JP5278226B2 - Alloy-saving high-strength hot-rolled steel sheet and manufacturing method thereof - Google Patents
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本発明は、特に、自動車部品などに用いられる５４０ＭＰａ以上の強度と優れた加工性を有する熱延鋼板及びその製造方法に関するものである。 The present invention particularly relates to a hot-rolled steel sheet having a strength of 540 MPa or more and excellent workability used for automobile parts and the like, and a method for producing the same.
鋼の強度を高めるには、Ｃや、Ｓｉ、Ｍｎなどの元素の添加による固溶強化、Ｔｉ、Ｎｂなどの析出物を利用した析出強化、金属組織を軟質のフェライトと硬質のマルテンサイトやベイナイトからなる複合組織とする組織強化が有効である。特に、自動車用部材は、軽量化や、安全性及び耐久性の向上が進められており、素材である鉄鋼材料の高強度化が要求されている。 To increase the strength of steel, solid solution strengthening by adding elements such as C, Si, and Mn, precipitation strengthening using precipitates such as Ti and Nb, and the metal structure of soft ferrite and hard martensite or bainite It is effective to strengthen the organization as a composite organization consisting of In particular, automobile components are being reduced in weight and improved in safety and durability, and a high strength steel material is required.
しかし、固溶強化は、析出強化や組織強化に比べて効果が小さく、自動車用部材の素材に求められるような高強度化は困難である。そのため、マルテンサイトなどの硬質相を組み合わせた複合組織鋼が開発されている。この複合組織鋼は、均一伸びにも優れるものの、フェライトと硬質相との硬度差に起因して、局部延性が低く、穴広げ性に劣る。 However, solid solution strengthening is less effective than precipitation strengthening and structure strengthening, and it is difficult to increase the strength as required for materials for automobile members. Therefore, a composite structure steel combining hard phases such as martensite has been developed. Although this composite structure steel is excellent in uniform elongation, the local ductility is low due to the difference in hardness between the ferrite and the hard phase, and the hole expandability is inferior.
これに対し、本来のフェライト相の均一組織の優れた変形能を維持したまま高強度化を図ろうとする技術開発が、近年再び検討され始めた。例えば、本発明者らの一部は、Ｔｉ、Ｎｂ、Ｍｏなどの炭化物形成元素を活用し、微細な炭化物を析出させ、フェライトを強化する方法を提案した（例えば、特許文献１〜３、参照）。また。Ｔｉを析出強化に利用した鋼板については、材質のばらつきを低減する方法も提案されている（例えば、特許文献４及び５、参照）。
On the other hand, technical development for increasing the strength while maintaining the excellent deformability of the original uniform structure of the ferrite phase has recently started to be examined again. For example, some of the present inventors have proposed a method of strengthening ferrite by precipitating fine carbides by utilizing carbide-forming elements such as Ti, Nb, and Mo (see, for example,
しかし、これらは比較的、多量の炭化物形成元素を添加するものである。これに対し、炭化物形成元素の添加を抑え、代わりにＡｌおよびＮによる析出強化を利用する方法が提案されている（例えば、特許文献６、参照）。 However, they add a relatively large amount of carbide forming elements. On the other hand, a method of suppressing the addition of carbide forming elements and using precipitation strengthening by Al and N instead has been proposed (see, for example, Patent Document 6).
特許文献６の鋼板は、ＡｌとＴｉ、Ｎｂなどの窒化物を利用するものであるが、省合金化という観点では、炭化物形成能の最も強いＴｉを主に利用し、他の炭化物形成元素の利用は極力低減することが望ましい。本発明は、Ｎｂ、Ｍｏ、Ｖなどの合金元素の添加を抑制し、少量のＴｉの添加によって効率的に析出強化させた、引張強度５４０〜６５０ＭＰａの高強度熱延鋼板及びその製造方法を提供することを目的とするものである。
The steel sheet of
本発明者らは、炭化物形成能が他の元素と比べて非常に高いＴｉを効率的に析出強化に利用するための、析出物のサイズ、および個数密度を見出し、これによって、最も少量の原子の利用で、多くの原子を利用したときと同じ強度を達成することが可能であることを明らかにした。添加元素の量を抑えることで、合金コストの低減が可能になるだけでなく、合金元素の添加に起因する加工性の低下も抑えることができる。 The inventors of the present invention have found the size and number density of precipitates for efficiently using Ti, which has a very high carbide forming ability compared to other elements, for precipitation strengthening. It was clarified that the use of can achieve the same strength as when many atoms were used. By suppressing the amount of the additive element, it is possible not only to reduce the alloy cost, but also to suppress a decrease in workability due to the addition of the alloy element.
本発明は、このような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。 This invention is made | formed based on such knowledge, The summary is as follows.
（１） 質量％で、
Ｃ：０．０２〜０．０８％、
Ｓｉ：０．０１〜１．５０％、
Ｍｎ： ０．１〜１．５％
Ｔｉ：０．０３〜０．０６％
を含有し、
Ｐ：０．１％以下、
Ｓ：０．００５％以下、
Ａｌ：０．５％以下、
Ｎ：０．００９％以下
に制限し、更に、Ｎｂ、Ｍｏ、Ｖの含有量の合計を０．０１％以下に制限し、残部がＦｅおよび不可避的不純物からなり、Ｃ量に対するＴｉ量の比が、
Ｔｉ／Ｃ：０．３７５〜１．６
であり、結晶粒内のＴｉＣ析出物の平均直径が０．８〜３ｎｍであり、平均個数密度が、１×１０17[個／ｃｍ3]以上であり、引張強度が５４０〜６５０ＭＰａであることを特徴とする省合金型高強度熱延鋼板。
(1) In mass%,
C: 0.02 to 0.08%,
Si: 0.01 to 1.50%,
Mn: 0.1 to 1.5%
Ti: 0.03-0.06%
Containing
P: 0.1% or less,
S: 0.005% or less,
Al: 0.5% or less,
N: Limiting to 0.009% or less, further limiting the total content of Nb, Mo, V to 0.01% or less, the balance being Fe and inevitable impurities, the ratio of Ti amount to C amount But,
Ti / C: 0.375 to 1.6
The average diameter of the TiC precipitates in the crystal grains is 0.8 to 3 nm, the average number density is 1 × 10 17 [pieces / cm 3 ] or more, and the tensile strength is 540 to 650 MPa. Alloy-saving high-strength hot-rolled steel sheet.
（２） 上記（１）に記載の高強度熱延鋼板の製造方法であって、上記（１）に記載の成分からなる鋼片を１２００℃以上に加熱し、最終加工温度ＦＴ［℃］を９００℃超として熱間加工を行い、２０℃／ｓ以上で５８０〜６４０℃の範囲内の温度ＭＴ1［℃］まで１次冷却し、続いて５℃／ｓ以下で、５１０〜６００℃の範囲内であり、前記ＭＴ1［℃］よりも低い温度ＭＴ2［℃］まで２次冷却し、巻取ることを特徴とする省合金型高強度熱延鋼板の製造方法。 (2) A method for producing a high-strength hot-rolled steel sheet as described in (1) above, wherein a steel slab comprising the component as described in (1) above is heated to 1200 ° C. or higher, and a final processing temperature FT [° C.] is set. Hot working is performed at over 900 ° C., and is first cooled to a temperature MT 1 [° C.] within a range of 580 to 640 ° C. at 20 ° C./s or higher, and subsequently at 510 ° C. to 600 ° C. at 5 ° C./s or lower. A method for producing an alloy-saving high-strength hot-rolled steel sheet, wherein the secondary cooling is performed to a temperature MT 2 [° C.] lower than the MT 1 [° C.], and winding is performed.
本発明によれば、より少ないかつ安価な合金元素の添加で効率的に強度を確保することができるため、産業上の貢献が極めて顕著である。 According to the present invention, the strength can be efficiently ensured with the addition of a less expensive and cheap alloy element, so that the industrial contribution is extremely remarkable.
本発明では低コストで所望の強度と加工性を確保するために極力合金元素は添加せず、析出強化を最大限活用するような鋼板の製造を目指した。そこで炭化物形成能が他の元素と比べて非常に高いＴｉを効率的に析出強化に利用することを検討した。
本発明者らは、ＴｉＣ析出物のサイズと析出物１個あたりの強化能について、種々の熱処理条件により、析出物のサイズおよび個数密度を変化させた析出強化鋼を製造して詳細に検討を行った。
In the present invention, in order to secure desired strength and workability at low cost, an alloy element is not added as much as possible, and the aim is to produce a steel sheet that makes the best use of precipitation strengthening. Therefore, we investigated the efficient use of Ti for precipitation strengthening, which has very high carbide forming ability compared to other elements.
The inventors of the present invention have studied in detail the production of precipitation-strengthened steel in which the size and number density of the precipitates are changed under various heat treatment conditions with respect to the size of TiC precipitates and the strengthening ability per precipitate. went.
まず、得られた析出強化鋼から、 平行部の直径が６ｍｍφ、長さが３２ｍｍの丸棒引張試験片を採取し、引張試験をＪＩＳ Ｚ ２２４１に準拠して行い、降伏強度を測定した。各試験片の降伏強度から、析出硬化していない試験片の降伏強度を差し引いて、析出強化量を求めた。 First, from the obtained precipitation-strengthened steel, a round bar tensile test piece having a parallel part diameter of 6 mmφ and a length of 32 mm was collected, and a tensile test was performed according to JIS Z 2241 to measure the yield strength. The yield strength of each test piece was determined by subtracting the yield strength of the test piece that had not been precipitation hardened.
また、ＴｉＣ析出物サイズ及びＴｉＣ析出物密度の測定を、三次元アトムプローブ測定法により、以下のようにして行った。
測定対象の試験片から、切断および電解研磨法により、必要に応じて電解研磨法とあわせて集束イオンビーム加工法を活用し、針状の試料を作製する。三次元アトムプローブ測定では、試料表面を一原子層ずつ蒸発させて積算したデータを、再構築して実空間での実際の原子の分布像として求めることができる。
The TiC precipitate size and TiC precipitate density were measured by the three-dimensional atom probe measurement method as follows.
A needle-like sample is prepared from a test specimen to be measured by cutting and electrolytic polishing using a focused ion beam processing method in combination with an electrolytic polishing method as necessary. In the three-dimensional atom probe measurement, data obtained by evaporating the sample surface by one atomic layer and integrating can be reconstructed and obtained as an actual atomic distribution image in real space.
観察されたＴｉＣ析出物の立体分布像の体積とＴｉＣ析出物の数からＴｉＣ析出物の個
数密度が求まる。また、ＴｉＣ析出物のサイズは、観察されたＴｉＣ析出物の構成原子数とＴｉＣの格子定数から、析出物を球状と仮定し算出した直径とし、任意に３０個以上のＴｉＣ析出物の直径を測定し、その平均値として求まる。なお、三次元アトムプローブ測定では、サイズが０．５ｎｍ未満の粒子を固溶状態と判断した。また、本発明の成分範囲の鋼材においては、サイズが５０ｎｍを超える粒子は個数密度が少なく、正確な評価が困難である。そのため、サイズが０．５〜５０ｎｍのＴｉＣ析出物を対象として、ＴｉＣサイズ及びＴｉＣ析出物密度を求めた。
The number density of TiC precipitates can be determined from the volume of the observed stereo distribution of TiC precipitates and the number of TiC precipitates. The size of the TiC precipitate is the diameter calculated from the observed number of constituent atoms of the TiC precipitate and the lattice constant of TiC, assuming that the precipitate is spherical, and arbitrarily the diameter of 30 or more TiC precipitates. Measure and find the average value. In the three-dimensional atom probe measurement, particles having a size of less than 0.5 nm were determined to be in a solid solution state. In addition, in the steel material having the component range of the present invention, particles having a size exceeding 50 nm have a small number density and are difficult to accurately evaluate. Therefore, TiC size and TiC precipitate density were determined for TiC precipitates having a size of 0.5 to 50 nm.
このようにして求めたＴｉＣ析出物のサイズ、個数密度、析出強化量から、析出物のサイズとその析出物１個あたりの析出強化量（転位をピンニングする抵抗力）との関係を計算で求めた。なお、計算式は、転位が粒子を切断するモデルの理論式に基づいている。 From the size, number density, and precipitation strengthening amount of the TiC precipitate thus obtained, the relationship between the size of the precipitate and the precipitation strengthening amount per one precipitate (resistance force for pinning dislocations) is obtained by calculation. It was. The calculation formula is based on a theoretical formula of a model in which dislocations cut particles.
次に、鋼材全体の析出強化量に及ぼすＴｉＣ析出物サイズの影響について検討を行った。その際、添加したＴｉによる鋼の最大の析出強化量を評価するために、鋼中に含まれるＴｉの全量が析出し、化学量論的なＴｉＣとなると仮定した。また、ＴｉＣ析出物のサイズが一定、即ち、ＴｉおよびＣの全原子数が一定であると仮定した。
そして、鋼のＴｉ量とＴｉＣ析出物のサイズから個数密度を求め、先に求めた析出物１個あたりの析出強化量と個数密度から鋼材の析出強化量を求めてＴｉＣ析出物のサイズと鋼材の析出強化量との関係を評価した。なお、鋼材の析出強化量は、析出物１個あたりの析出強化量の評価に用いた計算式を変形し、ＴｉＣ析出物のサイズから求めた個数密度によって評価した。
Next, the influence of the TiC precipitate size on the precipitation strengthening amount of the whole steel material was examined. At that time, in order to evaluate the maximum precipitation strengthening amount of the steel by the added Ti, it was assumed that the entire amount of Ti contained in the steel was precipitated and became stoichiometric TiC. It was also assumed that the size of the TiC precipitate was constant, that is, the total number of Ti and C atoms was constant.
Then, the number density is obtained from the Ti amount of the steel and the size of the TiC precipitate, and the precipitation strengthening amount of the steel material is obtained from the precipitation strengthening amount and the number density per precipitate obtained previously, and the size of the TiC precipitate and the steel material are obtained. The relationship with the precipitation strengthening amount of was evaluated. The precipitation strengthening amount of the steel material was evaluated by the number density obtained from the size of the TiC precipitate by modifying the calculation formula used for the evaluation of the precipitation strengthening amount per precipitate.
図１および図２に、Ｔｉ量を０．０３質量％および０．０６質量％としたときの、ＴｉＣ析出物のサイズと鋼材の析出強化量との関係を示す。
図１および図２に示すように、平均直径（ＴｉＣ析出物のサイズ）が０．８〜３ｎｍの範囲で、鋼材の析出強化量が特に高くなることがわかる。また、図１と図２とを比較すると、鋼材のＴｉ量が変化しても、粒子直径と析出強化量との相対的な関係は変化しないことがわかる。
FIG. 1 and FIG. 2 show the relationship between the size of the TiC precipitate and the precipitation strengthening amount of the steel when the Ti amount is 0.03% by mass and 0.06% by mass.
As shown in FIGS. 1 and 2, it can be seen that the precipitation strengthening amount of the steel material is particularly high when the average diameter (size of TiC precipitates) is in the range of 0.8 to 3 nm. Moreover, when FIG. 1 and FIG. 2 are compared, even if Ti amount of steel materials changes, it turns out that the relative relationship between a particle diameter and precipitation strengthening amount does not change.
したがって、図１および２の結果から、一定量のＴｉを添加した場合、ＴｉＣ析出物の平均直径を０．８〜３ｎｍの範囲とすることで、非常に効率良く析出強化を作用させられることがわかる。すなわち、ＴｉＣ析出物の平均直径を０．８〜３ｎｍの範囲とすれば、０．８ｎｍ未満及び３ｎｍ超の場合に比べて、強度を確保するために必要なＴｉ量を削減することができると考えられる。 Therefore, from the results of FIGS. 1 and 2, when a certain amount of Ti is added, precipitation strengthening can be applied very efficiently by setting the average diameter of TiC precipitates in the range of 0.8 to 3 nm. Recognize. That is, if the average diameter of the TiC precipitates is in the range of 0.8 to 3 nm, the amount of Ti necessary for securing the strength can be reduced as compared with the case of less than 0.8 nm and more than 3 nm. Conceivable.
以下、本発明について詳細に説明する。
Ｃは、本発明では、微細なＴｉＣ析出物を生じて析出強化に寄与する重要な元素であり、０．０２％以上の添加が必要である。一方、Ｃ量が０．０８％を超えると、粗大なセメンタイトが生じ、延性、特に、局部延性が低下する。
Hereinafter, the present invention will be described in detail.
In the present invention, C is an important element that generates fine TiC precipitates and contributes to precipitation strengthening, and addition of 0.02% or more is necessary. On the other hand, when the amount of C exceeds 0.08%, coarse cementite is generated, and ductility, particularly local ductility is lowered.
Ｓｉは、脱酸元素であり、０．０１％以上を添加する。また、Ｓｉは固溶強化に寄与する元素であるが、含有量が１．５０％を超えると加工性が劣化するため、Ｓｉ量の上限を１．５０％以下とする。 Si is a deoxidizing element, and 0.01% or more is added. Si is an element that contributes to solid solution strengthening, but if the content exceeds 1.50%, the workability deteriorates, so the upper limit of Si content is 1.50% or less.
Ｍｎは脱酸、脱硫に有効な元素であり、固溶強化にも寄与するため、０．２％以上を添加する。一方、Ｍｎ量が１．５％を超えると、偏析が生じやすくなり加工性が低下し、またコストが上昇するため好ましくない。 Mn is an element effective for deoxidation and desulfurization, and contributes to solid solution strengthening, so 0.2% or more is added. On the other hand, if the amount of Mn exceeds 1.5%, segregation is likely to occur, workability is reduced, and costs are increased, which is not preferable.
Ｔｉは、フェライトの粒内に微細なＴｉＣ析出物を析出し、析出強化に寄与する極めて重要な元素である。強度を上昇させるため、０．０３％以上を添加することが好ましい。一方、０．０６％を超えるＴｉを添加すると、ＴｉＣ析出物が粗大化しやすくなり、製造を難しくするため、本発明の析出物サイズおよび個数密度を達成するためには０．０６％以下とすることが好ましい。 Ti is an extremely important element that contributes to precipitation strengthening by precipitating fine TiC precipitates in ferrite grains. In order to increase the strength, 0.03% or more is preferably added. On the other hand, when Ti exceeding 0.06% is added, TiC precipitates are likely to be coarsened, making production difficult. Therefore, in order to achieve the precipitate size and number density of the present invention, the content is made 0.06% or less. It is preferable.
Ｐは、不純物であり、加工性や溶接性を損なうため、０．１％以下に制限する。特に、Ｐは粒界に偏析して延性を低下させるため、Ｐ量を０．０２％以下に制限することが好ましい。 P is an impurity and is limited to 0.1% or less because it impairs workability and weldability. In particular, since P segregates at the grain boundaries to lower the ductility, it is preferable to limit the amount of P to 0.02% or less.
Ｓは、不純物であり、特に、熱間加工性を損なうため、０．００５％以下に制限する。硫化物などの介在物による延性の低下を抑制するためには、Ｓ量を０．００２％以下に制限することが好ましい。 S is an impurity, and is particularly limited to 0.005% or less in order to impair hot workability. In order to suppress a decrease in ductility due to inclusions such as sulfides, it is preferable to limit the amount of S to 0.002% or less.
Ｎは、ＴｉＮを形成し、鋼の加工性を低下させるため、０．００９％以下に制限することが好ましい。
Ａｌは、脱酸剤であり、０．５％以下を含有させる。なお、Ａｌを過剰に添加すると窒化物を形成し、延性が低下するため、０．１５％以下に制限することが好ましい。
N forms TiN and lowers the workability of the steel, so it is preferable to limit it to 0.009% or less.
Al is a deoxidizer and contains 0.5% or less. Note that, if Al is added excessively, nitrides are formed and ductility is lowered. Therefore, it is preferable to limit to 0.15% or less.
Ｎｂ、Ｍｏ、ＶもＴｉと同様にフェライト結晶粒内に炭化物を析出する元素である。しかし、Ｎｂ、Ｍｏ、Ｖともに合金コストが高い割に見合った析出強化能はＴｉより小さいため、Ｎｂ、Ｍｏ、Ｖの合計量を０．０１％以下に制限する。 Nb, Mo, and V are also elements that precipitate carbide in the ferrite crystal grains, like Ti. However, since the precipitation strengthening ability corresponding to the high alloy cost of Nb, Mo, and V is smaller than that of Ti, the total amount of Nb, Mo, and V is limited to 0.01% or less.
また、ＣａおよびＲＥＭを介在物の形態を制御するために添加してもよく、含有量が合計で０．０１％以下とすることが好ましい。 Further, Ca and REM may be added to control the form of inclusions, and the total content is preferably 0.01% or less.
その他の不可避的不純物としては、例えば、スクラップから混入する可能性がある、Ｎｉ、Ｃｕ、Ｓｎが挙げられる。Ｎｉ、Ｃｕ、Ｓｎなどの含有量の許容範囲は、それぞれ０．０１％以下である。 Examples of other inevitable impurities include Ni, Cu, and Sn that may be mixed from scrap. The allowable ranges for the contents of Ni, Cu, Sn, etc. are each 0.01% or less.
また、本発明ではＴｉとＣとの含有量の比Ｔｉ／Ｃを１．６以下とすることが重要である。これは原子数の比率に換算するとＴｉ／Ｃが約０．４以下に相当する。従来の析出強化鋼では、ＴｉＣを析出させるために、Ｃに対してＴｉを過剰に含有させていた。しかし、本発明では、添加したＴｉを鋼中に固溶させず、ＴｉＣの析出を促進させ、析出強化に有効に寄与させるために、Ｔｉ量がＣに対して過剰にならないように添加する。 In the present invention, it is important that the content ratio Ti / C of Ti and C is 1.6 or less. This corresponds to Ti / C of about 0.4 or less when converted to the ratio of the number of atoms. In conventional precipitation strengthened steel, Ti is excessively contained with respect to C in order to precipitate TiC. However, in the present invention, the added Ti is not dissolved in the steel, the precipitation of TiC is promoted, and the Ti content is added so as not to be excessive with respect to C in order to effectively contribute to precipitation strengthening.
また、Ｔｉ／Ｃが１．６を超えると高温でＴｉＣが析出し易くなり、本発明の製造方法では、ＴｉＣ析出物の平均直径が３ｎｍを超え、平均個数密度が、１×１０17[個／ｃｍ3]未満になる。より好ましいＴｉ／Ｃの上限は１．０以下である。Ｔｉ／Ｃの下限値は、Ｔｉ量の下限値が０．０３％であり、Ｃ量の上限値が０．０８％であることから、０．３７５以上とする。 Further, when Ti / C exceeds 1.6, TiC is likely to precipitate at a high temperature. In the production method of the present invention, the average diameter of TiC precipitates exceeds 3 nm, and the average number density is 1 × 10 17 [pieces]. / Cm 3 ]. A more preferable upper limit of Ti / C is 1.0 or less. The lower limit value of Ti / C is set to 0.375 or more because the lower limit value of the Ti amount is 0.03% and the upper limit value of the C amount is 0.08%.
次に、本発明の高強度鋼の金属組織について説明する。
本発明の鋼板の金属組織は特に指定しないが、加工性の観点から実質的にフェライトとベイナイトとからなることが好ましい。
Next, the metal structure of the high strength steel of the present invention will be described.
Although the metal structure of the steel plate of the present invention is not particularly specified, it is preferably substantially composed of ferrite and bainite from the viewpoint of workability.
析出強化量は、析出物１個あたりの強化量（転位をピンニングする力）と、析出物の個数密度で決まると考えることができる。結晶粒内に析出したＴｉＣ析出物の平均直径は、０．８ｎｍ未満では、析出物１個当たりのピンニング力が弱く、析出強化への寄与が小さい。一方、ＴｉＣ析出物の平均直径が３ｎｍを超えると、析出物１個あたりの強化量の増加は飽和し、個数密度が減少するため、析出強化量が低下する。析出強化のためには、１．５〜３．０ｎｍのＴｉＣ析出物を利用することが更に好ましい。 The amount of precipitation strengthening can be considered to be determined by the amount of strengthening per precipitate (the force for pinning dislocations) and the number density of precipitates. When the average diameter of TiC precipitates precipitated in the crystal grains is less than 0.8 nm, the pinning force per precipitate is weak and the contribution to precipitation strengthening is small. On the other hand, when the average diameter of the TiC precipitate exceeds 3 nm, the increase in the amount of strengthening per precipitate is saturated and the number density decreases, so the amount of precipitation strengthening decreases. For precipitation strengthening, it is more preferable to use a 1.5 to 3.0 nm TiC precipitate.
結晶粒内に析出したＴｉＣ析出物の個数密度は、効率的に析出強化を活用するために、高いほうが好ましい。強度５４０ＭＰａ以上を達成するためには１×１０17個／ｃｍ3以上が必要である。好ましくは２×１０17個／ｃｍ3以上、さらに好ましくは４×１０17個／ｃｍ3以上である。
析出物の平均直径と平均個数密度の測定には、前述したように、三次元アトムプローブ測定法を用いる。三次元アトムプローブ法による測定では、針状の試料を用い、１つの結晶粒内の析出状態を観察する。測定に用いる試料数は３以上とし、平均直径は、３０個以上の析出物の直径を測定して求める。また、測定する析出物のサイズは、現状の三次元アトムプローブ測定では、サイズが０．５ｎｍ未満の粒子を固溶状態と判断し、正確に個数密度を算出できる上限が、本発明の成分範囲では、５０ｎｍであるので、０．５〜５０ｎｍのものを測定して、平均直径と平均個数密度を求める。
The number density of TiC precipitates precipitated in the crystal grains is preferably higher in order to efficiently utilize precipitation strengthening. In order to achieve a strength of 540 MPa or more, 1 × 10 17 pieces / cm 3 or more is required. Preferably it is 2 × 10 17 pieces / cm 3 or more, more preferably 4 × 10 17 pieces / cm 3 or more.
As described above, the three-dimensional atom probe measurement method is used to measure the average diameter and average number density of the precipitates. In the measurement by the three-dimensional atom probe method, a needle-like sample is used and the precipitation state in one crystal grain is observed. The number of samples used for measurement is 3 or more, and the average diameter is obtained by measuring the diameter of 30 or more precipitates. In addition, the size of the precipitate to be measured is determined by the current three-dimensional atom probe measurement, and the upper limit for accurately calculating the number density can be determined by determining particles having a size of less than 0.5 nm as a solid solution state. Then, since it is 50 nm, the thing of 0.5-50 nm is measured and an average diameter and an average number density are calculated | required.
また、本発明において上記ＴｉＣ析出物とは、炭化物だけでなく、炭化物中に窒素が若干混入した炭窒化物も含むものとする。また、ＴｉＣ析出物の中にＮｂ、Ｍｏ、Ｖが固溶したものも含むものとする。 In the present invention, the TiC precipitate includes not only carbides but also carbonitrides in which nitrogen is slightly mixed in the carbides. Moreover, the thing which Nb, Mo, and V dissolved in TiC precipitate is also included.
次に、本発明の高強度鋼の製造方法について説明する。
鋼を常法によって溶製、鋳造し、得られた鋼片を熱間圧延する。鋼片は、生産性の観点から、連続鋳造設備で製造することが好ましい。熱間圧延の加熱温度は、炭化物形成元素と炭素を十分に鋼材中に分解溶解させるため、１２００℃以上とする。鋳造後、鋼片を冷却して、１２００℃以上の温度で圧延を開始しても良い。１２００℃以下に冷却された鋼片を加熱する場合は、１時間以上の保持を行うことが好ましい。
Next, the manufacturing method of the high strength steel of this invention is demonstrated.
Steel is melted and cast by a conventional method, and the obtained steel piece is hot-rolled. The steel slab is preferably manufactured by continuous casting equipment from the viewpoint of productivity. The heating temperature of the hot rolling is set to 1200 ° C. or higher in order to sufficiently decompose and dissolve the carbide forming element and carbon in the steel material. After casting, the steel slab may be cooled and rolling may be started at a temperature of 1200 ° C. or higher. When heating a steel piece cooled to 1200 ° C. or lower, it is preferable to hold for at least 1 hour.
熱間加工の終了温度は、９００℃超とする。これは、高温でのＴｉＣ析出物の粗大化を抑制するためであり、熱間加工後は速やかに冷却を開始することが必要である。熱間加工の終了温度は、高温でのＴｉＣの析出を抑制するため、好ましくは９２０℃以上とする。熱間加工後は、１次冷却後、更に２次冷却を行う。 The end temperature of hot working is over 900 ° C. This is for suppressing the coarsening of the TiC precipitate at a high temperature, and it is necessary to start cooling promptly after hot working. The end temperature of hot working is preferably 920 ° C. or higher in order to suppress precipitation of TiC at a high temperature. After hot working, secondary cooling is further performed after primary cooling.
１次冷却の冷却速度は、２０℃／ｓ以上とする。これは、高温でのＴｉＣ析出物の析出、成長や、冷却中のフェライト変態を抑制するためである。高温でのＴｉＣの析出を抑制するためには、１次冷却の冷却速度を５０℃／ｓ以上とすることが好ましい。１次冷却の冷却速度の上限は規定しないが、１００℃／ｓ超にすると、１次冷却の停止温度の制御が困難になる。 The cooling rate of the primary cooling is set to 20 ° C./s or more. This is to suppress precipitation and growth of TiC precipitates at high temperatures and ferrite transformation during cooling. In order to suppress precipitation of TiC at a high temperature, it is preferable to set the cooling rate of primary cooling to 50 ° C./s or more. The upper limit of the cooling rate of the primary cooling is not specified, but if it exceeds 100 ° C./s, it becomes difficult to control the stop temperature of the primary cooling.
１次冷却の停止温度ＭＴ1［℃］は、高温でのＴｉＣ析出物の生成を抑制するため、５８０〜６４０℃の範囲内とすることが必要である。ＭＴ1が６４０℃を超えるとＴｉＣ析出物が粗大化し、効率的な析出強化が困難になる。一方、ＭＴ1が５８０℃未満になると、２次冷却でのＴｉＣ析出物の析出が不十分になる。 The primary cooling stop temperature MT 1 [° C.] needs to be in the range of 580 to 640 ° C. in order to suppress the formation of TiC precipitates at a high temperature. When MT 1 exceeds 640 ° C., TiC precipitates are coarsened, and efficient precipitation strengthening becomes difficult. On the other hand, when MT 1 is less than 580 ° C., precipitation of TiC precipitates in the secondary cooling becomes insufficient.
更に、２次冷却を行う。２次冷却の冷却速度は、ＴｉＣ析出物の形成を促進するため、５℃／ｓ以下とする。２次冷却の冷却速度の下限は特に規定しないが、製造コストの観点から、空冷が好ましい。 Further, secondary cooling is performed. The cooling rate of the secondary cooling is set to 5 ° C./s or less in order to promote the formation of TiC precipitates. The lower limit of the cooling rate of the secondary cooling is not particularly specified, but air cooling is preferable from the viewpoint of manufacturing cost.
２次冷却の停止温度ＭＴ2［℃］は、ＴｉＣ析出物の粗大化を防止し、微細なＴｉＣ析出物を増加させるため、５１０〜６００℃とすることが必要である。なお、２次冷却を施すため、ＭＴ2［℃］は、ＭＴ1［℃］よりも低くなる。ＴｉＣ析出物の粗大化を防ぎ十分な析出密度を得るためには１０℃以上低くするのが好ましい。ＭＴ2［℃］が６００℃を超えるとＴｉＣ析出物が粗大化し、効率的な析出強化が困難になる。一方、ＭＴ2［℃］が５１０℃未満になると、ＴｉＣ析出物の析出が不十分になる。 The secondary cooling stop temperature MT 2 [° C.] is required to be 510 to 600 ° C. in order to prevent coarsening of TiC precipitates and increase fine TiC precipitates. Since secondary cooling is performed, MT 2 [° C.] is lower than MT 1 [° C.]. In order to prevent coarsening of TiC precipitates and obtain a sufficient precipitation density, it is preferable to lower by 10 ° C. or more. When MT 2 [° C.] exceeds 600 ° C., TiC precipitates are coarsened, and efficient precipitation strengthening becomes difficult. On the other hand, when MT 2 [° C.] is less than 510 ° C., the precipitation of TiC precipitates becomes insufficient.
表１に示した成分組成を有する鋼を溶解し、鋳造して鋼片を作製した。表１の成分値は化学分析値で質量％である。次に、表２に示した製造条件で鋼片に熱間圧延を施し、熱延鋼板を製造した。 Steel having the composition shown in Table 1 was melted and cast to produce a steel slab. The component values in Table 1 are chemical analysis values and are mass%. Next, the steel pieces were hot-rolled under the production conditions shown in Table 2 to produce hot-rolled steel sheets.
これらの熱延鋼板から、ＪＩＳ Ｚ ２２０１に準拠して５号試験片を採取した。引張試験は、ＪＩＳ Ｚ ２２４１に準拠して行い、引張特性を評価した。 From these hot-rolled steel sheets, No. 5 test pieces were collected according to JIS Z 2201. The tensile test was performed according to JIS Z 2241 to evaluate the tensile properties.
ＴｉＣ析出物のサイズ及び密度の測定は、三次元アトムプローブ測定法によって行った。電解研磨法により針状の試料を作製し、観察されたＴｉＣ析出物の構成原子数とＴｉＣの格子定数から、析出物を球状と仮定し算出した直径をサイズとした。更に、３０個以上のＴｉＣ析出物の直径を測定し、その平均値を求めた。また測定に用いた試料の体積とＴｉＣ炭化物の数からＴｉＣ炭化物の個数密度を求めた。 The size and density of the TiC precipitate were measured by a three-dimensional atom probe measurement method. A needle-like sample was prepared by an electropolishing method, and the diameter calculated from the observed number of constituent atoms of the TiC precipitate and the lattice constant of TiC assuming that the precipitate was spherical was taken as the size. Furthermore, the diameter of 30 or more TiC precipitates was measured, and the average value was obtained. The number density of TiC carbides was determined from the volume of the sample used for the measurement and the number of TiC carbides.
結果を表２に示す。成分および製造条件が本発明の範囲内であれば、ＴｉＣ析出物のサイズが０．８〜３ｎｍ、個数密度が１×１０17以上となり、引張強度が５４０ＭＰａ以上になることがわかる。 The results are shown in Table 2. It can be seen that when the components and production conditions are within the scope of the present invention, the size of the TiC precipitate is 0.8 to 3 nm, the number density is 1 × 10 17 or more, and the tensile strength is 540 MPa or more.
一方、製造Ｎｏ．１５はＴｉ量が少なく、製造Ｎｏ．１６はＣ量が少なく、Ｔｉ／Ｃの比率も高いため、ＴｉＣ析出物の生成が不十分になり、ＴｉＣの個数密度および強度が低下した例である。製造Ｎｏ．１７はＣ量が多く、延性が低下した例である。製造Ｎｏ．１８は、多量のＭｏが添加されており、ＴｉＣ析出物のサイズおよび個数密度および強度は満たしているものの、合金コストが高くなり、また延性が低下した例である。 On the other hand, production No. No. 15 has a small amount of Ti. No. 16 is an example in which since the amount of C is small and the ratio of Ti / C is high, the generation of TiC precipitates is insufficient and the number density and strength of TiC are reduced. Production No. 17 is an example in which the amount of C is large and ductility is lowered. Production No. No. 18 is an example in which a large amount of Mo is added and the size, number density and strength of the TiC precipitates are satisfied, but the alloy cost is increased and the ductility is lowered.
製造Ｎｏ．２は一次冷却の冷却速度が遅く、製造Ｎｏ．６および製造Ｎｏ．１４は一次冷却の停止温度が高いため、ＴｉＣ析出物が大きくなり、ＴｉＣの個数密度および強度が低下した例である。 Production No. No. 2 has a slow primary cooling rate. 6 and production no. No. 14 is an example in which since the primary cooling stop temperature is high, the TiC precipitates become large and the number density and strength of TiC decrease.
製造Ｎｏ．４は一次冷却の停止温度が低く、製造Ｎｏ．８は二次冷却の冷却速度が速いため、ＴｉＣ析出物の生成が不十分になり、ＴｉＣの個数密度および強度が低下した例である。 Production No. No. 4 has a low primary cooling stop temperature. No. 8 is an example in which the cooling rate of the secondary cooling is high, so that TiC precipitates are not sufficiently generated, and the number density and strength of TiC are reduced.
製造Ｎｏ．９は二次冷却の停止温度が高く、ＴｉＣ析出物が大きくなり、ＴｉＣの個数密度および強度が低下した例である。製造Ｎｏ．１０は熱間加工の終了温度が低く、高温でのＴｉＣ析出物の粗大化が起きたと考えられ、ＴｉＣの個数密度および強度が低下した例である。 Production No. No. 9 is an example in which the secondary cooling stop temperature is high, TiC precipitates are increased, and the number density and strength of TiC are reduced. Production No. No. 10 is an example in which the hot working finish temperature is low, and TiC precipitates are coarsened at a high temperature, and the number density and strength of TiC are reduced.
Claims (2)
Ｃ：０．０２〜０．０８％、
Ｓｉ：０．０１〜１．５０％、
Ｍｎ： ０．１〜１．５％
Ｔｉ：０．０３〜０．０６％
を含有し、
Ｐ：０．１％以下、
Ｓ：０．００５％以下、
Ａｌ：０．５％以下、
Ｎ：０．００９％以下
に制限し、更に、Ｎｂ、Ｍｏ、Ｖの含有量の合計を０．０１％以下に制限し、残部がＦｅおよび不可避的不純物からなり、Ｃ量に対するＴｉ量の比が、
Ｔｉ／Ｃ：０．３７５〜１．６
であり、結晶粒内のＴｉＣ析出物の平均直径が０．８〜３ｎｍであり、平均個数密度が、１×１０17[個／ｃｍ3]以上であり、引張強度が５４０〜６５０ＭＰａであることを特徴とする省合金型高強度熱延鋼板。 % By mass
C: 0.02 to 0.08%,
Si: 0.01 to 1.50%,
Mn: 0.1 to 1.5%
Ti: 0.03-0.06%
Containing
P: 0.1% or less,
S: 0.005% or less,
Al: 0.5% or less,
N: Limiting to 0.009% or less, further limiting the total content of Nb, Mo, V to 0.01% or less, the balance being Fe and inevitable impurities, the ratio of Ti amount to C amount But,
Ti / C: 0.375 to 1.6
The average diameter of the TiC precipitates in the crystal grains is 0.8 to 3 nm, the average number density is 1 × 10 17 [pieces / cm 3 ] or more, and the tensile strength is 540 to 650 MPa. Alloy-saving high-strength hot-rolled steel sheet.
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