JP5509909B2 - Manufacturing method of high strength hot-rolled steel sheet - Google Patents
Manufacturing method of high strength hot-rolled steel sheet Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims description 55
- 239000010959 steel Substances 0.000 title claims description 55
- 238000004519 manufacturing process Methods 0.000 title claims description 14
- 229910001566 austenite Inorganic materials 0.000 claims description 40
- 229910000734 martensite Inorganic materials 0.000 claims description 39
- 238000001816 cooling Methods 0.000 claims description 31
- 229910000859 α-Fe Inorganic materials 0.000 claims description 22
- 239000000203 mixture Substances 0.000 claims description 17
- 238000004804 winding Methods 0.000 claims description 15
- 238000003303 reheating Methods 0.000 claims description 10
- 238000005098 hot rolling Methods 0.000 claims description 9
- 229910052802 copper Inorganic materials 0.000 claims description 7
- 229910052750 molybdenum Inorganic materials 0.000 claims description 7
- 229910052759 nickel Inorganic materials 0.000 claims description 7
- 229910052720 vanadium Inorganic materials 0.000 claims description 7
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 6
- 239000012535 impurity Substances 0.000 claims description 5
- 229910052758 niobium Inorganic materials 0.000 claims description 5
- 229910052742 iron Inorganic materials 0.000 claims description 3
- 229910052748 manganese Inorganic materials 0.000 claims description 2
- 230000000694 effects Effects 0.000 description 18
- 230000000717 retained effect Effects 0.000 description 17
- 238000005096 rolling process Methods 0.000 description 17
- 238000010438 heat treatment Methods 0.000 description 11
- 238000000034 method Methods 0.000 description 11
- 230000015572 biosynthetic process Effects 0.000 description 9
- 230000007423 decrease Effects 0.000 description 9
- 238000005728 strengthening Methods 0.000 description 7
- 230000009466 transformation Effects 0.000 description 7
- 229910052804 chromium Inorganic materials 0.000 description 4
- 238000009749 continuous casting Methods 0.000 description 4
- 239000000463 material Substances 0.000 description 4
- 229910001562 pearlite Inorganic materials 0.000 description 4
- 230000008569 process Effects 0.000 description 3
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 2
- 230000002411 adverse Effects 0.000 description 2
- 238000000137 annealing Methods 0.000 description 2
- 229910001563 bainite Inorganic materials 0.000 description 2
- 238000005266 casting Methods 0.000 description 2
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- 230000006866 deterioration Effects 0.000 description 2
- 238000010191 image analysis Methods 0.000 description 2
- 230000006872 improvement Effects 0.000 description 2
- 238000005461 lubrication Methods 0.000 description 2
- 150000001247 metal acetylides Chemical class 0.000 description 2
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- 238000005204 segregation Methods 0.000 description 2
- 238000012360 testing method Methods 0.000 description 2
- 238000011282 treatment Methods 0.000 description 2
- GRYLNZFGIOXLOG-UHFFFAOYSA-N Nitric acid Chemical compound O[N+]([O-])=O GRYLNZFGIOXLOG-UHFFFAOYSA-N 0.000 description 1
- 238000002441 X-ray diffraction Methods 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 229910001567 cementite Inorganic materials 0.000 description 1
- 239000011248 coating agent Substances 0.000 description 1
- 239000010960 cold rolled steel Substances 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 239000002131 composite material Substances 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
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- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 229910017604 nitric acid Inorganic materials 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
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- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 1
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- Heat Treatment Of Sheet Steel (AREA)
Description
本発明は、主に自動車、電気等の産業分野で使用される部材として好適な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 relates to a high-strength hot-rolled steel sheet excellent in workability suitable as a member mainly used in industrial fields such as automobiles and electricity, and a method for producing the same.
近年、地球環境保全の見地から、自動車の燃費向上が重要な課題となっている。これに伴い、車体材料の高強度化により薄肉化を図り、車体そのものを軽量化しようとする動きが活発となってきている。しかしながら、鋼板の高強度化は延性の低下、即ち成形加工性の低下を招く。このため、高強度と高加工性を併せ持つ材料の開発が望まれているのが現状である。 In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of global environmental conservation. Along with this, there is an active movement to reduce the thickness of the vehicle body by increasing the strength of the vehicle body material and to reduce the weight of the vehicle body itself. However, increasing the strength of the steel sheet causes a decrease in ductility, that is, a decrease in formability. For this reason, the present situation is that development of a material having both high strength and high workability is desired.
さらに製造コスト等の経済性に対する配慮も必要とされ、この点から冷延鋼板よりも熱延鋼板の方が有利である。 Furthermore, consideration is required for economics such as manufacturing costs, and in this respect, hot-rolled steel sheets are more advantageous than cold-rolled steel sheets.
このような要求に対して、これまでに成形性を考慮した高強度熱延鋼板が種々開発されている。強度−延性バランスの優れた鋼として、特許文献1〜3には、残留オーステナイトを活用した高加工性高強度熱延鋼板の製造方法が開示されている。しかし、これらの所謂TRIP鋼は伸び特性には優れるものの穴拡げ性が劣るという問題がある。穴拡げ性は加工穴部を拡張してフランジ成形させるときの加工性を示す指標で、伸び特性と共に高強度鋼板に要求される重要な特性である。 In response to such demands, various high strength hot-rolled steel sheets in consideration of formability have been developed so far. As steels having an excellent balance between strength and ductility, Patent Documents 1 to 3 disclose methods for producing high workability and high strength hot-rolled steel sheets using retained austenite. However, these so-called TRIP steels have a problem that the hole expandability is inferior although they are excellent in elongation characteristics. Hole expansibility is an index indicating workability when a processed hole is expanded to form a flange, and is an important characteristic required for high-strength steel sheets together with elongation characteristics.
上述したように、特許文献1〜3では、優れた伸び特性および伸びフランジ性を兼ね備える熱延鋼板は得られていない。 As described above, Patent Documents 1 to 3 do not provide a hot-rolled steel sheet having both excellent elongation characteristics and stretch flangeability.
本発明は、かかる事情に鑑み、590MPa以上の強度（ＴＳ）を有し、かつ、伸びおよび伸びフランジ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。 In view of such circumstances, an object of the present invention is to provide a high-strength hot-rolled steel sheet having a strength (TS) of 590 MPa or more and excellent in elongation and stretch flangeability, and a method for producing the same.
本発明者らは、上記した課題を達成し、伸びおよび伸びフランジ性に優れた高強度熱延鋼板を製造するため、鋼板の成分組成およびミクロ組織の観点から鋭意研究を重ねた。
その結果、フェライト相の面積率が20%以上、焼戻しマルテンサイト相の面積率が10〜60%、マルテンサイト相の面積率が0〜10%、残留オーステナイト相の体積率が3〜15%である組織を有することにより、伸びと伸びフランジ性を両立できることが分かった。
一般的に残留オーステナイト相が存在すると残留オーステナイト相のTRIP（変態誘起塑性）効果により延性が向上する。しかし、歪の付加により残留オーステナイト相が変態して生成するマルテンサイト相は非常に硬質なものとなり、その結果、主相であるフェライト相との硬度差が大きくなり伸びフランジ性が低下することが知られている。これに対し、本発明の成分および組織構成とすることで、高い伸びと高い伸びフランジ性が両立可能となる。残留オーステナイト相が存在しても高い伸びフランジ性が可能となる理由について詳細は不明であるが、残留オーステナイト相が焼戻しマルテンサイト相と共存することにより、残留オーステナイト相の穴拡げ性（伸びフランジ性）への悪影響が低減されるためと考えられる。
In order to achieve the above-described problems and to produce a high-strength hot-rolled steel sheet excellent in elongation and stretch flangeability, the present inventors have made extensive studies from the viewpoint of the component composition and microstructure of the steel sheet.
As a result, the area ratio of the ferrite phase is 20% or more, the area ratio of the tempered martensite phase is 10 to 60%, the area ratio of the martensite phase is 0 to 10%, and the volume ratio of the retained austenite phase is 3 to 15%. It has been found that having a certain structure can achieve both elongation and stretch flangeability.
In general, the presence of residual austenite phase improves ductility due to the TRIP (transformation-induced plasticity) effect of the residual austenite phase. However, the martensite phase produced by transformation of the retained austenite phase due to the addition of strain becomes very hard, and as a result, the hardness difference from the ferrite phase, which is the main phase, becomes large, and the stretch flangeability decreases. Are known. On the other hand, by setting it as the component and structure | tissue structure of this invention, it becomes compatible [high elongation and high stretch flangeability]. The details of why high stretch flangeability is possible even in the presence of residual austenite phase are unknown, but the residual austenite phase coexists with the tempered martensite phase, so that the hole expandability of stretched austenite phase (stretch flangeability) This is thought to be due to the reduced adverse effects on
本発明は、以上の知見に基づいてなされたものであり、その要旨は以下のとおりである。
[1]成分組成は、質量%で、Ｃ：0.05〜0.3％、Si：0.3〜2.5%、Mn：0.5〜3.5％、P：0.003〜0.100％、S：0.020％以下、Al：0.010〜0.5%を含み、残部が鉄および不可避的不純物からなり、組織は、フェライト相の面積率が20%以上、焼戻しマルテンサイト相の面積率が10〜60%、マルテンサイト相の面積率が0〜10%、残留オーステナイト相の体積率が3〜15%であることを特徴とする高強度熱延鋼板。
[2]さらに、成分組成として、質量%で、Cr：0.005〜2.00%、Mo：0.005〜2.00%、V：0.005〜2.00%、Ni：0.005〜2.00%、Cu：0.005〜2.00%から選ばれる1種または2種以上の元素を含有することを特徴とする前記[1]に記載の高強度熱延鋼板。
[3]さらに、成分組成として、質量%で、Ti：0.01〜0.20%、Nb：0.01〜0.20%から選ばれる1種または2種の元素を含有することを特徴とする前記[1]または前記[2]に記載の高強度熱延鋼板。
[4]さらに、成分組成として、質量%で、B：0.0002〜0.005%を含有することを特徴とする前記[1]〜前記[3]のいずれかに記載の高強度熱延鋼板。
[5]さらに、成分組成として、質量%で、Ca：0.001〜0.005%、REM：0.001〜0.005%から選ばれる1種または2種の元素を含有することを特徴とする前記[1]〜前記[4]のいずれかに記載の高強度熱延鋼板。
[6]前記[1]〜前記[5]のいずれかに記載の成分組成を有するスラブに、熱間圧延を施したのち、800〜600℃の温度範囲を10〜100℃/sの平均冷却速度で1回目の冷却を行い、次いで100〜300℃まで2回目の冷却を行い、100〜300℃の巻取り温度で巻取った後、300〜450℃の温度に再加熱し、該再加熱温度域で1〜600min保持し、次いで、冷却することを特徴とする高強度熱延鋼板の製造方法。
なお、本明細書において、鋼の成分を示す％は、すべて質量％である。また、本発明において、「高強度熱延鋼板」とは、引張強度ＴＳが590MPa以上である熱延鋼板である。
This invention is made | formed based on the above knowledge, The summary is as follows.
[1] Component composition is mass%, C: 0.05-0.3%, Si: 0.3-2.5%, Mn: 0.5-3.5%, P: 0.003-0.100%, S: 0.020% or less, Al: 0.010-0.5 The balance is composed of iron and inevitable impurities, and the structure has a ferrite phase area ratio of 20% or more, a tempered martensite phase area ratio of 10 to 60%, and a martensite phase area ratio of 0 to 10%. A high-strength hot-rolled steel sheet characterized in that the volume ratio of residual austenite phase is 3 to 15%.
[2] Furthermore, the component composition is selected from Cr: 0.005-2.00%, Mo: 0.005-2.00%, V: 0.005-2.00%, Ni: 0.005-2.00%, Cu: 0.005-2.00% in mass%. The high-strength hot-rolled steel sheet according to [1], containing one or more elements.
[3] The above [1] or the above, further comprising, as a component composition, by mass%, one or two elements selected from Ti: 0.01 to 0.20% and Nb: 0.01 to 0.20% The high-strength hot-rolled steel sheet according to [2].
[4] The high-strength hot-rolled steel sheet according to any one of [1] to [3], further containing B: 0.0002 to 0.005% by mass% as a component composition.
[5] Furthermore, as a component composition, it contains one or two elements selected from Ca: 0.001 to 0.005% and REM: 0.001 to 0.005% by mass%, [1] to the above The high-strength hot-rolled steel sheet according to any one of [4].
[6] After subjecting the slab having the component composition according to any one of [1] to [5] to hot rolling, an average cooling of 10 to 100 ° C./s in a temperature range of 800 to 600 ° C. 1st cooling at speed, then 2nd cooling to 100-300 ° C, winding at 100-300 ° C winding temperature, reheating to 300-450 ° C, the reheating A method for producing a high-strength hot-rolled steel sheet, which is maintained for 1 to 600 minutes in a temperature range and then cooled.
In addition, in this specification,% which shows the component of steel is mass% altogether. In the present invention, the “high-strength hot-rolled steel sheet” is a hot-rolled steel sheet having a tensile strength TS of 590 MPa or more.
本発明によれば、590MPa以上の引張強度ＴＳを有し、かつ、伸びおよび伸びフランジ性に優れた高強度熱延鋼板が得られる。本発明の高強度熱延鋼板を例えば自動車構造部材に適用することにより、自動車の軽量化と衝突安全性向上の両立を可能とし、自動車車体の高性能化に大きく寄与するという優れた効果を奏する。 According to the present invention, a high-strength hot-rolled steel sheet having a tensile strength TS of 590 MPa or more and excellent in elongation and stretch flangeability can be obtained. By applying the high-strength hot-rolled steel sheet of the present invention to, for example, an automobile structural member, it is possible to achieve both the reduction of the weight of the automobile and the improvement of the collision safety, and the excellent effect of greatly contributing to the improvement of the performance of the automobile body. .
以下に、本発明の詳細を説明する。 Details of the present invention will be described below.
1)成分組成
C：0.05〜0.3％
Cはオーステナイト相を安定化させる元素であり、フェライト相以外の相を生成しやすくする。また、鋼板強度を上昇させ、組織を複合化して強度と伸びのバランス（以下、ＴＳ×ＥＬと称することもある）を向上させる。これらの効果を得るためには、C量は0.05％以上とする。C量が0.05%未満では製造条件の最適化を図ったとしてもフェライト相以外の相の確保が難しく、ＴＳ×ＥＬが低下する。一方、C量が0.3％を超えると、溶接部および熱影響部の硬化が著しく、溶接部の機械的特性が劣化する。こうした観点からC量は0.05％以上0.3%以下の範囲とする。好ましくは0.08％以上0.15%以下である。
1) Component composition
C: 0.05-0.3%
C is an element that stabilizes the austenite phase and facilitates the formation of phases other than the ferrite phase. In addition, the steel sheet strength is increased and the structure is combined to improve the balance between strength and elongation (hereinafter sometimes referred to as TS × EL). In order to obtain these effects, the C content is 0.05% or more. If the C content is less than 0.05%, it is difficult to secure a phase other than the ferrite phase even if the production conditions are optimized, and TS × EL decreases. On the other hand, when the amount of C exceeds 0.3%, the welded part and the heat-affected zone are significantly hardened, and the mechanical properties of the welded part deteriorate. From such a viewpoint, the C content is in the range of 0.05% to 0.3%. Preferably they are 0.08% or more and 0.15% or less.
Si：0.3〜2.5％
Siは鋼の強化に有効な元素である。また、フェライト相生成元素であり、オーステナイト相中へのCの濃化促進および炭化物の生成を抑制することから、残留オーステナイト相の生成を促進する働きを有する。Si量が0.3%に満たないとその添加効果に乏しくなるので、下限は0.3%とする。ただし過剰な添加は、表面性状、溶接性を劣化させるので、上限は2.5%とする。好ましくは0.7%以上2.0%以下である。
Si: 0.3-2.5%
Si is an effective element for strengthening steel. Further, it is a ferrite phase forming element, and has the function of promoting the formation of a retained austenite phase because it promotes the concentration of C in the austenite phase and suppresses the formation of carbides. If the Si content is less than 0.3%, the effect of addition becomes poor, so the lower limit is made 0.3%. However, excessive addition degrades the surface properties and weldability, so the upper limit is 2.5%. Preferably they are 0.7% or more and 2.0% or less.
Mn：0.5〜3.5％
Mnは鋼の強化に有効な元素であり、焼戻しマルテンサイト相等の低温変態相の生成を促進する。このような作用は、Mn含有量が0.5％以上で認められる。ただし、Mnを3.5％を超えて過剰に添加すると、第二相分率の過剰な増加や固溶強化によるフェライト相の延性劣化が著しくなり成形性が低下する。従って、Mn量は0.5%以上3.5％以下とする。好ましくは1.5%以上3.0%以下である。
Mn: 0.5-3.5%
Mn is an element effective for strengthening steel, and promotes the formation of a low-temperature transformation phase such as a tempered martensite phase. Such an effect is observed when the Mn content is 0.5% or more. However, if Mn is added excessively exceeding 3.5%, the ductile deterioration of the ferrite phase due to excessive increase of the second phase fraction or solid solution strengthening becomes remarkable and formability is lowered. Therefore, the Mn content is 0.5% or more and 3.5% or less. Preferably they are 1.5% or more and 3.0% or less.
P：0.003〜0.100％
Ｐは鋼の強化に有効な元素であり、この効果は0.003%以上で得られる。しかし、0.100%を超えて過剰に添加すると粒界偏析により脆化を引き起こし、耐衝撃性を劣化させる。従って、Ｐ量は0.003%以上0.100%以下とする。
P: 0.003-0.100%
P is an element effective for strengthening steel, and this effect is obtained at 0.003% or more. However, excessive addition over 0.100% causes embrittlement due to grain boundary segregation and degrades impact resistance. Therefore, the P amount is set to 0.003% or more and 0.100% or less.
S：0.020％以下
SはMnSなどの介在物となって、耐衝撃特性の劣化や溶接部のメタルフローに沿った割れの原因になるので極力低い方が良いが、製造コストの面から0.020％以下とする。
S: 0.020% or less
S is an inclusion such as MnS, which causes deterioration of impact resistance and cracks along the metal flow of the weld, so it is better to be as low as possible, but it is 0.020% or less from the viewpoint of manufacturing cost.
Al：0.010〜0.5％
Alは脱酸剤として用いられ、脱酸工程で添加する。Al量が0.010%に満たないとその添加効果が乏しくなる。よって、下限は0.010%とする。一方、多量に添加すると連続鋳造時の鋼片割れ発生の危険性が高まり製造性を低下させる。よって、上限は0.5％とする。
Al: 0.010 to 0.5%
Al is used as a deoxidizer and is added in the deoxidation step. If the amount of Al is less than 0.010%, the effect of addition becomes poor. Therefore, the lower limit is 0.010%. On the other hand, if added in a large amount, the risk of steel piece cracking during continuous casting increases and productivity decreases. Therefore, the upper limit is 0.5%.
残部はFeおよび不可避的不純物である。
ただし、これらの成分元素に加えて、以下の合金元素を必要に応じて添加することができる。
The balance is Fe and inevitable impurities.
However, in addition to these component elements, the following alloy elements can be added as necessary.
Cr：0.005〜2.00%、Mo：0.005〜2.00%、V：0.005〜2.00%、Ni：0.005〜2.00%、Cu：0.005〜2.00%から選ばれる1種または2種以上
Cr、Mo、V、Ni、Cuは焼鈍温度からの冷却時にパーライト相の生成を抑制し、低温変態相の生成を促進し鋼の強化に有効に働く。この効果は、Cr、Mo、V、Ni、Cu の少なくとも1種を0.005%以上含有させることで得られる。しかし、Cr、Mo、V、Ni、Cu のそれぞれの成分が2.00%を超えるとその効果は飽和し、コストアップの要因となる。従ってCr、Mo、V、Ni、Cu量を添加する場合は、それぞれ0.005％以上2.00%以下とする。
One or more selected from Cr: 0.005-2.00%, Mo: 0.005-2.00%, V: 0.005-2.00%, Ni: 0.005-2.00%, Cu: 0.005-2.00%
Cr, Mo, V, Ni, and Cu effectively suppress the formation of pearlite phase during cooling from the annealing temperature, promote the formation of low-temperature transformation phase, and effectively work to strengthen steel. This effect can be obtained by adding 0.005% or more of at least one of Cr, Mo, V, Ni, and Cu. However, if each component of Cr, Mo, V, Ni, and Cu exceeds 2.00%, the effect is saturated and causes an increase in cost. Therefore, when adding Cr, Mo, V, Ni, and Cu, the content should be 0.005% or more and 2.00% or less, respectively.
Ti：0.01〜0.20%、Nb：0.01〜0.20%から選ばれる1種または2種
Ti、Nbは炭窒化物を形成し、鋼を析出強化により高強度化する作用を有する。このような効果は少なくとも1種を0.01%以上添加することで認められる。一方、それぞれの成分が0.20％を超えて含有しても、過度に高強度化し、延性が低下する。従って、Ti、Nbを添加する場合は、それぞれ0.01%以上0.20％以下とする。
1 or 2 types selected from Ti: 0.01-0.20%, Nb: 0.01-0.20%
Ti and Nb form carbonitrides and have the effect of strengthening steel by precipitation strengthening. Such an effect can be recognized by adding at least one of 0.01% or more. On the other hand, even if each component exceeds 0.20%, the strength is excessively increased and the ductility is lowered. Therefore, when adding Ti and Nb, the content is made 0.01% or more and 0.20% or less, respectively.
Ｂ：0.0002〜0.005%
Bはオーステナイト相粒界からのフェライト相の生成を抑制し強度を上昇させる作用を有する。その効果は0.0002%以上の添加で得られる。しかし、B量が0.005%を超えるとその効果は飽和し、コストアップの要因となる。従って、Ｂ量は0.0002%以上0.005%以下とする。
B: 0.0002 to 0.005%
B has the effect of suppressing the formation of a ferrite phase from the austenite grain boundary and increasing the strength. The effect can be obtained by adding 0.0002% or more. However, if the amount of B exceeds 0.005%, the effect is saturated, which increases the cost. Therefore, the B content is 0.0002% or more and 0.005% or less.
Ca：0.001〜0.005%、REM：0.001〜0.005%から選ばれる1種または2種
Ca、REMはいずれも硫化物の形態制御により加工性を改善する効果を有しており、必要に応じてCa、REMの1種または2種を0.001%以上含有させることができる。しかしながら過剰な添加は成形性を低下させる恐れがあるため、Ca、REMを添加する場合は、それぞれ0.005%以下とする。
Ca: One or two selected from 0.001 to 0.005%, REM: 0.001 to 0.005%
Both Ca and REM have an effect of improving workability by controlling the form of sulfides, and one or two of Ca and REM can be contained in an amount of 0.001% or more as required. However, excessive addition may reduce moldability, so when Ca and REM are added, the content is 0.005% or less.
2)ミクロ組織
フェライト相の面積率が20%以上
フェライト相の面積率が20%未満だとＴＳ×ＥＬが低下するため20%以上とする。好ましくは50%以上である。一方、フェライト相の面積率が87%を超えると必要量の焼戻しマルテンサイト相および残留オーステナイト相の確保ができなくなる。よって、フェライト相の面積率は87%以下が好ましい。
2) If the area ratio of the microstructure ferrite phase is 20% or more and the area ratio of the ferrite phase is less than 20%, TS × EL is lowered, so it is set to 20% or more. Preferably it is 50% or more. On the other hand, when the area ratio of the ferrite phase exceeds 87%, it becomes impossible to secure a necessary amount of tempered martensite phase and retained austenite phase. Therefore, the area ratio of the ferrite phase is preferably 87% or less.
焼戻しマルテンサイト相の面積率が10〜60%
焼戻しマルテンサイト相とはマルテンサイト相をAc1変態点以下の温度に加熱して得られる、転位密度の高いフェライト相とセメンタイト相との複合組織を示し、鋼の強化に有効に働く。また、焼戻しマルテンサイト相はマルテンサイト相に比べて穴拡げ性（伸びフランジ性）への悪影響が小さく、顕著な穴拡げ性（伸びフランジ性）の低下なしに強度を確保するのに有効な相である。以上の理由により、焼戻しマルテンサイト相の面積率は10％以上とする。焼戻しマルテンサイト相の面積率が10%未満では強度確保が困難となる。一方、その面積率が60%を超えるとＴＳ×ＥＬが低下するため、焼戻しマルテンサイト相の面積率は60%以下とする。
The area ratio of tempered martensite phase is 10-60%
The tempered martensite phase is a composite structure of a ferrite phase and a cementite phase with a high dislocation density obtained by heating the martensite phase to a temperature below the Ac1 transformation point, and effectively works to strengthen steel. In addition, the tempered martensite phase has a smaller adverse effect on hole expandability (stretch flangeability) than the martensite phase, and is an effective phase for ensuring strength without a significant decrease in hole expandability (stretch flangeability). It is. For the above reasons, the area ratio of the tempered martensite phase is 10% or more. If the area ratio of the tempered martensite phase is less than 10%, it is difficult to ensure the strength. On the other hand, if the area ratio exceeds 60%, TS × EL decreases, so the area ratio of the tempered martensite phase is 60% or less.
マルテンサイト相の面積率が0〜10%
マルテンサイト相の面積率が10%を超えると穴拡げ率（以下、λと称することがある）が顕著に低下するので、面積率の上限を10％とする。また、マルテンサイト相の面積率の下限は0％である。従って、マルテンサイト相の面積率は0％以上10%以下とする。
Martensite phase area ratio 0-10%
When the area ratio of the martensite phase exceeds 10%, the hole expansion ratio (hereinafter sometimes referred to as λ) is remarkably lowered, so the upper limit of the area ratio is set to 10%. Further, the lower limit of the area ratio of the martensite phase is 0%. Therefore, the area ratio of the martensite phase is 0% or more and 10% or less.
残留オーステナイト相の体積率が3〜15%
残留オーステナイト相は鋼の強化に寄与するだけでなく、鋼のＴＳ×ＥＬの向上にも有効に働く。このような効果は体積率で3%以上で得られる。一方、残留オーステナイト相の体積率が15%超えとなると穴拡げ性（伸びフランジ性）が低下する。従って、残留オーステナイト相の体積率は3％以上15%以下とする。
3-15% volume fraction of retained austenite phase
The residual austenite phase not only contributes to the strengthening of the steel, but also works effectively to improve the TS × EL of the steel. Such an effect is obtained at a volume ratio of 3% or more. On the other hand, when the volume ratio of the retained austenite phase exceeds 15%, the hole expandability (stretch flangeability) decreases. Therefore, the volume ratio of the retained austenite phase is 3% or more and 15% or less.
なお、上記ミクロ組織の構成が満足されれば本発明の目的を達成できるため、フェライト相、焼戻しマルテンサイト相、マルテンサイト相、残留オーステナイト相以外の相として、パーライト相およびベイナイト相を含むことができる。ただし、延性（伸び）および穴拡げ性（伸びフランジ性）確保の観点からパーライトおよびベイナイトはそれぞれ面積率で3%以下とすることが望ましい。 In addition, since the object of the present invention can be achieved if the above microstructure structure is satisfied, it may include a pearlite phase and a bainite phase as phases other than the ferrite phase, the tempered martensite phase, the martensite phase, and the retained austenite phase. it can. However, from the viewpoint of securing ductility (elongation) and hole expansibility (stretch flangeability), it is desirable that pearlite and bainite each have an area ratio of 3% or less.
なお、本発明におけるフェライト相、焼戻しマルテンサイト相およびマルテンサイト相の面積率とは、観察面積に占める各相の面積の割合のことである。上記各面積率は、例えば、鋼板の圧延方向に平行な板厚断面を研磨後、3%ナイタールで腐食し、SEM(走査電子顕微鏡)を用いて2000倍の倍率で10視野観察し、市販の画像処理ソフトを用いて求めることができる。また、残留オーステナイト相の体積率とは、板厚1/4面における残留オーステナイト相の｛111｝、｛200｝、｛220｝、｛311｝面とフェライト相の｛110｝、｛200｝、｛211｝面の全ての組合せについて求めたX線回折積分強度比の平均値である。 In addition, the area ratio of the ferrite phase, the tempered martensite phase, and the martensite phase in the present invention is the ratio of the area of each phase to the observation area. Each area ratio is, for example, after polishing a plate thickness section parallel to the rolling direction of the steel plate, corroded with 3% nital, observed 10 fields at a magnification of 2000 using a SEM (scanning electron microscope), commercially available It can be determined using image processing software. The volume fraction of the retained austenite phase is the {111}, {200}, {220}, {311} face of the retained austenite phase on the 1/4 thickness plane and {110}, {200} of the ferrite phase. It is an average value of X-ray diffraction integrated intensity ratios obtained for all combinations of {211} planes.
3)製造条件
本発明の高強度熱延鋼板は、例えば、上記の成分組成を有するスラブに、熱間圧延を施したのち、800〜600℃の温度範囲を10〜100℃/sの平均冷却速度で1回目の冷却を行い、次いで100〜300℃まで2回目の冷却を行い、100〜300℃の巻取り温度で巻取った後、300〜450℃の温度に再加熱し、該再加熱温度域で1〜600min保持し、次いで、冷却する。
以下、詳細に説明する。
3) Manufacturing conditions The high-strength hot-rolled steel sheet of the present invention is, for example, subjected to hot rolling on a slab having the above component composition, and then an average cooling of 10 to 100 ° C./s in a temperature range of 800 to 600 ° C. 1st cooling at speed, then 2nd cooling to 100-300 ° C, winding at 100-300 ° C winding temperature, reheating to 300-450 ° C, the reheating Hold in the temperature range for 1-600 min, then cool.
Details will be described below.
上記の成分組成に調整した鋼を転炉などで溶製し、連続鋳造法等でスラブとし、熱間圧延を行う。
使用する鋼スラブは、成分のマクロ偏析を防止するために連続鋳造法で製造するのが好ましいが、造塊法、薄スラブ鋳造法で製造してもよい。また、鋼スラブを製造したのち、いったん室温まで冷却し、その後再度加熱する従来法に加え、室温まで冷却しないで、温片のままで加熱炉に挿入する、あるいはわずかの保熱をおこなった後に直ちに圧延する直送圧延・直接圧延などの省エネルギープロセスも問題なく適用できる。
Steel adjusted to the above component composition is melted in a converter or the like, and is slabed by a continuous casting method or the like, and is hot-rolled.
The steel slab to be used is preferably produced by a continuous casting method in order to prevent macro segregation of components, but may be produced by an ingot casting method or a thin slab casting method. After manufacturing the steel slab, in addition to the conventional method of cooling to room temperature and then heating again, without cooling to room temperature, insert it into a heating furnace as it is, or carry out slight heat retention Energy saving processes such as direct feed rolling and direct rolling, which are rolled immediately, can be applied without any problem.
スラブ加熱温度：1100℃以上（好適条件）
スラブ加熱温度は、低温加熱がエネルギー的には好ましいが、加熱温度が1100℃未満では、炭化物が十分に固溶できなかったり、圧延荷重の増大による熱間圧延時のトラブル発生の危険が増大するなどの問題が生じる。そのため、スラブ加熱温度は1100℃以上が好ましい。なお、酸化の増加にともなうスケールロスの増大などから、スラブ加熱温度は1300℃以下とすることが好ましい。
なお、スラブ加熱温度を低くしても熱間圧延時のトラブルを防止するといった観点から、シートバーを加熱する、いわゆるシートバーヒーターを活用してもよい。
Slab heating temperature: 1100 ℃ or higher (preferred conditions)
As for the slab heating temperature, low-temperature heating is preferable in terms of energy, but if the heating temperature is less than 1100 ° C, the carbide cannot be sufficiently dissolved, or the risk of trouble occurring during hot rolling due to an increase in rolling load increases. Problems arise. Therefore, the slab heating temperature is preferably 1100 ° C. or higher. The slab heating temperature is preferably set to 1300 ° C. or lower because of an increase in scale loss accompanying an increase in oxidation.
From the viewpoint of preventing troubles during hot rolling even if the slab heating temperature is lowered, a so-called sheet bar heater that heats the sheet bar may be used.
仕上圧延終了温度：A3点以上（好適条件）
仕上圧延終了温度がA3点未満では、圧延中にα（フェライト）とγ（オーステナイト）が生成して、鋼板にバンド状組織が生成し易くなり、材料特性に異方性を生じさせたり、加工性を低下させる原因となる場合がある。このため、仕上圧延終了温度はA3変態点以上とすることが好ましい。
Finishing rolling finish temperature: A 3 points or more (preferred conditions)
If it is less than finish rolling temperature A 3 points, during rolling alpha (ferrite) and gamma (austenite) are generated, the band-like structure is liable to generate in the steel plate, or causing anisotropy in material properties, It may cause a decrease in workability. Therefore, finish rolling temperature is preferably set to A 3 transformation point or more.
次いで、800〜600℃の温度範囲を10〜100℃/sの平均冷却速度で1回目の冷却を行い、次いで100〜300℃まで2回目の冷却を行い、100〜300℃の巻取り温度で巻取る。
800〜600℃の温度範囲の平均冷却速度：10〜100℃/s
800〜600℃の温度範囲における平均冷却速度が10℃/s未満ではパーライト相が生成し、ＴＳ×ＥＬおよび穴拡げ性（伸びフランジ性）が低下する。従って、平均冷却速度は10℃/s以上とする。一方、平均冷却速度が100℃/sを超えるとフェライト相の生成が抑制され、面積率が20%以上のフェライト相が得られなくなる。以上より、800〜600℃の温度範囲における平均冷却速度は10〜100℃/sとする。
100〜300℃まで2回目の冷却、すなわち、600℃から巻取り温度までの冷却の平均冷却速度は特に規定しない。冷却ゾーンのライン長の制約などから、好ましくは5℃/s以上とする。
巻取り温度：100〜300℃
巻取り温度は本発明において最も重要な要件の一つである。巻取り時にオーステナイト相の一部がマルテンサイト相に変態し、残りは未変態のオーステナイト相となる。そこから再加熱・保持後、室温まで冷却することで、マルテンサイト相は焼戻しマルテンサイト相となり、未変態オーステナイト相は残留オーステナイト相またはマルテンサイト相となる。巻取り温度が300℃より高い温度では、冷却停止時のマルテンサイト変態が不十分で未変態オーステナイト相の量が多くなり、最終的なマルテンサイト相または残留オーステナイト相が過剰に生成し、穴拡げ性（伸びフランジ性）を低下させる。また、巻取り温度が100℃より低くなると、オーステナイト相がほとんどマルテンサイト相に変態し未変態オーステナイト量が減少し、体積率が3%以上の残留オーステナイト相が得られない。このため、巻取り温度は100〜300℃とする。
Next, the first cooling in the temperature range of 800 to 600 ° C. is performed at an average cooling rate of 10 to 100 ° C./s, then the second cooling is performed to 100 to 300 ° C., and the winding temperature is 100 to 300 ° C. Take up.
Average cooling rate in the temperature range of 800-600 ° C: 10-100 ° C / s
When the average cooling rate in the temperature range of 800 to 600 ° C. is less than 10 ° C./s, a pearlite phase is generated, and TS × EL and hole expandability (stretch flangeability) are deteriorated. Therefore, the average cooling rate is 10 ° C./s or more. On the other hand, when the average cooling rate exceeds 100 ° C./s, the formation of a ferrite phase is suppressed, and a ferrite phase having an area ratio of 20% or more cannot be obtained. From the above, the average cooling rate in the temperature range of 800 to 600 ° C. is 10 to 100 ° C./s.
The average cooling rate of the second cooling from 100 to 300 ° C., that is, the cooling from 600 ° C. to the coiling temperature is not particularly specified. Due to the limitation of the line length of the cooling zone, it is preferably 5 ° C./s or more.
Winding temperature: 100 ~ 300 ℃
The coiling temperature is one of the most important requirements in the present invention. During winding, a part of the austenite phase is transformed into a martensite phase, and the rest becomes an untransformed austenite phase. Then, after reheating and holding, and cooling to room temperature, the martensite phase becomes a tempered martensite phase, and the untransformed austenite phase becomes a retained austenite phase or a martensite phase. When the coiling temperature is higher than 300 ° C, the martensite transformation at the time of cooling stop is insufficient and the amount of untransformed austenite phase increases, resulting in excessive formation of the final martensite phase or residual austenite phase, resulting in hole expansion. (Elongation flangeability) is reduced. On the other hand, when the coiling temperature is lower than 100 ° C., the austenite phase is almost transformed into a martensite phase, the amount of untransformed austenite is reduced, and a retained austenite phase having a volume ratio of 3% or more cannot be obtained. For this reason, winding temperature shall be 100-300 degreeC.
なお、本発明における熱間圧延工程では、熱間圧延時の圧延荷重を低減するために仕上圧延の一部または全部を潤滑圧延としてもよい。潤滑圧延を行うことは、鋼板形状の均一化、材質の均一化の観点からも有効である。なお、潤滑圧延の際の摩擦係数は0.25〜0.10の範囲とすることが好ましい。また、相前後するシートバー同士を接合し、連続的に仕上圧延する連続圧延プロセスとすることが好ましい。連続圧延プロセスを適用することは、熱間圧延の操業安定性の観点からも好ましい。 In the hot rolling process of the present invention, part or all of finish rolling may be lubricated rolling in order to reduce the rolling load during hot rolling. Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. In addition, it is preferable to make the friction coefficient in the case of lubrication rolling into the range of 0.25-0.10. Moreover, it is preferable to set it as the continuous rolling process which joins the sheet | seat bars which precede and follow, and finish-rolls continuously. Applying the continuous rolling process is also preferable from the viewpoint of the operational stability of hot rolling.
巻取り後、300〜450℃の温度に再加熱し、該再加熱温度域で1〜600min保持する。
巻取り後、300〜450℃の温度範囲で1min以上600min以下保持することで、巻取り時に生成したマルテンサイト相が焼戻され焼戻しマルテンサイト相となり、穴拡げ性（伸びフランジ性）が向上し、さらに巻取り時にマルテンサイト相に変態しなかった未変態オーステナイト相が安定化され、最終的に体積率が3%以上の残留オーステナイト相が得られ、伸びが向上する。再加熱温度が300℃未満または保持時間が1min未満ではマルテンサイト相の焼戻しおよびオーステナイト相の安定化が不十分となり穴拡げ性（伸びフランジ性）および伸びが低下する。また再加熱温度が450℃を超える、あるいは保持時間が600minを超えると巻取り時の未変態オーステナイト相から炭化物が析出し、最終的に体積率が3%以上の残留オーステナイト相が得られなくなる。また、再加熱の方法は誘導加熱あるいはBAF焼鈍などいかなる方法を用いても良い。
After winding, it is reheated to a temperature of 300 to 450 ° C. and held for 1 to 600 minutes in the reheating temperature range.
After winding, by holding at a temperature range of 300 to 450 ° C for 1 min to 600 min, the martensite phase generated during winding is tempered to become a tempered martensite phase, improving hole expandability (stretch flangeability). Furthermore, the untransformed austenite phase that did not transform into the martensite phase during winding is stabilized, and finally a retained austenite phase with a volume fraction of 3% or more is obtained, and the elongation is improved. If the reheating temperature is less than 300 ° C. or the holding time is less than 1 minute, the tempering of the martensite phase and the stabilization of the austenite phase are insufficient, and the hole expandability (stretch flangeability) and elongation are lowered. If the reheating temperature exceeds 450 ° C. or the holding time exceeds 600 min, carbides precipitate from the untransformed austenite phase at the time of winding, and a residual austenite phase having a volume ratio of 3% or more cannot be obtained. Further, any method such as induction heating or BAF annealing may be used as the reheating method.
次いで、室温まで冷却する。室温までの冷却速度および冷却方法は特に規定せず、放冷、ガス冷却、水冷却およびその組合せ等、いかなる方法でも構わない。
なお、熱間圧延後の鋼板には、形状矯正、表面粗度等の調整のため調質圧延を加えてもよい。また、亜鉛めっきなどの各種めっき処理および樹脂あるいは油脂コーティング、各種塗装等の処理を施しても何ら不都合はない。
以上により、本発明の伸びおよび伸びフランジ性に優れた高強度熱延鋼板が得られる。
It is then cooled to room temperature. The cooling rate and cooling method to room temperature are not particularly defined, and any method such as cooling, gas cooling, water cooling, and combinations thereof may be used.
In addition, you may add temper rolling to the steel plate after a hot rolling for adjustments, such as shape correction and surface roughness. Moreover, there is no inconvenience even if various plating treatments such as galvanization and treatments such as resin or oil coating and various coatings are performed.
As described above, a high-strength hot-rolled steel sheet having excellent elongation and stretch flangeability according to the present invention can be obtained.
表1に示す成分組成を有し、残部がFeおよび不可避的不純物よりなる鋼（成分組成：Nは不可避的不純物）を転炉にて溶製し、連続鋳造法にて鋳片（スラブ）とした。得られた鋳片を表2に示す条件で板厚2.9mmまで熱間圧延した。 Steel with the composition shown in Table 1 and the balance consisting of Fe and unavoidable impurities (component composition: N is an unavoidable impurity) is melted in a converter, and slab (slab) is obtained by continuous casting. did. The obtained slab was hot-rolled to a thickness of 2.9 mm under the conditions shown in Table 2.
以上により得られた熱延鋼板について、断面ミクロ組織、引張特性および穴拡げ性（伸びフランジ性）を調査した。得られた結果を表3に示す。
なお、鋼板の断面ミクロ組織は3%ナイタール溶液（3%硝酸+エタノール）で組織を現出し、走査型電子顕微鏡（SEM）で2000倍の倍率で深さ方向板厚1/4位置を10視野観察して、撮影した組織写真を用いて、画像解析処理を行ない、フェライト相の分率（面積率）を定量化した（なお、画像解析処理は市販の画像処理ソフトを用いることができる）。
マルテンサイト相の面積率、焼戻しマルテンサイト相の面積率は、組織の細かさに応じて1000〜3000倍の適切な倍率でSEM写真を撮影し、画像処理ソフトで定量化した。
残留オーステナイト相の体積率は、鋼板を板厚方向の1/4面まで研磨し、この板厚1/4面の回折X線強度により求めた。入射Ｘ線にはMoKα線を使用し、残留オーステナイト相の｛111｝、｛200｝、｛220｝、｛311｝面とフェライト相の｛110｝、｛200｝、｛211｝面のピークの積分強度の全ての組み合わせについて強度比を求め、これらの平均値を残留オーステナイト相の体積率とした。
また、引張特性は、引張方向が鋼板の圧延方向と直角方向となるようサンプル採取したJIS5号試験片を用いて、JISZ2241に準拠した引張試験を行ない、ＴＳ（引張強さ）、ＥＬ（伸び）を測定し、強度と伸びの積（ＴＳ×ＥＬ）で表される強度と伸びのバランスの値を求めた。
さらに、λ（穴拡げ率）は日本鉄鋼連盟規格JFST1001に準じた穴拡げ試験を行い、測定した。
The hot-rolled steel sheet obtained as described above was examined for a cross-sectional microstructure, tensile properties, and hole expandability (stretch flangeability). The results obtained are shown in Table 3.
In addition, the cross-sectional microstructure of the steel sheet is revealed with a 3% nital solution (3% nitric acid + ethanol), and 10 views of the thickness direction 1/4 position at a magnification of 2000 times with a scanning electron microscope (SEM). The image analysis process was performed using the observed and photographed tissue photographs, and the fraction (area ratio) of the ferrite phase was quantified (in addition, commercially available image processing software can be used for the image analysis process).
The area ratio of the martensite phase and the area ratio of the tempered martensite phase were quantified with image processing software by taking SEM photographs at an appropriate magnification of 1000 to 3000 times depending on the fineness of the structure.
The volume fraction of the retained austenite phase was determined by diffracting X-ray intensities on the 1/4 plane of the plate thickness after polishing the steel plate to 1/4 plane in the plate thickness direction. For incident X-rays, MoKα rays are used, and the peaks of {111}, {200}, {220}, {311} in the retained austenite phase and {110}, {200}, {211} in the ferrite phase Intensity ratios were obtained for all combinations of integrated intensities, and the average value of these ratios was taken as the volume ratio of the retained austenite phase.
Tensile properties are determined by conducting a tensile test in accordance with JISZ2241 using JIS No. 5 test specimens sampled so that the tensile direction is perpendicular to the rolling direction of the steel sheet. TS (tensile strength), EL (elongation) Was measured, and the value of the balance between strength and elongation represented by the product of strength and elongation (TS × EL) was determined.
Further, λ (hole expansion rate) was measured by performing a hole expansion test according to JFST1001.
表3より、本発明例の鋼板はＴＳとＥＬのバランス（ＴＳ×ＥＬ）が21000MPa・%以上、穴拡げ率（λ）が75%以上であり、高強度と、優れた伸びおよび伸びフランジ性を示している。
一方、本発明の範囲をはずれる比較例の鋼板はＴＳとＥＬのバランス（ＴＳ×ＥＬ）が21000MPa・%未満および（または）λが75%未満となり、強度、伸びおよび伸びフランジ性のいずれかが劣っている。
From Table 3, the steel sheet of the present invention has a balance between TS and EL (TS × EL) of 21000 MPa ·% or more and a hole expansion ratio (λ) of 75% or more, high strength, excellent elongation and stretch flangeability. Is shown.
On the other hand, the steel plate of the comparative example which is out of the scope of the present invention has a balance between TS and EL (TS × EL) of less than 21000 MPa ·% and / or λ of less than 75%, and any one of strength, elongation and stretch flangeability is achieved. Inferior.
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