JP2010236047A - Steel sheet having high toughness and high tensile strength and excellent strength-elongation balance, and method for manufacturing the same - Google Patents
Steel sheet having high toughness and high tensile strength and excellent strength-elongation balance, and method for manufacturing the same Download PDFInfo
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Abstract
Description
本発明は、橋梁、貯蔵タンク、圧力容器およびラインパイプなど鉄鋼構造物の用途に供して好適な高張力鋼板およびその製造方法に関し、特に480MPa以上の降伏強度と優れた低温靭性を併せて付与することにより、強度−伸びバランスの有利な向上を図ろうとするものである。 The present invention relates to a high-tensile steel plate suitable for use in steel structures such as bridges, storage tanks, pressure vessels, and line pipes, and a method for producing the same, and particularly provides a yield strength of 480 MPa or more and excellent low-temperature toughness. Thus, the strength-elongation balance is advantageously improved.
橋梁、貯蔵タンク、圧力容器およびラインパイプなどの鉄鋼構造物に用いられる鋼板は、強度が高く、靭性に優れていることは勿論であるが、これらに加え、耐震性の観点から高い延性が求められる。一般に、建築用鋼材では、耐震性確保のために低降伏比化することで塑性変形能を高めている。
しかしながら、これらの鋼材は、２相域焼入れなどの手段により、マルテンサイトまたはベイナイト主体の組織中に軟質のフェライト組織を導入し、ミクロ的に不均一な組織とすることによって低降伏比を実現しているため、鋼材の降伏現象を早期に発生させることに繋がり、高い負荷がかかる構造物などで必要とされる降伏強度とのバランスをとることが難しく、また複雑な熱処理工程を必要とすることから、実用的な大量生産品としては必ずしも適当ではなかった。
Steel sheets used for steel structures such as bridges, storage tanks, pressure vessels and line pipes are of course high in strength and excellent in toughness, but in addition to these, high ductility is required from the viewpoint of earthquake resistance. It is done. In general, steel materials for construction increase plastic deformability by reducing the yield ratio to ensure earthquake resistance.
However, these steel materials achieve a low yield ratio by introducing a soft ferrite structure into a martensite- or bainite-based structure by means such as two-phase quenching and making it a micro-homogeneous structure. Therefore, it is difficult to balance the yield strength required for structures with high loads, which leads to the early occurrence of steel yielding, and requires a complicated heat treatment process. Therefore, it was not necessarily suitable as a practical mass-produced product.
一方で、一様伸びが高いほど耐震性が優れていることが知られていて、ラインパイプなどでは、全伸び（一様伸び＋局部伸び）が大きいことが要求される。これは、外部からの応力により変形が始まってから破壊するまでに変形する量が大きいことを意味しており、鋼材に対する安全性の指標となっている。 On the other hand, it is known that the higher the uniform elongation, the better the earthquake resistance, and the line pipe or the like is required to have a large total elongation (uniform elongation + local elongation). This means that the amount of deformation from the start of deformation due to external stress to the time of destruction is large, which is an indicator of safety for steel materials.
全伸びを大きくするための手段として、一様伸びと局部伸びのどちらかあるいは両者を良くすることが考えられる。引張試験片の標点距離が長いほど、全伸びに占める一様伸びの比率は大きくなるが、一般的に使用されている引張試験片の範囲内では、局部伸びの割合も40〜50％程度あることが多く、全伸びを大きくするためには、結果的に一様伸びと局部伸びのどちらかが小さくなることは好ましくない。 As a means for increasing the total elongation, it is conceivable to improve one or both of uniform elongation and local elongation. The longer the gauge distance of the tensile test piece, the larger the ratio of uniform elongation to the total elongation, but within the range of commonly used tensile test pieces, the percentage of local elongation is also about 40-50%. In many cases, in order to increase the total elongation, it is not preferable that either the uniform elongation or the local elongation is reduced as a result.
一般に、伸び（一様伸びを含む）の向上には、複相組織化が有効であると考えられている。その例として、特許文献１や特許文献２などが挙げられる。
特許文献１では、オーステナイトの再結晶温度域で圧延終了後、２相域での冷却を制御することによってフェライト＋マルテンサイト組織とする方法が示されている。
しかしながら、この方法では、一様伸びは向上するものの、フェライト粒が粗大化するために、低温靭性は良好とは言えない。また、ミクロ組織が不均一であることから、局部伸びが著しく低下するおそれもある。
In general, it is considered that multiphase organization is effective for improving elongation (including uniform elongation). Examples thereof include Patent Document 1 and Patent Document 2.
Patent Document 1 discloses a method of forming a ferrite + martensitic structure by controlling cooling in a two-phase region after rolling is completed in the austenite recrystallization temperature region.
However, in this method, although the uniform elongation is improved, the ferrite grains are coarsened, so that the low temperature toughness is not good. Further, since the microstructure is non-uniform, there is a risk that the local elongation is significantly reduced.
特許文献２では、残留オーステナイトを生成させて伸びを向上させる手法が示されている。薄鋼板などでは、残留オーステナイトを生成させたTRIP鋼等が実用化されているが、厚鋼板の分野では、実用化された例はない。その理由として、合金成分コストが高いことや、溶接性との両立が困難であることが挙げられる。 Patent Document 2 discloses a technique for improving elongation by generating retained austenite. For thin steel sheets, TRIP steel and the like that generate retained austenite have been put into practical use, but there has been no practical application in the field of thick steel sheets. The reason is that the alloy component cost is high and it is difficult to achieve compatibility with weldability.
また、一方で、Cu析出を利用することにより、伸びが向上することが報告されている。これは、軟質な強化粒子を使うことにより、強化粒子自体の塑性変形能が高いことから、ミクロ的な不均一変形が抑制されるためと考えられている。例えば、特許文献３に、その手法が示されている。
しかしながら、Cu析出強化を発現させるには、概ね１％以上のCu添加が必要であることから、製造コストおよび特性の安定性の観点から、実用鋼としての実現可能性は低い。
On the other hand, it has been reported that elongation is improved by utilizing Cu precipitation. This is considered to be because the use of soft reinforcing particles suppresses microscopic non-uniform deformation because the plastic deformability of the reinforcing particles themselves is high. For example, Patent Document 3 discloses the technique.
However, in order to develop Cu precipitation strengthening, approximately 1% or more of Cu needs to be added. Therefore, the feasibility as a practical steel is low from the viewpoint of manufacturing cost and stability of characteristics.
上述したとおり、従来の技術では、生産性の低下や製造コストの増大、さらには溶接性や靭性の低下などの問題を残していた。
本発明は、上記の現状に鑑み開発されたもので、鋼板の板厚中心部と表層部の組織を個別に制御することにより、生産性の低下や製造コストの増大を招くことなしに、480MPa以上の降伏強度と優れた低温靭性を有し、ひいては強度−伸びバランスに優れ、しかも板厚方向の硬さ分布が一様な高靱性高張力鋼板を、その有利な製造方法と共に提案することを目的とする。
As described above, the conventional techniques have left problems such as a decrease in productivity, an increase in manufacturing cost, and a decrease in weldability and toughness.
The present invention was developed in view of the above-mentioned present situation, and by individually controlling the structure of the center portion and the surface layer portion of the steel sheet, without causing a decrease in productivity and an increase in manufacturing cost, 480 MPa We propose a high-toughness, high-tensile steel sheet that has the above-mentioned yield strength and excellent low-temperature toughness, as well as excellent strength-elongation balance and uniform hardness distribution in the thickness direction, along with its advantageous manufacturing method. Objective.
さて、発明者らは、優れた低温靭性と480MPa以上の降伏強度を確保した上で、全厚引張試験片における全伸びを向上させる方法について、鋭意研究を進めた。
その結果、全厚引張試験片での全伸びの支配因子としては、板厚中心部および表裏層のミクロ組織、表裏層の硬さおよび試験片内での材質の均一性が挙げられ、これらを適正に制御することにより、所期した目的が有利に達成されることの知見を得た。
Now, the inventors have conducted extensive research on a method for improving the total elongation of a full-thickness tensile specimen while ensuring excellent low-temperature toughness and yield strength of 480 MPa or more.
As a result, the factors governing the total elongation of the full-thickness tensile specimen include the microstructure of the center of the plate thickness and the front and back layers, the hardness of the front and back layers, and the uniformity of the material within the specimen. The knowledge that the intended purpose is achieved advantageously by controlling appropriately was acquired.
すなわち、
(1) フェライト＋ベイナイト組織とすることにより伸びが向上する、
(2) 圧延で表裏層に導入された加工フェライトは伸びには不利である、
(3) TMCPプロセスにおいて冷却開始温度および冷却停止温度を制御することにより、板厚中心を単相組織にすることができ、これにより延性低下の原因となるマイクロボイドの発生を抑制できる
ことを明らかにした。
また、
(4) これらのミクロ組織制御により、比較的高い降伏強度が得られる
ことを見出した。
さらに、
(5) 表層のみを優先的に加熱する焼戻し処理を施すことにより、伸びが向上する
(6) また、表層を加熱することで、鋼板内の表面硬さが軽減し伸びが向上する
ことを見出した。
そして、かような鋼材は、
(7) ライン上に配置された加速冷却、加熱設備を駆使して一連の工程で造り込むことにより、高効率で得られる
ことを見出した。
本発明は、上記の知見に基づいて完成されたものである。
That is,
(1) Elongation improves by adopting ferrite + bainite structure.
(2) Processed ferrite introduced into the front and back layers by rolling is disadvantageous for elongation.
(3) By controlling the cooling start temperature and cooling stop temperature in the TMCP process, it is clear that the center of the plate thickness can be made into a single-phase structure, which can suppress the generation of microvoids that cause ductility reduction. did.
Also,
(4) It was found that a relatively high yield strength can be obtained by controlling the microstructure.
further,
(5) Elongation is improved by applying a tempering treatment that preferentially heats only the surface layer.
(6) Moreover, it discovered that the surface hardness in a steel plate reduced and elongation improved by heating a surface layer.
And such steel is
(7) It has been found that high efficiency can be obtained by building in a series of processes using accelerated cooling and heating equipment arranged on the line.
The present invention has been completed based on the above findings.
すなわち、本発明の要旨構成は次のとおりである。
１．質量％で、Ｃ：0.03〜0.18％、Si：0.01〜0.55％、Mn：0.5〜2.0％、Al：0.005〜0.1％およびＮ：0.0005〜0.005％を含有し、残部はFeおよび不可避的不純物の組成になり、ミクロ組織がフェライトとベイナイトの混合組織であって、板厚中心の上下１mmを含む領域のミクロ組織は、加工フェライトを含まず、ポリゴナルフェライトが面積率で５％以下のベイナイト主体の組織、一方、表裏面から板厚方向に1.5mmの領域のミクロ組織は、加工フェライトの面積率が５％以下、ポリゴナルフェライトの面積率が10％以上の、フェライトとベイナイトの混合組織になり、鋼板の表裏面下0.5mmより内部側における板厚方向の硬さ分布の最大値と最小値の差がビッカース硬さで30HV未満で、かつ板厚が20mm以上、降伏強度が480MPa以上であることを特徴とする強度−伸びバランスに優れた高靭性高張力鋼板。
That is, the gist configuration of the present invention is as follows.
1. In mass%, C: 0.03-0.18%, Si: 0.01-0.55%, Mn: 0.5-2.0%, Al: 0.005-0.1% and N: 0.0005-0.005%, the balance being Fe and inevitable impurities The composition is a mixed structure of ferrite and bainite, and the microstructure of the region including 1 mm above and below the center of the plate thickness does not contain processed ferrite, and polygonal ferrite is mainly composed of bainite with an area ratio of 5% or less. On the other hand, the microstructure of 1.5mm in the thickness direction from the front and back surfaces is a mixed structure of ferrite and bainite with an area ratio of processed ferrite of 5% or less and a polygonal ferrite area ratio of 10% or more. The difference between the maximum and minimum hardness distribution in the thickness direction from 0.5 mm below the front and back surfaces of the steel sheet is less than 30 HV in Vickers hardness, the thickness is 20 mm or more, and the yield strength is 480 MPa or more. Strength-elongation balun characterized by High toughness high tensile strength steel sheet excellent in.
２．前記鋼板が、さらに質量％で、Cu：0.8％以下、Ni：２％以下、Cr：１％以下、Mo：0.8％以下、Nb：0.05％以下、Ｖ：0.1％以下、Ti：0.025％以下、Ｂ：0.002％以下およびCa：0.005％以下のうちから選んだ一種または二種以上を含有する組成になることを特徴とする前記１に記載の強度−伸びバランスに優れた高靭性高張力鋼板。 2. The steel sheet is further in mass%, Cu: 0.8% or less, Ni: 2% or less, Cr: 1% or less, Mo: 0.8% or less, Nb: 0.05% or less, V: 0.1% or less, Ti: 0.025% or less B: 0.002% or less and Ca: 0.005% or less, the composition containing one or more kinds selected from the above, high toughness and high strength steel plate excellent in strength-elongation balance as described in 1 above .
３．質量％で、Ｃ：0.03〜0.18％、Si：0.01〜0.55％、Mn：0.5〜2.0％、Al：0.005〜0.1％およびＮ：0.0005〜0.005％を含有し、残部はFeおよび不可避的不純物の組成になるスラブを、1050〜1250℃の温度に加熱後、累積圧下率：50％以上、鋼板表面温度：Ａr3以上、Ａr3＋15℃以下の条件で熱間圧延を終了し、ついで板厚中心がＡr3以上の温度から加速冷却を開始し、鋼板平均温度が350℃以上550℃以下の温度域まで冷却したのち、空冷するものとし、該加速冷却の際、鋼板表面温度が300℃以上の温度域において、0.3秒以上の一時的に冷却されない期間を１回または２回以上、かつ合計時間が1.5秒以上、15秒以下の非冷却期間を設けることを特徴とする強度−伸びバランスに優れた高靭性高張力鋼板の製造方法。 3. In mass%, C: 0.03-0.18%, Si: 0.01-0.55%, Mn: 0.5-2.0%, Al: 0.005-0.1% and N: 0.0005-0.005%, the balance being Fe and inevitable impurities After heating the slab to be a composition to a temperature of 1050 to 1250 ° C., hot rolling was completed under conditions of cumulative reduction: 50% or more, steel sheet surface temperature: Ar 3 or more, Ar 3 + 15 ° C. or less, and then the plate thickness Accelerated cooling starts at a temperature of Ar 3 or higher at the center, and the steel plate average temperature is 350 ° C or higher and 550 ° C or lower, and then air-cooled. During this accelerated cooling, the steel plate surface temperature is 300 ° C or higher. In the temperature range, the strength-elongation balance is characterized by providing a non-cooling period of 0.3 seconds or more and a total cooling time of 1.5 seconds or more and 15 seconds or less for a period of 0.3 seconds or more that is not temporarily cooled. A method for producing excellent high toughness and high strength steel sheets.
４．前記非冷却期間を含む前記加速冷却後、さらに誘導加熱により、鋼板の板厚中心温度が650℃以下かつ表面の最高到達温度が580℃以上730℃以下に急速加熱する焼戻し処理を施すことを特徴とする上記３に記載の強度−伸びバランスに優れた高靭性高張力鋼板の製造方法。 4). After the accelerated cooling including the non-cooling period, a tempering process is performed by rapid heating to a plate thickness center temperature of the steel sheet of 650 ° C. or lower and a maximum surface temperature of 580 ° C. or higher and 730 ° C. or lower by induction heating. 4. A method for producing a high-toughness, high-tensile steel sheet excellent in strength-elongation balance as described in 3 above.
５．前記スラブが、さらに質量％で、Cu：0.8％以下、Ni：２％以下、Cr：１％以下、Mo：0.8％以下、Nb：0.05％以下、Ｖ：0.1％以下、Ti：0.025％以下、Ｂ：0.002％以下およびCa：0.005％以下のうちから選んだ一種または二種以上を含有する組成になることを特徴とする上記３または４に記載の強度−伸びバランスに優れた高靭性高張力鋼板。 5. The slab is further mass%, Cu: 0.8% or less, Ni: 2% or less, Cr: 1% or less, Mo: 0.8% or less, Nb: 0.05% or less, V: 0.1% or less, Ti: 0.025% or less B: 0.002% or less and Ca: 0.005% or less, the composition containing one or more selected from the group consisting of 2 or more, and high toughness with excellent strength-elongation balance as described in 3 or 4 above Tensile steel plate.
本発明によれば、生産性の低下や製造コストの増大を招くことなしに、高い降伏強度と優れた低温靭性を有し、ひいては強度−伸びバランスに優れ、しかも板厚方向の硬さ分布が一様な高靱性高張力鋼板を、安定して得ることができる。 According to the present invention, it has high yield strength and excellent low temperature toughness without causing a decrease in productivity and an increase in manufacturing cost, and thus has an excellent strength-elongation balance, and has a hardness distribution in the thickness direction. A uniform high toughness and high strength steel sheet can be obtained stably.
以下、本発明を具体的に説明する。
まず、本発明において、鋼の成分組成を前記の範囲に限定した理由について説明する。なお、成分に関する「％」表示は特に断らない限り質量％を意味するものとする。
Ｃ：0.03〜0.18％
Ｃは、高張力鋼板の母材強度の確保に必要な元素であるが、含有量が0.03％に満たないとCu，Ni，Cr，Moなどの焼入性向上元素の多量添加が必要となり、コスト高となるだけでなく、溶接性の劣化を招き、また大入熱溶接が施される場合には、溶接金属へのＣの希釈が少なくなり、継手強度の確保が困難となる。一方、Ｃ量が0.18％を超えると母材靭性および溶接性の劣化を招き、また溶接継手部靭性の劣化を招くため、Ｃ量は0.03〜0.18％の範囲に限定した。
The present invention will be specifically described below.
First, the reason why the component composition of steel is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.03-0.18%
C is an element necessary to ensure the strength of the base material of the high-tensile steel sheet. However, if the content is less than 0.03%, it is necessary to add a large amount of a hardenability improving element such as Cu, Ni, Cr, and Mo. Not only is the cost high, but also weldability is deteriorated, and when high heat input welding is performed, the dilution of C into the weld metal is reduced, and it is difficult to ensure the joint strength. On the other hand, if the C content exceeds 0.18%, the base metal toughness and weldability deteriorate, and the weld joint toughness deteriorates. Therefore, the C content is limited to a range of 0.03 to 0.18%.
Si：0.01〜0.55％
Siは、母材強度および溶接継手強度を確保する上で有用な元素なので、0.01％以上含有させるものとした。しかしながら、Si量が0.55％を超えると、溶接割れ感受性と溶接継手靭性の劣化を招く。そのため、Si量は0.01〜0.55％の範囲に限定した。
Si: 0.01-0.55%
Si is an element useful for ensuring the strength of the base metal and the welded joint, so it was included in an amount of 0.01% or more. However, if the Si content exceeds 0.55%, the weld crack sensitivity and weld joint toughness are deteriorated. Therefore, the Si content is limited to a range of 0.01 to 0.55%.
Mn：0.5〜2.0％
Mnは、母材強度および溶接継手強度を確保する上で有用なので、0.5％以上含有させるものとした。しかしながら、Mn量が2.0％を超えると溶接割れ感受性が劣化させるだけでなく、必要以上の焼入性をもたらし母材靭性および継手靭性を劣化させる。そのため、Mn量は0.5〜2.0％の範囲に限定した。
Mn: 0.5-2.0%
Mn is useful for ensuring the strength of the base metal and the welded joint, so it was added at 0.5% or more. However, if the amount of Mn exceeds 2.0%, not only the weld cracking sensitivity is deteriorated, but also hardenability more than necessary is brought about, and the base metal toughness and joint toughness are deteriorated. Therefore, the Mn content is limited to a range of 0.5 to 2.0%.
Al：0.005〜0.1％
Alは、鋼の脱酸剤として有用であるので、0.005％以上含有させる。また、結晶粒の微細化による母材靭性確保のためには0.01％以上の添加が好適である。しかしながら、Al量が0.1％を超えると母材靭性を損なうので、Alは0.005〜0.1％の範囲で含有させるものとした。
Al: 0.005-0.1%
Al is useful as a deoxidizer for steel, so 0.005% or more is contained. In addition, addition of 0.01% or more is suitable for securing the base material toughness by refining crystal grains. However, if the Al content exceeds 0.1%, the toughness of the base metal is impaired, so Al is included in the range of 0.005 to 0.1%.
Ｎ：0.0005〜0.005％
Ｎは、AlやNbなどと反応し析出物を形成することで結晶粒を微細化し、母材靭性を向上させる効果がある。しかしながら、含有量が0.0005％未満では結晶粒の微細化および強度確保に必要な析出物が形成されず、一方0.005％を超えるとむしろ母材および大入熱溶接継手の靭性を損なうので、Ｎは0.0005〜0.005％の範囲で含有させるものとした。
N: 0.0005-0.005%
N reacts with Al, Nb, and the like to form precipitates and thereby has the effect of refining crystal grains and improving the base material toughness. However, if the content is less than 0.0005%, precipitates necessary for refining the crystal grains and securing the strength are not formed. On the other hand, if the content exceeds 0.005%, the toughness of the base metal and the high heat input welded joint is impaired. It was made to contain in 0.0005 to 0.005% of range.
以上、基本成分について説明したが、本発明では、その他にも、Cu，Ni，Cr，Mo，Nb，Ｖ，Ti，ＢおよびCaのうちから選んだ一種または二種以上を、以下の範囲で適宜含有させることができる。 Although the basic components have been described above, in the present invention, one or more selected from Cu, Ni, Cr, Mo, Nb, V, Ti, B, and Ca are also included in the following ranges. It can be contained as appropriate.
Cu：0.8％以下、Ni：２％以下、Cr：１％以下、Mo：0.8％以下、Nb：0.05％以下、Ｖ：0.1％以下
本発明鋼において、特に引張強さ600MPa級以上の高張力鋼板を得る場合や、耐候性を必要とする場合には、Cu，Ni，Cr，Mo，NbおよひＶのうちから選んだ少なくとも一種を添加することが有利である。この場合、Cu、Ni、Ｃｒ、Moについては、いずれも多量の添加は高コストとなり、また、溶接性を低下させるため、それぞれ、Cuについては上限を0.8％、Crについては上限を１％、Niについては上限を２％、Moについては上限を0.8％とした。また、Nbは、母材強度確保に有効であるが、多量の添加は強化に寄与せず、逆に、溶接継手靭性を劣化させることから、添加する場合の上限は0.05％、好ましくは0.03％である。さらに、Ｖは、母材強度と溶接継手強度を確保する上で有効に作用するが、0.1％を超える添加は溶接割れ感受性を劣化させるので、上限を0.1％とした。
Cu: 0.8% or less, Ni: 2% or less, Cr: 1% or less, Mo: 0.8% or less, Nb: 0.05% or less, V: 0.1% or less In the present invention steel, particularly high tensile strength of 600 MPa class or more When obtaining a steel plate or when weather resistance is required, it is advantageous to add at least one selected from Cu, Ni, Cr, Mo, Nb and V. In this case, for Cu, Ni, Cr, and Mo, the addition of a large amount is expensive, and in order to reduce weldability, the upper limit is 0.8% for Cu and the upper limit is 1% for Cr. For Ni, the upper limit was 2%, and for Mo, the upper limit was 0.8%. Nb is effective in securing the strength of the base metal, but adding a large amount does not contribute to strengthening, and conversely deteriorates the weld joint toughness, so the upper limit when added is 0.05%, preferably 0.03%. It is. Furthermore, V acts effectively in securing the base metal strength and weld joint strength, but addition exceeding 0.1% degrades the weld crack sensitivity, so the upper limit was made 0.1%.
Ti：0.025％以下、Ｂ：0.002％以下
Tiは、ミクロ組織の細粒化およびＢ添加鋼の場合には焼入性に有効なＢを確保するために添加するが、0.025％を超える添加は母材靭性を損ねることから、Ti量は0.025％以下とした。また、Ｂは、ごく微量の添加で焼入性を高める効果が得られるが、過剰に添加するとＢＮを形成し逆に焼入性の低下を招き、また溶接熱影響部が著しく硬化するため、Ｂの上限は0.002％とした。
Ti: 0.025% or less, B: 0.002% or less
Ti is added to refine the microstructure and to ensure B effective in hardenability in the case of B-added steel, but addition over 0.025% impairs the base metal toughness. 0.025% or less. In addition, B has an effect of improving hardenability by adding a very small amount, but if added excessively, BN is formed and conversely the hardenability is lowered, and the weld heat affected zone is remarkably hardened. The upper limit of B was 0.002%.
Ca：0.005％以下
Caは、靭性を劣化させるMnＳの析出形態を変化させて、その悪影響を緩和する作用があるが、過剰の添加は焼入性の低下を招くため、上限は0.005％とした。
Ca: 0.005% or less
Ca changes the precipitation form of MnS, which degrades toughness, and has the effect of mitigating the adverse effects. However, excessive addition causes a decrease in hardenability, so the upper limit was made 0.005%.
残部は、Feおよび不可避的不純物である。
ここに、不可避的不純物としては、Ｐ，Ｓなどが考えられるが、健全な母材および溶接継手を得るためには、いずれも0.015％以下に抑制することが望ましい。
なお、本発明の効果を損なわない範囲であれば、上記以外の成分の含有、たとえば、靱性改善を目的として、０．００５０％以下のMg及び／または０．０２％以下のREM（希土類金属）の含有、を拒むものではない。
The balance is Fe and inevitable impurities.
Here, P, S, etc. are conceivable as inevitable impurities, but in order to obtain a sound base metal and a welded joint, it is desirable to suppress both to 0.015% or less.
In addition, within the range not impairing the effects of the present invention, the content of components other than the above, for example, 0.0050% or less of Mg and / or 0.02% or less of REM (rare earth metal) for the purpose of improving toughness. It does not refuse to contain.
次に、本発明において、鋼組織を前記のように限定した理由について説明する。
本発明のミクロ組織は、フェライトとベイナイトの混合組織であるが、本発明では、鋼板の中心部と表層部とで組織を個別に制御する。
板厚中心部のミクロ組織：ベイナイト主体組織
鋼材の全厚引張試験において最高荷重到達後鋼板の中心部からボイドが発生し、それらが成長、連結することにより破断に至る。従って、板厚中心部でのボイドの発生を抑制するためには、ボイドの発生源である異相組織の界面を低減する必要があり、そのためには、板厚中心の上下１mmを含む領域のミクロ組織を、加工フェライトを含まず、かつポリゴナルフェライトが面積率で５％以下のベイナイト主体組織とすることが重要である。なお、面積率とは、鋼板断面のミクロ組織から測定される領域内の平均の面積分率を示す。
Next, the reason why the steel structure is limited as described above in the present invention will be described.
Although the microstructure of the present invention is a mixed structure of ferrite and bainite, in the present invention, the structure is individually controlled at the center portion and the surface layer portion of the steel sheet.
Microstructure at the center of the plate thickness: Main structure of bainite In the full thickness tensile test of the steel material, after reaching the maximum load, voids are generated from the center of the steel plate, and they grow and connect to break. Therefore, in order to suppress the occurrence of voids at the center of the plate thickness, it is necessary to reduce the interface of the heterogeneous structure that is the source of the voids. It is important that the structure is a bainite-based structure that does not include processed ferrite and that polygonal ferrite has an area ratio of 5% or less. In addition, an area ratio shows the average area fraction in the area | region measured from the microstructure of a steel plate cross section.
ここに、板厚中心部の組織制御を行うべき領域を、板厚中心の上下１mmを含む領域としたのは、少なくとも板厚中心の上下１mm以内の領域を上記したように組織制御すれば、ボイドの発生が効果的に抑制されるためである。
また、加工フェライトは伸びに対して不利な組織であるので、この領域には加工フェライトは存在させないことにした。
さらに、この領域におけるポリゴナルフェライトの量が面積率で５％を超えると、ポリゴナルフェライトとベイナイトの強度差のために、界面でボイドが発生しやすく伸びの低下を招くので、ポリゴナルフェライトの量は面積率で５％以下に制限した。
Here, the region to be subjected to the structure control at the center of the plate thickness is the region including 1 mm above and below the center of the plate thickness. If the region within 1 mm above and below the center of the plate thickness is controlled as described above, This is because the generation of voids is effectively suppressed.
In addition, since the processed ferrite has a structure which is disadvantageous to elongation, the processed ferrite is not allowed to exist in this region.
Furthermore, if the amount of polygonal ferrite in this region exceeds 5% in terms of area ratio, voids are likely to occur at the interface due to the difference in strength between polygonal ferrite and bainite. The amount was limited to 5% or less in terms of area ratio.
表層部のミクロ組織：フェライトとベイナイトの混合組織
高い一様伸びを確保するためには、加工フェライトを抑制すると共に、延性に優れるポリゴナルフェライトの導入が有効であり、同時に表裏層の硬さを低下させることで延性も向上する。
ただし、ポリゴナルフェライトが面積率で10％未満では、一様伸びや延性の改善効果が小さいので、表裏面から板厚方向に1.5mmの鋼板表層部のミクロ組織は、加工フェライトの面積率が５％以下、ポリゴナルフェライトの面積率が10％以上、好ましくは20％以上、さらに好ましくは30％以上の、フェライトとベイナイトの混合組織とする。
Microstructure of surface layer: Mixed structure of ferrite and bainite In order to ensure high uniform elongation, it is effective to suppress the processed ferrite and introduce polygonal ferrite with excellent ductility, and at the same time reduce the hardness of the front and back layers. The ductility is also improved by lowering.
However, when polygonal ferrite is less than 10% in area ratio, the effect of improving uniform elongation and ductility is small, so the microstructure of the steel sheet surface layer 1.5mm from the front and back surfaces to the sheet thickness direction has an area ratio of processed ferrite. The mixed structure of ferrite and bainite is 5% or less, and the area ratio of polygonal ferrite is 10% or more, preferably 20% or more, more preferably 30% or more.
図１(a)，(b)にそれぞれ、加工フェライトとポリゴナルフェライトの写真を比較して示す。
加工フェライトは、フェライト変態後に圧延により歪が加えられるため、偏平な形状となる。一方、ポリゴナルフェライトは、フェライト変態後に圧延により歪が加えられることがないので、比較的等軸な形状である。従って、加工フェライトとポリゴナルフェライトは、内部の転位密度が異なるなどの違いがあるが、その形状の違いから両者を判別することができる。
本発明では、３％ナイタールで腐食したサンプルの光学顕微鏡観察写真でフェライトのアスペクト比（長軸／短軸）を測定し、このアスペクト比が3.0以上のものを加工フェライト、3.0未満のものをポリゴナルフェライトと定義する。
FIGS. 1 (a) and 1 (b) show photographs of processed ferrite and polygonal ferrite, respectively.
The processed ferrite has a flat shape because strain is applied by rolling after ferrite transformation. Polygonal ferrite, on the other hand, has a relatively equiaxed shape because no strain is applied by rolling after ferrite transformation. Therefore, although the processed ferrite and the polygonal ferrite have a difference such as a difference in internal dislocation density, both can be discriminated from the difference in shape.
In the present invention, the aspect ratio (major axis / minor axis) of the ferrite is measured with an optical microscopic photograph of a sample corroded with 3% nital. It is defined as null ferrite.
板厚方向の硬さ分布の最大値と最小値の差：ビッカース硬さで30HV未満
板厚方向の硬さ分布が大きいと、硬さが大きい部分の伸びが他の硬さの小さい部分の伸びに比べて小さいため、同じ強度で比較した場合に伸びが低くなるなどの問題が生じるので、この硬さ分布を極力小さくすることが望ましい。
そこで、本発明では、板厚方向の硬さ分布の最大値と最小値の差を、ビッカース硬さで30HV未満に抑制するものとした。
ここに、このような一様な硬さ分布を得るには、後述する製造工程において、本発明に従う加速冷却後、誘導加熱を利用した焼戻し処理を施せば良い。
なお、従来の板厚方向の硬さ分布の最大値と最小値の差は、ビッカース硬さでせいぜい50HV程度までしか低減できていなかった。
Difference between maximum and minimum hardness distribution in the plate thickness direction: Vickers hardness of less than 30HV If the hardness distribution in the plate thickness direction is large, the elongation of the portion with the greater hardness is the elongation of the portion with the smaller hardness. Therefore, it is desirable to make the hardness distribution as small as possible because problems such as low elongation occur when compared at the same strength.
Therefore, in the present invention, the difference between the maximum value and the minimum value of the hardness distribution in the thickness direction is suppressed to less than 30 HV in terms of Vickers hardness.
Here, in order to obtain such a uniform hardness distribution, a tempering process using induction heating may be performed after accelerated cooling according to the present invention in the manufacturing process described later.
Note that the difference between the maximum value and the minimum value of the hardness distribution in the thickness direction in the past could only be reduced to about 50 HV at most by the Vickers hardness.
なお、本発明では、対象とする鋼板の板厚を20mm以上とするが、その理由は次のとおりである。
すなわち、本発明のミクロ組織を有する鋼板を本発明の製造方法で実現するためには、以下に述べるとおり、冷却開始時に表層および板厚中心の温度を規定する必要があるが、板厚が20mm未満では現実的に制御が困難であるため、板厚は20mm以上とした。
In the present invention, the thickness of the target steel sheet is set to 20 mm or more, for the following reason.
That is, in order to realize the steel sheet having the microstructure of the present invention by the manufacturing method of the present invention, as described below, it is necessary to define the temperature of the surface layer and the center of the sheet thickness at the start of cooling, but the sheet thickness is 20 mm. If it is less than this, it is difficult to control practically, so the plate thickness is set to 20 mm or more.
次に、本発明の製造方法について説明する。
前記した成分組成になる溶鋼を、転炉や電気炉等の公知の炉を用いて溶製した後、連続鋳造法や造塊−分塊法でスラブとする。
ついで、得られたスラブを、1050〜1250℃の温度に加熱後、累積圧下率：50％以上、鋼板表面温度：Ａr3以上、Ａr3＋15℃以下の条件で熱間圧延を終了し、ついで板厚中心がＡr3以上の温度から加速冷却を行い、鋼板平均温度が350℃以上550℃以下の温度域まで冷却したのち、空冷することにより、本発明で所期した強度−伸びバランスに優れた高靭性高張力鋼板を製造する。
以下、製造条件を上記の範囲に限定した理由について説明する。
Next, the manufacturing method of this invention is demonstrated.
The molten steel having the above-described component composition is melted using a known furnace such as a converter or an electric furnace, and then formed into a slab by a continuous casting method or an ingot-bundling method.
Next, after the obtained slab was heated to a temperature of 1050 to 1250 ° C., the hot rolling was completed under the conditions of the cumulative rolling reduction: 50% or more, the steel sheet surface temperature: Ar 3 or more, and Ar 3 + 15 ° C. or less. The center of thickness is accelerated from a temperature of Ar 3 or higher, and the average temperature of the steel plate is cooled to a temperature range of 350 ° C to 550 ° C, and then air-cooled, providing excellent strength-elongation balance in the present invention. Manufactured high toughness and high strength steel sheet.
Hereinafter, the reason why the manufacturing conditions are limited to the above range will be described.
加熱温度：1050〜1250℃
スラブ加熱は、鋼中の成分を均一化とMo，Nb，Ｖなどの析出強化元素を固溶させるために少なくとも1050℃を確保する必要があるが、加熱温度があまりに高くなると、結晶粒が粗大化し板厚中心においてはマイクロボイドの発生を助長することに加え、母材の靭性劣化を招くため、1050〜1250℃の範囲に限定した。好ましくは1200℃以下である。
Heating temperature: 1050-1250 ° C
In slab heating, it is necessary to secure at least 1050 ° C in order to homogenize the components in the steel and dissolve precipitation strengthening elements such as Mo, Nb, and V. However, if the heating temperature becomes too high, the crystal grains become coarse In the center of the thickness, in addition to promoting the generation of microvoids, the toughness of the base material is deteriorated, so the temperature is limited to the range of 1050 to 1250 ° C. Preferably it is 1200 degrees C or less.
熱間圧延における累積圧下率：50％以上
熱間圧延によりオーステナイト粒の微細化を図ると共に、後工程での加速冷却により、ベイナイト変態の促進およびフェライト粒の微細化を図るためには、熱間圧延における累積圧下率を50％以上とする必要がある。また、母材の靭性を向上させ、より安定に確保する観点からは、1050℃以下900℃以上の温度域で20％以上の累積圧下を付与することが望ましい。これにより、オーステナイト（γ）粒の再結晶に伴って組織が細粒化し、母材の靭性を向上かつ安定化させる。これと同じ効果の面からは、各圧延パス毎の圧下量を５％以上、好ましくは10％以上とすることが望ましい。
Cumulative rolling reduction in hot rolling: 50% or more In order to reduce the austenite grain size by hot rolling and to accelerate bainite transformation and refine the ferrite grain size by accelerated cooling in the subsequent process, The cumulative rolling reduction in rolling needs to be 50% or more. From the viewpoint of improving the toughness of the base material and ensuring more stability, it is desirable to apply a cumulative reduction of 20% or more in a temperature range of 1050 ° C. or lower and 900 ° C. or higher. Thereby, a structure | tissue refines | miniaturizes with recrystallization of austenite ((gamma)) grain, and the toughness of a base material is improved and stabilized. From the aspect of the same effect, it is desirable that the reduction amount for each rolling pass is 5% or more, preferably 10% or more.
圧延終了時の鋼板表面温度：Ａr3以上、Ａr3＋15℃以下
加工フェライトを抑制する上で、最も重要な制御項目である。圧延をＡr3変態点よりもより低い温度で終了すると、初析フェライトを加工することになり、転位を含む加工フェライトが生成するので、圧延終了時における鋼板表面温度はＡr3以上とする。一方、圧延終了温度がＡr3以上であれば加工フェライトの生成は抑制できるものの、高温すぎると結晶粒が粗大化し、靭性の低下や伸びの低下を招く。よって、圧延終了時における鋼板表面温度はＡr3＋15℃以下とする。
Steel plate surface temperature at the end of rolling: Ar 3 or higher, Ar 3 + 15 ° C. or lower This is the most important control item in suppressing processed ferrite. When rolling is completed at a temperature lower than the Ar 3 transformation point, pro-eutectoid ferrite is processed, and processed ferrite containing dislocations is generated. Therefore, the steel sheet surface temperature at the end of rolling is set to Ar 3 or higher. On the other hand, if the rolling end temperature is Ar 3 or higher, the formation of processed ferrite can be suppressed, but if the temperature is too high, the crystal grains become coarse, leading to a decrease in toughness and a decrease in elongation. Therefore, the steel sheet surface temperature at the end of rolling is set to Ar 3 + 15 ° C. or lower.
なお、Ａr3点は、例えば、次に示す関係式を用いて算出することができる。
Ａr3（℃）＝910−310[%Ｃ]−80[%Mn]−20[%Cu]−15[%Cr]−55[%Ni]−80[%Mo] 但し、[%Ｍ]は、Ｍ元素の含有量（質量％）を表す。
The Ar 3 point can be calculated using, for example, the following relational expression.
Ar 3 (° C.) = 910−310 [% C] −80 [% Mn] −20 [% Cu] −15 [% Cr] −55 [% Ni] −80 [% Mo] where [% M] is , Represents the content (mass%) of the M element.
加速冷却の開始温度：板厚中心がＡr3以上
圧延終了温度が上記した温度域の場合、圧延終了後すぐに表層部からフェライト変態が進行する。したがって、表層部においては、直ちに加速冷却を行っても目標とするフェライト分率を確保することが可能であるが、板厚中心部では、ベイナイト主体の組織とする必要があるため、加速冷却開始前のフェライト変態を抑制することを目的として、この加速冷却は板厚中心がＡr3以上の温度から行うこととした。好適な加速冷却の開始温度は、Ａr3＋5〜50℃の範囲である。
ここに、板厚中心温度は、板厚、表面温度および冷却条件等が与えられた場合に、シミュレーション計算等により求められるものを用いることができる。
Accelerated cooling start temperature: The center of the plate thickness is Ar 3 or more. When the rolling end temperature is in the temperature range described above, ferrite transformation proceeds from the surface layer portion immediately after the end of rolling. Therefore, in the surface layer portion, it is possible to secure the target ferrite fraction even if accelerated cooling is performed immediately, but since it is necessary to have a bainite-based structure at the center of the plate thickness, accelerated cooling starts. In order to suppress the previous ferrite transformation, this accelerated cooling is performed from a temperature at which the thickness center is Ar 3 or higher. A suitable starting temperature for accelerated cooling is in the range of Ar 3 + 5-50 ° C.
Here, the plate thickness center temperature can be obtained by simulation calculation or the like when plate thickness, surface temperature, cooling conditions, and the like are given.
ここで、上記した加速冷却の具体的な冷却速度としては、4℃/s以上程度とすることが好ましい。というのは、冷却速度が4℃/ｓに満たないと、冷却途中に一部にフェライトが生成し、強度低下するためである。 Here, the specific cooling rate of the above-described accelerated cooling is preferably about 4 ° C./s or more. This is because if the cooling rate is less than 4 ° C./s, ferrite is partially formed during cooling and the strength is lowered.
加速冷却の停止温度：鋼板平均温度で350℃以上550℃以下
冷却停止温度が鋼板平均温度で350℃未満になると、加速冷却によりマルテンサイトが生成し、靭性が劣化する。一方、冷却停止温度が鋼板平均温度で550℃超では、ベイナイト変態が十分進行しないため、高張力鋼板としての強度を確保するのが困難となるだけでなく、粗大なパーライト組織が生成し、延性が低下する。従って、板厚中心をベイナイト主体組織とするために、冷却停止温度は鋼板平均温度で350℃以上550℃以下の範囲とする。加速冷却終了後は、後述の誘導加熱を実施する場合を除き、空冷することが望ましい。
Accelerated cooling stop temperature: 350 ° C. or higher and 550 ° C. or lower at the steel plate average temperature When the cooling stop temperature falls below 350 ° C. at the steel plate average temperature, martensite is generated by accelerated cooling and the toughness deteriorates. On the other hand, when the cooling stop temperature is higher than 550 ° C at the average temperature of the steel sheet, the bainite transformation does not proceed sufficiently. Decreases. Therefore, in order to make the sheet thickness center a bainite main structure, the cooling stop temperature is in the range of 350 ° C. or more and 550 ° C. or less in terms of the steel plate average temperature. After the accelerated cooling is finished, it is desirable to cool by air except when the induction heating described later is performed.
ここで、冷却時の温度を板厚方向の平均温度で規定した理由は、鋼板の板厚が大きい場合や冷却速度が速い場合には、板厚方向の各部位で温度履歴が異なってしまい、基準が明確でなくなることを防ぐために、鋼材の全体的な材質として最も良く関係する平均温度を基準としたのである。
なお、平均温度は、板厚、表面温度および冷却条件等が与えられた場合に、シミュレーション計算等により求められるものを用いることができる。例えば、差分法を用い、板厚方向の温度分布を平均化することにより得られた温度を平均温度とすることができる。
Here, the reason for prescribing the temperature during cooling by the average temperature in the plate thickness direction is that when the plate thickness of the steel plate is large or the cooling rate is fast, the temperature history is different in each part in the plate thickness direction, In order to prevent the standard from becoming unclear, the average temperature, which is the most relevant to the overall quality of the steel, was used as the standard.
As the average temperature, a value obtained by simulation calculation or the like when a plate thickness, a surface temperature, a cooling condition, or the like is given can be used. For example, the temperature obtained by averaging the temperature distribution in the plate thickness direction using the difference method can be used as the average temperature.
加速冷却時の非冷却期間：合計時間が1.5秒以上、15秒以下
熱間圧延後の加速冷却過程における連続冷却の際に、非冷却期間を設けることにより、表裏層に比べ高温である板厚内部からの熱により表裏層は復熱し、これにより表裏層のみの硬さが低下する。その際、鋼板の中央部に近くなるほど、非冷却期間を設けることによる復熱の影響は小さく、鋼板の中央部およびその周辺では、冷却熱履歴の変化は小さく、冷却速度の低下はほとんど無いかあるいはごく僅かに抑えることができるため、硬さはほとんど低下しない。従って、全厚としての強度を大きく低下させることなく、また熱間圧延後の冷却に要する時間は変わらないため、生産性を低下させることなしに、強度−伸びバランスに優れた高張力鋼板を得ることができる。
Non-cooling period during accelerated cooling: Total time is 1.5 seconds or more and 15 seconds or less. The plate thickness is higher than the front and back layers by providing a non-cooling period during continuous cooling in the accelerated cooling process after hot rolling. The heat from the inside causes the front and back layers to recuperate, thereby reducing the hardness of only the front and back layers. At that time, the closer to the center of the steel plate, the less the effect of recuperation due to the provision of the non-cooling period, and the change in cooling heat history is small at the center of the steel plate and its surroundings, and there is almost no decrease in cooling rate Or since it can suppress very slightly, hardness hardly falls. Therefore, the strength as a whole thickness is not greatly reduced, and the time required for cooling after hot rolling does not change, so that a high-tensile steel sheet having an excellent strength-elongation balance can be obtained without reducing productivity. be able to.
ここに、１回当たりの非冷却期間が0.3秒より短い場合、表裏層硬さの低下が十分でなく、期待する効果が得られないため、１回当たりの非冷却期間は0.3秒以上とする。好ましくは0.8秒以上である。
また、非冷却期間の長さと回数は、製品板厚やサイズ、強度レベルに応じて設定することができる。しかしながら、合計した非冷却時間が短すぎると、表裏層硬さの低下が十分でなく、期待する効果が得られず、一方長過ぎると、板厚中心部およびその周辺の冷却速度が低下することにより、通常の直接焼入れに比べて強度が低下するだけでなく、生産性の低下を招く。従って、非冷却期間の合計時間は1.5秒以上、15秒以下とする。好ましくは３秒以上、13秒以下である。
さらに、上記したような非冷却時間を設ける温度域については、鋼板の温度が低い場合は表裏層の復熱が小さくなって期待される効果が十分得られなくなるため、鋼板の表面温度が300℃以上の温度域とする。
Here, when the non-cooling period per time is shorter than 0.3 seconds, the decrease in front and back layer hardness is not sufficient, and the expected effect cannot be obtained, so the non-cooling period per time is 0.3 seconds or more. . Preferably it is 0.8 second or more.
Further, the length and number of times of the non-cooling period can be set according to the product plate thickness, size, and strength level. However, if the total non-cooling time is too short, the hardness of the front and back layers will not be sufficiently lowered and the expected effect will not be obtained, while if it is too long, the cooling rate at the center of the plate thickness and its surroundings will be reduced. As a result, not only the strength is lowered but also productivity is lowered as compared with the normal direct quenching. Therefore, the total time of the non-cooling period is 1.5 seconds or more and 15 seconds or less. Preferably, it is 3 seconds or more and 13 seconds or less.
Furthermore, for the temperature range providing the non-cooling time as described above, if the temperature of the steel sheet is low, the recuperation of the front and back layers becomes small and the expected effect cannot be obtained sufficiently, so the surface temperature of the steel sheet is 300 ° C. The above temperature range.
また、本発明では、上記した非冷却期間を含む加速冷却の後、さらに誘導加熱により、鋼板の板厚中心温度が650℃以下かつ表面の最高到達温度が580℃以上730℃以下に急速加熱する焼戻し処理を施すことが有利である。
上述した冷却方法により、従来に比べて表層硬さは低下し、伸びは向上するが、鋼板表面のスケールの性状による加速冷却時の冷却速度ばらつきなどに起因して、同一鋼板内でも表面の硬さにばらつきは存在する。引張試験片の平行部にこのようなばらつきが存在することは、伸びの低下を招く。そこで、表面を加熱することにより、同一鋼板内での表面の硬さのばらつきを軽減するのである。
In the present invention, after accelerated cooling including the above-described non-cooling period, the center thickness of the steel sheet is rapidly heated to 650 ° C. or less and the highest surface temperature reaches 580 ° C. or more and 730 ° C. or less by induction heating. It is advantageous to apply a tempering treatment.
Although the surface hardness is reduced and the elongation is improved by the above-described cooling method, the surface hardness is increased even within the same steel sheet due to variations in the cooling rate during accelerated cooling due to the scale properties of the steel sheet surface. There is variation in the size. The presence of such a variation in the parallel portion of the tensile test piece causes a decrease in elongation. Therefore, by heating the surface, variations in surface hardness within the same steel sheet are reduced.
この場合の加熱温度は、板厚中心部あるいは全厚での強度が目標の強度となる適正な温度とする必要があるが、表面の最高到達温度が580℃未満では、表層部の硬さの低減効果や加工フェライトの回復効果が十分でなく、一方730℃を超えると鋼板内部の温度上昇により、全厚としての強度が大幅に低下するおそれがあるだけでなく、炭化物の粗大化により靭性が低下する。また、板厚中心部の温度が650℃超では靭性が低下するおそれがある。従って、この焼戻し処理における加熱温度は、中心温度で650℃以下、鋼板の表面温度で580℃以上、好ましくは620℃以上、730℃以下とした。ここで、板厚中心温度とは、誘導加熱後、鋼板内部の温度分布がほぼ均一になった時の温度を指す。なお、誘導加熱後の冷却条件は特に規定するものではなく、通常の空冷でよいが、積極的に冷却してもさしつかえない。 The heating temperature in this case needs to be an appropriate temperature at which the strength at the center of the plate thickness or at the total thickness becomes the target strength, but if the maximum surface temperature is less than 580 ° C, the hardness of the surface layer portion The reduction effect and recovery effect of processed ferrite are not sufficient.On the other hand, if the temperature exceeds 730 ° C, not only may the strength as a whole thickness decrease significantly due to the temperature rise inside the steel sheet, but also the toughness will increase due to the coarsening of carbides. descend. Further, if the temperature at the center of the plate thickness exceeds 650 ° C., the toughness may decrease. Therefore, the heating temperature in this tempering treatment was set to 650 ° C. or lower at the center temperature and 580 ° C. or higher, preferably 620 ° C. or higher and 730 ° C. or lower as the surface temperature of the steel sheet. Here, the plate thickness center temperature refers to the temperature when the temperature distribution inside the steel plate becomes substantially uniform after induction heating. In addition, the cooling conditions after induction heating are not particularly specified, and normal air cooling may be used, but active cooling may be performed.
表層を優先的に加熱する方法としては、通常のガス燃焼による加熱に代えて、誘導加熱を用い、鋼板表層部に誘導電流を集中させることによって、鋼板内部に比べて表層部の温度が高くなる温度分布を与えることができる。この誘導加熱処理は、オンラインでもオフラインでも構わないが、エネルギーコストの観点からは、焼入れ直後に加熱が可能なオンラインとすることが有利である。
また、誘導加熱を用いることにより、従来に比べ短時間で焼戻し処理ができるため、生産性が向上するのと同時に、鋼板表層と板厚中心部との硬度差がさらに小さくすることができる。すなわち、鋼板の表裏面下0.5mmより内部側における板厚方向の硬さ分布の最大値と最小値の差をビッカース硬さで30HV未満とすることができる。
かくして、480MPa以上の降伏強度と優れた低温靱性を有する、強度−伸びバランスに優れた高靭性高張力鋼板の安定した製造が可能となる。
As a method for preferentially heating the surface layer, the temperature of the surface layer portion becomes higher than that inside the steel plate by using induction heating instead of heating by normal gas combustion and concentrating the induction current on the steel plate surface layer portion. A temperature distribution can be given. This induction heating treatment may be online or offline, but from the viewpoint of energy cost, it is advantageous to make it online that can be heated immediately after quenching.
Further, by using induction heating, the tempering treatment can be performed in a shorter time than in the prior art, so that productivity can be improved and at the same time the hardness difference between the steel sheet surface layer and the plate thickness center can be further reduced. That is, the difference between the maximum value and the minimum value of the hardness distribution in the thickness direction on the inner side from 0.5 mm below the front and back surfaces of the steel sheet can be made less than 30 HV in terms of Vickers hardness.
Thus, it is possible to stably produce a high-toughness, high-tensile steel plate having a yield strength of 480 MPa or more and excellent low-temperature toughness and excellent strength-elongation balance.
表１に示す成分組成になる鋼を溶製し、鋼塊を作製したのち、表２に示す製造条件にて所定の板厚に熱間圧延後、同じく表２に示す種々の条件で供試鋼を製造した。鋼記号Ｃ、鋼記号Ｋはそれぞれ、Ｃ量、Mn量が本発明の適正範囲外の比較鋼であるが、その他の鋼種は成分組成が本発明の適正範囲を満足する適合鋼である。
鋼板中心部のポリゴナルフェライトの面積率は、板厚中央の上下１mmの領域において、３％ナイタールで腐食した400倍の光学顕微鏡写真をランダムに５枚撮影し、画像解析によりポリゴナルフェライト分率を算出した。また、表層部の加工フェライトおよびポリゴナルフェライト分率は、表層直下から0.3mm間隔で400倍の光学顕微鏡写真を５枚撮影し、画像解析により加工フェライトとポリゴナルフェライト分率を算出した。
また、母材の機械的性質の評価として、JIS 5号引張試験片を用いた全厚引張試験、ビッカース硬さによる板厚方向の硬さ分布測定、および1/2ｔ位置でのシャルピー衝撃試験を行った。伸びはＴＳと相関関係があることから、伸びの評価としてＴＳ×Ｅｌ（全伸び）の値を用い、この値が大きいほど、強度−伸びバランス（ＴＳ×Ｅｌ）が優れると評価した。ＴＳ×Ｅｌは、板厚：25mmで30000MPa・％以上、板厚：35mmで35000MPa・％以上を、またvＴsは、−70℃以下を目標値とした。
The steel having the composition shown in Table 1 was melted to produce a steel ingot, and after hot rolling to a predetermined plate thickness under the manufacturing conditions shown in Table 2, the test was conducted under various conditions shown in Table 2 as well. Steel was produced. Steel symbol C and steel symbol K are comparative steels in which the C content and Mn content are outside the proper ranges of the present invention, but the other steel types are compatible steels whose component compositions satisfy the proper ranges of the present invention.
The area ratio of polygonal ferrite in the center of the steel sheet was measured by randomly analyzing five optical micrographs of 400 times corroded with 3% nital in the area of 1 mm above and below the center of the plate thickness, and analyzing the polygonal ferrite fraction. Was calculated. Further, the processed ferrite and polygonal ferrite fractions in the surface layer portion were obtained by taking five optical micrographs 400 times at 0.3 mm intervals from directly below the surface layer, and calculating the processed ferrite and polygonal ferrite fractions by image analysis.
In addition, as an evaluation of the mechanical properties of the base metal, a full-thickness tensile test using JIS No. 5 tensile test pieces, hardness distribution measurement in the thickness direction by Vickers hardness, and Charpy impact test at 1 / 2t position went. Since elongation has a correlation with TS, the value of TS × El (total elongation) was used as an evaluation of elongation, and the greater this value, the better the strength-elongation balance (TS × El). TS × El was set to 30000 MPa ·% or more when the plate thickness was 25 mm, 35000 MPa ·% or more when the plate thickness was 35 mm, and the target value of vTs was −70 ° C. or less.
各供試鋼のミクロ組織および機械的性質について調べた結果を、表３示す。
表３中、No.１〜３、９〜11、13、15〜21は発明例であり、No.４〜８、12、14、22は比較例である。なお、No.９〜12に関しては、圧延後、誘導加熱を用いた焼戻し処理を施した。
Table 3 shows the results of examining the microstructure and mechanical properties of each test steel.
In Table 3, Nos. 1-3, 9-11, 13, 15-21 are invention examples, and Nos. 4-8, 12, 14, 22 are comparative examples. In addition, about No. 9-12, the tempering process using induction heating was performed after rolling.
本発明に従い得られた発明例はいずれも、降伏強度（ＹＳ）が480MPa以上、vＴsが−70℃以下で、ＴＳ×Ｅｌが30000MPa・％以上という優れた特性が得られている。
これに対し、比較例のうち、No.1及びNo.2は、加速冷却時に非冷却期間を設けなかったため、板厚方向の硬さ分布が小さくならなかった。
No.9は、加速冷却時の非冷却期間が長すぎたために、強度が低下した。
No.13は、加速冷却時に非冷却期間を設けた温度域が低すぎたために、表面硬さ分布の低下効果が得られていない。
No.17は、加速冷却開始温度が低く、板厚中心部のフェライト分率が高いために、強度が低く、靱性も低値である。
No.18は、圧延終了温度が高かったために、表層でポリゴナルフェライトが十分に確保されておらず、またNo.19は、圧延終了温度が低く冷却開始温度が守られていなかったために、加工フェライトが導入されて表層部が硬くなり、強度−伸びバランスが低かった。
No.20は、冷却停止温度が低かったために、表層が硬化し、表層と中心部の硬度差が大きく、伸びが低値であり、靱性も低下した。
No.21は、冷却停止温度が高かったために、板厚中心部で粗大なパーライトが生成し、フェライト及びパーライトを主体とする組織となり、ベイナイト主体の組織とはならなかったため、強度及び靱性が低下した。
No.25は、誘導加熱処理における温度が高すぎたために、強度が低下し、靱性も低下した。
No.27およびNo.35は、成分が請求範囲外であったために、目標とする降伏応力が得られなかった。
All of the inventive examples obtained according to the present invention have excellent characteristics such as yield strength (YS) of 480 MPa or more, vTs of −70 ° C. or less, and TS × El of 30000 MPa ·% or more.
On the other hand, among the comparative examples, No. 1 and No. 2 did not have a non-cooling period during accelerated cooling, so the hardness distribution in the thickness direction did not become small.
In No. 9, the strength decreased because the non-cooling period during accelerated cooling was too long.
In No. 13, the temperature range in which the non-cooling period was provided at the time of accelerated cooling was too low, so the effect of reducing the surface hardness distribution was not obtained.
No. 17 has a low accelerated cooling start temperature and a high ferrite fraction at the center of the plate thickness, and therefore has low strength and low toughness.
In No.18, the rolling end temperature was high, so polygonal ferrite was not sufficiently secured in the surface layer, and No.19 was processed because the rolling end temperature was low and the cooling start temperature was not kept. Ferrite was introduced, the surface layer portion became hard, and the strength-elongation balance was low.
In No. 20, since the cooling stop temperature was low, the surface layer was cured, the hardness difference between the surface layer and the central portion was large, the elongation was low, and the toughness was also decreased.
In No. 21, because the cooling stop temperature was high, coarse pearlite was generated at the center of the plate thickness, and it became a structure mainly composed of ferrite and pearlite, but did not become a structure mainly composed of bainite, so the strength and toughness decreased. did.
In No. 25, since the temperature in the induction heat treatment was too high, the strength decreased and the toughness also decreased.
In No. 27 and No. 35, the target yield stress was not obtained because the component was outside the claimed range.
本発明に従って得られる高張力鋼板は、ＹＳ：480MPa以上という高い降伏強度と、vＴs：−70℃以下という優れた低温靱性と共に、ＴＳ×Ｅｌ：30000MPa・％以上（板厚：25mmの場合）という優れた強度−伸びバランス、さらには板厚方向の硬さ分布の最大値と最小値の差がビッカース硬さで30HV未満という一様な硬さ分布をそなえているので、橋梁や貯蔵タンク、圧力容器、ラインパイプなどの鉄鋼構造物の用途に供して極めて有用である。 The high-tensile steel plate obtained according to the present invention has a high yield strength of YS: 480 MPa or more and an excellent low-temperature toughness of vTs: −70 ° C. or less, and TS × El: 30000 MPa ·% or more (when the plate thickness is 25 mm). Excellent strength-elongation balance, as well as the difference between the maximum and minimum hardness distributions in the plate thickness direction, Vickers hardness is uniform less than 30HV, so bridges, storage tanks, pressure It is extremely useful for use in steel structures such as containers and line pipes.
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