JP4848586B2 - Method for producing high-tensile hot-rolled steel sheet for processing having microstructure - Google Patents

Description

【００２５】
Al：0.10mass％以下
Alは、脱酸剤として作用すると共に、結晶粒を微細化する元素であり、必要に応じて含有させることができる。しかしながら、Al量が0.10mass％を超えると、酸化物系介在物が増加して清浄度が低下し、表面欠陥を増加させる。このため、Alは0.10mass％以下とするのが好ましい。より好ましくは 0.005〜0.07mass％である。
【００２６】
Ti：0.05〜0.5 mass％
Tiは、さらに微細化を図るための元素である。固溶Tiは、Nbと同様に仕上圧延後の再結晶を遅らせる元素として知られている。しかしながら、さらに詳細な検討を行った結果、Tiを含有する低炭素鋼を熱間圧延するに当たり、低温で加熱するとTiＣが加熱中に溶解せずに残留するが、これは上記の再結晶抑制効果に悪影響を及ぼさないこと、しかもTiＣの存在が、オーステナイトの結晶粒の成長を抑制し、結晶粒をより微細化できること、さらにこのような加工前のオーステナイト粒の微細化によって熱延中に動的再結晶が発現し、変形抵抗の低下、オーステナイト粒成長の抑制などが生じることが判明した。
この効果を発揮させるためには、Tiは0.05mass％以上必要であり、それ未満だとTiＣの析出が不充分である。しかしながら、0.5 mass％を超える添加は、Ｃを本発明の上限値で添加したとしても多すぎて、固溶Tiによる過度の強化が進むだけでなく、TiＣの量が多くなり、逆に成形性が損なわれることから、Tiは0.05〜0.5 mass％が好ましい。
なお、上記の範囲でTiを含有させる場合には、加熱温度は1150℃以下とする必要がある。
【００２７】
なお、本発明では、不純物元素のうち特にＳは、MnＳ等の非金属介在物を形成し、延性を低下させ、穴拡げ加工性（バーリング性）を劣化させるため、できるだけ低減することが望ましい元素である。バーリング性の観点からは 0.005mass％まで許容できる。このためＳは0.005 mass％以下とするのが好ましい。より好ましくは0.0015mass％以下である。
【００２８】
次に、本発明に従う製造条件について説明する。
加熱温度
熱間圧延は、溶製素材を一旦冷却したのち再加熱する再加熱圧延としても、直送圧延やホットチャージローリングとしても良い。また、薄スラブ鋳片のような連続鋳造された薄スラブを直接熱間圧延してもよい。再加熱する場合には、初期オーステナイト粒を微細化するために、1150℃以下に加熱するのが望ましい。また直送圧延する場合も、1150℃以下まで冷却したのち圧延を開始するのが、オーステナイト粒の微細化を促進する上で好ましい。
また、特にTiを添加した場合には、加熱温度は、TiＣの析出温度を考慮して、1150℃以下にする必要がある。
【００２９】
仕上げ圧延温度：Ａr₃〜（Ａr₃＋100 ℃）
仕上げ圧延温度がＡr₃点を下回ると、加工中にフェライトが生成し、これに起因して特に熱延板の表層部が粗大粒となったり、強度や伸びの板面内異方性が大きくなるので好ましくない。一方（Ａr₃＋100 ℃）を超えると導入した加工歪みが加工中に回復して、目的とする加工歪みの蓄積が達成できず、また粒成長が生じてオーステナイト粒径が大きくなるので好ましくない。よって、仕上げ圧延温度はＡr₃〜（Ａr₃＋100 ℃）の範囲に限定した。
【００３０】
圧延後の冷却開始時間：0.5 秒以内
圧延後の高温状態で 0.5秒を超えて保持されると、オーステナイトの粒成長が生じてフェライト変態前の粒径が大きくなり、その後いくら低温で変態させたとしても 3.5μm 以下のフェライト粒を得ることができなくなる。従って、圧延後の冷却開示時間は 0.5秒以内とした。
【００３１】
巻取り温度
コイルの巻取り温度は 350〜600 ℃とする。巻き取り後の冷却速度は特に限定されるものではなく、製造しようとする鋼板に応じて適宜定めれば良い。しかしながら、巻取り温度が高いと、パーライト主体の組織となってフェライト粒の粒成長が起こり易くなり、一方巻取り温度が低いと第２相がマルテンサイト主体の組織となって伸びフランジ性が劣化する。従って、コイルの巻取り温度は 350〜600 ℃の範囲に限定したのである。
【００３２】
最終スタンドの圧下歪みＥと 600℃までの平均冷却速度CRとの関係
前述したとおり、
Ａ＝88CR^-0.6＋0.2 (42.8(mass%Si)−28.26(mass%Mn)−190.6 (mass%Ｃ))＋9.8
とする時、微細なフェライト粒を得るためには、Ｅ／log(ｄ／Ａ) を０〜3.0の範囲に調整する必要がある。
そこで、本発明では、ＥとCRについて、次式(1) の関係を満足させることにしたのである。
Ｅ／log(ｄ／Ａ) ＝０〜3.0 --- (1)
ここで、ｄ：仕上げ圧延前のオーステナイト粒径（μm)
なお、Ａ値が負の値となる場合はＥ／log(ｄ／Ａ) は計算不能となり、本発明の範囲外である。
また、冷却速度を制御すべき温度範囲の下限を 600℃としたのは、一般に 600℃以上の領域でフェライト変態が終了するからである。
【００３３】
図１(a), (b), (c) にそれぞれ、成分組成、加熱温度および仕上げ温度は本発明の適正範囲を満足する材料について、Ｅ／log(ｄ／Ａ) を種々に変化させた場合のTS×λ、TS×Elおよびフェライト粒径ｄαについて調べた結果を示す。
同図に示したとおり、Ｅ／log(ｄ／Ａ) を０〜3.0 の範囲に制御することによって、微細なフェライト組織が得られ、TS×λ、TS×Elも高い、優れた加工性を有する熱延鋼板が得られることが分かる。特に、結晶粒径については、Tiを添加することにより、さらなる微細化が達成されていることが分かる。
これに対し、Ｅ／log(ｄ／Ａ) が０未満、または 3.0を超えると、フェライト変態温度が高くなり、フェライト核生成よりも、粒成長の方が優先的となり、微細組織が得られない。微細組織が得られない場合は、十分なフェライト変態が行えずに第２相の分率が大きくなったり、硬質な第２相の粒径が大きくなって、穴拡げ性や、伸び特性を低下させてしまう。
【００３４】
かくして、フェライトを主相として、パーライトやベイナイトを第２相とする微細組織の複相鋼板が得られるのであるが、主相であるフェライト相の分率は85％以上とすることが好ましい。というのは、フェライト相分率が85％未満になると、硬質な第２相が増えると共に第２相の大きさも増大し、マクロな破壊の起点となって加工性が低下するからである。
また、かかるフェライト相の粒径は 3.5μm 以下とすることが好ましい。というのは、フェライト粒径が 3.5μm を超えると、これに伴って第２相の大きさも増大し、やはりマクロな破壊の起点となって所望する加工性が得られなくなるからである。
【００３５】
一方、第２相としては、まず相の種類については、パーライト、ベイナイトまたは残留オーステナイトが好ましく、またその分率は15％以下とすることが好ましい。というのは、硬質なマルテンサイトが存在したり、第２相の分率が15％を超えると、マクロな破壊の起点となって加工性が低下するからである。なお、本発明では、第２相の粒径が小さく、パーライトとセメンタイトを明確に識別することは困難であるため、本発明では、セメンタイトも含め、パーライトとして表すこととする。
また、かかる第２相の粒径は２μm 以下とすることが好ましい。というのは、第２相の粒径が２μm を超えると、やはりマクロな破壊の起点となって所望する加工性が得られなくなるからである。
【００３６】
【実施例】
表１に示す成分組成になるスラブを、表２に示す条件で熱間圧延し板厚：3.2mmの熱延板とした。
なお、表２中、オーステナイト粒径（γ粒径）は、仕上げ圧延前のシートバーを急冷してピクリン酸によるエッチングで組織観察し、１mm×１mmの視野で10箇所測定してその平均を求めた結果である。
かくして得られた熱延板のJIS 5 号試験片による引張特性、フェライト粒径および穴拡げ率について調べた結果を表３に示す。
また、表３には、第２相の分率、粒径および相の種類について調査した結果も併せて示す。
なお、フェライト粒径は 0.1mm×0.1mm の視野で10箇所測定してその平均値で表示した。また、穴拡げ試験は、日本鉄鋼連盟規格 JFST 1001に準じ、採取した試験片に12.5％のクリアランスで10mmφの穴を打ち抜き、頂角：60°の円錐ポンチを打ち抜き後のせん断面にかえりのある側の反対側から挿入して穴を拡げる成形を行い、亀裂が板厚を貫通した時の穴径ｄを求め、次式に基づき穴拡げ率λを計算した。
λ（％）＝｛（ｄ−ｄ₀)／ｄ₀ ｝× 100
ここで、ｄ：試験後の穴径（mm）
ｄ₀ ：試験前の穴径（mm）
さらに、強度−伸びバランスはTS×Elで、強度−穴拡げバランスTS×λで評価した。
【００３７】
【表１】

【００３８】
【表２】

【００３９】
【表３】

【００４０】
表３から明らかなように、本発明に従い得られた熱延板はいずれも、TS×λが60000MPa・％以上、TS×Elが22000MPa・％以上という優れた加工性が得られている。
これに対し、No.2は、圧延後の冷却開始時間が長い場合であるが、この場合には冷却開始までの間に組織的な変化（回復や再結晶）が生じてしまうため、熱延板の粒径が粗粒となり、特に穴拡げ性が低下した。
No.3は、仕上げ圧延温度が高いために、圧延歪みと冷却速度のバランスが不適切となり、熱延板の粒径が粗粒となった。また、第２相の粒径も大きくなり、強度−伸びバランス、強度−穴拡げバランスとも低下した。
No.6は、Tiを 0.100mass％添加して材料について加熱温度の上昇により、オーステナイト粒径が大きくなってしまった結果、No.3と同様に、熱延板の粒径が粗粒となり、強度−伸びバランス、強度−穴拡げバランスが低下した。
No.7は、成分に対する圧下歪みと冷却速度が整合しないために粒径が大きくなった。
No.11 〜13は、成分組成あるいはさらに仕上げ温度が適正範囲から外れているために、所望の特性を得ることができなかった。すなわち、No.11 は、Ti量が上限を超えて含有されているため固溶Tiによって延性を担う軟質であるはずのフェライト粒が硬質化し、強度−伸びバランス、強度−穴拡げバランスが低下した。また、No.12 では、Mn量が多いためにフェライト変態温度が低くなり、仕上げ温度が変態点に対して高すぎ、変態前に加工歪みが回復するので、粒が粗大化し、強度−伸びバランス、強度−穴拡げバランスが低下した。さらに、No.13 では、Ｃ量が多いため、変態点が低くなり、フェライト変態を十分に進行させることができなかった。このため、大きく、硬質な第２相の存在により、特に穴拡げ性が低下した。
【００４１】
【発明の効果】
かくして、本発明によれば、結晶組織を効果的に微細化して、加工性特に延性とバーリング性に優れた高張力熱延鋼板を安定して得ることができる。
【図面の簡単な説明】
【図１】 Ｅ／log(ｄ／Ａ) を種々に変化させた場合のTS×λ、TS×Elおよびフェライト粒径ｄαの変化状況を示したグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention is a hot-rolled steel sheet suitable for use in automobiles, home appliances, mechanical structures, and buildings, particularly for processing that has an ultrafine grain structure as it is hot-rolled and has excellent ductility and burring properties. The present invention relates to a method for producing a high-tensile hot-rolled steel sheet.
[0002]
[Prior art]
In recent years, in the automobile industry, as one of measures for improving the fuel consumption of automobiles, there is an increasing demand for reducing the weight of the vehicle body. In order to reduce the weight of the vehicle body, the use of high-strength, high-strength steel sheets that can be thinned is directed. However, in general, workability such as ductility and burring properties decreases with increasing strength. For this reason, there has been a demand for a hot-rolled high-tensile steel sheet that suppresses such deterioration of workability.
[0003]
Conventionally, as a high-tensile steel sheet having excellent workability, a dual-phase steel sheet (for example, Japanese Patent Laid-Open No. 54-11425) has been proposed in which ferrite is the main phase and martensite or bainite is the second phase. This dual-phase steel sheet has an excellent balance between strength and ductility, has high workability, and is suitable for automobile parts, but has a problem in that it is inferior in burring workability. This problem was also true for TRIP (Transformation Induced Plasticity) steel.
[0004]
Therefore, as a method for solving the above problem, for example, in Japanese Patent Application Laid-Open No. 57-145965, a C-Si-Mn steel with a reduced alloying element is used. A method for producing a high-tensile hot-rolled steel sheet having a composite structure of ferrite and bainite by adjusting the winding temperature is disclosed. JP-A-60-181231 discloses a method for producing a high-tensile hot-rolled steel sheet having a bainite-based structure by adjusting the total rolling reduction of finish rolling and the cooling rate after rolling.
[0005]
However, in the high-tensile hot-rolled steel sheet having a ferrite-bainite composite structure described in JP-A-57-145965, the burring workability is improved, but a high level of strength-ductility as requested by an automobile manufacturer. I couldn't get the balance.
In addition, the steel sheet described in JP-A-60-181231 has improved burring workability than before, but the balance between strength and hole expansion workability of this steel sheet is tensile strength (TS) × hole. The expansion workability (λ) was only about 47000 MPa ·%. Here, λ is a hole expansion rate (%) described later. Furthermore, the balance between strength and ductility is TS x Elongation (El) of about 20000 MPa ·%, which does not satisfy the high-level characteristics required by automobile manufacturers.
[0006]
On the other hand, apart from the above method, a method of refining the structure has been proposed as a method for obtaining high strength while ensuring both ductility and burring characteristics. For example, Japanese Patent Application Laid-Open No. 58-123823 discloses a high-tensile heat having fine grains by promoting a strain-induced transformation from austenite (γ) to ferrite (α) by applying large pressure to austenite grains. A method of manufacturing a rolled steel sheet is disclosed.
JP-A-9-143570 discloses a method for producing a high-tensile hot-rolled steel sheet having fine grains by heating a steel containing Ti and Nb at a low temperature of 1100 ° C. or lower and then performing hot rolling. It is disclosed.
[0007]
However, although the method described in JP-A-58-123823 can obtain a fine-grained structure, the resulting structure is a structure extended in the rolling direction. There was a problem of increased directionality. In automotive steel plates for press forming, the forming limit is governed by the characteristic level in the direction of the least ductility, so it is difficult to ensure high pressability with steel plates with high anisotropy.
Further, in the method described in JP-A-9-143570, an isotropic and fine structure is formed, and an excellent strength-ductility balance and strength-hole expansion balance can be obtained. Instead, problems remained where high strength could not be obtained.
[0008]
By the way, the refinement of crystal grains exhibits the effect of improving the strength-ductility balance of the ferrite itself. However, the local elongation may decrease if the size is made smaller than necessary. Further, the refinement of ferrite brings about the refinement of the second phase (bainite, martensite, pearlite, etc.), which contributes to the improvement of moldability. In particular, the fineness of the second phase is effective for local ductility such as hole expansibility.
And it is thought by the above-mentioned two effects that the fine-grained steel has excellent formability.
[0009]
In order to maximize the above two miniaturization effects, it is considered that there are suitable mother phase particle size, second phase fraction, phase type and particle size. In this sense, conventionally, a method for accurately controlling the ferrite grain size and the second phase has been demanded.
Conventionally, most of the proposed methods of grain refinement require the addition of Ti, Nb, etc., and these elements suppress recrystallization during rolling, especially when applied to high-tensile steel sheets. There was a problem that the rolling load was excessive. Also, a fine granulation technique for cooling immediately after rolling has been disclosed. Although such a fine granulation technique is effective under a specific component and a reduction ratio, the same processing conditions are applied to different component systems. When applied to, fine graining was not always achieved.
[0010]
[Problems to be solved by the invention]
For this reason, the technique which can show cooling conditions with appropriate reduction according to steel composition was calculated | required. In this case, a technique for miniaturization without adding Ti and Nb, and a technique that can be expected to have a further miniaturization effect when Ti and Nb are added have been desired.
[0011]
The present invention advantageously responds to the above-described demand, and an object thereof is to propose an advantageous method for producing a high-tensile hot-rolled steel sheet excellent in workability, particularly ductility and burring properties.
Here, the target material levels in the present invention are as follows.
TS × λ ≧ 60000 MPa ・%
TS × El ≧ 22000 MPa ・%
[0012]
[Means for Solving the Problems]
Now, as a result of intensive studies to achieve the above object, the inventors have not only reduced the ferrite grain size by prescribing the cooling conditions according to the component system, but also the second phase. It has been found that the fraction can be reduced and the particle size can be reduced, and the hardness of the second phase can be controlled.
For example, if a steel containing a large amount of a hardenability-improving element such as Mn is cooled too quickly, the ferrite transformation does not proceed sufficiently, the fraction of the second phase becomes high and hardens at the same time, and the workability decreases. There was a case. In addition, when the excessive reduction is performed, the ferrite transformation temperature becomes high, and there is a case where the fine grain is not formed by the grain growth.
[0013]
In this regard, the present invention balances the generation of ferrite nucleation sites, the grain growth rate, and the low-temperature transformation of the second phase by balancing the transformation temperature of steel with the austenite grain size and degree of work before processing. And succeeded in solving the above problems.
Specifically, according to the present invention, the fraction of the second phase can be suppressed to about 15% or less, and the generation of martensite that has a large hardness difference from the main phase ferrite and lowers the hole expandability is suppressed. can do. In addition, if the fraction of the second phase can be suppressed to about 15% or less, the second phase will be finely and evenly dispersed as the main phase becomes finer, and the origin of large ductile fracture disappears. The processability, such as extending and hole expansibility, is improved.
[0014]
Another feature of the present invention is that the addition of Nb, which has been frequently used in the past, is not essential. By balancing the generation of ferrite nucleation sites and the grain growth rate in the manner described above, a ferrite grain size of 3.5 μm or less, which is equivalent to or better than that conventionally obtained with Nb-added steel, can be obtained. . Addition of Nb suppresses recrystallization after hot rolling and transforms at a low temperature to increase the number of ferrite nucleation sites, but it is necessary to transform from unrecrystallized expanded γ grains, In addition to the material problem of increased anisotropy, there was an operational problem of increased rolling load.
According to the present invention, there is no such problem, and a material with small anisotropy can be produced without excessive load.
[0015]
In addition, when Ti is added, γ grain growth can be effectively suppressed without excessive rolling load by dynamic recrystallization. In this case as well, the generation of ferrite nucleation sites and the grain growth rate are controlled. By balancing, fine ferrite grains can be obtained much more stably than in the past.
[0016]
That is, the gist configuration of the present invention is as follows.
1. C: 0.2 mass% or less,
Si: 2.0 mass% or less,
Mn: 3.0 mass% or less and
Al: The steel containing 0.10 mass% or less and Ti is suppressed to less than 0.05 mass%, and the balance of the steel slab composed of Fe and inevitable impurities is hot-rolled, and the finishing temperature is Ar ₃ to (Ar ₃ +100 ° C). ), Rolling is started within 0.5 seconds after rolling and rolled up in a temperature range of 350 to 600 ° C. The rolling strain of the final stand is E, and the average cooling rate up to 600 ° C is CR (° C / s). Then, a method for producing a high-tensile hot-rolled steel sheet for processing having a microstructure with a ferrite grain size of 3.5 μm or less, which satisfies the relationship of the following formula (1) for E and CR:
A = 88CR ^-0.6 +0.2 (42.8 (mass% Si) −28.26 (mass% Mn) −190.6 (mass% C))
+9.8
E / log (d / A) = 0.630 to 3.0 --- (1)
Where d: austenite grain size (μm) before finish rolling
[0017]
2. C: 0.2 mass% or less,
Si: 2.0 mass% or less,
Mn: 3.0 mass% or less and
Al: Contains 0.10 mass% or less, and
Ti: 0.05-0.5 mass%
In the hot rolling of the steel slab consisting of Fe and inevitable impurities, the balance is heated at 1150 ° C or less, the finishing temperature is Ar ₃ to (Ar ₃ + 100 ° C), 0.5 seconds after rolling Within a temperature range of 350 to 600 ° C, and when the strain at the final stand is E, and the average cooling rate up to 600 ° C is CR (° C / s), A method for producing a high-tensile hot-rolled steel sheet for processing having a microstructure with a ferrite grain size of 3.5 μm or less, which satisfies the relationship of the following formula (1):
A = 88CR ^-0.6 +0.2 (42.8 (mass% Si) −28.26 (mass% Mn) −190.6 (mass% C))
+9.8
E / log (d / A) = 0.630 to 3.0 --- (1)
Where d: austenite grain size (μm) before finish rolling
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be specifically described.
In order to obtain a fine ferrite structure, it is important to transform near the nose of the ferrite transformation curve.
The nose position of the ferrite transformation curve varies depending on the component, austenite grain size, and rolling reduction. Among the components, Mn and C delay the ferrite transformation, and Si has a promoting effect. The smaller the austenite grain size, the more the ferrite transformation is promoted. And the greater the processing strain, the more the ferrite transformation is promoted. The above-mentioned conditions are conditions in which the ferrite transformation promoting effect and the delay effect are well balanced and transformation is performed near the nose of the ferrite transformation.
The value A is a critical crystal grain size that can be transformed in the vicinity of the nose of the ferrite transformation when the cooling rate (CR) and components are not processed (the strain is zero). When the austenite grain size is smaller than this critical grain size, ferrite transformation is promoted by refinement, the ferrite transformation temperature is increased, and grain growth is prioritized over ferrite nucleation, and a microstructure is obtained. Absent. Moreover, when A value is a negative value, a ferrite transformation cannot be performed at a cooling rate that can be achieved by normal rolling, and the main phase becomes a low-temperature transformation phase.
[0019]
As a result of examining the relationship between the A value and the actual austenite grain size and investigating the relationship with the amount of strain in rolling, it is fine to adjust the E / log (d / A) to a range of 0 to 3.0. It was investigated that this was the condition for obtaining a ferrite structure.
Here, the rolling reduction E of the final stand is a true strain amount calculated by E = ln (t ₀ / t) from the plate thicknesses t ₀ and t before and after passing through the stand, and a generally used reduction rate e = 1−1. There is a relationship of E = ln {1 / (1−e)} with respect to (t / t ₀ ).
When the ratio of E / log (d / A) is less than 0, it means that the austenite grain size becomes smaller than the critical grain size, the ferrite transformation temperature becomes higher, and grain growth is more preferable than ferrite nucleation. It becomes preferential and a fine structure cannot be obtained.
In addition, even if it exceeds 3.0, the ferrite transformation temperature becomes higher due to strain induction because the rolling exceeds the appropriate amount of strain expected from the cooling conditions, γ grain size, and components, and grain growth is more important than ferrite nucleation. Becomes a priority and a fine structure cannot be obtained. If a fine structure cannot be obtained, sufficient ferrite transformation cannot be performed and the second phase fraction becomes high or the particle size of the hard second phase becomes large, deteriorating the hole expansibility and elongation characteristics. End up.
However, this requires a sufficiently small matrix phase particle size. For this reason, it is important to limit the heating temperature, finishing temperature, and further the component composition within the scope of the present invention.
[0020]
First, the reason why the component composition of the material is limited to the above range in the present invention will be described.
C: 0.2 mass% or less C is an inexpensive strengthening component and contains a necessary amount according to desired steel plate strength. However, if the amount of C exceeds 0.2 mass%, not only the workability deteriorates but also the weldability deteriorates. On the other hand, if the amount of C exceeds 0.2 mass%, the second phase fraction tends to increase beyond 15%. For this reason, C was limited to 0.2 mass% or less. More preferably, it is the range of 0.05-0.15 mass%.
[0021]
Si: 2.0 mass% or less, Mn: 3.0 mass% or less
Si and Mn are very important elements for controlling the strength of the steel and the second phase. In particular, Si can reinforce the matrix ferrite without reducing formability. Further, during the transformation from austenite to ferrite, the carbon is discharged to austenite to generate residual austenite and to improve the formability. On the other hand, Mn has an effect of lowering the transformation point, and contributes to refinement of ferrite by causing ferrite transformation at a lower temperature. In addition, segregation to the grain boundary has an effect of suppressing grain growth. However, if Si is added in excess of 2.0 mass%, and Mn is added in excess of 3.0 mass%, weldability and chemical conversion properties are deteriorated, which is not preferable.
Therefore, it is necessary to set Si: 2.0 mass% or less and Mn: 3.0 mass% or less.
In consideration of obtaining a hot-rolled steel sheet having a tensile strength of 590 MPa or more, it is necessary to utilize the solid solution strengthening ability of Si and Mn and the control function of the second phase. In this sense, Si is desirably contained in an amount of 0.01 mass% or more, more preferably 0.1 mass% or more, while Mn is contained in an amount of 0.05 mass% or more, more preferably 0.5 mass% or more.
[0022]
Addition of the above-mentioned useful components is necessary for achieving both formability and high strength, but rolling conditions need to be changed in accordance with the addition amount in order to form a structure. The effect particularly affects the ferrite transformation temperature.
Therefore, when the influence on the transformation temperature of each element was taken into the A value and the relationship with the cooling rate was determined, the above formula A was obtained.
[0023]
As described above, the essential components have been described. However, in the present invention, other elements described below can be appropriately contained.
P: 0.5 mass% or less P is useful as a reinforcing component and can be added according to the desired steel plate strength. However, excessive addition segregates at the grain boundaries and causes embrittlement. For this reason, it is preferable that P is 0.5 mass% or less. In addition, since excessive reduction becomes high cost, Preferably it is 0.001-0.2 mass%, More preferably, it is 0.005-0.2 mass%.
When P is 0.05 mass% or more, the influence on the transformation point cannot be ignored. In this case, the following formula is used as the A value.
[0024]
Cr: 0.1-2.0 mass%, Mo: 0.1-2.0 mass%, Ni: 0.1-2.0 mass%
It is also effective to add elements such as Cr, Mo, and Ni in terms of solid solution strengthening of ferrite and suppression of grain growth. Even if Cr, Mo, and Ni are added in excess of 2.0 mass%, the effect is saturated, so this is the upper limit from an economic viewpoint. Moreover, since the effect is hardly exhibited at less than 0.1 mass%, it is preferably 0.1 to 2.0 mass%. In the case of adding P, it is possible to control by adding the change amount of the transformation point of the element (change of transformation point per 1 mass%) to the above-mentioned formula A together with P. Specifically, the following equation is used as the A value.

[0025]
Al: 0.10 mass% or less
Al is an element that acts as a deoxidizer and refines the crystal grains, and can be contained as necessary. However, if the Al content exceeds 0.10 mass%, oxide inclusions increase, the cleanliness decreases, and surface defects increase. For this reason, it is preferable to make Al into 0.10 mass% or less. More preferably, it is 0.005-0.07 mass%.
[0026]
Ti: 0.05-0.5 mass%
Ti is an element for further miniaturization. Solid solution Ti is known as an element that delays recrystallization after finish rolling in the same manner as Nb. However, as a result of further detailed investigation, when hot rolling low carbon steel containing Ti, TiC remains undissolved during heating when heated at a low temperature. In addition, the presence of TiC suppresses the growth of austenite crystal grains and makes the crystal grains finer, and further, during the hot rolling due to such refinement of austenite grains before processing. It was found that recrystallization occurred, resulting in a decrease in deformation resistance and suppression of austenite grain growth.
In order to exert this effect, 0.05 mass% or more of Ti is necessary, and if it is less than that, precipitation of TiC is insufficient. However, the addition exceeding 0.5 mass% is too much even if C is added at the upper limit of the present invention, and not only excessive strengthening by solute Ti proceeds, but also the amount of TiC increases, conversely the formability. Since Ti is impaired, 0.05 to 0.5 mass% of Ti is preferable.
In addition, when Ti is contained in the above range, the heating temperature needs to be 1150 ° C. or less.
[0027]
In the present invention, among the impurity elements, particularly S is an element that should be reduced as much as possible in order to form non-metallic inclusions such as MnS, reduce ductility, and deteriorate hole expansion workability (burring properties). It is. From the viewpoint of burring, 0.005 mass% is acceptable. For this reason, S is preferably 0.005 mass% or less. More preferably, it is 0.0015 mass% or less.
[0028]
Next, manufacturing conditions according to the present invention will be described.
The heating temperature hot rolling may be reheating rolling in which the molten material is once cooled and then reheated, or may be direct feed rolling or hot charge rolling. Further, a continuously cast thin slab such as a thin slab cast may be directly hot-rolled. When reheating, it is desirable to heat to 1150 ° C. or lower in order to refine the initial austenite grains. Also in the case of direct rolling, it is preferable to start rolling after cooling to 1150 ° C. or less in order to promote the refinement of austenite grains.
In particular, when Ti is added, the heating temperature needs to be 1150 ° C. or less in consideration of the precipitation temperature of TiC.
[0029]
Finishing rolling temperature: Ar ₃ to (Ar ₃ + 100 ° C)
When the finish rolling temperature is lower than the Ar ₃ point, ferrite is generated during processing, resulting in coarse grain in the surface layer portion of the hot-rolled sheet, and large in-plane anisotropy of strength and elongation. This is not preferable. On the other hand, if it exceeds (Ar ₃ + 100 ° C.), the introduced processing strain is recovered during processing, the target processing strain cannot be accumulated, and grain growth occurs and the austenite grain size becomes large. Therefore, the finish rolling temperature is limited to a range of Ar ₃ to (Ar ₃ + 100 ° C.).
[0030]
Cooling start time after rolling: Within 0.5 seconds When held at high temperature after rolling for more than 0.5 seconds, austenite grain growth occurs, the grain size before ferrite transformation increases, and then transformation is performed at a low temperature. However, ferrite grains of 3.5 μm or less cannot be obtained. Therefore, the cooling disclosure time after rolling is set within 0.5 seconds.
[0031]
Winding temperature The coil winding temperature shall be 350-600 ° C. The cooling rate after winding is not particularly limited, and may be appropriately determined according to the steel sheet to be manufactured. However, if the coiling temperature is high, the pearlite-based structure tends to cause ferrite grain growth, whereas if the coiling temperature is low, the second phase is composed mainly of martensite and the stretch flangeability deteriorates. To do. Therefore, the coil winding temperature is limited to the range of 350 to 600 ° C.
[0032]
Relationship between the rolling strain E of the final stand and the average cooling rate CR up to 600 ° C
A = 88CR ^-0.6 +0.2 (42.8 (mass% Si) −28.26 (mass% Mn) −190.6 (mass% C)) + 9.8
In order to obtain fine ferrite grains, it is necessary to adjust E / log (d / A) to a range of 0 to 3.0.
Therefore, in the present invention, the relationship of the following formula (1) is satisfied for E and CR.
E / log (d / A) = 0-3.0 --- (1)
Where d: austenite grain size (μm) before finish rolling
When the A value is a negative value, E / log (d / A) cannot be calculated and is outside the scope of the present invention.
The reason why the lower limit of the temperature range in which the cooling rate should be controlled is 600 ° C. is that ferrite transformation generally ends in the region of 600 ° C. or higher.
[0033]
1 (a), 1 (b), and 1 (c), E / log (d / A) was variously changed for materials satisfying the proper range of the present invention in terms of component composition, heating temperature, and finishing temperature. The results of investigation on TS × λ, TS × El, and ferrite grain size dα in the case are shown.
As shown in the figure, by controlling E / log (d / A) in the range of 0 to 3.0, a fine ferrite structure can be obtained, and TS × λ and TS × El are high, and excellent workability is achieved. It turns out that the hot-rolled steel plate which has is obtained. In particular, regarding the crystal grain size, it can be seen that further refinement is achieved by adding Ti.
On the other hand, when E / log (d / A) is less than 0 or exceeds 3.0, the ferrite transformation temperature becomes high, and grain growth is prioritized over ferrite nucleation, and a fine structure cannot be obtained. . If a fine structure cannot be obtained, sufficient ferrite transformation cannot be performed and the fraction of the second phase is increased, or the particle size of the hard second phase is increased, resulting in decreased hole expansibility and elongation characteristics. I will let you.
[0034]
Thus, a microstructured double phase steel sheet having ferrite as the main phase and pearlite or bainite as the second phase can be obtained, but the fraction of the ferrite phase as the main phase is preferably 85% or more. This is because when the ferrite phase fraction is less than 85%, the number of hard second phases increases and the size of the second phase also increases, and the workability deteriorates as a starting point for macro fracture.
Further, the particle size of the ferrite phase is preferably 3.5 μm or less. This is because if the ferrite grain size exceeds 3.5 μm, the size of the second phase also increases accordingly, and the desired workability cannot be obtained as a starting point of macro fracture.
[0035]
On the other hand, as the second phase, the type of phase is preferably pearlite, bainite or retained austenite, and the fraction is preferably 15% or less. This is because if hard martensite is present or if the fraction of the second phase exceeds 15%, the workability deteriorates as a starting point for macro fracture. In the present invention, since the particle size of the second phase is small and it is difficult to clearly distinguish between pearlite and cementite, in the present invention, it is expressed as pearlite including cementite.
The particle size of the second phase is preferably 2 μm or less. This is because if the particle size of the second phase exceeds 2 μm, the desired workability cannot be obtained as a starting point for macro fracture.
[0036]
【Example】
The slab having the component composition shown in Table 1 was hot-rolled under the conditions shown in Table 2 to obtain a hot-rolled sheet having a sheet thickness of 3.2 mm.
In Table 2, the austenite particle size (γ particle size) is obtained by quenching the sheet bar before finish rolling, observing the structure by etching with picric acid, measuring 10 points in a 1 mm × 1 mm field of view, and calculating the average. It is a result.
Table 3 shows the results obtained by examining the tensile properties, the ferrite particle diameter, and the hole expansion ratio of the hot-rolled sheet obtained as described above using JIS No. 5 test pieces.
Table 3 also shows the results of investigation on the fraction of the second phase, the particle size, and the type of phase.
The ferrite grain size was measured at 10 locations with a field of view of 0.1 mm × 0.1 mm and displayed as the average value. In addition, the hole expansion test conforms to JFST 1001 of the Japan Iron and Steel Federation, punching a 10mmφ hole with a 12.5% clearance in the sampled specimen, and burring the sheared surface after punching a conical punch with an apex angle of 60 ° Inserting from the opposite side was performed to expand the hole, the hole diameter d when the crack penetrated the plate thickness was determined, and the hole expansion rate λ was calculated based on the following equation.
λ (%) = {(d−d ₀ ) / d ₀ } × 100
Where d: hole diameter after test (mm)
d ₀ : Hole diameter before test (mm)
Further, the strength-elongation balance was evaluated by TS × El, and the strength-hole expansion balance TS × λ.
[0037]
[Table 1]

[0038]
[Table 2]

[0039]
[Table 3]

[0040]
As is clear from Table 3, all the hot-rolled sheets obtained according to the present invention have excellent workability such that TS × λ is 60000 MPa ·% or more and TS × El is 22000 MPa ·% or more.
On the other hand, No. 2 is a case where the cooling start time after rolling is long, but in this case, a structural change (recovery and recrystallization) occurs before the start of cooling. The grain size of the plate became coarse, and in particular the hole expandability decreased.
In No. 3, the finish rolling temperature was high, so the balance between rolling strain and cooling rate became inadequate, and the grain size of the hot rolled sheet became coarse. In addition, the particle size of the second phase also increased, and both the strength-elongation balance and the strength-hole expansion balance decreased.
As for No.6, as a result of increasing the heating temperature for the material by adding 0.100 mass% of Ti and increasing the austenite grain size, as in No.3, the grain size of the hot rolled sheet becomes coarse, The strength-elongation balance and the strength-hole expansion balance decreased.
In No. 7, the grain size increased because the rolling strain for the components and the cooling rate did not match.
Nos. 11 to 13 could not obtain desired characteristics because the component composition or the finishing temperature was out of the proper range. That is, in No.11, since the Ti amount exceeds the upper limit, the ferrite grains that should be soft due to the solid solution Ti became hardened, and the strength-elongation balance and the strength-hole expansion balance were lowered. . In No. 12, since the Mn content is large, the ferrite transformation temperature is low, the finishing temperature is too high with respect to the transformation point, and the work strain is recovered before transformation, so the grains become coarse and the strength-elongation balance , Strength-hole expansion balance decreased. Further, in No. 13, since the amount of C was large, the transformation point was low, and the ferrite transformation could not be sufficiently advanced. For this reason, the hole expandability was particularly lowered due to the presence of the large and hard second phase.
[0041]
【The invention's effect】
Thus, according to the present invention, a high-strength hot-rolled steel sheet excellent in workability, particularly ductility and burring properties can be stably obtained by effectively refining the crystal structure.
[Brief description of the drawings]
FIG. 1 is a graph showing changes in TS × λ, TS × El, and ferrite grain size dα when E / log (d / A) is variously changed.

Claims (2)

C: 0.2 mass% or less,
Si: 2.0 mass% or less,
Mn: 3.0 mass% or less and
Al: The steel containing 0.10 mass% or less and Ti is suppressed to less than 0.05 mass%, and the balance of the steel slab composed of Fe and inevitable impurities is hot-rolled, and the finishing temperature is Ar ₃ to (Ar ₃ +100 ° C). ), Rolling is started within 0.5 seconds after rolling and rolled up in a temperature range of 350 to 600 ° C. The rolling strain of the final stand is E, and the average cooling rate up to 600 ° C is CR (° C / s). Then, a method for producing a high-tensile hot-rolled steel sheet for processing having a microstructure with a ferrite grain size of 3.5 μm or less, which satisfies the relationship of the following formula (1) for E and CR:
A = 88CR ^-0.6 +0.2 (42.8 (mass% Si) −28.26 (mass% Mn) −190.6 (mass% C))
+9.8
E / log (d / A) = 0.630 to 3.0 --- (1)
Where d: austenite grain size (μm) before finish rolling C: 0.2 mass% or less,
Si: 2.0 mass% or less,
Mn: 3.0 mass% or less and
Al: Contains 0.10 mass% or less, and
Ti: 0.05-0.5 mass%
In the hot rolling of the steel slab consisting of Fe and inevitable impurities, the balance is heated at 1150 ° C or less, the finishing temperature is Ar ₃ to (Ar ₃ + 100 ° C), 0.5 seconds after rolling Within a temperature range of 350 to 600 ° C, and when the strain at the final stand is E, and the average cooling rate up to 600 ° C is CR (° C / s), A method for producing a high-tensile hot-rolled steel sheet for processing having a microstructure with a ferrite grain size of 3.5 μm or less, which satisfies the relationship of the following formula (1):
A = 88CR ^-0.6 +0.2 (42.8 (mass% Si) −28.26 (mass% Mn) −190.6 (mass% C))
+9.8
E / log (d / A) = 0.630 to 3.0 --- (1)
Where d: austenite grain size (μm) before finish rolling

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