WO2015041118A1 - Thin steel plate having excellent fatigue properties and production method therefor - Google Patents
Thin steel plate having excellent fatigue properties and production method therefor Download PDFInfo
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
Definitions
- the present invention mainly relates to a thick steel plate used as a structural material for ships, buildings, bridges, construction machines, and the like, and a method for manufacturing the same. More specifically, the present invention relates to a thick steel plate having a tensile strength of 490 MPa or more and less than 650 MPa and excellent fatigue characteristics, and a method for producing the same.
- a technique for improving fatigue characteristics is required for large structures, but a technique for reducing the crack growth rate (a technique for improving crack growth characteristics) is also required. This is because even if a fatigue crack occurs, if the crack growth rate is low, it is possible to find and repair the damaged part before failure. Therefore, there is a demand for a steel sheet that improves the fatigue characteristics (fatigue strength) of the steel material itself and improves the fatigue crack growth characteristics (decreases the fatigue crack growth rate).
- Cu and Ni are elements necessary for increasing the C activity and generating fine precipitates.
- it is necessary to contain at least one of 0.1% or more.
- it is 0.15% or more, More preferably, it is 0.2% or more.
- the Cu and Ni contents are excessive, the precipitates become coarse and fatigue characteristics cannot be improved, and other characteristics such as toughness are deteriorated. From such a viewpoint, it is necessary that both be 1.0% or less, preferably 0.8% or less, more preferably 0.6% or less.
- Tensile strength TS was measured by taking a tensile test piece having a thickness of 4 mm and a gauge distance of 35 mm from a surface depth of 2 to 6 mm of each steel sheet and performing a tensile test according to JIS Z2241 (2011).
- Test No. 39 is an example using a steel piece (steel type X) with a large Kp value, the bainite fraction is reduced, the effective crystal grain size is increased, and the pearlite fraction in the remaining structure is reduced, The fatigue characteristics deteriorated.
Abstract
Description
圧延方向に平行な縦断面において、鋼板表面から深さ3mmの観察位置で測定したときに、金属組織が下記(a)~(d)の要件を満足し、析出物が下記(A)の要件を満足するところに特徴がある。
0.01≦[Nb]+2[Ti]+2[V]≦0.10 …(1)
([Nb]、[Ti]および[V]は、それぞれNb、TiおよびVの鋼板中の質量%基準での含有量を示す。)
0≦([Cu]+[Ni])-2[Si]≦1.0 …(2)
([Cu]、[Ni]および[Si]は、それぞれCu、NiおよびSiの鋼板中の質量%基準での含有量を示す。)
[Mn]+1.5[Cr]+2[Mo]≦2.4 …(3)
([Mn]、[Cr]および[Mo]は、それぞれMn、CrおよびMoの鋼板中の質量%基準での含有量を示す。)
(a)金属組織がベイナイト組織と、残部組織とから構成され、全組織中、ベイナイト分率が80面積%以上である。
(b)隣り合う結晶の方位差が15°以上の大角粒界に基づいてベイナイトの結晶粒を決定したとき、当該結晶粒の板厚方向の平均長さが7μm以下である。
(c)前記残部組織の円相当直径が3.0μm以下である。
(d)前記残部組織中、パーライト分率が80面積%以上である。
(A)Nb,TiおよびVの少なくともいずれかを含む円相当直径が20nm以下の析出物の個数が100個/μm2以上である。 The thick steel plate of the present invention that has solved the above problems is, by mass%, C: 0.03-0.12%, Si: more than 0% and not more than 0.3%, Mn: 1.0-2.0% B: 0.0005 to 0.005% each and one or more selected from Cu: 0.1 to 1.0% and Ni: 0.1 to 1.0%, and V: 0% Containing 0.05% or less, Nb: more than 0% and 0.05% or less, and Ti: more than 0% and 0.05% or less, and these elements are represented by the following (1) Satisfying the relationship of formula (3), the balance being thick steel plates of iron and inevitable impurities,
In a longitudinal section parallel to the rolling direction, the metal structure satisfies the following requirements (a) to (d) when measured at an observation position at a depth of 3 mm from the surface of the steel sheet, and the precipitates satisfy the following requirements (A). There is a feature in satisfying.
0.01 ≦ [Nb] +2 [Ti] +2 [V] ≦ 0.10 (1)
([Nb], [Ti], and [V] indicate the content of Nb, Ti, and V, respectively, on a mass% basis in the steel sheet.)
0 ≦ ([Cu] + [Ni]) − 2 [Si] ≦ 1.0 (2)
([Cu], [Ni] and [Si] indicate the contents of Cu, Ni and Si on the basis of mass% in the steel sheet, respectively.)
[Mn] +1.5 [Cr] +2 [Mo] ≦ 2.4 (3)
([Mn], [Cr], and [Mo] indicate the content of Mn, Cr, and Mo on a mass% basis in the steel sheet, respectively.)
(A) A metal structure is comprised from a bainite structure and a remainder structure, and a bainite fraction is 80 area% or more in all the structures.
(B) When bainite crystal grains are determined based on a large-angle grain boundary in which the orientation difference between adjacent crystals is 15 ° or more, the average length of the crystal grains in the plate thickness direction is 7 μm or less.
(C) A circle equivalent diameter of the remaining tissue is 3.0 μm or less.
(D) The pearlite fraction is 80 area% or more in the remaining structure.
(A) The number of precipitates having an equivalent circle diameter of 20 nm or less containing at least one of Nb, Ti and V is 100 / μm 2 or more.
(e)金属組織がベイナイト組織と、このベイナイトよりも硬質な残部組織とから構成され、全組織中、ベイナイト分率が80面積%以上である。
(f)隣り合う結晶の方位差が15°以上の大角粒界に基づいてベイナイトの結晶粒を決定したとき、当該結晶粒の板厚方向の平均長さが7μm以下である。
(g)前記硬質残部組織の円相当直径が3μm以下である。 In addition, as a result of conducting a crack growth test and a structure observation on various steel sheets, the present inventors have found that the structure at a position of 1/4 of the sheet thickness (also referred to as a t / 4 position) in a longitudinal section parallel to the rolling direction. It was found that by controlling the form so as to satisfy the following requirements (e) to (g), a steel sheet having excellent crack growth characteristics in addition to fatigue characteristics can be obtained.
(E) The metal structure is composed of a bainite structure and a remaining structure harder than the bainite, and the bainite fraction is 80 area% or more in the entire structure.
(F) When bainite crystal grains are determined based on a large-angle grain boundary in which the orientation difference between adjacent crystals is 15 ° or more, the average length of the crystal grains in the plate thickness direction is 7 μm or less.
(G) The equivalent circular diameter of the hard residual structure is 3 μm or less.
加熱温度:1000~1200℃
全熱間圧延工程の累積圧下率:70%以上
仕上げ圧延中における、Ar3変態点+150℃~Ar3変態点+50℃の温度範囲での累積圧下率:50%以上
仕上げ圧延終了温度:Ar3変態点+30℃以上の温度
仕上げ圧延終了温度から600℃までの平均冷却速度:10℃/秒以下 On the other hand, the production method of the present invention that has achieved the above object is characterized by hot rolling a steel slab having the chemical composition as described above under the following conditions. In this manufacturing method, the cooling stop temperature is preferably 550 ° C. or higher.
Heating temperature: 1000-1200 ° C
Cumulative rolling reduction of the total hot rolling step: 70% during the finish rolling, Ar 3 transformation point + 150 ℃ ~ Ar 3 cumulative rolling reduction in the temperature range of transformation point + 50 ° C. - 50% or more finish rolling end temperature: Ar 3 Temperature of transformation + 30 ° C or higher Average cooling rate from finish rolling finish temperature to 600 ° C: 10 ° C / second or less
金属組織に占めるベイナイト分率を80面積%以上確保し、且つ隣り合う結晶の方位差が15°以上の大角粒界に基づいてベイナイトの結晶粒を決定したとき、当該結晶粒の板厚方向の平均長さ(以下、「有効結晶粒径」と呼ぶことがある)を7μm以下とする。ベイナイト分率は好ましくは85面積%以上、より好ましくは90面積%以上である。有効結晶粒径は、好ましくは6μm以下であり、より好ましくは5μm以下である。 (Organization)
When the bainite fraction in the metal structure is secured at 80 area% or more and the crystal grains of bainite are determined on the basis of the large-angle grain boundaries where the orientation difference between adjacent crystals is 15 ° or more, The average length (hereinafter sometimes referred to as “effective crystal grain size”) is 7 μm or less. The bainite fraction is preferably 85 area% or more, more preferably 90 area% or more. The effective crystal grain size is preferably 6 μm or less, more preferably 5 μm or less.
ベイナイト組織以外の組織(残部組織)を、円相当直径で3.0μm以下とし、残部組織に占めるパーライト分率を80面積%以上とする必要がある。残部組織の円相当直径を3.0μm以下としたのは、残部組織の平均サイズが3.0μmを上回ると、靱性等、他の特性を大きく低下させる恐れがあるためである。残部組織は、また、残部組織を硬質なマルテンサイトやマルテンサイト-オーステナイト混合組織(MA)にすることで、後段寿命の向上は期待できる。その一方でこれらの組織は、変態時に多量の可動転位がベイナイト組織中に導入されるため前段寿命を大きく低下させ、全寿命をも低下させる恐れがある。これに対し、残部組織を、変態時に剪断変形を生じないパーライト主体とすることによって、亀裂発生寿命の低下を防ぐことが可能になる。残部組織の円相当直径の好ましい上限は2.5μm以下(より好ましくは2.0μm以下)であり、好ましい下限は概ね0.5μm以上である。 (Remaining organization)
It is necessary that the structure (remaining structure) other than the bainite structure is 3.0 μm or less in terms of equivalent circle diameter, and the pearlite fraction in the remaining structure is 80 area% or more. The reason why the equivalent circle diameter of the remaining structure is set to 3.0 μm or less is that when the average size of the remaining structure exceeds 3.0 μm, other characteristics such as toughness may be greatly deteriorated. The remaining structure can also be expected to improve the life of the latter stage by making the remaining structure hard martensite or martensite-austenite mixed structure (MA). On the other hand, in these structures, since a large amount of movable dislocations is introduced into the bainite structure during transformation, there is a possibility that the life of the previous stage is greatly reduced and the entire life is also reduced. On the other hand, it is possible to prevent a decrease in crack generation life by making the remaining structure a main body of pearlite that does not cause shear deformation during transformation. A preferable upper limit of the equivalent circle diameter of the remaining tissue is 2.5 μm or less (more preferably 2.0 μm or less), and a preferable lower limit is approximately 0.5 μm or more.
厚鋼板の疲労特性を向上させるためには、Nb,TiおよびVの少なくともいずれかを含む円相当直径が20nm以下の析出物(炭化物および炭窒化物)を鋼板中に微細分散させる必要がある。析出物による強化(析出強化)は、析出物をより微細に且つ多量に分散させることが有効であることが一般的である。また、析出物が転位により剪断されない場合は、析出物サイズが微細であるほうが析出強化の効果が大きくなる。ここで上記析出物(炭化物および炭窒化物)の円相当直径を20nm以下と規定したのは、析出強化を活用しつつ、靱性等の他の特性を低下させる粗大な析出物が生じることを抑制するという観点からである。 (Precipitate)
In order to improve the fatigue characteristics of the thick steel plate, it is necessary to finely disperse precipitates (carbide and carbonitride) having an equivalent circle diameter of 20 nm or less including at least one of Nb, Ti and V in the steel plate. In general, for strengthening by precipitation (precipitation strengthening), it is effective to finely disperse the precipitate in a large amount. In addition, when the precipitate is not sheared by dislocation, the effect of precipitation strengthening is greater when the precipitate size is finer. The reason why the equivalent circle diameter of the above-mentioned precipitates (carbides and carbonitrides) is defined as 20 nm or less is to suppress the formation of coarse precipitates that deteriorate other properties such as toughness while utilizing precipitation strengthening. From the viewpoint of doing.
0.01≦[Nb]+2[Ti]+2[V]≦0.10 …(1)
([Nb]、[Ti]および[V]は、それぞれNb、TiおよびVの鋼板中の含有量(質量%)を示す。) The inventors of the present invention designed the alloy by paying attention to the ability to form precipitates by C (this is called “C activity”) so that fine precipitates can be secured stably at low cost. did. From this point of view, it is necessary to appropriately control the amount of the elements constituting the precipitate, and specifically, the following contents of Nb, Ti and V must satisfy the relationship of the formula (1).
0.01 ≦ [Nb] +2 [Ti] +2 [V] ≦ 0.10 (1)
([Nb], [Ti] and [V] indicate the contents (mass%) of Nb, Ti and V in the steel sheet, respectively.)
Cは、母材(鋼板)の強度を確保するために重要な元素であり、併せて析出物を構成する元素である。こうした効果を有効に発揮させるために、C量は0.03%以上と定めた。C量は、好ましくは0.04%以上であり、より好ましくは0.05%以上である。一方、C量が過剰になると、残部組織が粗大且つ過剰に生じるため疲労特性が低下する。そこでC量は0.12%以下と定めた。C量は、好ましくは0.10%以下であり、より好ましくは0.08%以下である。 (C: 0.03-0.12%)
C is an important element for securing the strength of the base material (steel plate), and is an element constituting the precipitate. In order to effectively exhibit such effects, the C content is determined to be 0.03% or more. The amount of C is preferably 0.04% or more, and more preferably 0.05% or more. On the other hand, when the amount of C is excessive, the remaining structure is coarse and excessively generated, so that fatigue characteristics are deteriorated. Therefore, the C amount is set to 0.12% or less. The amount of C is preferably 0.10% or less, and more preferably 0.08% or less.
Siは、母材(鋼板)の強度を確保するために必要な元素であるが、同時にC活量を低下させる元素であるため、その添加量を0.3%以下とする必要がある。Si量は、好ましくは0.25%以下であり、より好ましくは0.2%以下である。尚、上記のような作用を発揮させるためには、Si量は0.01%以上であることが好ましく、より好ましくは0.05%以上である。 (Si: 0.3% or less (excluding 0%))
Si is an element necessary for ensuring the strength of the base material (steel plate), but at the same time, it is an element that lowers the C activity, so that the amount added must be 0.3% or less. The amount of Si is preferably 0.25% or less, and more preferably 0.2% or less. In addition, in order to exhibit the above effects, the Si amount is preferably 0.01% or more, more preferably 0.05% or more.
Mnは、ベイナイト組織を得るために焼入れ性を確保する上で重要な元素である。こうした作用を有効に発揮させるため、Mn量は1.0%以上とする必要がある。Mn量は、好ましくは1.2%以上であり、より好ましくは1.4%以上である。しかし、Mn量が過剰になると、残部組織中に占めるパーライト分率が低下するため十分な疲労特性が得られない。そのため、Mn量は2.0%以下とする必要がある。Mn量は、好ましくは1.8%以下であり、より好ましくは1.6%以下である。 (Mn: 1.0-2.0%)
Mn is an important element in securing hardenability in order to obtain a bainite structure. In order to exhibit such an action effectively, the amount of Mn needs to be 1.0% or more. The amount of Mn is preferably 1.2% or more, more preferably 1.4% or more. However, when the amount of Mn is excessive, the pearlite fraction in the remaining structure is reduced, so that sufficient fatigue characteristics cannot be obtained. Therefore, the amount of Mn needs to be 2.0% or less. The amount of Mn is preferably 1.8% or less, more preferably 1.6% or less.
Bは、焼入れ性を向上させる元素であり、またフェライト変態を抑制してベイナイト組織を生じさせやすくする元素である。こうした効果を発揮させるためには、Bは0.0005%以上含有させる必要がある。B量は、好ましくは0.001%以上であり、より好ましくは0.002%以上である。しかしながら、B含有量が過剰になると、十分な析出物が得られず疲労特性向上効果が得られないため、0.005%以下とする必要がある。B含有量の好ましい上限は0.004%以下であり、より好ましくは0.003%以下である。 (B: 0.0005-0.005%)
B is an element that improves hardenability, and is an element that suppresses ferrite transformation and easily causes a bainite structure. In order to exhibit such an effect, B needs to be contained by 0.0005% or more. The amount of B is preferably 0.001% or more, and more preferably 0.002% or more. However, if the B content is excessive, sufficient precipitates cannot be obtained and the effect of improving fatigue characteristics cannot be obtained, so 0.005% or less is necessary. The upper limit with preferable B content is 0.004% or less, More preferably, it is 0.003% or less.
CuおよびNiは、C活量を増加させて微細な析出物を生じさせるために必要な元素である。こうした作用を発揮させるためには、少なくともいずれかを0.1%以上含有させる必要がある。好ましくは0.15%以上であり、より好ましくは0.2%以上である。しかしながら、Cu,Niの含有量が過剰になると、析出物が粗大化して疲労特性の向上が得られないばかりか、靱性等、他の特性を悪化させる。こうした観点から、いずれも1.0%以下とする必要があり、好ましくは0.8%以下、より好ましくは0.6%以下である。 (One or more selected from Cu: 0.1 to 1.0% and Ni: 0.1 to 1.0%)
Cu and Ni are elements necessary for increasing the C activity and generating fine precipitates. In order to exert such an effect, it is necessary to contain at least one of 0.1% or more. Preferably it is 0.15% or more, More preferably, it is 0.2% or more. However, if the Cu and Ni contents are excessive, the precipitates become coarse and fatigue characteristics cannot be improved, and other characteristics such as toughness are deteriorated. From such a viewpoint, it is necessary that both be 1.0% or less, preferably 0.8% or less, more preferably 0.6% or less.
0≦([Cu]+[Ni])-2[Si]≦1.0 …(2)
([Cu]、[Ni]および[Si]は、それぞれCu、NiおよびSiの鋼板中の含有量(質量%)を示す。) Since Cu, Ni, and Si are elements that affect the C activity, in order to stably obtain fine precipitates, these contents should be set so as to satisfy the relationship of the following formula (2). Need to control.
0 ≦ ([Cu] + [Ni]) − 2 [Si] ≦ 1.0 (2)
([Cu], [Ni] and [Si] indicate the contents (mass%) of Cu, Ni and Si in the steel sheet, respectively.)
V、NbおよびTiは、焼入れ性の向上によりベイナイト組織の確保に加えて析出物を生じさせるために必要な元素である。しかしながら、過剰に含有させると析出物が粗大化し、十分な疲労向上効果が得られないため、夫々0.05%以下に制限する必要がある。これらの元素の好ましい下限は、上記(1)式の関係で自ずと決定される。 (V: 0.05% or less (not including 0%), Nb: 0.05% or less (not including 0%) and Ti: 0.05% or less (not including 0%) One or more)
V, Nb, and Ti are elements necessary for producing precipitates in addition to securing a bainite structure by improving hardenability. However, if it is excessively contained, the precipitates become coarse and a sufficient fatigue improvement effect cannot be obtained. Therefore, it is necessary to limit each to 0.05% or less. The preferable lower limit of these elements is naturally determined by the relationship of the above formula (1).
[Mn]+1.5[Cr]+2[Mo]≦2.4 …(3)
([Mn]、[Cr]および[Mn]は、それぞれMn、CrおよびMoの鋼板中の含有量(質量%)を示す。) In order to make the remaining structure a pearlite-based structure and prevent a decrease in crack generation life, it is necessary to appropriately control the relationship among the contents of Mn, Cr and Mo. Mn, Cr and Mo must satisfy the relationship of the following formula (3).
[Mn] +1.5 [Cr] +2 [Mo] ≦ 2.4 (3)
([Mn], [Cr] and [Mn] indicate the contents (mass%) of Mn, Cr and Mo in the steel sheet, respectively.)
Caは、鋼中の介在物(例えばMnS等)の形状の異方性を低減する元素であり、介在物が破壊の起点となることを防ぎ、疲労特性を向上させるのに有効な元素である。こうした作用を有効に発揮させるには、0.0005%以上含有させることが好ましい。より好ましくは0.001%以上である。しかしながら、Ca含有量が過剰になると、鋼中の清浄度を低下させ、却って疲労特性を悪化させるだけではなく、靱性等、他の特性を低下させる恐れがある。そのため、Ca含有量は0.005%以下とすることが好ましく、より好ましくは0.003%以下である。 (Ca: 0.005% or less (excluding 0%))
Ca is an element that reduces the anisotropy of the shape of inclusions (for example, MnS, etc.) in steel, and is an element that is effective in preventing inclusions from becoming the starting point of fracture and improving fatigue properties. . In order to exhibit such an action effectively, it is preferable to contain 0.0005% or more. More preferably, it is 0.001% or more. However, when the Ca content is excessive, not only the cleanliness in the steel is lowered and the fatigue properties are deteriorated, but other properties such as toughness may be lowered. Therefore, the Ca content is preferably 0.005% or less, and more preferably 0.003% or less.
Alは、脱酸材として有用な元素であり、こうした作用を発揮させるには0.01%以上含有させることが好ましい。より好ましくは0.02%以上である。但し、Al含有量が過剰になると、疲労特性だけではなく、靱性等を低下させるために、0.10%以下とすることが好ましく、より好ましくは0.04%以下である。 (Al: 0.10% or less (excluding 0%))
Al is an element useful as a deoxidizer, and it is preferable to contain 0.01% or more in order to exhibit such an action. More preferably, it is 0.02% or more. However, when the Al content is excessive, in order to reduce not only fatigue properties but also toughness and the like, the content is preferably 0.10% or less, more preferably 0.04% or less.
Nは、固溶強化により強度を向上させる効果があるので、必要によって積極的に含有させる。しかしながら、N含有量が過剰になると、粗大な窒化物が生じ、疲労特性を低下させる。こうした観点から、N含有量は0.010%以下とすることが好ましく、より好ましくは0.008%以下である。尚、Nによる効果を有効に発揮させるためには、その含有量は0.0035%以上とすることが好ましく、より好ましくは0.0040%以上である。 (N: 0.010% or less (excluding 0%))
Since N has an effect of improving strength by solid solution strengthening, it is positively incorporated as necessary. However, when the N content is excessive, coarse nitrides are formed and the fatigue characteristics are deteriorated. From such a viewpoint, the N content is preferably set to 0.010% or less, and more preferably 0.008% or less. In order to effectively exhibit the effect of N, the content is preferably 0.0035% or more, more preferably 0.0040% or more.
Crは、Mnと同様の効果を有する元素であり、ベイナイト組織を安定的に得ることができる。このような作用を発揮させるためには、Crは0.1%以上含有させことが好ましい。より好ましくは0.5%以上である。しかしながら、Cr含有量が過剰になると、残部組織中のパーライト分率を低減させて疲労特性が低下する恐れがあるため、2%以下とすることが好ましい。Cr含有量のより好ましい上限は1.7%以下である。 (Cr: 2% or less (excluding 0%))
Cr is an element having the same effect as Mn, and can stably obtain a bainite structure. In order to exert such an effect, it is preferable to contain Cr by 0.1% or more. More preferably, it is 0.5% or more. However, if the Cr content is excessive, the pearlite fraction in the remaining structure may be reduced and the fatigue characteristics may be deteriorated, so 2% or less is preferable. The upper limit with more preferable Cr content is 1.7% or less.
Moは、Mnと同様の効果を有する元素であり、ベイナイト組織を安定的に得ることができる。このような作用を発揮させるためには、Moは0.005%以上含有させることが好ましく、より好ましくは0.01%以上、更に好ましくは0.03%以上である。しかしながら、Mo含有量が過剰になると、残部組織中のパーライト分率を低減させて疲労特性が低下するので、その含有量は1%以下とすることが好ましい。Mo含有量のより好ましい上限は、0.7%以下であり、更に好ましくは0.5%以下である。 (Mo: 1% or less (excluding 0%))
Mo is an element having an effect similar to that of Mn, and a bainite structure can be stably obtained. In order to exhibit such an action, Mo is preferably contained in an amount of 0.005% or more, more preferably 0.01% or more, and further preferably 0.03% or more. However, if the Mo content is excessive, the pearlite fraction in the remaining structure is reduced and the fatigue characteristics are lowered, so the content is preferably 1% or less. The upper limit with more preferable Mo content is 0.7% or less, More preferably, it is 0.5% or less.
加熱温度:1000~1200℃
全熱間圧延工程の累積圧下率:70%以上
仕上げ圧延中における、Ar3変態点+150℃~Ar3変態点+50℃の温度範囲での累積圧下率:50%以上
仕上げ圧延終了温度:Ar3変態点+30℃以上の温度
仕上げ圧延終了温度から600℃までの平均冷却速度:10℃/秒以下 The thick steel plate of the present invention satisfies the above-mentioned requirements, and its manufacturing method is not particularly limited. In the steel plate series manufacturing process of hot rolling after melting and casting the steel, In order to obtain precipitates with improved properties, steel slabs having the chemical composition described above (for example, slabs) are used, the heating temperature before hot rolling, the cumulative rolling reduction during hot rolling, and finish rolling. It is preferable to control the temperature, finish rolling reduction, finish rolling end temperature, cooling rate after hot rolling, and cooling stop temperature as follows.
Heating temperature: 1000-1200 ° C
Cumulative rolling reduction in all hot rolling processes: 70% or more Cumulative rolling reduction in the temperature range of Ar 3 transformation point + 150 ° C. to Ar 3 transformation point + 50 ° C. during finishing rolling: 50% or more Finish rolling finishing temperature: Ar 3 Transformation point + 30 ° C or higher Average cooling rate from finish rolling finish temperature to 600 ° C: 10 ° C / second or less
累積圧下率=(t0-t1)/t2×100 …(4)
[(4)式中、t0は表面から3mmの位置の温度が圧延温度範囲にあるときの鋼片の圧延開始厚み(mm)、t1は表面から3mmの位置の温度が圧延温度範囲にあるときの鋼片の圧延終了厚み(mm)、t2は圧延前の鋼片(例えばスラブ)の厚みを、夫々示す。]
Ar3変態点=910-230×[C]+25×[Si]-74×[Mn]-56×[Cu]-16×[Ni]-9×[Cr]-5×[Mo]-1620×[Nb]…(5)
但し、[C]、[Si]、[Mn]、[Cu]、「Ni]、[Cr]、[Mo]および[Nb]は、夫々C、Si、Mn、Cu、Ni、Cr、MoおよびNbの含有量(質量%)を示す。 The “cumulative rolling reduction” is a value calculated from the following equation (4). The Ar 3 transformation point employs a value obtained by the following equation (5) (the values shown in Tables 1 and 2 described later are also the same).
Cumulative rolling reduction = (t 0 −t 1 ) / t 2 × 100 (4)
[In the formula (4), t 0 is the rolling start thickness (mm) of the steel slab when the temperature at the position 3 mm from the surface is in the rolling temperature range, and t 1 is the temperature at the position 3 mm from the surface in the rolling temperature range. The rolling end thickness (mm) of the steel slab at a certain time, and t 2 indicate the thickness of the steel slab (for example, slab) before rolling. ]
Ar 3 transformation point = 910-230 × [C] + 25 × [Si] −74 × [Mn] −56 × [Cu] −16 × [Ni] −9 × [Cr] −5 × [Mo] −1620 × [Nb] (5)
However, [C], [Si], [Mn], [Cu], “Ni”, [Cr], [Mo] and [Nb] are C, Si, Mn, Cu, Ni, Cr, Mo and The Nb content (% by mass) is shown.
板厚のt/4となる位置の金属組織に占めるベイナイト分率を80面積%以上確保し、且つ隣り合う結晶方位差が15°以上の大角粒界に基づいてベイナイトの結晶粒を決定したとき、当該結晶粒の板厚方向の平均長さ(平均有効結晶粒径)を7μm以下とする。ベイナイト分率は好ましくは85面積%以上、より好ましくは90面積%以上である。平均有効結晶粒径は、好ましくは6μm以下であり、より好ましくは5μm以下である。 (Structure inside the steel plate)
When bainite crystal grains are determined on the basis of large-angle grain boundaries in which the bainite fraction in the metal structure at the position of t / 4 of the plate thickness is 80% by area or more and the adjacent crystal orientation difference is 15 ° or more. The average length of the crystal grains in the plate thickness direction (average effective crystal grain size) is 7 μm or less. The bainite fraction is preferably 85 area% or more, more preferably 90 area% or more. The average effective crystal grain size is preferably 6 μm or less, more preferably 5 μm or less.
ベイナイト組織以外の組織(残部組織)のうち、硬質の残部組織(硬質残部組織)を円相当直径で3μm以下とし、ベイナイトよりも硬質なものとすることが好ましい。硬質残部組織の円相当直径を3μm以下としたのは、残部組織の平均サイズが3μmを上回ると、靱性等、他の特性を大きく低下させる恐れがある。残部組織は基本的にマルテンサイト、MAを含むものでありこれらの硬質残部組織は亀裂進展速度を低下させることが可能となる。 (Remaining structure inside the steel plate)
Of the structures (remaining structures) other than the bainite structure, the hard remaining structure (hard remaining structure) is preferably 3 μm or less in terms of the equivalent circle diameter, and is preferably harder than bainite. The reason why the equivalent circle diameter of the hard residual structure is set to 3 μm or less is that when the average size of the residual structure exceeds 3 μm, other characteristics such as toughness may be greatly deteriorated. The remaining structure basically includes martensite and MA, and these hard remaining structures can reduce the crack growth rate.
全熱間圧延工程の累積圧下率:80%以上
未再結晶温度域での圧下率:70%未満 In order to ensure such a structure form, it is preferable to control the cumulative reduction ratio during hot rolling and the reduction ratio in the non-recrystallization temperature range as follows.
Cumulative rolling reduction in all hot rolling processes: 80% or more Rolling reduction in non-recrystallization temperature range: less than 70%
下記表1および表2に示す化学成分組成の鋼(鋼種A~Z)を、通常の溶製法に従って溶製し鋳造した後、下記表3に示す各種条件(圧延条件No.a~p)にて熱間圧延を行い、厚さ18~20mmの鋼板を得た。尚、表3において、「未再結晶温度域圧下率」とは、Ar3変態点+150℃~Ar3変態点+50℃の温度範囲での圧下率(累積圧下率)である。また表3に示した「未再結晶温度域圧下率」は設計値であり、仕上げ圧延終了温度がAr3変態点+150℃よりも高くなる(即ち、未再結晶温度域到達前に圧延を完了する)場合には、未再結晶温度域圧下率は0%(即ち、未再結晶温度域での圧下なし)となる(例えば、表5の試験No.26)。 Example 1
After the steels (steel types A to Z) having the chemical composition shown in Table 1 and Table 2 were melted and cast according to a normal melting method, they were subjected to various conditions (rolling conditions No. a to p) shown in Table 3 below. Thus, hot rolling was performed to obtain a steel plate having a thickness of 18 to 20 mm. In Table 3, “non-recrystallization temperature range reduction ratio” is the reduction ratio (cumulative reduction ratio) in the temperature range of Ar 3 transformation point + 150 ° C. to Ar 3 transformation point + 50 ° C. In addition, “Non-recrystallization temperature range reduction ratio” shown in Table 3 is a design value, and the finish rolling finish temperature becomes higher than Ar 3 transformation point + 150 ° C. (that is, rolling is completed before reaching the non-recrystallization temperature range). In this case, the unrecrystallization temperature range reduction rate is 0% (that is, no reduction in the non-recrystallization temperature range) (for example, test No. 26 in Table 5).
鋼板表面から深さ3mm位置の鋼板の圧延方向に平行で且つ鋼板の表面に対して垂直な面が露出するようにサンプルを切り出し、これを#150~#1000までの湿式エメリー紙を用いて研磨し、その後に研磨剤としてダイヤモンド研磨剤を用いて鏡面研磨仕上げした。この鏡面試験片を、2%硝酸-エタノール溶液(ナイタール溶液)でエッチングした後、150μm×200μmの視野を観察倍率400倍で観察し、画像解析にてベイナイト分率(面積%)を測定した。合計で5視野のベイナイト分率を求めて、その平均値を採用した。 (Bainite fraction)
A sample is cut out so that a plane parallel to the rolling direction of the steel sheet at a depth of 3 mm from the steel sheet surface and perpendicular to the surface of the steel sheet is exposed, and this is polished using wet emery paper from # 150 to # 1000. Then, mirror polishing was performed using a diamond abrasive as an abrasive. After this mirror surface test piece was etched with a 2% nitric acid-ethanol solution (Nital solution), a 150 μm × 200 μm field of view was observed at an observation magnification of 400 times, and the bainite fraction (area%) was measured by image analysis. A total of 5 bainite fractions were obtained and the average value was adopted.
鋼板表層から深さ3mm位置の鋼板の圧延方向に平行な断面において、SEM(Scanning Electron Microscape:走査型電子顕微鏡)-EBSP(Electron Backscatter Pattern:電子後方散乱解析像法)によって有効結晶粒径(大角粒界径)を測定した。具体的には、TEX SEM Laboratries社のEBSP装置(商品名:「OIM」)をSEMと組み合わせて用い、傾角(結晶方位差)が15°以上の境界を結晶粒界として有効結晶粒径を測定した。このときの測定条件は、測定領域:200μm×200μm、測定ステップ:0.5μm間隔とし、測定方位の信頼性を示すコンフィデンス・インデックス(Confidence Index)が0.1よりも小さい測定点は解析対象から除外した。このようにして求められる結晶粒界について、板厚方向に100箇所の切断長さを測定し、その平均値を有効結晶粒径とした。但し、有効結晶粒径が2.0μm以下は測定ノイズとして判断し、除外した。 (Effective crystal grain size)
In a cross section parallel to the rolling direction of a steel sheet at a depth of 3 mm from the steel sheet surface layer, the effective crystal grain size (large angle) is measured by SEM (Scanning Electron Microscape) -EBSP (Electron Backscatter Pattern). Grain boundary diameter) was measured. Specifically, using an EBSP device (trade name: “OIM”) manufactured by TEX SEM Laboratories in combination with SEM, the effective crystal grain size is measured with a boundary having an inclination (crystal orientation difference) of 15 ° or more as the grain boundary. did. The measurement conditions at this time are: measurement area: 200 μm × 200 μm, measurement step: 0.5 μm interval, and measurement points whose confidence index (Confidence Index) indicating the reliability of the measurement direction is smaller than 0.1 are analyzed. Excluded. With respect to the crystal grain boundaries thus obtained, the cutting lengths at 100 locations in the plate thickness direction were measured, and the average value was taken as the effective crystal grain size. However, an effective crystal grain size of 2.0 μm or less was judged as measurement noise and excluded.
残部組織のサイズと、残部組織中のパーライト分率は、上記ベイナイト分率の測定と同様の方法によってサンプルを切り出し、研磨、エッチングを行った後、SEMにて観察倍率1000倍で観察し、画像解析にて残部組織のサイズ(円相当直径)と、残部組織中のパーライト分率を求めた。両者とも5視野の平均値を採用した。 (Remaining tissue size and pearlite fraction)
The size of the remaining structure and the pearlite fraction in the remaining structure were cut out by a method similar to the measurement of the bainite fraction, polished and etched, and then observed with an SEM at an observation magnification of 1000 times. The size of the remaining structure (equivalent circle diameter) and the pearlite fraction in the remaining structure were determined by analysis. Both adopted the average value of 5 fields of view.
各鋼板の表層深さ2~6mm位置から板厚4mm、標点距離35mmの引張試験片を採取し、JIS Z2241(2011)にしたがって引張試験を行うことによって、引張強さTSを測定した。 (Tensile strength)
Tensile strength TS was measured by taking a tensile test piece having a thickness of 4 mm and a gauge distance of 35 mm from a surface depth of 2 to 6 mm of each steel sheet and performing a tensile test according to JIS Z2241 (2011).
疲労特性は、板厚表層2~6mm位置から4mm厚の鋼材を切り出し、図1に示すような試験片を作製して行った。尚、試験片表面はエメリー紙にて#1200まで研磨を行って、表面状態の影響を除去した。得られた試験片について、インストロン社製電気油圧サーボ式疲労試験機を用いて、以下の条件で疲労試験を行った。 (Fatigue properties)
The fatigue characteristics were obtained by cutting out a steel material having a thickness of 4 mm from the 2 to 6 mm position of the plate thickness surface layer and preparing a test piece as shown in FIG. The surface of the test piece was polished to # 1200 with emery paper to remove the influence of the surface state. About the obtained test piece, the fatigue test was done on condition of the following using the electrohydraulic servo type fatigue testing machine made from Instron.
制御方法:荷重制御
制御波形:正弦波
応力比:R=-1
試験速度:20Hz
試験終了サイクル数:5000000回 Test environment: Room temperature, in air Control method: Load control Control waveform: Sine wave Stress ratio: R = -1
Test speed: 20Hz
Number of test end cycles: 5000000 times
鋼板表面から3mm位置より採取したサンプルから、抽出レプリカ法によって作製した試験片について、透過型電子顕微鏡(TEM:Transmission Electron Microscape)にて、観察倍率150000倍、観察視野750nm×625nm、観察箇所5視野を観察し、画像解析によりその視野中のNb,Ti,Vのいずれかを含む析出物の面積を測定し、この面積から各析出物の円相当直径を算出した。尚、Nb,Ti,Vのいずれかを含むことはEDX(Energy Dispersive X-ray spectrometry:エネルギー分散型X線分析)によって判別した。円相当直径が20nm以下となる析出物の1μm2あたりに換算することで個数密度とした。 (Number density and size of precipitates)
A test piece prepared by the extraction replica method from a sample collected from a position 3 mm from the surface of the steel sheet was observed with a transmission electron microscope (TEM) at an observation magnification of 150,000 times, an observation field of view 750 nm × 625 nm, and an observation part 5 fields of view. The area of the precipitate containing any of Nb, Ti, and V in the visual field was measured by image analysis, and the equivalent circle diameter of each precipitate was calculated from this area. The inclusion of Nb, Ti, or V was determined by EDX (Energy Dispersive X-ray spectrometry). The number density was determined by converting per 1 μm 2 of the precipitate having an equivalent circle diameter of 20 nm or less.
表4に示した試験No.1~19の各鋼板について、板厚の1/4となる位置のベイナイト分率、有効結晶粒径、第2相のサイズについて、実施例1に示した方法と同様にして評価した。試験片の採取方法については、板厚の1/4となる位置とする以外は、上記と同様である。また、これらの鋼板につき、下記の方法によって亀裂進展速度を測定した。 (Example 2)
Test No. shown in Table 4 For each of the steel plates 1 to 19, the bainite fraction, the effective crystal grain size, and the size of the second phase at a position corresponding to 1/4 of the plate thickness were evaluated in the same manner as in the method shown in Example 1. The method of collecting the test piece is the same as described above except that the position is a quarter of the plate thickness. Moreover, about these steel plates, the crack growth rate was measured by the following method.
ASTM E647に準拠し、コンパクト試験片を用い、電気油圧サーボ式疲労試験機にて下記の条件で疲労亀裂進展試験を行い、亀裂進展速度を測定した。尚、コンパクト試験片は、板厚の1/2となる位置から採取し、図2に示す形状のものを用いた。また、亀裂長さはコンプライアンス法を用いた。
試験環境:室温、大気中
制御方法:荷重制御
制御波形:正弦波
応力比:R=-1
試験速度:5~20Hz (Crack growth rate)
In accordance with ASTM E647, using a compact test piece, a fatigue crack growth test was performed using an electrohydraulic servo fatigue tester under the following conditions, and the crack growth rate was measured. In addition, the compact test piece was extract | collected from the position used as 1/2 of board thickness, and the thing of the shape shown in FIG. 2 was used. Moreover, the compliance method was used for the crack length.
Test environment: Room temperature, in air Control method: Load control Control waveform: Sine wave Stress ratio: R = -1
Test speed: 5-20Hz
da/dn=C(ΔK)m …(6)
[(1)式中、a:亀裂長さ、n:繰り返し数、C,m:材料、荷重等の条件で決まる定数を夫々示す。] At this time, the value in the stable growth region ΔK = 20 (MPa · m 1/2 ) where the Paris law defined by the following equation (6) is satisfied was evaluated as a representative value. A crack having a crack growth rate of 5.0 × 10 −5 mm / cycle or less when ΔK = 20 (MPa · m 1/2 ) is considered to have excellent crack growth characteristics.
da / dn = C (ΔK) m (6)
[In formula (1), a: crack length, n: number of repetitions, C, m: constants determined by conditions such as material and load, respectively. ]
なお、本出願は、2013年9月20日付けで出願された日本特許出願(特願2013-195590)及び2014年1月30日付けで出願された日本特許出願(特願2014-015524)に基づいており、その全体が引用により援用される。 Although the invention has been described in detail with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
This application is based on a Japanese patent application filed on September 20, 2013 (Japanese Patent Application No. 2013-195590) and a Japanese patent application filed on January 30, 2014 (Japanese Patent Application No. 2014-015524). Which is incorporated by reference in its entirety.
Claims (5)
- 質量%で、C:0.03~0.12%、Si:0%超0.3%以下、Mn:1.0~2.0%、B:0.0005~0.005%を夫々含有し、且つCu:0.1~1.0%およびNi:0.1~1.0%から選ばれる1種以上と、V:0%超0.05%以下、Nb:0%超0.05%以下およびTi:0%超0.05%以下よりなる群から選ばれる1種以上とを含有し、これらの元素が下記(1)~(3)式の関係を満足する、残部が鉄および不可避的不純物の厚鋼板であり、
圧延方向に平行な縦断面において鋼板表面から深さ3mmの観察位置で測定したときに、金属組織が下記(a)~(d)の要件を満足し、析出物が下記(A)の要件を満足することを特徴とする疲労特性に優れた厚鋼板。
0.01≦[Nb]+2[Ti]+2[V]≦0.10 …(1)
([Nb]、[Ti]および[V]は、それぞれNb、TiおよびVの鋼板中の質量%基準での含有量を示す。)
0≦([Cu]+[Ni])-2[Si]≦1.0 …(2)
([Cu]、[Ni]および[Si]は、それぞれCu、NiおよびSiの鋼板中の質量%基準での含有量を示す。)
[Mn]+1.5[Cr]+2[Mo]≦2.4 …(3)
([Mn]、[Cr]および[Mo]は、それぞれMn、CrおよびMoの鋼板中の質量%基準での含有量を示す。)
(a)金属組織がベイナイト組織と、残部組織とから構成され、全組織中、ベイナイト分率が80面積%以上である。
(b)隣り合う結晶の方位差が15°以上の大角粒界に基づいてベイナイトの結晶粒を決定したとき、当該結晶粒の板厚方向の平均長さが7μm以下である。
(c)前記残部組織の円相当直径が3.0μm以下である。
(d)前記残部組織中、パーライト分率が80面積%以上である。
(A)Nb、TiおよびVの少なくともいずれかを含む円相当直径が20nm以下の析出物の個数が100個/μm2以上である。 In mass%, C: 0.03-0.12%, Si: more than 0% and 0.3% or less, Mn: 1.0-2.0%, B: 0.0005-0.005%, respectively And at least one selected from Cu: 0.1 to 1.0% and Ni: 0.1 to 1.0%, V: more than 0% and 0.05% or less, Nb: more than 0%, and 0.0. 0.5% or less and Ti: one or more selected from the group consisting of more than 0% and 0.05% or less, and these elements satisfy the following formulas (1) to (3), with the balance being iron And inevitable impurities thick steel plate,
When measured at an observation position at a depth of 3 mm from the steel sheet surface in a longitudinal section parallel to the rolling direction, the metal structure satisfies the following requirements (a) to (d), and the precipitate satisfies the following requirements (A). Thick steel plate with excellent fatigue characteristics characterized by satisfaction.
0.01 ≦ [Nb] +2 [Ti] +2 [V] ≦ 0.10 (1)
([Nb], [Ti], and [V] indicate the content of Nb, Ti, and V, respectively, on a mass% basis in the steel sheet.)
0 ≦ ([Cu] + [Ni]) − 2 [Si] ≦ 1.0 (2)
([Cu], [Ni] and [Si] indicate the contents of Cu, Ni and Si on the basis of mass% in the steel sheet, respectively.)
[Mn] +1.5 [Cr] +2 [Mo] ≦ 2.4 (3)
([Mn], [Cr], and [Mo] indicate the content of Mn, Cr, and Mo on a mass% basis in the steel sheet, respectively.)
(A) A metal structure is comprised from a bainite structure and a remainder structure, and a bainite fraction is 80 area% or more in all the structures.
(B) When bainite crystal grains are determined based on a large-angle grain boundary in which the orientation difference between adjacent crystals is 15 ° or more, the average length of the crystal grains in the plate thickness direction is 7 μm or less.
(C) A circle equivalent diameter of the remaining tissue is 3.0 μm or less.
(D) The pearlite fraction is 80 area% or more in the remaining structure.
(A) The number of precipitates having an equivalent circle diameter of 20 nm or less containing at least one of Nb, Ti and V is 100 / μm 2 or more. - 更に、質量%で、Ca:0%超0.005%以下、Al:0%超0.10%以下、N:0%超0.010%以下、Cr:0%超2%以下およびMo:0%超1%以下よりなる群から選ばれる1種以上を含有するものである請求項1に記載の厚鋼板。 Further, in mass%, Ca: more than 0% to 0.005% or less, Al: more than 0% to 0.10% or less, N: more than 0% to 0.010% or less, Cr: more than 0% to 2% or less, and Mo: The thick steel plate according to claim 1, comprising at least one selected from the group consisting of more than 0% and not more than 1%.
- 圧延方向に平行な縦断面において、板厚の1/4位置となる位置を観察したときに、金属組織が下記(e)~(g)の要件を満足するものである請求項1または2に記載の厚鋼板。
(e)金属組織がベイナイト組織と、このベイナイトよりも硬質な残部組織とから構成され、全組織中、ベイナイト分率が80面積%以上である。
(f)隣り合う結晶の方位差が15°以上の大角粒界に基づいてベイナイトの結晶粒を決定したとき、当該結晶粒の板厚方向の平均長さが7μm以下である。
(g)前記硬質残部組織の円相当直径が3μm以下である。 3. The metal structure satisfies the following requirements (e) to (g) when a position that is a quarter of the plate thickness is observed in a longitudinal section parallel to the rolling direction: The described thick steel plate.
(E) The metal structure is composed of a bainite structure and a remaining structure harder than the bainite, and the bainite fraction is 80 area% or more in the entire structure.
(F) When bainite crystal grains are determined based on a large-angle grain boundary in which the orientation difference between adjacent crystals is 15 ° or more, the average length of the crystal grains in the plate thickness direction is 7 μm or less.
(G) The equivalent circular diameter of the hard residual structure is 3 μm or less. - 請求項1または2に記載の化学成分組成の鋼片を下記の条件で熱間圧延することを特徴とする疲労特性に優れた厚鋼板の製造方法。
加熱温度:1000~1200℃
全熱間圧延工程の累積圧下率:70%以上
仕上げ圧延中における、Ar3変態点+150℃~Ar3変態点+50℃の温度範囲での累積圧下率:50%以上
仕上げ圧延終了温度:Ar3変態点+30℃以上の温度
仕上げ圧延終了温度から600℃までの平均冷却速度:10℃/秒以下 A method for producing a thick steel plate having excellent fatigue characteristics, wherein the steel slab having the chemical composition according to claim 1 or 2 is hot-rolled under the following conditions.
Heating temperature: 1000-1200 ° C
Cumulative rolling reduction in all hot rolling processes: 70% or more Cumulative rolling reduction in the temperature range of Ar 3 transformation point + 150 ° C. to Ar 3 transformation point + 50 ° C. during finishing rolling: 50% or more Finish rolling finishing temperature: Ar 3 Transformation point + 30 ° C or higher Average cooling rate from finish rolling finish temperature to 600 ° C: 10 ° C / second or less - 冷却停止温度が550℃以上である請求項4に記載の厚鋼板の製造方法。 The method for producing a thick steel plate according to claim 4, wherein the cooling stop temperature is 550 ° C or higher.
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EP3502295A4 (en) * | 2016-08-19 | 2020-01-08 | Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.) | Thick steel plate and production method therefor |
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KR20240021260A (en) * | 2021-07-16 | 2024-02-16 | 제이에프이 스틸 가부시키가이샤 | Heavy steel plate and manufacturing method of thick steel plate |
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