JP6879323B2 - Manufacturing method of thick steel sheet with excellent fatigue characteristics - Google Patents
Manufacturing method of thick steel sheet with excellent fatigue characteristics Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims description 60
- 239000010959 steel Substances 0.000 title claims description 60
- 238000004519 manufacturing process Methods 0.000 title claims description 11
- 238000005096 rolling process Methods 0.000 claims description 17
- 238000005204 segregation Methods 0.000 claims description 15
- 238000009749 continuous casting Methods 0.000 claims description 12
- 238000000034 method Methods 0.000 claims description 6
- 238000005098 hot rolling Methods 0.000 claims description 3
- 238000001816 cooling Methods 0.000 claims 1
- 238000012360 testing method Methods 0.000 description 8
- 238000005275 alloying Methods 0.000 description 7
- 208000010392 Bone Fractures Diseases 0.000 description 6
- 206010017076 Fracture Diseases 0.000 description 6
- 238000009661 fatigue test Methods 0.000 description 6
- 239000000956 alloy Substances 0.000 description 5
- 229910045601 alloy Inorganic materials 0.000 description 5
- 238000005516 engineering process Methods 0.000 description 5
- 230000000052 comparative effect Effects 0.000 description 4
- 230000000694 effects Effects 0.000 description 4
- 239000011159 matrix material Substances 0.000 description 4
- 230000008719 thickening Effects 0.000 description 4
- 230000015572 biosynthetic process Effects 0.000 description 2
- 238000005266 casting Methods 0.000 description 2
- 238000007796 conventional method Methods 0.000 description 1
- 238000005520 cutting process Methods 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 239000002994 raw material Substances 0.000 description 1
- 238000011946 reduction process Methods 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 239000002436 steel type Substances 0.000 description 1
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Description
本発明は、船舶、海洋構造物、橋梁、建築物等に使用する構造用材料として好適な厚鋼板に関するものである。本発明は、特に、板厚が50mm以上かつ引張強さが490〜720MPaである厚鋼板の疲労特性の改善に有効である。 The present invention relates to a thick steel plate suitable as a structural material used for ships, marine structures, bridges, buildings and the like. The present invention is particularly effective in improving the fatigue characteristics of a thick steel sheet having a plate thickness of 50 mm or more and a tensile strength of 490 to 720 MPa.
近年、船舶や海洋構造物、橋梁、建築物等の構造物が大型化していくに連れて、使用される厚鋼板の強度と板厚の増加が求められている。大型の構造物の信頼性および健全性を確保する観点から、疲労破壊を防止するための疲労特性の向上は最も重要な課題の一つであり、溶接部のみならず厚鋼板自体の疲労特性を向上する技術が検討されている。 In recent years, as the size of structures such as ships, marine structures, bridges, and buildings has increased, it has been required to increase the strength and thickness of the thick steel plates used. From the viewpoint of ensuring the reliability and soundness of large structures, improving fatigue characteristics to prevent fatigue fracture is one of the most important issues. Technology to improve is being considered.
一般に、厚鋼板の疲労特性に関して、溶接部等の高い応力集中領域が存在しない場合、厚鋼板の強度が高いほど、疲労強度が向上することが知られている。一方で、高強度の厚鋼板では、厚鋼板の組織（以下、母相という）中に存在する介在物等を起点として疲労亀裂が発生し易いことも判明している（非特許文献１参照）。つまり、介在物と母相との界面に生じる応力集中が、疲労亀裂を発生させる原因となっている。 Generally, regarding the fatigue characteristics of a thick steel sheet, it is known that the higher the strength of the thick steel sheet, the higher the fatigue strength when there is no high stress concentration region such as a welded portion. On the other hand, it has also been found that fatigue cracks are likely to occur in high-strength thick steel sheets starting from inclusions and the like existing in the structure of the thick steel sheet (hereinafter referred to as the matrix) (see Non-Patent Document 1). .. That is, the stress concentration generated at the interface between the inclusions and the matrix causes fatigue cracks.
この現象を詳しく研究すると、高強度の厚鋼板が優れた疲労特性を発揮するためには、母相を均一で介在物のない組織とする必要があることが分かる。 A detailed study of this phenomenon reveals that in order for a high-strength thick steel sheet to exhibit excellent fatigue characteristics, it is necessary to have a uniform matrix with no inclusions.
しかし、従来の厚鋼板の製造技術、とりわけ板厚50mm以上かつ引張強さ490〜720MPaである厚鋼板の製造技術では、素材となるスラブを連続鋳造する工程にて中心偏析（すなわち合金元素が中心部で濃化する現象）が発生するという問題、および、スラブを圧延する工程にてスラブ表面とスラブ内部の温度や塑性変形量に差が生じるという問題が残されており、その問題を解消する技術は確立されていない。 However, in the conventional thick steel plate manufacturing technology, especially in the thick steel plate manufacturing technology with a plate thickness of 50 mm or more and a tensile strength of 490 to 720 MPa, central segregation (that is, alloy elements are the center) in the process of continuously casting the slab as the raw material. There remains a problem that a phenomenon of thickening in the part occurs) and a difference in temperature and plastic deformation amount between the slab surface and the inside of the slab in the process of rolling the slab, and this problem is solved. The technology has not been established.
つまり、従来の技術では、圧延された厚鋼板の板厚方向の中央部にも合金元素の濃化領域が形成されるのを防止できず、その領域にて濃化した合金元素が介在物となって析出し、厚鋼板の疲労特性を劣化させる原因となっている。 That is, in the conventional technique, it is not possible to prevent the formation of a concentrated alloy element region in the central portion of the rolled thick steel sheet in the plate thickness direction, and the alloy element concentrated in that region becomes an inclusion. This causes deterioration of the fatigue characteristics of thick steel sheets.
本発明は、従来の技術の問題点を解消し、優れた疲労特性を有する厚鋼板、およびその製造方法を提供することを目的とする。 An object of the present invention is to solve the problems of the prior art and to provide a thick steel sheet having excellent fatigue characteristics and a method for producing the same.
本発明者は、スラブを連続鋳造する工程で不可避的に発生する中心偏析と、スラブを圧延する工程を経て得られた厚鋼板に残留する合金元素の濃化領域との関係について調査した。その結果、スラブの圧延工程において板厚方向の中央部に加えられる塑性変形が不十分な場合に、スラブの中心偏析に起因する合金元素の濃化領域が厚鋼板にも残留することが分かった。そして、
(a)厚鋼板に残留する合金元素の濃化領域にて介在物が生成される、
(b)その介在物と厚鋼板の母相との界面から疲労亀裂が発生する、
(c)厚鋼板の疲労特性を向上するためには、圧延工程にて合金元素の濃化領域を消滅させる必要がある
ことを見出した。
The present inventor investigated the relationship between the central segregation inevitably generated in the step of continuously casting the slab and the concentrated region of the alloy element remaining in the thick steel sheet obtained in the step of rolling the slab. As a result, it was found that when the plastic deformation applied to the central part in the plate thickness direction in the rolling process of the slab is insufficient, the concentrated region of the alloy element due to the central segregation of the slab remains in the thick steel sheet. .. And
(a) Inclusions are formed in the concentrated region of alloying elements remaining on the thick steel sheet.
(b) Fatigue cracks occur from the interface between the inclusions and the matrix of the thick steel sheet.
(c) It was found that in order to improve the fatigue characteristics of thick steel sheets, it is necessary to eliminate the concentrated region of alloying elements in the rolling process.
次に、合金元素の濃化領域を消滅するための圧延技術について詳細に研究した結果、
(d)板厚方向の中央部は圧延工程における塑性変形が加わり難いので、スラブを圧延した後も中心偏析が厚鋼板の板厚方向中央部に合金元素の濃化領域となって残留する
ことが判明し、さらに、
(e)連続鋳造されたスラブに発生する中心偏析を、圧延工程に供する前にスラブの板厚方向中央部から外れた位置に移動させることによって、圧延工程にて十分な塑性変形が合金元素の濃化領域に加わる
ので、合金元素の濃化領域が消滅するという知見を得た。
Next, as a result of detailed research on rolling technology for eliminating the concentrated region of alloying elements,
(d) Since the central part in the plate thickness direction is unlikely to be subjected to plastic deformation in the rolling process, the central segregation remains in the central part in the plate thickness direction of the thick steel sheet as a concentrated region of alloying elements even after rolling the slab. Turned out, and in addition
(e) By moving the central segregation generated in the continuously cast slab to a position deviated from the central part in the plate thickness direction of the slab before it is subjected to the rolling process, sufficient plastic deformation in the rolling process is achieved by the alloying elements. It was found that the concentrated region of the alloying element disappears because it joins the concentrated region.
本発明は、このような知見に基づいてなされたものである。
すなわち本発明は、連続鋳造によって得られたスラブを熱間圧延することによって製造される厚鋼板であって、スラブの板厚方向に対向する互いに平行な２面のうちの片面に減厚処理を施してスラブの板厚を減少させた後に、熱間圧延を行なって製造された厚鋼板である。
The present invention has been made based on such findings.
That is, the present invention is a thick steel sheet produced by hot rolling a slab obtained by continuous casting, and one of two parallel surfaces of the slab facing each other in the plate thickness direction is subjected to a thickness reduction treatment. It is a thick steel sheet manufactured by hot rolling after reducing the plate thickness of the slab.
本発明の厚鋼板においては、連続鋳造によって得られたスラブの板厚ＴCASTED（mm）と、減厚処理を施した後のスラブの板厚ＴPLANED（mm）との差ΔＴ＝ＴCASTED−ＴPLANEDが
ΔＴ≧５mm
を満たすことが好ましい。厚鋼板は、板厚が50mm以上かつ引張強さが490〜720MPaであることが好ましい。
In the thick steel plate of the present invention, the difference between the slab plate thickness T CASTED (mm) obtained by continuous casting and the slab plate thickness T PLANED (mm) after the thickness reduction treatment ΔT = T CASTED − T PLANED is ΔT ≧ 5mm
It is preferable to satisfy. The thick steel plate preferably has a plate thickness of 50 mm or more and a tensile strength of 490 to 720 MPa.
また本発明は、連続鋳造によって得られたスラブを熱間圧延して厚鋼板を製造する厚鋼板の製造方法において、スラブを冷却した後、スラブの板厚方向に対向する互いに平行な２面のうちの片面に減厚処理を施し、次いでスラブを加熱し、引き続きスラブを熱間圧延する厚鋼板の製造方法である。 Further, the present invention is a method for manufacturing a thick steel plate in which a slab obtained by continuous casting is hot-rolled to produce a thick steel plate. This is a method for producing a thick steel sheet in which one side of the steel sheet is thickened, then the slab is heated, and then the slab is hot-rolled.
本発明の厚鋼板の製造方法においては、連続鋳造によって得られたスラブの板厚ＴCASTED（mm）と、減厚処理を施した後のスラブの板厚ＴPLANED（mm）との差ΔＴ＝ＴCASTED−ＴPLANEDが
ΔＴ≧５mm
を満たすことが好ましい。厚鋼板は、板厚が50mm以上かつ引張強さが490〜720MPaであることが好ましい。
In the method for producing a thick steel sheet of the present invention, the difference between the slab thickness T CASTED (mm) obtained by continuous casting and the slab thickness T PLANED (mm) after the thickness reduction treatment ΔT = T CASTED −T PLANED is ΔT ≧ 5mm
It is preferable to satisfy. The thick steel plate preferably has a plate thickness of 50 mm or more and a tensile strength of 490 to 720 MPa.
本発明によれば、優れた疲労特性を有する厚鋼板を得ることができる。特に板厚が50mm以上、引張強さが490〜720MPaの厚鋼板においても、疲労特性を向上することができるので、産業上格段の効果を奏する。 According to the present invention, a thick steel sheet having excellent fatigue characteristics can be obtained. In particular, even thick steel sheets with a plate thickness of 50 mm or more and a tensile strength of 490 to 720 MPa can improve fatigue characteristics, which is extremely effective in industry.
図１は、連続鋳造によって得られたスラブの板厚ＴCASTED（mm）と、減厚処理を施した後のスラブの板厚ＴPLANED（mm）とを示す断面図である。 Figure 1 is a cross-sectional view illustrating a thickness T Casted slab obtained by continuous casting (mm), and a thickness T PLANED slabs was subjected to a thickness reduction process (mm).
図１に示すように、連続鋳造によって得られたスラブ１の板厚はＴCASTEDである。スラブ１の、板厚方向に対向する互いに平行な２面のうちの片面に減厚処理を施すことによって、板厚はＴPLANEDとなる。以下では、減厚処理を施した後のスラブの符号を３とする。 As shown in FIG. 1, the plate thickness of the slab 1 obtained by continuous casting is T CASTED . By applying the thickness reduction treatment to one of the two parallel surfaces of the slab 1 facing each other in the plate thickness direction, the plate thickness becomes T PLANED . In the following, the code of the slab after the thickening treatment is set to 3.
連続鋳造に起因する中心偏析は、スラブ１の板厚方向の中心線２近辺に発生する。そのスラブ１を圧延工程に供すると、板厚方向の中心線２近辺に塑性変形が加わり難いので、圧延工程を終了した厚鋼板（図示せず）の板厚方向の中心線近辺に合金元素の濃化領域が残存する。
Center segregation due to continuous casting occurs in the vicinity of the
ところがスラブ１を圧延に供する前に減厚処理すると、得られたスラブ３の板厚方向の中心線４は、中心偏析が残留する中心線２とは異なる位置に移動する。そのスラブ３を圧延工程に供すると、板厚方向の中心線４には塑性変形が加わり難いが、中心偏析が残留する中心線２には十分な塑性変形を加えることができる。
However, if the slab 1 is subjected to the thickness reduction treatment before being subjected to rolling, the
したがって圧延工程にて、十分な塑性変形を中心偏析の部位に加えることが可能となり、合金元素の濃化領域を消滅させることができる。こうして介在物の生成を防止し、ひいては疲労亀裂の発生を防止できる。 Therefore, in the rolling process, sufficient plastic deformation can be applied to the central segregation site, and the concentrated region of the alloying element can be eliminated. In this way, the formation of inclusions can be prevented, and thus the occurrence of fatigue cracks can be prevented.
スラブ１の板厚ＴCASTED（mm）とスラブ３の板厚ＴPLANED（mm）との差ΔＴ（＝ＴCASTED−ＴPLANED）が小さすぎる場合は、スラブ３の中心線４に極めて近い位置に中心偏析が存在することになるので、スラブ３を圧延する工程において塑性変形を中心偏析の部位に加えることが困難になる。そのため、ΔＴは５mm以上（ΔＴ≧５mm）が好ましい。
If the difference between the thickness T Casted slab 1 (mm) and the slab 3 having a thickness T PLANED (mm) ΔT (= T CASTED -T PLANED) is too small, very close to the
また、ΔＴが大きすぎる場合は、減厚処理に長時間を要するので、加工コストの上昇を招く。したがって、15mm≧ΔＴ≧５mmが一層好ましい。 Further, if ΔT is too large, it takes a long time for the thickening process, which causes an increase in processing cost. Therefore, 15 mm ≧ ΔT ≧ 5 mm is more preferable.
本発明によれば、厚鋼板の鋼種や寸法に関わらず、厚鋼板の疲労特性を向上する効果が得られる。特に、板厚50mm以上かつ引張強さ490〜720MPaである厚鋼板では、疲労特性を向上する技術が確立されておらず、本発明を適用することによって多大な効果を発揮することが可能となる。 According to the present invention, the effect of improving the fatigue characteristics of a thick steel sheet can be obtained regardless of the steel type and dimensions of the thick steel sheet. In particular, for thick steel sheets having a plate thickness of 50 mm or more and a tensile strength of 490 to 720 MPa, a technique for improving fatigue characteristics has not been established, and it is possible to exert a great effect by applying the present invention. ..
連続鋳造によって製造した板厚300mmのスラブを用いて、発明の効果を調査した。本調査で利用した圧延機に圧延可能な板厚の制約があったため、まずスラブの両面を80mmずつ切削加工し、板厚140mmの加工スラブを得た。続いて、加工スラブに減厚処理として切削加工および研削加工を施して、板厚120〜140mmの鋼板とし、その鋼板を圧延に供して板厚60mmの鋼板を得た。鋼板を得るにあたって、スラブの中心偏析が、鋼板の板厚方向60mm（すなわち板厚方向の中心線）の位置から種々ずれるように切削および研削した。これを発明例とする。 The effect of the invention was investigated using a slab with a plate thickness of 300 mm manufactured by continuous casting. Since the rolling mill used in this survey had restrictions on the plate thickness that can be rolled, first, both sides of the slab were machined by 80 mm to obtain a processed slab with a plate thickness of 140 mm. Subsequently, the processed slab was subjected to cutting and grinding as a thickness reduction treatment to obtain a steel plate having a plate thickness of 120 to 140 mm, and the steel plate was subjected to rolling to obtain a steel plate having a plate thickness of 60 mm. In obtaining the steel sheet, the center segregation of the slab was cut and ground so as to be variously deviated from the position of the steel sheet in the plate thickness direction of 60 mm (that is, the center line in the plate thickness direction). This is an example of the invention.
また比較のために、板厚120mmの鋼板を得るにあたって、加工スラブの板厚方向70mmの位置にある中心偏析が、鋼板の板厚方向60mm（すなわち板厚方向の中心線）の位置に一致するように切削および研削した。これを比較例とする。この比較例の鋼板も、圧延に供して板厚60mmの鋼板を得た。 For comparison, when obtaining a steel plate with a plate thickness of 120 mm, the center segregation at the position of 70 mm in the plate thickness direction of the processed slab coincides with the position of the steel plate in the plate thickness direction of 60 mm (that is, the center line in the plate thickness direction). Cut and ground as such. This is a comparative example. The steel sheet of this comparative example was also rolled to obtain a steel sheet having a thickness of 60 mm.
これらの鋼板から、夫々、疲労試験の試験片（図２参照）を採取し、試験片の長手方向に荷重を繰り返し付与して、試験片が破断するまで疲労試験を行なった。なお、図２中の数値の単位はmmである。試験片は、その長手方向が鋼板の上下両表面に対して平行、かつ鋼板の左右両側面に対して垂直となるように採取した。 A test piece (see FIG. 2) for a fatigue test was taken from each of these steel sheets, a load was repeatedly applied in the longitudinal direction of the test piece, and a fatigue test was performed until the test piece broke. The unit of the numerical value in FIG. 2 is mm. The test piece was collected so that its longitudinal direction was parallel to both the upper and lower surfaces of the steel sheet and perpendicular to both the left and right side surfaces of the steel sheet.
疲労試験においては、破断した試験片の破面に現われる模様（いわゆるビーチマーク）によって疲労亀裂の進展挙動を調査するので、応力範囲400MPa（応力比0.1）と応力範囲222MPa（応力比0.5）を2000回ずつ交互に切り替えて荷重を付与した。 In the fatigue test, the growth behavior of fatigue cracks is investigated by the pattern (so-called beach mark) that appears on the fracture surface of the fractured test piece, so the stress range of 400 MPa (stress ratio 0.1) and the stress range of 222 MPa (stress ratio 0.5) are set to 2000. The load was applied by alternately switching each time.
疲労試験によって得られた破断寿命のデータを表１に示す。なお破断寿命は、試験片の疲労亀裂の発生から疲労亀裂の進展を経て、破断の最終段階である延性破壊に到るまでの間に、負荷を付与した回数である。 Table 1 shows the fracture life data obtained by the fatigue test. The fracture life is the number of times a load is applied from the occurrence of fatigue cracks in the test piece to the progress of fatigue cracks to the final stage of fracture, ductile fracture.
表１から明らかなように、発明例の鋼板は、比較例の鋼板よりも破断寿命が長くなっており、疲労特性が優れていることが分かる。 As is clear from Table 1, the steel sheet of the invention example has a longer breaking life than the steel sheet of the comparative example, and it can be seen that the fatigue characteristics are excellent.
また、疲労試験によって破断した試験片の破面を観察すると、発明例の鋼板と比較例の鋼板の疲労亀裂は、いずれもスラブの中心偏析領域を起点として発生していた。つまり、スラブの板厚方向の中心線に加わる塑性変形が、鋼板の疲労特性に影響を及ぼしたと考えられる。 Further, when observing the fracture surface of the test piece fractured by the fatigue test, fatigue cracks of the steel plate of the invention example and the steel plate of the comparative example were both generated from the central segregation region of the slab. That is, it is considered that the plastic deformation applied to the center line in the plate thickness direction of the slab affected the fatigue characteristics of the steel sheet.
１ 連続鋳造によって得られたスラブ
２ スラブ１の板厚方向の中心線
３ 減厚処理を施した後のスラブ
４ スラブ３の板厚方向の中心線
1 Slab obtained by
Claims (3)
ΔＴ≧５mm
を満たすことを特徴とする請求項１に記載の厚鋼板の製造方法。 The difference between the plate thickness T CASTED (mm) of the slab obtained by the continuous casting and the plate thickness T PLANED (mm) of the slab after the thickness reduction treatment is ΔT = T CASTED −T PLANED is ΔT. ≧ 5mm
The method for manufacturing a thick steel sheet according to claim 1, wherein the thick steel sheet is satisfied.
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