JP5615100B2 - High strength high workability hot rolled steel sheet and method for producing the same - Google Patents
High strength high workability hot rolled steel sheet and method for producing the same Download PDFInfo
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請求項に係る発明は、高い引張強度をもちながらも異方性がなく優れた加工性を有し残留オーステナイトによるTRIP効果を利用した比較的低強度（590MPa級）から超高強度（1370MPa級）の熱延鋼板と、その製造方法に関するものである。 The claimed invention has a high tensile strength, no anisotropy, excellent workability, and relatively low strength (590 MPa class) to ultra high strength (1370 MPa class) using the TRIP effect of retained austenite This invention relates to a hot-rolled steel sheet and a manufacturing method thereof.
高強度高加工性材料としては、主に自動車用途向けに次世代型鋼板としてデユアルフェイズ（DP）鋼板や変態誘起塑性（TRIP）を利用した複相組織鋼板が開発されており、薄肉軽量化の要請に応えつつある。また、更なる高強度化への取り組みとしては鋼板加工時に素材を加熱し熱間でプレス成形してマルテンサイト組織化するホットスタンプ法なる加工方法も実用段階に達している。 As high-strength, high-workability materials, dual-phase (DP) steel sheets and multiphase steel sheets using transformation-induced plasticity (TRIP) have been developed as next-generation steel sheets mainly for automotive applications. We are responding to requests. In addition, as an effort to further increase the strength, a processing method called a hot stamp method, in which a raw material is heated at the time of processing a steel sheet and is hot pressed to form a martensite structure, has reached a practical stage.
この種、高強度材料の開発の中心は、鉄鋼材料の組織として高い強度を持つベイナイトやマルテンサイトといった組織をいかに利用するか又は利用できるかが問題であり、その材料特性、製造方法、トータルコストなどが多角的に検討され開発が進められている。 The core of the development of this type of high-strength material is how to use or be able to use a structure such as bainite or martensite, which has a high strength as the structure of steel materials, and its material properties, manufacturing method, and total cost Are being studied and developed from various perspectives.
現在までに、開発された高強度高加工性材料の中では、残留オーステナイトを利用したTRIP型鋼板が最もその特性バランス（例えば引張強度×伸びの値）が優れている事が確認されているが、実用強度は７８０MPa級までに限られている。また、更なる高強度化を図る試みとして、フェライトを含まないTRIP型鋼板としてTBF鋼やTAM鋼といった新たなタイプの鋼板の開発も進められている。そのような点は下記の非特許文献１に記載されている。
そしてこの材料特性上優位なTRIP型鋼板の開発は、今後も、更なる高強度化と総合パフォーマンスの向上が進められていくと思われる。
To date, it has been confirmed that, among the developed high-strength, high-workability materials, TRIP steel sheets using retained austenite have the best balance of properties (for example, tensile strength x elongation value). The practical strength is limited to 780 MPa class. In addition, as an attempt to further increase the strength, the development of new types of steel sheets such as TBF steel and TAM steel as TRIP steel sheets that do not contain ferrite is being promoted. Such a point is described in Non-Patent Document 1 below.
The development of TRIP-type steel sheets that are superior in terms of material properties is expected to continue to increase strength and improve overall performance.
図１に、ハイテン開発に関わる研究論文その他文献に示された成分に関し、Si量で層別し、MnとCrの含有量をプロットしたものを示す。研究開発対象はいずれもMnの含有量がCr量より多いものが殆どで、特に、TRIP型としてSiを１％以上含有したものについては、主要元素がMnとなっているものしか見当たらない。
下記に示す非特許文献２・３についても状況は同じであり、従来までの開発ではＳｉ−Ｍｎ系でTRIP型鋼板が研究実用化されている。
また、Crの変態挙動遅延効果に着目した下記の特許文献１についても、TRIP型鋼板に対する展開は考慮されていない（低Si系材料）。
Fig. 1 shows the contents of Mn and Cr plotted according to the amount of Si for the components shown in research papers and other documents related to Hi-Ten development. Most of the R & D targets have a Mn content higher than the Cr content. In particular, for TRIP type containing 1% or more of Si, only the main element is Mn.
The situation is the same for Non-Patent Documents 2 and 3 shown below, and in the conventional development, TRIP-type steel sheets based on the Si-Mn system have been researched and put into practical use.
Also, the following Patent Document 1 focusing on the effect of delaying the transformation behavior of Cr does not take into account the development of TRIP type steel sheets (low-Si material).
従来のTRIP型鋼板の製造方法は、熱間圧延＋冷間圧延＋熱処理の工程に傾注しており、鋼板製造時のエネルギー消費（CO2の排出）や最大製造可能板厚（最大2.3ｍｍ前後）に課題が残る。
また、従来型Ｓｉ−Ｍｎ系TRIP型鋼板では、変態が急速に進行し、それが原因で引張強度等の特性にバラつきが生じがちである。
Conventional TRIP type steel plate manufacturing methods are focused on the process of hot rolling + cold rolling + heat treatment, energy consumption (CO2 emission) and maximum manufacturable thickness (up to around 2.3mm) during steel plate manufacturing. The problem remains.
Further, in the conventional type Si-Mn TRIP type steel sheet, the transformation proceeds rapidly, which tends to cause variations in properties such as tensile strength.
本発明は、上記の点を考慮して行ったもので、高強度高加工性材料として最も優位なTRIP型鋼板を比較的低強度（590MPa級）から超高強度（1370MPa級）まで、板厚1.0ｍｍ〜6.0ｍｍまでの範囲で熱間圧延ままの工程で安定して製造する手段を提供することを目的とする。 The present invention has been made in consideration of the above points. The thickness of TRIP steel sheet, which is the most superior as a high-strength, high-workability material, from relatively low strength (590 MPa class) to ultra-high strength (1370 MPa class) It aims at providing the means to manufacture stably in the process of hot rolling in the range of 1.0 mm-6.0 mm.
発明者らは、従来型Ｓｉ−Ｍｎ系TRIP型鋼板に対し、成分系に関する研究調査を鋭意行った。その結果、ベイナイト変態の遅延効果を有するＣｒを有効利用する新タイプのＳｉ−Ｃｒ系TRIP型鋼板が、熱間圧延ままで（つまり冷間圧延を行わずに）この種高強度高加工性鋼板を製造する場合には、製造時の温度許容範囲が広く、品質の安定性の観点から非常に有効である事を見出した。つまりこの種成分系を適正に熱間圧延する事で、冷間圧延時の電力消費や熱処理時の燃料消費などと言ったCO2の排出要因を完全に取り除くことが可能で、更に、この種高強度鋼板は板厚が厚くなると、その分冷間圧延以降の処理設備もそれに見合う能力（パワー）が必要となる為、現在の設備能力では２．３ｍｍ前後が限界となっているが、熱間圧延ままであれば、その制限も大幅に改善でき適用範囲を拡げることが可能となる。また、従来型熱延鋼板では特性不安定の問題から、その強度レンジの限界が７８０ＭＰaであったが、新タイプでは更に高強度側への展開が図れる事が明らかになった。その詳細を以下に示す。 The inventors diligently conducted research on component systems for conventional Si-Mn TRIP steel sheets. As a result, a new type of Si-Cr-based TRIP steel sheet that effectively uses Cr, which has the effect of delaying bainite transformation, is such a high-strength, high-workability steel sheet that remains hot rolled (that is, without cold rolling). It was found that the temperature tolerance during production is wide and is very effective from the viewpoint of quality stability. In other words, by appropriately hot rolling this seed component system, it is possible to completely eliminate CO2 emission factors such as power consumption during cold rolling and fuel consumption during heat treatment. As the steel sheet becomes thicker, the processing equipment after cold rolling needs to have a capacity (power) corresponding to that, so the current equipment capacity is limited to around 2.3 mm. If rolled, the restriction can be greatly improved and the application range can be expanded. In addition, the conventional hot-rolled steel sheet has a problem of characteristic instability, so the limit of its strength range is 780 MPa, but it has become clear that the new type can be developed to a higher strength side. Details are shown below.
発明による高強度高加工性熱延鋼板は、従来から残留オーステナイトの確保に必要とされているＳｉに加え、Ｃｒを１％以上含有させる事を大きな特徴としており、この添加により、Ｍｎ系で発生する急速な変態と冷却ムラによる特性のバラツキを回避することができる。
詳細には、オーステナイトフォーマーであるＣを除く主要な化学成分が、Si，Cr，Mo，Alを含むフェライトフォーマー元素であるとともに、金属組織として残留オーステナイトを含む高強度高加工性熱延鋼板であり、望ましくは、下記の（１）式を満足するものである。
0.42 ≦ C＋0.24×（Cr＋Mo）≦ 0.68 ・・・・・・・・・・・・・・（１）
この成分系を適切に用いる事で、未変態オーステナイトの変態挙動が緩慢（温度に対する感受性が鈍くなる）となり、熱間圧延中に生じる様々な冷却ムラ（温度偏差）に対しても特性が変動しにくい良加工性TRIP型高強度鋼板を、熱間圧延のみで（冷間圧延を行うことなく）超高強度かつ厚肉サイズまで製造可能となるのである。
The high-strength, high-workability hot-rolled steel sheet according to the invention is characterized by the inclusion of 1% or more of Cr in addition to Si that has been conventionally required for securing retained austenite. This makes it possible to avoid variations in characteristics due to rapid transformation and cooling unevenness.
Specifically, the main chemical components excluding C, which is an austenite former, are ferrite former elements including Si, Cr, Mo, Al, and a high-strength, high-workability hot-rolled steel sheet containing retained austenite as a metal structure. Preferably, the following expression (1) is satisfied.
0.42 ≦ C + 0.24 × (Cr + Mo) ≦ 0.68 (1)
By properly using this component system, the transformation behavior of untransformed austenite becomes slow (sensitivity to temperature), and the characteristics fluctuate even for various cooling irregularities (temperature deviation) that occur during hot rolling. This makes it possible to manufacture hard-working TRIP-type high-strength steel sheets that are difficult to manufacture by ultra-high strength and thick-wall size only by hot rolling (without performing cold rolling).
図２は確認実験の結果を示すもので、
Ａ；0.18Ｃ-1.20Ｓｉ-1.80Ｍｎ-0.10Ｃｒ（Ｓｉ-Ｍｎ系）
Ｂ；0.18Ｃ-1.00Ｓｉ-0.31Ｍｎ-1.90Ｃｒ（Ｓｉ-Ｃｒ系）
のサンプルをＡe3点以上（950℃）から各熱処理温度に急冷後２時間保持し水焼入れした時の引張強度を示したものである。鋼種Ａが各温度でベイナイト変態を終了し低温側ほど強度の上がる通常の変態（特性）挙動を示すのに対し、鋼種Ｂは、高温側の各温度で強度が上昇していることがわかる。これは、鋼種Ｂでは、オーステナイトの変態が遅延し未変態のまま存在する（変態が遅延してオーステナイトのままでいる為、最終の水焼入れでマルテンサイト化し強度が上昇する）ためであると考えられる。
Figure 2 shows the results of the confirmation experiment.
A: 0.18C-1.20Si-1.80Mn-0.10Cr (Si-Mn system)
B: 0.18C-1.00Si-0.31Mn-1.90Cr (Si-Cr system)
This sample shows the tensile strength when the sample is kept at Ae3 point or higher (950 ° C.) at each heat treatment temperature and then quenched for 2 hours after quenching. It can be seen that steel type A completes the bainite transformation at each temperature and exhibits a normal transformation (characteristic) behavior in which the strength increases as the temperature decreases, whereas steel type B increases in strength at each temperature on the high temperature side. This is considered to be because, in Steel B, the transformation of austenite is delayed and remains untransformed (since the transformation is delayed and remains austenite, it becomes martensite and the strength increases in the final water quenching). It is done.
図３は、熱延鋼板と冷延鋼板（再加熱連続焼鈍時）の冷却パターンを模式的に示したものである。冷延鋼板の場合は恒温保持の時間が短く（数分）、この間にベイナイト変態を急速に進行させる必要があるが、熱延鋼板の場合は巻取りから常温までの冷却時間は非常に長い（数十時間）ため、徐々にベイナイト変態を進行させるのが望ましい。そうでないと、熱間圧延の場合、折角この間に充分にＣ濃縮した未変態オーステナイトもベイナイト変態を開始してしまい、有効な残留オーステナイトを得る事が出来ないと共に、鋼板内の特性変動を増幅してしまう。この熱延鋼板の冷却パターンに即してベイナイト変態を遅延させる効果をもたらすには、Ｃｒ量が１％以上必要である。そのような量のＣｒを有すると、未変態オーステナイトに徐々にＣ濃縮が進んで行くため、有効な残留オーステナイトを充分得られると共に、実質的に冷却後の組織にはＣ濃化不十分に起因するマルテンサイトの発生も抑制できる。 FIG. 3 schematically shows a cooling pattern of a hot-rolled steel sheet and a cold-rolled steel sheet (during continuous reheating). In the case of a cold-rolled steel sheet, the constant temperature holding time is short (several minutes), and during this time it is necessary to rapidly advance the bainite transformation, but in the case of a hot-rolled steel sheet, the cooling time from winding to room temperature is very long ( Therefore, it is desirable to gradually advance the bainite transformation. Otherwise, in the case of hot rolling, untransformed austenite that is sufficiently C-concentrated during this time also starts bainite transformation, so that effective retained austenite cannot be obtained, and characteristic fluctuations in the steel plate are amplified. End up. In order to bring about an effect of delaying the bainite transformation in accordance with the cooling pattern of the hot-rolled steel sheet, the amount of Cr is required to be 1% or more. With such an amount of Cr, C concentration gradually proceeds to untransformed austenite, so that effective residual austenite can be obtained sufficiently, and the structure after cooling is substantially caused by insufficient C concentration. Generation of martensite can be suppressed.
図４（a）・（b）は上記熱間圧延時の冷却条件を模擬的に再現する為に行ったラボ実験の結果である。上記（図２）と同じサンプルＡ及びＢを９５０℃に加熱し、各巻取り模擬温度に設定したソルトバスへ急冷後、温調制御により１５℃／時間でゆっくりと常温まで冷却した時の強度（図４（a））と伸び（図４（b））の値を示す。
Ｓｉ-Ｍｎ系の鋼種Ａは巻取り模擬温度に対し引張強度変化が大きくなるが、Ｓｉ-Ｃｒ系の鋼種Ｂは強度変化が小さく、かつ、伸びの値も鋼種Ａと同等になる事がわかる。
この効果により、Ｓｉ-Ｃｒ系の熱延ＴＲＩＰ鋼板は、伸びを担保したまま安定した強度を確保する事が可能で、従来、製造が難しかった９８０ＭＰａ超の熱延ＴＲＩＰ鋼板も製造可能となるのであり、この場合の組織設計としてはフェライト＋ベイナイト＋残留オーステナイト、又は、ベイナイト＋残留オーステナイトの二者択一が可能となる。
4 (a) and 4 (b) show the results of a laboratory experiment conducted to simulate the cooling conditions during the hot rolling. The same samples A and B as above (Fig. 2) were heated to 950 ° C, rapidly cooled to a salt bath set at each winding simulation temperature, and then the strength when slowly cooled to room temperature at 15 ° C / hour by temperature control ( FIG. 4 (a)) and elongation (FIG. 4 (b)) are shown.
It can be seen that the Si-Mn type steel type A has a large change in tensile strength with respect to the coiling simulation temperature, but the Si-Cr type steel type B has a small change in strength and the elongation value is equivalent to that of the steel type A. .
Because of this effect, the Si—Cr hot rolled TRIP steel sheet can ensure a stable strength while ensuring elongation, and a hot rolled TRIP steel sheet of over 980 MPa, which has been difficult to manufacture, can be manufactured. In this case, as the structure design, either ferrite + bainite + retained austenite or bainite + retained austenite can be selected.
発明による高強度高加工性熱延鋼板の成分範囲は、 C: 0.06〜0.25％、Si: 1.0〜3.0％、Cr: 1.0〜2.0％、Mo: 0.01〜0.30％、Al: 0.01〜0.5％を含み、残部は鉄および不可避的不純物の組成とするのがよい。またさらに、Mn: 0.1〜1.0％、Ni: 0.01〜0.50、Ti: 0.02〜0.20％、Nb: 0.02〜0.10％、V:0.02〜0.20％、B：0.0001〜0.0030％のうち１種以上を含有したものも好ましい。
こうした適切な種類と量の化学成分を含むこととすれば、上記のようにマルテンサイト組織を実質的に有していない(つまり全体における同組織の比率が２％程度以下の)、望ましい機械的性質を発揮するＴＲＩＰ型高強度高加工性熱延鋼板とすることが容易である。
引張強度のレベルについては、Ｃ及びＭｎその他の量を適宜調整する事で590MPa以上1470MPa未満に作り分けする事が可能である。低強度側を設計する場合や高速軸圧潰性能の向上のため等で、軟質なフェライト組織を用いた鋼板としたい場合には、下記の式を満足する成分配合が好ましい。
6.46*C-0.26*Si+1.30*Mn+0.49*Ni+0.89*Cr+3.03*Mo＜2.57 ・・・・・・・（２）
なお、各成分の作用については後述する。
The component ranges of the high strength and high workability hot rolled steel sheet according to the invention are as follows: C: 0.06 to 0.25%, Si: 1.0 to 3.0%, Cr: 1.0 to 2.0%, Mo: 0.01 to 0.30%, Al: 0.01 to 0.5% Including the balance of iron and inevitable impurities is preferable. Further, Mn: 0.1 to 1.0%, Ni: 0.01 to 0.50, Ti: 0.02 to 0.20%, Nb: 0.02 to 0.10%, V: 0.02 to 0.20%, B: Contains one or more of 0.0001 to 0.0030% Those made are also preferred.
Assuming that these appropriate types and amounts of chemical components are included, as described above, it has substantially no martensite structure (that is, the ratio of the same structure in the whole is about 2% or less), and desirable mechanical It is easy to make a TRIP type high-strength, high-workability hot-rolled steel sheet that exhibits its properties.
About the level of tensile strength, it is possible to make it separately from 590 MPa to less than 1470 MPa by appropriately adjusting C, Mn and other amounts. In the case of designing a low-strength side or for improving the high-speed axial crush performance, etc., when it is desired to make a steel sheet using a soft ferrite structure, a component blend that satisfies the following formula is preferable.
6.46 * C-0.26 * Si + 1.30 * Mn + 0.49 * Ni + 0.89 * Cr + 3.03 * Mo <2.57 (2)
In addition, the effect | action of each component is mentioned later.
高強度高加工性熱延鋼板として、上記の組織を有するとともに、板厚が1.0mmから6.0mm、引張強度TS（MPa）が590 MPaから1470MPaで、TSと伸び値EL（％）との積TS×ELが20000（MPa・％）以上であるものが好ましい。
そのような鋼板は、上述の組織を有していて高い強度と良い伸び特性とを兼ね備えるものだからである。
A high-strength, high-workability hot-rolled steel sheet that has the above structure, a thickness of 1.0 mm to 6.0 mm, a tensile strength TS (MPa) of 590 MPa to 1470 MPa, and the product of TS and elongation value EL (%) It is preferable that TS × EL is 20000 (MPa ·%) or more.
This is because such a steel sheet has the above-described structure and has both high strength and good elongation characteristics.
上記高強度高加工性熱延鋼板の製造方法に係る発明は、上記成分範囲で1200℃以上の鋼材（素材スラブ）を粗圧延した後、複数スタンドを有する熱間圧延機によって、Ae3温度以上の単相域で最終仕上圧延を完了することを特徴とする。
また、さらに、仕上圧延の圧延完了後、20秒以内でBs 以下Ms+50℃以上で巻き取ることとするのが好ましい。なお、Ae3（α⇔γ平衡変態温度）は下記の（３）式、Bs（ベイナイト変態開始温度）は（４）式、Ms（マルテンサイト変態開始温度）は（５）式で示す。
Ae3=919-266*C+38*Si-28*Mn-27*Ni-11*Cr+12*Mo・・・・・・・・・・・ (3)
Bs＝649-83*C-19*Si-26*Mn-30*Ni-21*Cr-29*Mo・・・・・・・・・・・・（4）
Ms=539-423*C-30.4*Mn-17.7*Ni-12.1*Cr-7.5*Mo・・・・・・・・・・・・（5）
この製造方法によれば、高温の単相域で圧延されるため、圧延方向及び圧延直角方向の材質異方性が低減出来、さらなる加工性の向上が図れるのである。
また、この製法によれば、従来フェライトを利用する際などに用いられている二段冷却などの水冷制御も必要ではなく、その温度管理が容易である。更に、２０秒以内と短時間でコイル状に巻き取る為、板状で受ける強制冷却中の外乱影響を極力抑える事が可能である。発明者らの製造試験によると、後述のように、こうした条件によって上述の高強度高加工性熱延鋼板を得ることができ、冷間圧延を施すことなく高い強度と良好な伸び特性とを享受できる。
The invention relating to the above-described method for producing a high-strength, high-workability hot-rolled steel sheet is obtained by roughly rolling a steel material (material slab) at 1200 ° C. or higher in the above component range, and then by using a hot rolling mill having a plurality of stands. The final finish rolling is completed in a single phase region.
Further, it is preferable to wind up at Bs or lower and Ms + 50 ° C. or higher within 20 seconds after completion of finish rolling. Ae3 (α⇔γ equilibrium transformation temperature) is represented by the following formula (3), Bs (bainite transformation start temperature) is represented by formula (4), and Ms (martensite transformation start temperature) is represented by formula (5).
Ae3 = 919-266 * C + 38 * Si-28 * Mn-27 * Ni-11 * Cr + 12 * Mo ... (3)
Bs = 649-83 * C-19 * Si-26 * Mn-30 * Ni-21 * Cr-29 * Mo (4)
Ms = 539-423 * C-30.4 * Mn-17.7 * Ni-12.1 * Cr-7.5 * Mo ... (5)
According to this manufacturing method, since rolling is performed in a high-temperature single-phase region, material anisotropy in the rolling direction and the direction perpendicular to the rolling direction can be reduced, and further workability can be improved.
Further, according to this manufacturing method, water cooling control such as two-stage cooling conventionally used when using ferrite is not necessary, and the temperature management is easy. Further, since the coil is wound in a short time within 20 seconds, it is possible to suppress the influence of disturbance during forced cooling received as a plate as much as possible. According to the inventors' production test, as described later, the above-described high-strength, high-workability hot-rolled steel sheet can be obtained under these conditions, and high strength and good elongation characteristics can be obtained without cold rolling. it can.
発明の高強度高加工性熱延鋼板は、フェライト及びベイナイト又はベイナイトに、残留オーステナイトが多量に微細に分散した状態で混在するため、互いに相反する特性である強度と加工特性を兼備した熱延鋼板となる。 The high-strength, high-workability hot-rolled steel sheet of the invention is a hot-rolled steel sheet that has both strength and workability characteristics that are contradictory to each other because ferrite and bainite or bainite are mixed in a large amount of finely dispersed retained austenite. It becomes.
発明の製造方法によれば、上記した高強度高加工性熱延鋼板を円滑に製造することが出来る。 According to the manufacturing method of the invention, the above-described high-strength, high-workability hot-rolled steel sheet can be manufactured smoothly.
以下、590MPa以上の引張り強度をもちながらも、優れた加工性を有する薄鋼板とその製造方法について、実施の形態を示す。
鋼板の成分系として、C: 0.06〜0.25％、Si: 1.0〜3.0％、Cr: 1.0〜2.0％、Mo: 0.01〜0.30％、Al: 0.01〜0.5％を含み、残部は鉄および不可避的不純物の組成である。さらに、Mn: 0.1〜1.0％、Ni: 0.01〜0.50、Ti: 0.02〜0.20％、Nb: 0.02〜0.10％、V:0.02〜0.20％、B：0.0001〜0.0030％のうちいずれか１種以上を含有するものも好ましい。
CrとMnとのみについて発明の鋼板の成分範囲を図示すると、図１における細線の□枠内に入る。
なお、ここで述べる薄鋼板とは、板厚が1.0から6.0mmの鋼板のことである。製造する鋼板は、主として自動車、家電製品、電子機器製品、等の高い加工性と強度が必要な部品に使用することが出来る。その他、鋼管用の素材としても適用が可能である。
Hereinafter, embodiments of a thin steel plate having excellent workability while having a tensile strength of 590 MPa or more and a manufacturing method thereof will be described.
As steel plate component system, C: 0.06-0.25%, Si: 1.0-3.0%, Cr: 1.0-2.0%, Mo: 0.01-0.30%, Al: 0.01-0.5%, the balance being iron and inevitable impurities Of the composition. Furthermore, Mn: 0.1 to 1.0%, Ni: 0.01 to 0.50, Ti: 0.02 to 0.20%, Nb: 0.02 to 0.10%, V: 0.02 to 0.20%, B: 0.0001 to 0.0030% What is contained is also preferable.
If the component range of the steel sheet of the invention is illustrated with respect to only Cr and Mn, it falls within the □ frame of the thin line in FIG.
The thin steel plate described here is a steel plate having a thickness of 1.0 to 6.0 mm. The steel sheet to be produced can be used mainly for parts that require high workability and strength, such as automobiles, home appliances, and electronic equipment products. In addition, it can be applied as a material for steel pipes.
まず、鋼板の成分について述べる。
炭素（C）としては、0.06〜0.25％の範囲の量とした。下限は冷却中に発生するフェライト相に伴うＣ濃縮を考慮しても残留オーステナイトを有効利用する為には最低限必要な量であり、上限は溶接性の観点から0.25％とした。
First, the components of the steel sheet will be described.
As carbon (C), the amount was in the range of 0.06 to 0.25%. The lower limit is the minimum amount necessary for effective utilization of retained austenite even considering the C concentration associated with the ferrite phase generated during cooling, and the upper limit is set to 0.25% from the viewpoint of weldability.
シリコン（Si）量は、1.0〜3.0％の範囲とする。シリコンは固溶強化による強度の向上効果も有する。さらに、残留オーステナイトの安定化のために活用する。シリコン量は、1.0％以上であれば、本発明の複合組織と材質特性が得られる。シリコン量は多いほど、残留オーステナイト量を増やすことができると同時に、その安定性を促す。しかし、3.0％以上のシリコン量になると、強度延性バランスの特性が飽和するので、シリコン量の上限を3.0％とする。 The amount of silicon (Si) is in the range of 1.0 to 3.0%. Silicon also has an effect of improving strength by solid solution strengthening. Furthermore, it is utilized for stabilization of retained austenite. If the amount of silicon is 1.0% or more, the composite structure and material characteristics of the present invention can be obtained. As the amount of silicon increases, the amount of retained austenite can be increased and at the same time the stability thereof is promoted. However, if the silicon content is 3.0% or more, the strength and ductility balance characteristics are saturated, so the upper limit of the silicon content is 3.0%.
クロム（Cr）量は、1.0〜2.0％の範囲とする。クロム量は1.0％未満になると、巻取り後の変態遅延効果が小さくベイナイト変態が進行してしまい安定したＴＲＩＰ型熱延鋼板とすることが出来ない。クロム量が2.0％を超えると、変態遅延効果が大きくなりすぎ充分なＣ濃縮が得られない状態で室温まで冷却され、マルテンサイト組織を生じてしまい良好な強度-延性バランスが得られない。 The amount of chromium (Cr) is in the range of 1.0 to 2.0%. If the chromium content is less than 1.0%, the effect of delaying transformation after winding is small, and the bainite transformation proceeds and a stable TRIP hot rolled steel sheet cannot be obtained. When the amount of chromium exceeds 2.0%, the effect of delaying transformation becomes too great, and cooling is performed to room temperature in a state where sufficient C concentration cannot be obtained, resulting in a martensite structure and a good strength-ductility balance cannot be obtained.
モリブデン（Mo）は、耐遅れ破壊特性を重視する場合に有効であるが、故意に添加を行えば大幅なコストの上昇を招くため、その範囲を0.01〜0.3％とした。また、Ｃｒと同様な変態遅延効果を有するため、多量添加はマルテンサイトの発生を促す結果となる。 Molybdenum (Mo) is effective when emphasizing delayed fracture resistance, but if added intentionally, it causes a significant cost increase, so the range was made 0.01 to 0.3%. Further, since it has a transformation delay effect similar to that of Cr, addition of a large amount results in promoting the generation of martensite.
ここで、先のベイナイト変態に対する遅延効果は鋼板のC量にも左右されるため、本請求範囲のC量との関係では、
0.42 ≦ C＋0.24＊（Cr＋Mo）≦ 0.68 ・・・・・・・・・・・・・・（１）
で制限される範囲にする事が、残留オーステナイトを充分に確保しつつ、実質的にマルテンサイトを含まない良好な特性を安定して得る上でより好ましい。
Here, since the delay effect on the previous bainite transformation also depends on the C amount of the steel sheet, in relation to the C amount in the claims,
0.42 ≦ C + 0.24 * (Cr + Mo) ≦ 0.68 (1)
Is more preferable in order to stably obtain good characteristics substantially free of martensite while sufficiently securing retained austenite.
アルミニウム（Al）量は、Siと同様な効果を持つ事が知られているため、更なる加工性の向上の目的に添加することが出来るが、0.5％以上になると製鋼上の問題が生じるため上限を0.5％とした。 The amount of aluminum (Al) is known to have the same effect as Si, so it can be added for the purpose of further improving workability. However, if it exceeds 0.5%, problems in steelmaking will occur. The upper limit was 0.5%.
マンガン（Mn）量は0.1〜1.0％の範囲とし、ニッケル(Ni)量は0.01〜0.5％の範囲とする。マンガン量が0.1%未満になると、製鋼上での製造が困難になるので0.1％以上とする。マンガン、ニッケルとも強度調整用その他の目的で添加するのが好ましいが、共に過度の添加は焼入れ性が向上しすぎ熱延強制冷却時の外乱などによる特性変動が大きくなるので上限をそれぞれ1.0％と0.5％とする Manganese (Mn) content is in the range of 0.1 to 1.0%, and nickel (Ni) content is in the range of 0.01 to 0.5%. If the amount of manganese is less than 0.1%, it becomes difficult to manufacture on steel making, so 0.1% or more. Both manganese and nickel are preferably added for strength adjustment and other purposes, but excessive addition of both increases the hardenability too much and increases the characteristic fluctuation due to disturbances during forced hot cooling, so the upper limit is 1.0% respectively. 0.5%
チタン（Ti）、ニオブ（Nb）、バナジウム（Ｖ）、ボロン（B）は、熱延工程における結晶粒の微細化効果を有している。
これら元素は、フェライト粒や残留オーステナイト粒を微細に分散させるために有効な元素であり、製造上の許容範囲で適量を１種又は２種添加したものも好ましい。
Titanium (Ti), niobium (Nb), vanadium (V), and boron (B) have a crystal grain refinement effect in the hot rolling process.
These elements are effective elements for finely dispersing ferrite grains and residual austenite grains, and those added with one or two appropriate amounts within the allowable range in production are also preferable.
上記の基準成分に調整したスラブ（被圧延鋼材）は、再加熱してから熱間圧延をおこなうか、もしくは鋳造後直ちに熱間圧延をおこなうものとする。熱間圧延を施すにあたっては、粗圧延の後、複数スタンドを有する熱間圧延機によって、圧延完了温度がAe3点以上にする事が異方性低減の観点から重要であり、強圧下率で高歪みによる仕上げ圧延での加工発熱が利用できる圧延設備が好ましい。 The slab (steel material to be rolled) adjusted to the above-described reference component is either hot-rolled after reheating or hot-rolled immediately after casting. When performing hot rolling, it is important from the viewpoint of reducing anisotropy that the rolling completion temperature should be at least Ae3 using a hot rolling mill with multiple stands after rough rolling. A rolling facility that can utilize the heat generated by finish rolling due to strain is preferable.
圧延終了温度がAe3温度以上で熱間圧延を完了し、圧延完了後20秒以内で巻取ることが熱延強制冷却時の外乱抑止上好ましい。圧延完了後巻取られるまでの間は板状態のままであり、強制冷却水の水量ムラ、板上面水の水切りムラ、板幅方向エッジの温度ドロップなどの外乱を受けやすいが、それら外乱を排除するには短時間でコイル状に巻取るのが好ましい。
巻取り温度はBs 以下,Ms+50℃以上の温度範囲にすることにより、Ｃｒの変態遅延効果を有効に利用し、オーステナイトを残留させ、マルテンサイトの発生を抑制する。
ここで巻取り温度をBs 以下、Ms+50℃以上としたが、Ms+50℃以下の温度範囲ではＣｒによる変態遅延効果に伴うＣ濃縮が不完全と成りやすく、マルテンサイト組織が生成しやすくなる。またBs 以上の温度では組織が粗大化し、かつ特性上最も有害なパーライトが出現する可能性が高まる為その範囲を限定する。
Hot rolling is completed when the rolling end temperature is equal to or higher than the Ae3 temperature, and winding is preferably performed within 20 seconds after completion of rolling in order to suppress disturbance during hot rolling forced cooling. It remains in the plate state until rolling after completion of rolling, and it is susceptible to disturbances such as uneven cooling water volume, uneven draining of the top surface water, and temperature drop at the edge in the width direction of the plate, but these disturbances are eliminated. For this purpose, it is preferable to take up the coil in a short time.
By setting the coiling temperature to a temperature range of Bs or less and Ms + 50 ° C. or more, the transformation delay effect of Cr is effectively used, austenite remains, and martensite generation is suppressed.
Here, the coiling temperature is set to Bs or lower and Ms + 50 ° C or higher. However, in the temperature range of Ms + 50 ° C or lower, C enrichment due to the transformation delay effect by Cr tends to be incomplete, and a martensite structure is likely to be formed. Become. In addition, at temperatures above Bs, the structure becomes coarse, and the possibility of appearance of the most harmful pearlite is increased, so the range is limited.
図５の（a）では、EBSD法を用いて、体心立方構造のフェライトもしくはベイナイト相と面心立方構造のオーステナイト相を色分けした本発明鋼の組織断面（結晶構造分布像）を示した。白色で示した残留オーステナイト組織は２．０μm以下に微細かつ均一に分散していることが観察出来る。（表２に示す実施事例No９）
また、図５（b）は、Ｃ濃化が不十分となりマルテンサイトが多量に生じた場合の分布像を示したものであり、黒色のマルテンサイト領域が残留オーステナイトと絡むように生じていて最終的にC濃化不十分な領域がマルテンサイト化したのが判る。（表２に示す実施事例No３）
本発明は、以上の知見に基づき開発されたものである。
FIG. 5 (a) shows a structural cross section (crystal structure distribution image) of the steel of the present invention in which the ferrite or bainite phase of the body-centered cubic structure and the austenite phase of the face-centered cubic structure are color-coded using the EBSD method. It can be observed that the retained austenite structure shown in white is finely and uniformly dispersed to 2.0 μm or less. (Implementation example No. 9 shown in Table 2)
FIG. 5 (b) shows a distribution image when the C concentration is insufficient and a large amount of martensite is generated. The black martensite region is generated so as to be entangled with the retained austenite. It can be seen that the area with insufficient C concentration has become martensite. (Implementation example No. 3 shown in Table 2)
The present invention has been developed based on the above findings.
以下に発明の実施例を説明する。
表１に示す化学成分（重量％）を有する溶鋼を、連続鋳造法もしくは鍛造法によりスラブ（圧延素材）とした。続いてこれらのスラブを再加熱し、熱間圧延を行い、熱延鋼板とした。
Examples of the invention will be described below.
Molten steel having chemical components (% by weight) shown in Table 1 was made into a slab (rolled material) by a continuous casting method or a forging method. Subsequently, these slabs were reheated and hot-rolled to obtain hot-rolled steel sheets.
表1に示す鋼種B,C,D,E,Fは実施例の範囲（化学成分について好ましい条件を満たす範囲）に属するもので、鋼種A,G,H,Iは比較例である。
開発例のＢ〜Ｆに示す通り、従来型Ｓｉ-Ｍｎ系TRIP鋼板用素材をＳｉ-Ｃｒ系に置き換えた成分系の他に、更にＣ等の添加量を増した高強度側の成分系も含まれる。鋼種Ｂ〜Ｆは、前述の（１）式、すなわち
0.42 ≦ C＋0.24＊（Cr＋Mo）≦ 0.68 ・・・・・・・・・・・・・・（１）
を満たす成分のもので、うち鋼種Ｃ・Ｄは、前述の（２）式、すなわち
6.46*C-0.26*Si+1.30*Mn+0.49*Ni+0.89*Cr+3.03*Mo＜2.57 ・・・・・・・（２）
をも満たすものである。
比較例の鋼種Aは従来型Ｓｉ-Ｍｎ系の成分でマンガン（Ｍｎ）とクロム（Ｃｒ）の量が本発明の範囲から外れている。
比較例の鋼種Gも炭素量（Ｃ）の低い場合の例で、同じくマンガン（Ｍｎ）とクロム（Ｃｒ）の量が本発明の範囲から外れている。
比較例の鋼種Ｈはシリコン（Ｓｉ）が本発明の範囲から外れている例である。
比較例の鋼種Iは炭素量（C）が高く（１）式の上限を外れている例である。
As shown in B to F of the development example, in addition to the component system in which the material for the conventional Si-Mn TRIP steel sheet is replaced with the Si-Cr system, there is also a component system on the high-strength side in which the addition amount of C and the like is further increased. included. Steel types B to F are the above-described formula (1), that is,
0.42 ≦ C + 0.24 * (Cr + Mo) ≦ 0.68 (1)
Of which the steel grades C and D are the above-mentioned formula (2), that is,
6.46 * C-0.26 * Si + 1.30 * Mn + 0.49 * Ni + 0.89 * Cr + 3.03 * Mo <2.57 (2)
Is also satisfied.
Steel type A of the comparative example is a conventional Si—Mn-based component, and the amounts of manganese (Mn) and chromium (Cr) are out of the scope of the present invention.
The steel type G of the comparative example is also an example in which the carbon amount (C) is low, and the amounts of manganese (Mn) and chromium (Cr) are also out of the scope of the present invention.
The steel type H of the comparative example is an example in which silicon (Si) is out of the scope of the present invention.
Steel type I of the comparative example is an example having a high carbon content (C) and deviating from the upper limit of formula (1).
表２に熱間圧延条件とその材料特性を示す。
表２のＮｏ１は、従来型Ｓｉ-Ｍｎ系の鋼種Ａを用いて発明の範囲内で熱間圧延を行ったものである。残留オーステナイトの比率が低く、引張強さ（ＴＳ）＊伸び（Ｅｌ）の値が不十分である。Ｎｏ２はＳｉ-Ｃｒ系の鋼種Ｂを用いたものでマルテンサイトの生成は殆ど認められず良好なＴＳ＊Ｅｌの値が得られている。Ｎｏ３は同じく鋼種ＢをＭｓ+５０℃以下で巻き取ったもので、高強度化するもののマルテンサイトが生じ、伸びの値が劣化する例である。Ｎｏ４及び５は板厚の異なる同じく鋼種Ｂを発明にしたがって熱間圧延した結果で、冷却能力などの点で巻取り温度が高い場合でもロバスト性が高く、良好な結果が得られている。
Ｎｏ６、７は低強度側（低Ｃ）の実施事例であり、フェライト相を含むＴＲＩＰ鋼板で所期の特性が得られている。但し、Ｎｏ８に示す通り、巻取り温度をＢｓ点よりかなり高くした場合には、パーライトが生成し著しい伸びの劣化を招く。
Ｎｏ９〜１１は高Ｓｉ含有鋼で高強度側を狙ったものであり、より優れたＴＳ＊Ｅｌの値を達成できる。但し、Ｎｏ１０に示す通り、熱延仕上温度をＡｅ３点以下の二相域にすると圧延方向と直角方向引張りでの伸び値の差ΔＥｌが大きくなり成形上の観点からは好ましくない。
Ｎｏ１２は低Ｃでの従来型Ｓｉ-Ｍｎ系でＮｏ１３はＳｉ量が範囲を外れたものであるが、両者とも充分な残留オーステナイトが得られず特性上も見劣る結果となる。
Ｎｏ１４は、Cが高く（１）式の上限を超えている為、変態遅延効果過大で多量のマルテンサイトが残存した例である。
今回の結果から、高Ｓｉ鋼程特性が向上する知見や成分系でフェライト相の有無に関する差異が生じているが、これらは、本開発鋼の根幹に関わるものではない。
但し、フェライト相の有無については実現強度に大きな影響を及ぼす為、（２）式を用いて複合組織に対する設計判断（フェライト相の要否）をする必要がある。
No. 1 in Table 2 is obtained by hot rolling within the scope of the invention using a conventional Si-Mn steel type A. The ratio of retained austenite is low and the value of tensile strength (TS) * elongation (El) is insufficient. No. 2 uses Si—Cr steel type B, and almost no martensite is observed, and a good TS * El value is obtained. No. 3 is also an example in which the steel type B is wound up at Ms + 50 ° C. or less, and martensite is generated although the strength is increased, and the elongation value is deteriorated. Nos. 4 and 5 are the results of hot rolling the same steel type B with different plate thicknesses according to the invention. Even when the coiling temperature is high in terms of cooling capacity, the robustness is high and good results are obtained.
Nos. 6 and 7 are examples of implementation on the low strength side (low C), and the desired characteristics are obtained with a TRIP steel sheet containing a ferrite phase. However, as shown in No. 8, when the coiling temperature is considerably higher than the Bs point, pearlite is generated and the elongation is significantly deteriorated.
Nos. 9 to 11 are high Si-containing steels aimed at the high strength side, and can achieve superior TS * El values. However, as shown in No. 10, when the hot rolling finishing temperature is set to a two-phase region of Ae3 or less, the difference ΔEl in elongation value between the rolling direction and the perpendicular direction tension becomes large, which is not preferable from the viewpoint of molding.
No12 is a conventional Si-Mn system at a low C, and No13 has a Si content outside the range, but in both cases, sufficient retained austenite cannot be obtained, resulting in poor properties.
No. 14 is an example in which a large amount of martensite remains due to excessive transformation delay effect because C is high and exceeds the upper limit of formula (1).
From these results, the difference in the presence or absence of the ferrite phase in the knowledge and the component system in which the characteristics of the high-Si steel are improved is not related to the foundation of the newly developed steel.
However, since the presence or absence of the ferrite phase has a great influence on the realization strength, it is necessary to make a design judgment (necessity of the ferrite phase) for the composite structure using the equation (2).
Claims (4)
質量％でMn: 0.1〜1.0％、Ni: 0.01〜0.50、Ti: 0.02〜0.20％、Nb: 0.02〜0.10％、V: 0.02〜0.20％、B: 0.0001〜0.0030％のうち１種以上をさらに含有し、残部は鉄および不可避的不純物の組成にてなること、By mass% Mn: 0.1-1.0%, Ni: 0.01-0.50, Ti: 0.02-0.20%, Nb: 0.02-0.10%, V: 0.02-0.20%, B: 0.0001-0.0030% Containing, the balance being composed of iron and inevitable impurities,
および、質量％で下記成分範囲式を満足することを特徴とする高強度高加工性熱延鋼板。And the high intensity | strength high workability hot-rolled steel plate characterized by satisfying the following component range formula by mass%.
6.46*C-0.26*Si+1.30*Mn+0.49*Ni+0.89*Cr+3.03*Mo＜2.576.46 * C-0.26 * Si + 1.30 * Mn + 0.49 * Ni + 0.89 * Cr + 3.03 * Mo <2.57
0.42 ≦ C＋0.24×（Cr＋Mo）≦ 0.68 Furthermore, the high intensity | strength high workability hot-rolled steel plate of Claim 1 characterized by satisfying the following component range formula by the mass%.
0.42 ≦ C + 0.24 × (Cr + Mo) ≦ 0.68
上記成分範囲にある1200℃以上の鋼材を粗圧延した後、複数スタンドを有する熱間圧延機によって、Ae3温度以上のオーステナイト域で最終仕上圧延を完了し、２０秒以内にＭｓ点+50℃以上Ｂｓ点以下で巻き取ることを特徴とする高強度高加工性熱延鋼板の製造方法。
ただし、Ae3温度、Ｂｓ点およびＭｓ点は、下記の温度（℃）を示す。
Ae3=919-266*C+38*Si-28*Mn-27*Ni-11*Cr+12*Mo
Bs＝649-83*C-19*Si-26*Mn-30*Ni-21*Cr-29*Mo
Ms=539-423*C-30.4*Mn-17.7*Ni-12.1*Cr-7.5*Mo
A method for producing a high-strength hot-rolled steel sheet according to any one of claims 1 to 3 ,
After roughly rolling a steel material of 1200 ° C or higher in the above component range, the final finish rolling is completed in an austenite region of Ae3 temperature or higher by a hot rolling mill having a plurality of stands, and Ms point + 50 ° C or higher within 20 seconds. A method for producing a high-strength, high-workability hot-rolled steel sheet, which is wound up below the Bs point.
However, Ae3 temperature, Bs point, and Ms point show the following temperature (degreeC).
Ae3 = 919-266 * C + 38 * Si-28 * Mn-27 * Ni-11 * Cr + 12 * Mo
Bs = 649-83 * C-19 * Si-26 * Mn-30 * Ni-21 * Cr-29 * Mo
Ms = 539-423 * C-30.4 * Mn-17.7 * Ni-12.1 * Cr-7.5 * Mo
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