JP5476735B2 - High-strength hot-rolled steel sheet excellent in workability and manufacturing method thereof - Google Patents
High-strength hot-rolled steel sheet excellent in workability and manufacturing method thereof Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims description 56
- 239000010959 steel Substances 0.000 title claims description 56
- 238000004519 manufacturing process Methods 0.000 title claims description 8
- 238000001816 cooling Methods 0.000 claims description 85
- 229910000859 α-Fe Inorganic materials 0.000 claims description 38
- 229910001563 bainite Inorganic materials 0.000 claims description 21
- 230000009466 transformation Effects 0.000 claims description 18
- 238000009835 boiling Methods 0.000 claims description 14
- 238000004804 winding Methods 0.000 claims description 12
- 229910001568 polygonal ferrite Inorganic materials 0.000 claims description 10
- 238000005098 hot rolling Methods 0.000 claims description 9
- 239000000203 mixture Substances 0.000 claims description 9
- 229910052758 niobium Inorganic materials 0.000 claims description 9
- 229910052720 vanadium Inorganic materials 0.000 claims description 9
- 229910052799 carbon Inorganic materials 0.000 claims description 5
- 229910052748 manganese Inorganic materials 0.000 claims description 5
- 239000012535 impurity Substances 0.000 claims description 4
- 229910052750 molybdenum Inorganic materials 0.000 claims description 4
- 229910052757 nitrogen Inorganic materials 0.000 claims description 4
- 229910052804 chromium Inorganic materials 0.000 claims description 3
- 229910052698 phosphorus Inorganic materials 0.000 claims description 3
- 229910052721 tungsten Inorganic materials 0.000 claims description 3
- 230000007423 decrease Effects 0.000 description 9
- 238000000034 method Methods 0.000 description 8
- 230000000694 effects Effects 0.000 description 7
- 239000000463 material Substances 0.000 description 7
- 150000001247 metal acetylides Chemical class 0.000 description 6
- 238000005096 rolling process Methods 0.000 description 6
- 239000000725 suspension Substances 0.000 description 6
- 229910052719 titanium Inorganic materials 0.000 description 6
- 238000005728 strengthening Methods 0.000 description 5
- 229910001566 austenite Inorganic materials 0.000 description 4
- 238000005516 engineering process Methods 0.000 description 4
- 239000002245 particle Substances 0.000 description 4
- 238000005266 casting Methods 0.000 description 3
- 229910000734 martensite Inorganic materials 0.000 description 3
- 229910001562 pearlite Inorganic materials 0.000 description 3
- 238000012545 processing Methods 0.000 description 3
- 238000009864 tensile test Methods 0.000 description 3
- 238000012360 testing method Methods 0.000 description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- 150000004767 nitrides Chemical class 0.000 description 2
- 238000005554 pickling Methods 0.000 description 2
- 238000001556 precipitation Methods 0.000 description 2
- 239000006104 solid solution Substances 0.000 description 2
- 230000000007 visual effect Effects 0.000 description 2
- -1 Ti is not sufficient Chemical class 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 238000007710 freezing Methods 0.000 description 1
- 230000008014 freezing Effects 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 238000010191 image analysis Methods 0.000 description 1
- 230000001771 impaired effect Effects 0.000 description 1
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- 238000000691 measurement method Methods 0.000 description 1
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- 230000008018 melting Effects 0.000 description 1
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- 230000007704 transition Effects 0.000 description 1
- 238000009849 vacuum degassing Methods 0.000 description 1
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Description
本発明は、主として自動車用部材、例えば車体のメンバーやフレームなどの構造部材やサスペンションなどの足まわり部材に用いられる高強度熱延鋼板、特に、引張強度TSが590〜700MPa程度の加工性に優れた高強度熱延鋼板およびその製造方法に関する。 The present invention is a high-strength hot-rolled steel sheet mainly used for automobile members, for example, structural members such as body members and frames, and suspension members such as suspensions, and in particular, excellent workability with a tensile strength TS of about 590 to 700 MPa. The present invention relates to a high strength hot-rolled steel sheet and a method for producing the same.
近年、自動車車体の軽量化を図るために高強度鋼板の利用が積極的に行われている。なかでも、車体の構造部材やサスペンションなどの足まわり部材のような優れた表面品質が要求されない部材には、経済性に優れた高強度熱延鋼板が多用されつつあり、TSが590〜700MPa程度の高強度鋼板が要求されるようになっている。また、このような高強度鋼板においても、良好な伸び特性、伸びフランジ性が要求される。 In recent years, high-strength steel sheets have been actively used to reduce the weight of automobile bodies. In particular, high strength hot-rolled steel sheets with excellent economic efficiency are being used frequently for parts that do not require excellent surface quality, such as structural members of vehicle bodies and suspension parts such as suspensions, and TS is about 590 to 700 MPa. High-strength steel sheets are required. In addition, such high-strength steel plates are also required to have good stretch characteristics and stretch flangeability.
従来から、高強度熱延鋼板の高強度化には、a) フェライト相中にマルテンサイト相あるいはベイナイト相などを生成させた変態組織強化法、b) Ti、Nb、Ｖなどの炭窒化物を形成させた析出強化法、c) a)とb)を併用した強化法が利用されており、要求される特性に応じて種々の高強度熱延鋼板が開発されている。例えば、安価な汎用の鋼板として析出強化された熱延鋼板(HSLA)が、伸び性が要求される鋼板としてフェライト相とマルテンサイト相からなるミクロ組織を有する複合組織鋼板(DP鋼板)が、伸びフランジ性が要求される鋼板としてベイナイト相を活用した鋼板が挙げられる。なかでも、特許文献1には、質量%で、C:0.010〜0.10%、Si:0.50〜1.50%、Mn:0.50〜2.00%、P:0.01〜0.15%、S:0.005%以下、N:0.001〜0.005%を含み、かつTi:0.005 〜0.03%、Ｖ:0.005 〜0.03%、Nb:0.01〜0.06%のうちの少なくとも一種を含有し、残部がFeおよび不可避的不純物の成分組成からなり、体積率80〜97%で、かつ平均粒径10μm以下のフェライト相と、残部としてベイナイト相を主体とする低温変態相からなる伸び性、形状凍結性、伸びフランジ性などの加工性に優れるTSが540〜590MPaの高強度熱延鋼板が提案されている。 Conventionally, to increase the strength of high strength hot rolled steel sheet, a) transformation structure strengthening method in which martensite phase or bainite phase is generated in ferrite phase, b) carbonitride such as Ti, Nb, V, etc. The formed precipitation strengthening method and the strengthening method using c) a) and b) in combination are used, and various high-strength hot-rolled steel sheets have been developed according to the required properties. For example, precipitation-strengthened hot-rolled steel sheets (HSLA) are used as inexpensive general-purpose steel sheets, and composite-structure steel sheets (DP steel sheets) having a microstructure composed of a ferrite phase and a martensite phase are required as elongation steel sheets. A steel sheet utilizing a bainite phase is exemplified as a steel sheet that requires flangeability. Among them, in Patent Document 1, in mass%, C: 0.010 to 0.10%, Si: 0.50 to 1.50%, Mn: 0.50 to 2.00%, P: 0.01 to 0.15%, S: 0.005% or less, N: 0.001 Containing 0.005% and containing at least one of Ti: 0.005 to 0.03%, V: 0.005 to 0.03%, Nb: 0.01 to 0.06%, the balance being composed of Fe and inevitable impurities, and volume TS with excellent workability such as extensibility, shape freezing property, stretch flangeability, etc., consisting of a ferrite phase with an average particle size of 10 μm or less and a low-temperature transformation phase mainly composed of a bainite phase as the balance. A high-strength hot-rolled steel sheet of ~ 590 MPa has been proposed.
しかしながら、特許文献1に記載の技術はTS:540〜590MPaを目標とする技術であり、前記したようなTS590〜700MPaの高強度化に対応した良好な伸び、伸びフランジ性を確保できる技術が求められていた。 However, the technology described in Patent Document 1 is a technology targeting TS: 540 to 590 MPa, and a technology that can ensure good elongation and stretch flangeability corresponding to the high strength of TS590 to 700 MPa as described above is required. It was done.
本発明は、伸び性として30%以上の全伸びElと、伸びフランジ性として120%以上の穴拡げ率λとを有し、TSが590〜700MPaの加工性に優れた高強度熱延鋼板およびその製造方法を提供することを目的とする。 The present invention has a high-strength hot-rolled steel sheet having a total elongation El of 30% or more as stretchability and a hole expansion ratio λ of 120% or more as stretch flangeability and excellent workability with TS of 590 to 700 MPa. It aims at providing the manufacturing method.
本発明者等は、上記の目的を達成すべく鋭意検討を行った結果、以下の知見を得た。 As a result of intensive studies to achieve the above object, the present inventors have obtained the following knowledge.
i) 組成の適正化を図り、組織全体に占める面積率でフェライト相を90%以上、ベイナイト相を2〜9%とし、かつフェライト相の平均粒径を5μm以下、フェライト相全体に占めるポリゴナルフェライト相の面積率を20%未満としたミクロ組織にすることにより、30%以上のEl、120%以上のλ、590〜700MPaのTSを達成できる。 i) By optimizing the composition, the area ratio in the entire structure is 90% or more for the ferrite phase, 2 to 9% for the bainite phase, and the average grain size of the ferrite phase is 5 μm or less. By using a microstructure in which the area ratio of the ferrite phase is less than 20%, 30% or more El, 120% or more λ, and TS of 590 to 700 MPa can be achieved.
ii) こうしたミクロ組織は、熱間圧延後、空冷を挟んで100℃/s以上の平均冷却速度で二段冷却することにより得られる。 ii) Such a microstructure can be obtained by performing two-stage cooling at an average cooling rate of 100 ° C./s or higher with air cooling after hot rolling.
本発明は、このような知見に基づきなされたもので、質量%で、C:0.05〜0.12%、Si:0.3〜1.0%、Mn:0.7〜1.3%、P:0.03%以下、S:0.005%以下、Al:0.005〜0.1%、N:0.01%以下を含有し、かつTi:0.005〜0.1%、Nb:0.005〜0.1%、V:0.005〜0.1%の中から選択された一種または二種以上を含有し、残部がFeおよび不可避的不純物からなる組成を有し、組織全体に占める面積率でフェライト相が90%以上、ベイナイト相が2〜9%であり、かつ前記フェライト相の平均粒径が5μm以下、前記フェライト相全体に占めるポリゴナルフェライト相の面積率が20%未満であるミクロ組織を有することを特徴とする加工性に優れた高強度熱延鋼板を提供する。 The present invention was made based on such knowledge, and in mass%, C: 0.05 to 0.12%, Si: 0.3 to 1.0%, Mn: 0.7 to 1.3%, P: 0.03% or less, S: 0.005% Hereinafter, Al: 0.005 to 0.1%, N: 0.01% or less, and Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1%, V: 0.005 to 0.1% selected from one or more In which the balance is Fe and inevitable impurities, the area ratio of the entire structure is 90% or more of the ferrite phase, 2 to 9% of the bainite phase, and the average particle diameter of the ferrite phase A high-strength hot-rolled steel sheet excellent in workability, characterized by having a microstructure in which the area ratio of the polygonal ferrite phase occupying the entire ferrite phase is less than 20%, is 5 μm or less.
本発明の高強度熱延鋼板には、さらに、質量%で、Cr:0.005〜0.1%、Mo:0.005〜0.1%、W:0.005〜0.1%の中から選択された一種または二種以上や、Ca:0.0005〜0.01%、REM:0.0005〜0.03%の中から選択された一種または二種を、個別にあるいは同時に含有させることが好ましい。 In the high-strength hot-rolled steel sheet of the present invention, in addition, by mass%, Cr: 0.005-0.1%, Mo: 0.005-0.1%, W: 0.005-0.1%, or one or more selected from One or two selected from Ca: 0.0005 to 0.01% and REM: 0.0005 to 0.03% are preferably contained individually or simultaneously.
本発明の高強度熱延鋼板は、上記の組成を有する鋼スラブを、Ar3変態点〜(Ar3変態点+100)℃の仕上温度で熱間圧延後、100℃/s以上の平均冷却速度で550〜620℃の冷却停止温度まで一次冷却し、1.5〜8.0s空冷後、100℃/s以上の平均冷却速度で二次冷却し、450〜550℃の巻取温度で巻取る方法により製造できる。 The high-strength hot-rolled steel sheet of the present invention is obtained by subjecting a steel slab having the above composition to an average cooling of 100 ° C./s or more after hot rolling at a finishing temperature of Ar 3 transformation point to (Ar 3 transformation point + 100) ° C. By primary cooling to a cooling stop temperature of 550 to 620 ° C at a speed, air cooling for 1.5 to 8.0 s, then secondary cooling at an average cooling rate of 100 ° C / s or more, and winding at a winding temperature of 450 to 550 ° C Can be manufactured.
このとき、二次冷却を核沸騰冷却となる条件で行うことが好ましい。 At this time, it is preferable to perform secondary cooling on the conditions used as nucleate boiling cooling.
本発明により、30%以上のElと、120%以上の穴拡げ率λとを有し、TSが590〜700MPaの加工性に優れた高強度熱延鋼板を製造できるようになった。本発明の高強度熱延鋼板は、自動車における車体のメンバーやフレームなどの構造部材やサスペンションなどの足まわり部材の軽量化に好適である。 According to the present invention, it has become possible to produce a high-strength hot-rolled steel sheet having an El of 30% or more and a hole expansion ratio λ of 120% or more and an excellent workability of TS of 590 to 700 MPa. The high-strength hot-rolled steel sheet of the present invention is suitable for reducing the weight of structural members such as body members and frames and suspension members such as suspensions in automobiles.
以下、本発明を具体的に説明する。なお、組成に関する「%」表示は特に断らない限り「質量%」を意味するものとする。 Hereinafter, the present invention will be specifically described. Note that “%” in relation to the composition means “% by mass” unless otherwise specified.
1) 組成
C:0.05〜0.12%
Cは、ベイナイト相を生成させ、必要な強度を確保するのに効果的な元素である。590MPa以上のTSを得るためには、C量を0.05%以上とする必要がある。一方、C量が0.12%を超えると、Elやλが低下する。したがって、C量は0.05〜0.12%とする。より好ましくは0.05〜0.09%である。
1) Composition
C: 0.05-0.12%
C is an element effective for generating a bainite phase and ensuring the necessary strength. In order to obtain TS of 590 MPa or more, the C content needs to be 0.05% or more. On the other hand, when the C content exceeds 0.12%, El and λ decrease. Therefore, the C content is 0.05 to 0.12%. More preferably, it is 0.05 to 0.09%.
Si:0.3〜1.0%
Siは、固溶強化により強度を上昇させるのに必要な元素である。Si量が0.3%未満では590MPa以上のTSを得るために高価な合金元素の添加量を増やす必要がある。一方、Si量が1.0%を超えると表面性状の著しい低下を招く。したがって、Si量は0.3〜1.0%とする。より好ましくは0.5〜0.9%である。
Si: 0.3-1.0%
Si is an element necessary for increasing the strength by solid solution strengthening. When the Si content is less than 0.3%, it is necessary to increase the amount of the expensive alloy element added in order to obtain TS of 590 MPa or more. On the other hand, if the amount of Si exceeds 1.0%, the surface properties are significantly lowered. Therefore, the Si content is 0.3 to 1.0%. More preferably, it is 0.5 to 0.9%.
Mn:0.7〜1.3%
Mnは、固溶強化およびベイナイト相の生成に有効な元素である。590MPa以上のTSを得るためにはMn量を0.7%以上とする必要がある。一方、Mn量が1.3%を超えると溶接性が低下する。したがって、Mn量は0.7〜1.3%とする。より好ましくは0.8〜1.2%である。
Mn: 0.7-1.3%
Mn is an effective element for solid solution strengthening and bainite phase generation. In order to obtain TS of 590 MPa or more, the Mn content needs to be 0.7% or more. On the other hand, if the Mn content exceeds 1.3%, the weldability decreases. Therefore, the Mn content is 0.7 to 1.3%. More preferably, it is 0.8 to 1.2%.
P:0.03%以下
P量が0.03%を超えると偏析によるElやλの低下を招く。したがって、Pは0.03%以下とする。
P: 0.03% or less
If the amount of P exceeds 0.03%, El and λ are reduced due to segregation. Therefore, P is set to 0.03% or less.
S:0.005%以下
Sは、MnおよびTiと硫化物を形成してElやλを低下させるとともに、高強度化に有効なMnやTi量の低下を招く。したがって、S量は0.005%以下とする。より好ましくは0.003%以下である。
S: 0.005% or less
S forms sulfides with Mn and Ti to reduce El and λ, and causes a decrease in the amount of Mn and Ti effective for increasing the strength. Therefore, the S content is 0.005% or less. More preferably, it is 0.003% or less.
Al:0.005〜0.1%
Alは、鋼の脱酸剤として重要な元素であり、それにはAl量を0.005%以上とする必要がある。一方、Al量が0.1%を超えると鋳造が難しくなったり、鋼中に多量の介在物が残存し材質や表面性状の低下を招く。したがって、Al量は0.005〜0.1%とする。
Al: 0.005-0.1%
Al is an important element as a deoxidizer for steel, and the Al content needs to be 0.005% or more. On the other hand, if the Al content exceeds 0.1%, casting becomes difficult, or a large amount of inclusions remain in the steel, causing deterioration of the material and surface properties. Therefore, the Al content is 0.005 to 0.1%.
N:0.01%以下
N量が0.01%を超えると、製造工程で多量の窒化物を生成し熱間延性を劣化させるので有害である。したがって、N量は0.01%以下とする。
N: 0.01% or less
If the amount of N exceeds 0.01%, a large amount of nitride is generated in the manufacturing process and the hot ductility is deteriorated, which is harmful. Therefore, the N content is 0.01% or less.
Ti:0.005〜0.1%、Nb:0.005〜0.1%、V:0.005〜0.1%の中から選択された一種または二種以上
Ti、Nb、Vは、その一部がCやNと結合し微細な炭化物や窒化物を形成し、高強度化に寄与する元素である。こうした効果を得るにはTi、Nb、Vの中から選択された一種または二種以上を含有させる必要があり、各元素の量は0.005%以上とする必要がある。一方、各元素の量が0.1%を超えるとElやλの低下を招く。したがって、Ti量は0.005〜0.1%、Nb:0.005〜0.1%、V:0.005〜0.1%とする。なお、Ti、Nb、V量は0.06%以下とすることが好ましく、さらに好ましくは0.01〜0.05%とする。
One or more selected from Ti: 0.005-0.1%, Nb: 0.005-0.1%, V: 0.005-0.1%
Ti, Nb, and V are elements that partly combine with C and N to form fine carbides and nitrides and contribute to high strength. In order to obtain such an effect, it is necessary to contain one or more selected from Ti, Nb, and V, and the amount of each element needs to be 0.005% or more. On the other hand, when the amount of each element exceeds 0.1%, the El and λ are reduced. Therefore, the Ti amount is 0.005 to 0.1%, Nb: 0.005 to 0.1%, and V: 0.005 to 0.1%. The Ti, Nb, and V amounts are preferably 0.06% or less, and more preferably 0.01 to 0.05%.
残部はFeおよび不可避的不純物であるが、以下の理由により、Cr:0.005〜0.1%、Mo:0.005〜0.1%、W:0.005〜0.1%の中から選択された一種または二種以上や、Ca:0.0005〜0.01%、REM:0.0005〜0.03%の中から選択された一種または二種を、個別にあるいは同時に含有させることが好ましい。 The balance is Fe and inevitable impurities, but for the following reasons, one or more selected from Cr: 0.005-0.1%, Mo: 0.005-0.1%, W: 0.005-0.1%, Ca One or two selected from: 0.0005 to 0.01% and REM: 0.0005 to 0.03% are preferably contained individually or simultaneously.
Cr:0.005〜0.1%
Crは、Cと結合し微細な炭化物を形成して高強度化に寄与する元素である。Cr量が0.005%未満ではその効果が小さく、0.1%を超えるとElやλの低下を招く。したがって、Cr量は0.005〜0.1%とすることが好ましい。
Cr: 0.005-0.1%
Cr is an element that combines with C to form fine carbides and contributes to high strength. If the Cr content is less than 0.005%, the effect is small, and if it exceeds 0.1%, El and λ are lowered. Therefore, the Cr content is preferably 0.005 to 0.1%.
Mo:0.005〜0.1%
Moは、Crと同様、Cと結合し微細な炭化物を形成して高強度化に寄与する元素である。Mo量が0.005%未満ではその効果が小さく、0.1%を超えるとElやλの低下を招く。したがって、Mo量は0.005〜0.1%とすることが好ましい。
Mo: 0.005-0.1%
Mo, like Cr, is an element that combines with C to form fine carbides and contributes to high strength. If the amount of Mo is less than 0.005%, the effect is small, and if it exceeds 0.1%, El and λ decrease. Therefore, the Mo amount is preferably 0.005 to 0.1%.
W:0.005〜0.1%
Wは、CrやMo同様、Cと結合し微細な炭化物を形成して高強度化に寄与する元素である。W量が0.005%未満ではその効果が小さく、0.1%を超えるとElやλの低下を招く。したがって、W量は0.005〜0.1%とすることが好ましい。
W: 0.005-0.1%
W, like Cr and Mo, is an element that combines with C to form fine carbides and contributes to high strength. If the amount of W is less than 0.005%, the effect is small, and if it exceeds 0.1%, El and λ decrease. Therefore, the W amount is preferably 0.005 to 0.1%.
Ca:0.0005〜0.01%、REM:0.0005〜0.03%
CaやREMは、介在物の形態制御に有効な元素であり、それぞれ単独で、あるいは共存してElやλの向上に寄与する。こうした効果を得るには、CaやREM量を0.0005%以上とすることが好ましい。一方、Ca量が0.01%を超えたり、REM量が0.03%を超えると鋼中介在物が増加し材質が劣化する。したがって、Ca量は0.0005〜0.01%、REM量は0.0005〜0.03%とすることが好ましい。
Ca: 0.0005-0.01%, REM: 0.0005-0.03%
Ca and REM are effective elements for controlling the morphology of inclusions and contribute to the improvement of El and λ, either alone or in combination. In order to obtain such an effect, the Ca or REM content is preferably 0.0005% or more. On the other hand, when the Ca content exceeds 0.01% or the REM content exceeds 0.03%, inclusions in the steel increase and the material deteriorates. Therefore, the Ca content is preferably 0.0005 to 0.01% and the REM content is preferably 0.0005 to 0.03%.
2) ミクロ組織
図1に示すように、本発明の高強度熱延鋼板のミクロ組織は、アシキュラーフェライト相中に矢印1で示すベイナイト相とあるいはさらに矢印2で示すポリゴナルフェライト相が分散した組織となっている。590〜700MPaのTSと30%以上のElを確保するには、組織全体に占める面積率でフェライト相を90%以上、ベイナイト相を2〜9%とし、かつフェライト相の平均粒径を5μm以下にする必要がある。また、フェライト相全体に占めるポリゴナルフェライト相の面積率を20%未満とする、すなわちアシキュラーフェライト相の面積率を80%超えとすることによりフェライト相とベイナイト相との硬度差が小さくなり、伸びフランジ性の一層の向上を図れ、120%以上のλが得られる。なお、フェライト相とベイナイト相以外にパーライト相や残留オーステナイト相などのその他の相が存在しても、その割合が組織全体に占める面積率で5%以下であれば、本発明の効果を損なうことはない。
2) Microstructure As shown in FIG. 1, the microstructure of the high-strength hot-rolled steel sheet of the present invention has a bainite phase indicated by arrow 1 and a polygonal ferrite phase indicated by arrow 2 dispersed in the acicular ferrite phase. It is an organization. To secure TS of 590 to 700 MPa and El of 30% or more, the ferrite phase is 90% or more, the bainite phase is 2 to 9%, and the average grain size of the ferrite phase is 5 μm or less. It is necessary to. In addition, the hardness difference between the ferrite phase and the bainite phase is reduced by setting the area ratio of the polygonal ferrite phase in the entire ferrite phase to less than 20%, that is, by making the area ratio of the acicular ferrite phase more than 80%. The stretch flangeability can be further improved, and a λ of 120% or more can be obtained. In addition to the ferrite phase and the bainite phase, even if other phases such as a pearlite phase and a retained austenite phase exist, the effect of the present invention is impaired if the ratio is 5% or less in the area ratio of the entire structure. There is no.
ここで、上記のフェライト相、ベイナイト相、あるいはその他の相の面積率は、走査型電子顕微鏡(SEM)用試験片を採取し、圧延方向に平行な板厚断面を研磨後、ナイタール腐食し、板厚中央部を倍率1000倍でSEM写真を10視野で撮影し、フェライト相、ベイナイト相、その他の相を画像処理により抽出し、画像解析処理によりフェライト相、ベイナイト相、その他の相の面積および観察視野の面積を測定して、(各相の面積)/(観察視野の面積)×100(%)より算出した。また、フェライト相全体に占めるポリゴナルフェライト相の面積率は、上記と同様にして、ポリゴナルフェライト相の面積を求め、(ポリゴナルフェライト相の面積)/(観察視野の面積-ベイナイト相の面積-その他の相の面積)×100(%)より算出した。さらに、フェライト相の平均粒径はJISG0552(1998)付属書2に準拠して、切片計測法に従い求めた。 Here, the area ratio of the above ferrite phase, bainite phase, or other phase is obtained by taking a specimen for a scanning electron microscope (SEM), polishing the plate thickness section parallel to the rolling direction, and then corroding the nital. SEM photographs were taken at 10x magnification at the center of the plate thickness at a magnification of 1000, the ferrite phase, bainite phase, and other phases were extracted by image processing, and the area of the ferrite phase, bainite phase, and other phases by image analysis processing The area of the observation visual field was measured and calculated from (area of each phase) / (area of the observation visual field) × 100 (%). In addition, the area ratio of the polygonal ferrite phase in the entire ferrite phase is obtained in the same manner as described above, and the area of the polygonal ferrite phase is obtained as follows: (Area of the polygonal ferrite phase) / (Area of observation field-Area of bainite phase) -Area of other phases) x 100 (%). Furthermore, the average particle diameter of the ferrite phase was determined according to the section measurement method in accordance with JISG0552 (1998) Appendix 2.
3) 製造条件
本発明の高強度熱延鋼板は、例えば、上記の組成を有する鋼スラブを、Ar3変態点〜(Ar3変態点+100)℃の仕上温度で熱間圧延後、100℃/s以上の平均冷却速度で550〜620℃の冷却停止温度まで一次冷却し、1.5〜8.0s空冷後、100℃/s以上の平均冷却速度で二次冷却し、450〜550℃の巻取温度で巻取る方法により製造できる。
3) Manufacturing conditions The high-strength hot-rolled steel sheet of the present invention is, for example, a steel slab having the above composition after hot rolling at a finishing temperature of Ar 3 transformation point to (Ar 3 transformation point +100) ° C. at 100 ° C. Primary cooling to a cooling stop temperature of 550 to 620 ° C at an average cooling rate of at least 150 / s, air cooling for 1.5 to 8.0s, followed by secondary cooling at an average cooling rate of at least 100 ° C / s and winding at 450 to 550 ° C It can be manufactured by a method of winding at temperature.
熱間圧延の仕上温度:Ar3変態点〜(Ar3変態点+100)℃
仕上温度がAr3変態点未満ではフェライト相とオーステナイト相の二相域圧延になり、鋼板の表層部に粗大粒や混粒が生じてElやλの低下を招く。一方、仕上温度が(Ar3変態点+100)℃を超えるとオーステナイト相の粒が粗大化するため、熱間圧延後に所望の微細なアシキュラーフェライト組織が得られず、590MPa以上のTSと120%以上のλの両立が困難となる。したがって、仕上温度はAr3変態点〜(Ar3変態点+100)℃とする。
Hot rolling finishing temperature: Ar 3 transformation point to (Ar 3 transformation point +100) ° C
When the finishing temperature is lower than the Ar 3 transformation point, the two-phase rolling of the ferrite phase and the austenite phase occurs, and coarse grains and mixed grains are generated in the surface layer portion of the steel sheet, leading to a decrease in El and λ. On the other hand, when the finishing temperature exceeds (Ar 3 transformation point +100) ° C., the grains of the austenite phase become coarse, so that a desired fine acicular ferrite structure cannot be obtained after hot rolling, and TS of 590 MPa or more and 120 Coexistence of λ of more than% becomes difficult. Therefore, the finishing temperature is Ar 3 transformation point to (Ar 3 transformation point + 100) ° C.
なお、Ar3変態点は、例えば、冷却速度10℃/sの加工フォーマスタ実験で熱膨張曲線を求め、その変化点により求めることができる。 The Ar 3 transformation point can be obtained, for example, by obtaining a thermal expansion curve by a processing formaster experiment at a cooling rate of 10 ° C./s and by using the change point.
熱間圧延後の一次冷却条件:平均冷却速度100℃/s以上、冷却停止温度550〜620℃
熱間圧延後の一次冷却の平均冷却速度が100℃/s未満では高温域からフェライト変態が開始され、ポリゴナルフェライト相が形成されやすく、すなわちアシキュラーフェライト相の生成が困難となり、590MPa以上のTSが得られない。したがって、一次冷却の平均冷却速度は100℃/s以上とする。一次冷却の平均冷却速度の上限は、次に延べる冷却停止温度の精度が確保されれば特に規定されないが、現状の冷却技術を考慮すると700℃/s以下が好ましい。なお、一次冷却の方法は、特に限定する必要はなく、例えば、公知のラミナー冷却による水冷も利用できる。
Primary cooling conditions after hot rolling: average cooling rate of 100 ° C / s or more, cooling stop temperature of 550-620 ° C
If the average cooling rate of primary cooling after hot rolling is less than 100 ° C / s, ferrite transformation starts from a high temperature range, and a polygonal ferrite phase is likely to be formed, that is, it is difficult to form an acicular ferrite phase, and the 590 MPa or more TS cannot be obtained. Therefore, the average cooling rate of the primary cooling is set to 100 ° C./s or more. The upper limit of the average cooling rate of the primary cooling is not particularly defined as long as the accuracy of the next cooling stop temperature is secured, but is preferably 700 ° C./s or less in consideration of the current cooling technology. The primary cooling method is not particularly limited, and for example, water cooling by known laminar cooling can also be used.
平均冷却速度100℃/s以上で行う一次冷却は550〜620℃の冷却停止温度で停止させる必要があるが、これは550℃未満ではTiなどの微細な炭化物の形成が十分でなく、590MPa以上のTSを得るのが困難になり、また、620℃を超えるとアシキュラーフェライト相が生成せず、フェライト相の粗大化やパーライト相の析出が顕著になり、Elやλが低下したり、590MPa以上のTSが得られなくなるためである。 Primary cooling performed at an average cooling rate of 100 ° C / s or higher needs to be stopped at a cooling stop temperature of 550 to 620 ° C, but this is less than 550 ° C, and formation of fine carbides such as Ti is not sufficient, and 590 MPa or higher TS is difficult to obtain, and when it exceeds 620 ° C, no acicular ferrite phase is formed, and ferrite phase coarsening and pearlite phase precipitation become prominent, El and λ decrease, and 590 MPa This is because the above TS cannot be obtained.
一次冷却後の空冷時間:1.5〜8.0s
一次冷却後の空冷時間は、所望のミクロ組織を達成するために極めて重要である。特に、アシキュラーフェライト相とベイナイト相の適正な造り込みのために、一次冷却を行った後は、冷却を停止して空冷とする。空冷時間が1.5s未満ではベイナイト相の生成量が過剰になってElやλが低下し、8.0sを超えると炭化物が粗大化したりフェライト相がポリゴナル化し、590MPa以上のTSが得られなくなる。したがって、一次冷却後の空冷時間は1.5〜8.0sとする。より好ましくは4〜7sである。
Air cooling time after primary cooling: 1.5-8.0s
The air cooling time after the primary cooling is extremely important to achieve the desired microstructure. In particular, after the primary cooling is performed in order to properly build the acicular ferrite phase and the bainite phase, the cooling is stopped and air cooling is performed. If the air cooling time is less than 1.5 s, the amount of bainite phase produced becomes excessive and El and λ decrease. If it exceeds 8.0 s, carbides become coarse or the ferrite phase becomes polygonal, and TS of 590 MPa or more cannot be obtained. Therefore, the air cooling time after the primary cooling is 1.5 to 8.0 s. More preferably, it is 4-7s.
空冷後の二次冷却条件:冷却速度100℃/s以上
空冷後は、空冷中に調整されたフェライト相の生成量が変動しないように、平均冷却速度100℃/s以上で巻取温度まで二次冷却する必要がある。なお、二次冷却の方法も、特に限定する必要はなく、例えば、公知のラミナー冷却による水冷を利用できる。
Secondary cooling conditions after air cooling: Cooling rate of 100 ° C / s or more After air cooling, the average cooling rate of 100 ° C / s or more is required to reach the coiling temperature so that the amount of ferrite phase adjusted during air cooling does not fluctuate. Next cooling is necessary. The secondary cooling method is not particularly limited, and for example, water cooling by known laminar cooling can be used.
特性のばらつきを小さくするために、二次冷却を核沸騰冷却となる条件で冷却することが好ましい。特に、500℃以下の温度域で水冷する場合に膜沸騰から核沸騰への遷移沸騰が起こり易いので、この温度域を核沸騰冷却となる条件で冷却することが好ましい。 In order to reduce the variation in characteristics, it is preferable to cool the secondary cooling under the condition of nucleate boiling cooling. In particular, when water cooling is performed at a temperature range of 500 ° C. or lower, transition boiling from film boiling to nucleate boiling is likely to occur. Therefore, it is preferable to cool this temperature range under conditions that provide nucleate boiling cooling.
巻取温度:450〜550℃
二次冷却後まで維持されたオーステナイト相をベイナイト相に変態させるために、450〜550℃の巻取温度で巻取る必要がある。これは、巻取温度が450℃未満ではベイナイト相より硬質なマルテンサイト相が生成し、また、550℃を超えるとパーライト相が生成して、Elやλが低下するためである。
Winding temperature: 450-550 ° C
In order to transform the austenite phase maintained until after the secondary cooling into the bainite phase, it is necessary to wind at a winding temperature of 450 to 550 ° C. This is because a martensite phase harder than the bainite phase is produced when the coiling temperature is less than 450 ° C., and a pearlite phase is produced when the coiling temperature exceeds 550 ° C., resulting in a decrease in El and λ.
その他の製造条件は通常の条件で行える。例えば、所望の組成を有する鋼は転炉や電気炉などで溶製後、真空脱ガス炉にて二次精錬を行って製造される。その後の鋳造は、生産性や品質上の点から連続鋳造法で行うのが好ましい。鋳造されるスラブは、厚みが200〜300mm程度の通常のスラブであっても、厚み30mm程度の薄スラブであってもよい。薄スラブにすれば粗圧延を省略できる。鋳造後のスラブは、そのまま直送熱間圧延しても、加熱炉で再加熱後熱間圧延してもよい。 Other manufacturing conditions can be performed under normal conditions. For example, steel having a desired composition is manufactured by performing secondary refining in a vacuum degassing furnace after melting in a converter or an electric furnace. The subsequent casting is preferably performed by a continuous casting method from the viewpoint of productivity and quality. The cast slab may be a normal slab having a thickness of about 200 to 300 mm or a thin slab having a thickness of about 30 mm. If a thin slab is used, rough rolling can be omitted. The slab after casting may be directly hot-rolled as it is or may be hot-rolled after being reheated in a heating furnace.
表1に示す組成とAr3変態点の鋼スラブNo.A〜Iを、1250℃に加熱し、表2に示す仕上温度で熱間圧延して板厚3mmの熱延鋼板とした後、表2に示すような一次冷却条件、空冷時間、二次冷却条件により冷却し、表2に示す巻取温度でコイル状に巻取った。なお、二次冷却において核沸騰冷却は行わなかった。また、表1のAr3変態点は前記の方法により求めた。 Steel slabs Nos. A to I having the composition and Ar 3 transformation point shown in Table 1 were heated to 1250 ° C. and hot-rolled at a finishing temperature shown in Table 2 to obtain a hot-rolled steel sheet having a thickness of 3 mm. The sample was cooled under the primary cooling conditions, air cooling time, and secondary cooling conditions as shown in FIG. 2, and wound into a coil at the winding temperature shown in Table 2. In the secondary cooling, nucleate boiling cooling was not performed. The Ar 3 transformation point in Table 1 was determined by the above method.
そして、巻取り後の熱延鋼板を酸洗後、コイルの長手方向中央部かつ幅方向中央部から組織観察用試料を採取し、上記の方法で組織全体に占めるフェライト相、ベイナイト相、その他の相の面積率、フェライト相の平均粒径、フェライト相全体に占めるポリゴナルフェライト相の面積率を求めた。また、組織観察用試料の採取位置近辺よりJIS 5号引張試験片(圧延方向に直角方向)および穴広げ試験用試験片(130mm角)を採取し、次のような方法によりTS、Elおよびλを求めた。
TS、El:3本の引張試験片に歪み速度10mm/minで引張試験を行ってTSとElを求め、3本の平均値をTS、Elとした。
λ:試験片中央に10mmφの穴を打ち抜いた後、60°円錐ポンチをバリと反対側から押し上げ、亀裂が板厚を貫通した時点での穴径dmmを測定し、次式より算出し、3個の平均値によりλを評価した。
λ(%)=[(d-10)/10]×100
結果を表3に示す。本発明例では、TSが590〜700MPaであり、かつElが30%以上のEl、λが120%以上で加工性に優れていることがわかる。
And after pickling the hot-rolled steel sheet after winding, a sample for observing the structure is collected from the central part in the longitudinal direction and the width direction of the coil, and the ferrite phase, bainite phase, The area ratio of the phase, the average particle diameter of the ferrite phase, and the area ratio of the polygonal ferrite phase in the entire ferrite phase were determined. In addition, JIS No. 5 tensile test specimen (perpendicular to the rolling direction) and hole expansion test specimen (130 mm square) were collected from the vicinity of the sampling position for the structure observation sample, and TS, El, and λ were obtained by the following method. Asked.
TS, El: Three tensile test pieces were subjected to a tensile test at a strain rate of 10 mm / min to obtain TS and El, and the average value of the three was taken as TS and El.
λ: After punching a 10mmφ hole in the center of the test piece, the 60 ° conical punch was pushed up from the opposite side of the burr, and the hole diameter dmm when the crack penetrated the plate thickness was measured. Λ was evaluated by the average value.
λ (%) = [(d-10) / 10] × 100
The results are shown in Table 3. In the examples of the present invention, it can be seen that TS is 590 to 700 MPa, El is El of 30% or more, and λ is 120% or more, which is excellent in workability.
さらに、二次冷却を、核沸騰冷却となる条件で行った場合のコイル内の材質変動への効果を検討するため、表1の鋼スラブCを、1250℃に加熱し、表4に示す仕上温度で熱間圧延して板厚3mmの熱延鋼板とした後、表4に示す一次冷却条件、空冷時間、二次冷却条件により冷却し、表4に示す巻取温度でコイル状に巻取った(鋼板No.18)。ここで、二次冷却は核沸騰冷却となる条件で行った。なお、表4には、鋼板No.18と同様にコイル内材質変動を調査した鋼板No.5の条件も合わせて示した。 Furthermore, in order to investigate the effect on material fluctuations in the coil when secondary cooling is performed under the conditions of nucleate boiling cooling, the steel slab C in Table 1 is heated to 1250 ° C and the finish shown in Table 4 is achieved. After hot rolling at a temperature to obtain a hot-rolled steel sheet having a thickness of 3 mm, the steel sheet is cooled in accordance with the primary cooling conditions, air cooling time, and secondary cooling conditions shown in Table 4, and wound into a coil at the winding temperature shown in Table 4. (Steel plate No. 18). Here, the secondary cooling was performed under the condition of nucleate boiling cooling. Table 4 also shows the conditions of steel plate No. 5 in which the variation in material in the coil was investigated in the same manner as steel plate No. 18.
そして、巻取り後の熱延鋼板を酸洗後、上記と同様にして、組織観察を行い、TS、El、λを求めた。また、上記核沸騰冷却を行った鋼板No.18および比較として核沸騰冷却を行わなかった鋼板No.5について、下記方法にてコイル内の材質変動を調査した。
コイル内の材質変動:コイル先端部から長手方向に100、200、400、600、700m入った各位置で、圧延方向に平行な方向を試験片の長手方向として、鋼板の幅方向に、幅方向の両端25mmの内側から25本の試験片を等間隔に採取し、合計125本のJIS 5号引張試験片(圧延方向に平行な方向が引張方向)を採取し、上記と同様な方法でTSを求め、その標準偏差σを算出した。
Then, after pickling the hot-rolled steel sheet after winding, the structure was observed in the same manner as described above to determine TS, El, and λ. In addition, regarding the steel plate No. 18 that was subjected to the nucleate boiling cooling and the steel plate No. 5 that was not subjected to the nucleate boiling cooling, the material variation in the coil was investigated by the following method.
Material variation in the coil: At each position 100, 200, 400, 600, 700m in the longitudinal direction from the coil tip, the direction parallel to the rolling direction is the longitudinal direction of the test piece, and the width direction of the steel sheet Collect 25 specimens at regular intervals from the inside of 25 mm at both ends, and collect a total of 125 JIS No. 5 tensile specimens (the direction parallel to the rolling direction is the tensile direction). And the standard deviation σ was calculated.
結果を表5に示す。本発明例である鋼板No.18および鋼板No.5とも、TSが590〜700MPaであり、かつElが30%以上、λが120%以上で加工性に優れており、さらに、二次冷却を核沸騰冷却となる条件で行った場合(鋼板No.18)のTSの標準偏差σは10MPaと小さく、二次冷却を核沸騰冷却となる条件とはしなかった場合(鋼板No.5)に比べ、コイル内の材質変動が小さいことがわかる。 The results are shown in Table 5. Both steel plate No. 18 and steel plate No. 5 which are examples of the present invention have excellent workability when TS is 590 to 700 MPa, El is 30% or more, and λ is 120% or more. The standard deviation σ of TS when nucleate boiling cooling is performed (steel plate No. 18) is as small as 10 MPa, and when secondary cooling is not used for nucleate boiling cooling (steel plate No. 5). In comparison, it can be seen that the material fluctuation in the coil is small.
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| JP5549238B2 (en) * | 2010-01-22 | 2014-07-16 | 新日鐵住金株式会社 | Cold rolled steel sheet and method for producing the same |
| JP5423737B2 (en) * | 2010-08-10 | 2014-02-19 | Ｊｆｅスチール株式会社 | High-strength hot-rolled steel sheet excellent in workability and manufacturing method thereof |
| WO2015099222A1 (en) * | 2013-12-26 | 2015-07-02 | 주식회사 포스코 | Hot-rolled steel plate having excellent welding property and burring property and method for manufacturing same |
| KR101917469B1 (en) * | 2016-12-23 | 2018-11-09 | 주식회사 포스코 | High strength hot rolled steel sheet having low deviation of mechanical property and excellent surface quality and method for manufacturing the same |
| CN113373375B (en) * | 2021-05-26 | 2022-07-19 | 攀钢集团攀枝花钢铁研究院有限公司 | 600 MPa-grade hot-rolled automobile beam steel strip with high fatigue performance and preparation method thereof |
| CN113322413B (en) * | 2021-05-28 | 2022-07-19 | 攀钢集团攀枝花钢铁研究院有限公司 | High-fatigue-performance 900 MPa-grade hot-rolled automobile crossbeam steel strip and preparation method thereof |
| CN113308646B (en) * | 2021-05-28 | 2022-07-19 | 攀钢集团攀枝花钢铁研究院有限公司 | High-fatigue-performance 700 MPa-grade hot-rolled automobile crossbeam steel strip and preparation method thereof |
| CN113322416B (en) * | 2021-05-31 | 2022-07-19 | 攀钢集团攀枝花钢铁研究院有限公司 | 800 MPa-grade hot-rolled automobile beam steel strip with high fatigue performance and preparation method thereof |
| KR102667657B1 (en) * | 2021-09-29 | 2024-05-23 | 현대제철 주식회사 | Hot-rolled steel sheet and method of manufacturing the same |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| JPS57145965A (en) * | 1981-03-04 | 1982-09-09 | Kobe Steel Ltd | High strength hot rolled steel plate and its manufacture |
| JPS59129725A (en) * | 1983-01-17 | 1984-07-26 | Kobe Steel Ltd | Production of hot rolled high tension steel sheet having excellent cold workability |
| JP4445365B2 (en) * | 2004-10-06 | 2010-04-07 | 新日本製鐵株式会社 | Manufacturing method of high-strength thin steel sheet with excellent elongation and hole expandability |
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