the aspect ratio of the prior γ grains is 3 or more. is there.
[0024]
  (3) The following(2)The high-tensile steel according to (1), wherein Vs represented by the formula is in the range of 0.10 to 0.42%.
[0025]
      (2): Vs = C + (Mn / 5) + 5P- (Ni / 10)-(Mo / 15) + (Cu / 10)
  Here, the element symbol represents the content (% by weight) in the steel.
[0026]
  (4)above (2)The high-tensile steel according to (2), wherein Vs represented by the formula is in the range of 0.10 to 0.42%.
[0027]
  (5) The steel slab is heated to 1000 to 1250 ° C. and rolled, and the cumulative reduction rate in the non-recrystallization temperature range of γ is 50% or more, Ar₃Rolling ends at the point or higher, Ar₃The method for producing high-tensile steel according to (2) or (4), wherein cooling is performed to 500 ° C. or less at a cooling rate of 10 to 45 ° C./s at the center of the wall thickness from the point.
[0028]
  (6) Furthermore, Ac₁The manufacturing method of said (5) which adds tempering below a point.
[0029]
  The high-tensile steel mentioned above mainly refers to a thick steel plate, but is not limited to a thick steel plate, and may be a hot-rolled steel plate or a bar steel. The above-mentioned high-strength steels are not only applicable to those containing alloy elements in the above-mentioned range, but also to steels containing trace amounts of well-known elements that do not significantly change the performance of the steel.
[0030]
  The metal structure (c) must be satisfied in the average value at the respective positions of the thick surface layer part, 1 / 4t part and 1 / 2t part.
[0031]
  The remaining phases other than the mixed structure of martensite and lower bainite are residual γ, upper bainite, and other phases. When the residual γ is included in the metal structure, the X-ray diffraction profile is analyzed and quantified, and the ratio is usually negligible.
[0032]
  In order to obtain the volume ratio of the mixed structure of martensite and lower bainite, thin film transmission observation or extraction replica observation is performed using an electron microscope. In particular, the extracted replica is useful because the difference in the precipitation form of carbide (cementite) in martensite or lower bainite can be clearly identified and observed over a relatively wide area without being limited to the local area.
[0033]
  In order to obtain the average value of the ratio of the mixed structure of martensite and lower bainite in the total metal structure using this extracted replica, it is desirable to take an average of about 10 to 30 fields of view about 2000 times. Although thin film transmission observation allows precise measurement, the magnification must be increased. For this reason, since the area | region of 1 visual field is narrow, it is desirable to set the number of visual fields which take an average value to 50-100.
[0034]
  The old γ grain boundary refers to the grain boundary of unrecrystallized γ grains immediately before transformation into the above mixed structure. When the mixed structure forms as the main phase (unless pro-eutectoid ferrite is formed), Are also clearly identified. Similarly, the aspect ratio of the old γ grain boundary is also an average value. The aspect ratio refers to (length of old γ grains in rolling direction (major axis)) / (length of old γ grains in thickness direction (minor axis)).
[0035]
  The “non-recrystallization temperature range” in the above (5) refers to a temperature range in which recrystallization of crystals deformed by rolling does not occur clearly. In the case of the Nb-containing steel according to the present invention having TS900 MPa or more, it indicates a temperature range of 950 ° C. or less. Therefore, the “cumulative rolling reduction in the non-recrystallization temperature range” refers to (plate thickness at 950 ° C.−finished plate thickness) / (plate thickness at 950 ° C.).
DETAILED DESCRIPTION OF THE INVENTION
[0036]
  Next, the reason why the present invention is limited as described above will be described in detail. In the following description, the high-tensile steel is assumed to be a thick steel plate or a hot-rolled steel plate.
[0037]
  1. Alloy elements
  “%” Indicating the alloy element content is"mass%"Is displayed.
[0038]
  C: 0.02-0.1%
  C is an element effective for increasing the strength, and is 0.02% or more in order to obtain a strength of TS900 MPa or more in the steel in the range of the present invention. On the other hand, if it exceeds 0.1%, not only the arrestability and generation characteristics of the base metal are deteriorated, but also the weldability at the site is remarkably deteriorated, so the upper limit is made 0.1%. To further improve both strength and arrestability, 0.04 to 0.085% is desirable.
[0039]
  Si: 0.6% or less
  Si has a high deoxidizing effect. When Si is set to 0, the loss of Al increases during deoxidation. Therefore, it is usually desirable that the lower limit is, for example, about 0.01% to the extent that deoxidation remains. However, if it exceeds 0.6%, not only the toughness of the HAZ is lowered but also the workability is deteriorated, so the upper limit is made 0.6%. In order to further improve the toughness of the HAZ, the content is preferably set to 0.3% or less. If TS can be sufficiently secured by other elements, it is desirable that the TS be 0.1% or less.
[0040]
  Mn:
  Since Mn is effective for increasing the strength, it is set to 0.2% or more in order to ensure the strength. On the other hand, if it exceeds 2.5%, the arrestability of the base metal and the generation characteristics of HAZ deteriorate, so in the case of high strength steel of TS900 MPa or more, Mn is limited to 2.5% or less. Further, excessive Mn promotes center segregation during casting, and particularly in the case of high-strength steel targeted by the present invention, it causes weld cracks and hydrogen defects, so Mn exceeding 2.5% Must be avoided.
[0041]
  Further, when Mn is limited to less than 1.7%, the center segregation is remarkably reduced. Therefore, when used in an environment where there is a strong risk of hydrogen-induced cracking selectively occurring at the center segregation portion, 1. It is desirable to make it less than 7%. In the case of steel used for a line pipe, it is rather normal to make Mn less than 1.7%. When used for other structures, it is more advantageous in terms of economy to make Mn 1.7 to 2.5%.
[0042]
  Ni: more than 1.2%, 2.5% or less
  Ni is effective for increasing strength, and at the same time, is necessary for improving toughness, particularly arrestability. Further, it exerts a particularly remarkable effect in controlling the precipitation form of carbides of HAZ and improving the toughness of HAZ. For this reason, Ni is made a range exceeding 1.2%. On the other hand, if it exceeds 2.5%, the sheet thickness used for the line pipe is excessively baked and lower bainite is not generated, and the effect of dividing the metal structure by the lower bainite cannot be obtained. For this reason, the base metal toughness is not improved, so 2.5% or less.
[0043]
  Nb: 0.01 to 0.1%
  Nb is effective for refining γ grains in the thermomechanical treatment and is set to 0.01% or more. On the other hand, if it exceeds 0.1%, not only the toughness of the HAZ is deteriorated but also the weldability at the site is remarkably deteriorated, so the upper limit is made 0.1%. In order to improve both the refinement of the metal structure of the base metal and the HAZ toughness, it is desirable that the content be 0.02 to 0.05%.
[0044]
  Ti: 0.005 to 0.03%
  Ti is effective in preventing the growth of γ grains during slab heating, and is made 0.005% or more. In particular, in Nb-containing steel, a small amount of Ti of 0.005% or more is effective in suppressing cracks on the surface of a continuously cast slab promoted by Nb. On the other hand, if it exceeds 0.03%, TiN becomes coarse and the effect of refining γ grains disappears, so the content is made 0.03% or less.
[0045]
  N: 0.001 to 0.006%
  N combines with Ti to form TiN, and has the effect of suppressing the growth of γ grains during slab reheating and welding. In order to obtain such an effect, the lower limit is made 0.001%. On the other hand, an increase in the amount of N causes deterioration of slab quality and deterioration of HAZ toughness due to an increase in solid solution N, so the upper limit is made 0.006%.
[0046]
  Al: 0.1% or less
  Al is added to molten steel as a deoxidizing material. Al contained in the solidified steel forms solute Al, AlN, etc. as solAl except for Al in the oxide. Among these, since AlN effectively acts on the refinement of the structure, it is desirable to contain 0.005% or more in order to improve the base material toughness. However, excess Al causes coarsening of inclusions such as oxides, harms the cleanliness of the steel, and deteriorates the toughness of the HAZ, so the upper limit is made 0.1%. In order to ensure good generation characteristics of HAZ, the preferable upper limit value is 0.06%, more preferably 0.05%.
[0047]
  Cu: 0 to 0.6%
  Cu may not be included. However, since Cu is effective in increasing the strength, it is included in the case of steel that secures the strength by lowering C for an environment where weld cracking is likely to occur. If it is less than 0.2%, the effect of increasing the strength is small. On the other hand, if it exceeds 0.6%, the toughness deteriorates, so the upper limit is made 0.6%. Furthermore, when aiming at toughness improvement, it is desirable to set it as 0.4% or less.
[0048]
  Cr: 0 to 0.8%
  Cr may not be included. However, since Cr is effective in increasing the strength, it is included when C must be lowered to improve weldability. If the content is less than 0.15%, the effect is not sufficiently exhibited. On the other hand, if it exceeds 0.8%, the toughness deteriorates, so the upper limit is made 0.8%. In order to further improve the balance of toughness and strength, it is desirable to be 0.3 to 0.7%.
[0049]
  Mo: 0 to 0.6%
  Mo may not be included. However, since it is effective for increasing the strength, it is included in the case of low C. If the content is less than 0.1%, the effect is small. On the other hand, if it exceeds 0.6%, the toughness deteriorates, so the upper limit is made 0.6%. In order to keep both strength and toughness in a favorable range, it is desirable that the content be 0.3 to 0.5%.
[0050]
  V: 0 to 0.1%
  V may not be included. However, V increases the strength without significantly increasing the hardenability, so V is included when securing the strength without increasing the hardenability. If the content is less than 0.01%, the effect is small. On the other hand, if it exceeds 0.1%, the toughness deteriorates, so the upper limit is made 0.1%. In order to provide both good toughness and strength, the content is desirably 0.01 to 0.06%.
[0051]
  Ca: 0 to 0.006%
  Ca may not be contained. However, since Ca forms a sulfate with Mn, S, O, etc. and refines the crystal grains of HAZ, it is desirable to include it particularly when improving the generation characteristics of the weld. If the content is less than 0.001%, the effect is small. On the other hand, if it exceeds 0.006%, nonmetallic inclusions in the steel increase and cause internal defects, so the content is made 0.006% or less.
[0052]
  B and Ceq (hardenability)
  It is necessary to adjust the hardenability in order to fill the metal structure (c) in the portion from the thick surface layer portion to the central portion. The effect on the hardenability of C, Mn, Cu, Ni, Cr, Mo and V is taken into the carbon equivalent Ceq and evaluated by the carbon equivalent Ceq. In the present invention, B is not incorporated in Ceq, but it is considered because it contributes greatly to hardenability in a small amount. Among other elements, Nb improves the hardenability in a solid solution state, but when manufactured by a normal processing heat treatment, Nb (CN) precipitates during hot rolling and the Nb content is 0.01 to 0. In the range of 1%, the solid solution Nb concentration does not vary greatly. In the present invention, since all steels contain Nb in the above range, it is not necessary to take out Nb as a variable factor of hardenability in the present invention. Further, Si contributes little to hardenability, and the same can be said.
[0053]
  When B is 0.0004% or less, the effect of improving the hardenability is not exhibited. Therefore, when the effect of improving the hardenability of B is ensured, the content of B exceeds 0.0004%. On the other hand, if it exceeds 0.0025%, the toughness of the HAZ deteriorates remarkably, so 0.0025% is made the upper limit. In order to sufficiently secure both the toughness and hardenability of the HAZ, the content is preferably 0.0005 to 0.002%. B: In the case of more than 0.0004% and 0.0025% or less, steel that does not exhibit the effect of B in order to avoid excessive hardenability and forming a metal structure that is too hard (“B-free steel”) However, the carbon equivalent is lower than B: in the range of 0 to 0.0004%. That is, the range of carbon equivalent Ceq is 0.4 to 0.58%. When Ceq is less than 0.4%, TS900 MPa cannot be secured even if the effect of improving the hardenability of B is sufficiently obtained, so Ceq is set to 0.4% or more. On the other hand, if Ceq exceeds 0.58%, the effect of B and the hardenability become excessive and the toughness deteriorates, so the content is made 0.58% or less. The above conditions for B and Ceq are the condition (b) in the invention (1).
[0054]
  Since B does not have the effect of improving hardenability in HAZ, it does not harden as much as Ceq is lowered and lowers weld crack sensitivity. However, since B tends to increase the average length in the growth direction of martensite and lower bainite and reduce toughness, the weld crack sensitivity may be slightly increased, and the toughness is extremely good. When it is intended, it shall be B-free steel. That is, B is set to a range of 0 to 0.0004%. In the case of B-free steel, Ceq is set in the range of 0.53 to 0.7% in order to ensure the hardenability of the base material. If Ceq is less than 0.53%, the hardenability is insufficient and a TS of 900 MPa or more cannot be secured. On the other hand, if it exceeds 0.7, the quenching is excessive and the arrestability deteriorates, so 0.7% is the upper limit. To do. The condition regarding B and Ceq in B-free steel is condition (a) in the invention (1).
[0055]
  Vs: 0.10 to 0.42%
  In order to obtain extremely good stopping characteristics, such as when used in a cold region, in addition to the limitations of the individual alloy elements described above, the present invention reduces central segregation. To that end, the above(2) formulaVs defined in the above is within the range of 0.10 to 0.42%. When Vs exceeds 0.42%, strong center segregation is generated at the center of the continuously cast slab. When Vs exceeds 0.42%, when manufactured by a continuous casting method, high-tensile steel of TS900 MPa or more is less likely to have good center toughness due to center segregation. On the other hand, when Vs is less than 0.10%, although center segregation is small, it is difficult to ensure TS900 MPa or more, so the lower limit of Vs is 0.10%.
[0056]
  P: 0.015% or less, S: 0.003% or less
  Among the inevitable impurity elements, the content of P and S has a significant effect on the toughness of steel, so it is necessary to reduce the content. The reduction of P reduces the center segregation of the slab and suppresses brittle fracture caused by the brittle grain boundary. S becomes MnS, precipitates in the steel, stretches by rolling, and adversely affects toughness. In order to suppress these adverse effects, P is 0.015% or less, and S is 0.003% or less. Other unavoidable impurities are desirably low, but if they are reduced too much, the cost increases, so that they may be at a normal level.
[0057]
  Other elements:
  In addition to the above, rare earth elements (La, Ce, Y, Nd, etc.), Zr, W, etc. may be contained in minute amounts.
[0058]
  2. Metal texture
  If the steel containing the above chemical composition is subjected to the usual work heat treatment or heat treatment as described above, a high strength steel of TS900 MPa or more having the target performance can be obtained. In order to further improve the performance, not only the chemical composition but also the high-strength steel having the metal structure (c) is used.
[0059]
  2-1) Mixed structure of martensite and lower bainite:
  In order to achieve both excellent strength and toughness of the base material, the metal structure is assumed to be “mixed structure of martensite and lower bainite” (hereinafter referred to as “mixed structure”). This mixed structure is 90% or more by volume ratio. The term “lower bainite” as used herein refers to the end face of the ferrite inside the lath-shaped bainitic ferrite (the surface of the tip of the lath-shaped bainitic ferrite growing at a constant angle in γ) and an angle of 60 degrees. The structure in which fine cementite is dispersed and precipitated. There is only one precipitation surface of fine cementite inside one bainitic ferrite. Tempered martensite is also a structure in which cementite precipitates in the martensite lath, but is different in that there are four variants.
[0060]
  The reason why the mixed tissue is 90% or more by volume is that the target arrestability, that is, 85% FATT of DWTT is −30 ° C. or lower. The reason why the toughness of the mixed structure is excellent is that the lower bainite formed on the high temperature side prior to martensite during quenching becomes a “wall” and subdivides the γ grains, which matches the martensite packet (the fracture surface unit of brittle fracture). ) To suppress growth.
[0061]
  In the low-carbon steel targeted by the present invention, the brittle fracture surface is composed of a cleaved fracture surface without plastic deformation and a ductile fracture surface with plastic deformation surrounding it thinly, and is called a pseudo-cleavage fracture surface. Taking the surrounding ductile fracture surface as the boundary, the average size of the cleavage fracture surface is called “fracture surface unit”. The smaller the fracture surface unit, the better the generation characteristics and arrestability.
[0062]
  When the volume fraction of the lower bainite is less than 2%, the above-described effect of dividing the structure of the lower bainite cannot be obtained. Therefore, the structure is not sufficiently refined due to the mixed structure, and the toughness is reduced. To do. On the other hand, if the ratio of the lower bainite, which is lower in strength than martensite, becomes too high, the strength of the entire steel decreases. Therefore, in order to satisfy the strength of TS 900 MPa or more, the lower bainite is desirably 75% by volume or less.
[0063]
  2-2) Aspect ratio of old γ grains
  Furthermore, in order to further improve the toughness after obtaining a mixed structure satisfying the strength, it is desirable to disperse the lower bainite. For this purpose, transformation is performed from an unrecrystallized state of γ, that is, a state in which dislocations accumulated by reduction are present at a high density. This state is a state in which the nucleation sites of the lower bainite exist at high density. For this reason, a lower bainite can be produced | generated from (gamma) grain boundary and many sites in a grain. In order to make such an effect appear reliably, the aspect ratio (flatness) of non-recrystallized γ (old γ grain) is set to 3 or more.
[0064]
  3. Production method
  Next, the manufacturing method will be described in detail. The manufacturing method of this invention can be positioned as the manufacturing method which provides metal structure (c) to said invention (1) or (3).
[0065]
  The most important point in this production method is to nucleate lower bainite and martensite not only in the former γ grain boundaries but also in the γ grains in which the accumulated dislocations introduced by hot rolling are preserved.
[0066]
  (A) Hot rolling
  The slab heating temperature is set to 1250 ° C. or less in order to prevent coarsening of γ crystal grains during heating, while solid solution of Nb effective for refinement of crystal grains during rolling and precipitation strengthening after rolling. And 1000 ° C. or higher. In order to nucleate the lower bainite from inside the γ grains and to suppress the growth of the lower bainite, high-density dislocations are necessary. For that purpose, rolling should be performed at 50% or more in the γ non-recrystallization temperature range. Is essential. On the other hand, if the reduction ratio in the non-recrystallization temperature range of γ exceeds 90%, the anisotropy of the mechanical properties becomes remarkable. Therefore, the reduction ratio in the non-recrystallization temperature range is desirably 90% or less.
[0067]
  Rolling finish temperature is Ar₃Below the point, a strong rolling texture remains and anisotropy occurs in the mechanical properties.₃It is set as the above rolling finishing temperature.
[0068]
  (B) Cooling
  In order to suppress the formation of upper bainite which deteriorates toughness, Ar₃It must be cooled at a constant cooling rate from above the point. The cooling rate after rolling is necessary to make the ratio of each structure appropriate. In the case of a steel plate, it is 10 to 10 in the center of the plate thickness, and in the case of a general steel material in the center of the wall thickness. The range is 45 ° C./s. When the cooling rate is less than 10 ° C./s, the upper bainite is generated or the ratio of the lower bainite exceeds 75%, and the strength and toughness, particularly the arrestability are deteriorated. On the other hand, when it exceeds 45 ° C./s, the lower bainite is not generated and the structure is composed only of martensite, and the toughness, particularly the arrestability is deteriorated.
[0069]
  The cooling stop temperature is 500 ° C. or lower. This is because when the cooling stop temperature exceeds 500 ° C., upper bainite is generated, and the mixed structure (c) cannot be obtained. Cooling may be performed up to room temperature, but if the hydrogen concentration is high in the steelmaking stage and the possibility of occurrence of hydrogen defects is high, it is desirable to cool to about 200 ° C. and then remove for dehydrogenation. . Or it is desirable to cool to about 200 degreeC and insert in a dehydrogenation annealing furnace, without making it less than 200 degreeC, or to perform the tempering process mentioned later. This is because hydrogen defects in the process of cooling after rolling mostly occur below 200 ° C.
[0070]
  (C) Tempering
  The steel produced by the above production method may be used as it is, or when extremely high arrestability is required.₁Temper below the point.
【Example】
[0071]
  Next, the effects of the present invention will be described with reference to examples.
[0072]
  Tables 1 and 2 show the chemical composition of the steel subjected to the experiment. The test steel is a thick steel plate having a thickness of 12 to 35 mm obtained by subjecting a slab obtained by melting and casting steel having the chemical composition shown in the table to a heat treatment under various conditions.Table 3 shows the C of each steel. eq Indicate.
[0073]
[Table 1]

[0074]
[Table 2]

[0075]
[Table 3]

[0076]
  Table 4These are tables showing conditions for thermomechanical treatment (hot rolling, cooling and tempering). As described above, the non-recrystallization temperature range of the steel is 950 ° C. or lower. Ar₃The point is in the range of 500-600 ° C.
[0077]
[Table 4]

[0078]
  Also,Table 5Shows the metal structure at the center of the plate thickness of the thick steel plate manufactured under these conditions.
[0079]
[Table 5]

[0080]
  Test pieces were collected from the center of the plate thickness of these steel plates and used for the next test.
[0081]
  Regarding the base material strength, YS and TS were determined by a tensile test (test piece: JIS Z 2204 No. 4, test method: JIS Z 2241). The toughness of the base material was measured by a 2 mm V notch Charpy impact test (test piece: JIS Z 2202 No. 4, test method: JIS Z 2242) and a DWTT test.
[0082]
  The DWTT test is an arrest evaluation test that is very commonly known in the line pipe industry. A press notch is made in the raw specimen with a knife edge, an impact load is applied with a falling weight or a large hammer, a brittle crack is generated from the notch, the fracture surface after fracture is observed, and the ductile fracture surface is broken into a brittle fracture surface. Evaluate only at the temperature at which the mode transitions. In an effective test, a brittle fracture surface occurs from the bottom of the press notch, but then changed to a ductile fracture surface (which requires a large amount of energy for the propagation of ductile cracks), and the ductile fracture surface is 85% of the entire fracture surface. In the case of the above ratio (85% FATT), it is judged that arrestability exists at the test temperature. If a brittle crack does not occur from the bottom of the notch, it is not considered an effective test. In such a case, carburizing treatment or the like is performed on the bottom of the notch to further embrittle it so that a brittle crack is generated from the notch bottom. In this example, a brittle fracture surface was generated from the bottom of the notch that had been press-notched in any steel.
[0083]
  On the other hand, the 2 mm V notch Charpy impact test is said to be a toughness evaluation test that mainly evaluates the generation characteristics but also includes arrestability. In the 2 mm V notch Charpy impact test of the base material, the absorbed energy at a test temperature of −40 ° C. was determined.
[0084]
  For toughness testing of welded joints, 2 mm V notch Charpy specimens were sampled from specimens cooled to 800-500 ° C. at a maximum heating temperature of 1350 ° C. and a heat input equivalent to 40,000 J / cm using a welding heat cycle reproduction tester. Then, it used for the test and evaluated by the absorbed energy of -20 degreeC. As described above, this evaluation is mainly an evaluation of generation characteristics.
[0085]
  In-situ weldability was evaluated by the y-groove weld cracking test (JIS Z 3158). Since these weld cracks are almost determined by the chemical composition and the metal structure of the base metal is not affected, a steel plate having a thickness of 25 mm was manufactured for the steels shown in Tables 1 and 2 at a heating temperature of 1150 ° C. and a finishing temperature of 900 ° C. Raw thickness y-groove weld crack specimens were collected from these thick steel plates and used for the test. The welding material is a commercially available 100 kg high-tensile hand welding rod, and after leaving it in the atmosphere at a temperature of 20 ° C. and a humidity of 75% so as to be about 1.5 cc / 100 g of hydrogen, heat input is 1.7 kJ / mm. The beads were placed under the conditions described above, cooled to room temperature, and then examined for cracks in accordance with JIS Z 3158.
[0086]
  Table 6Is a list showing these test results.
[0087]
[Table 6]

[0088]
  Test numbers X1 to X10 which are comparative examples are steel alloy elements, C excess (X1), Si excess (X2), Mn excess (X3), Cu excess (X4), Ni excess (X5), Cr excess ( Since X6), Mo excess (X7), V excess (X8), Ti excess (X9), and Al excess (X10), the toughness of the base material, especially arrestability, did not reach the target except for X10. In X10, the toughness certainly reached the target value, but the strength did not satisfy 900 MPa.
[0089]
  In Comparative Examples X11 and X12, Ceq was excessive and excessive, respectively, but X11 had low toughness and weld cracking, and X12 had low strength and toughness due to insufficient hardenability.
[0090]
  Although the chemical composition of the steel falls within the scope of the present invention, the bases of Comparative Examples Y1, Y2, Y6 and Y10, in which the hot rolling or cooling conditions are outside the range of the appropriate conventional method and greatly outside the range of the metal structure (c) The toughness was very unsatisfactory.
[0091]
  On the other hand, in the present invention example, an absorption energy of 200 J or more was obtained in a strength of TS 900 MPa or more and a Charpy impact test at −40 ° C. In DWTT, which was the greatest concern, 85% FATT was −40 ° C. or lower, indicating extremely satisfactory arrestability. Furthermore, the weld performance and on-site weldability were also good.
【The invention's effect】
[0092]
  According to the present invention, it is possible to obtain a high-tensile steel having a tensile strength of 900 MPa or more and excellent toughness, particularly arrestability. As a result, it was possible to dramatically improve the construction efficiency when laying pipelines and the transportation efficiency during operation while ensuring sufficient safety.

Claims (6)

In mass% , C: 0.02 to 0.1%, Si: 0.6% or less, Mn: 0.2 to 2.5%, Ni: more than 1.2%, 2.5% or less, Nb : 0.01 to 0.1%, Ti: 0.005 to 0.03%, N: 0.001 to 0.006%, Al: 0.1% or less, Cu: 0 to 0.6%, Cr : 0-0.8%, Mo: 0-0.6%, V: 0-0.1%, Ca: 0-0.006%, the balance consists of Fe and inevitable impurities, and the following conditions (a ) Or (b), and among the inevitable impurities, P is 0.015% or less and S is 0.003% or less, and a high strength steel having a tensile strength of 900 MPa or more.
(a) Further, B is in the range of 0 to 0.0004%, and the carbon equivalent Ceq of the following formula (1) is 0.53 to 0.7%.
(b) Further, B includes more than 0.0004% and 0.0025% or less, and the carbon equivalent Ceq of the following formula (1) is 0.4 to 0.58%.
(1) : Ceq = C + (Mn / 6) + {(Cu + Ni) / 15} + {(Cr + Mo + V) / 5}
Here, the element symbol represents the content (% by weight) in the steel. The high-tensile steel according to claim 1, wherein the metal structure satisfies the following (c).
(c) The mixed structure of martensite and lower bainite is 90% by volume or more in the metal structure, of which the lower bainite is 2% by volume or more, and the aspect ratio of the prior austenite grains is 3 or more. is there. The high-tensile steel according to claim 1, wherein Vs represented by the following formula (2) is in the range of 0.10 to 0.42%.
(2) : Vs = C + (Mn / 5) + 5P- (Ni / 10)-(Mo / 15) + (Cu / 10)
Here, the element symbol represents the content (% by weight) in the steel. The high-tensile steel according to claim 2, wherein Vs represented by the following formula (2) is in the range of 0.10 to 0.42%.
(2) : Vs = C + (Mn / 5) + 5P- (Ni / 10)-(Mo / 15) + (Cu / 10)
Here, the element symbol represents the content (% by weight) in the steel. The steel slab was rolled after heating to 1000 to 1250 ° C., the cumulative reduction rate in the pre-recrystallization temperature zone of γ is 50% or more, and terminates the rolling at Ar ₃ point or more, the thickness of _three or more points Ar The manufacturing method of the high-tensile steel according to claim 2 or 4, wherein the cooling is performed to 500 ° C or lower at a cooling rate of 10 to 45 ° C / s at the center. Furthermore, the manufacturing method of the said Claim 5 which adds tempering in Ac ₁ point or less.