JP3965792B2 - Manufacturing method of steel sheet with excellent surface properties - Google Patents

Description

【００３２】
このブロックに対する酸化実験を、加熱装置を有するチャンバー内において行った。Ｎ₂ 雰囲気中において、サンプルを１２００〜１２９０℃に加熱した後、酸素分圧を１〜１０％の範囲で種々変化させた雰囲気ガスに置換し、３０〜９０分均熱してスケールを生成させた。なお、雰囲気ガスは、上記酸素のほか、ＣＯ₂ 分圧が７〜８％、Ｈ₂ Ｏ分圧が１４〜１８％で、残りはＮ₂ とした。
【００３３】
スケール生成後のサンプルを炉内から取り出した後、すぐに水冷することによって、均熱直後の粒状酸化物の生成状態を凍結した。水冷後のサンプルの断面検鏡を行い、粒状酸化物の深さを測定した。更に、スケール生成に伴う地鉄の減肉量（焼き減り量）も併せて測定した。
【００３４】
図１に、地鉄の焼減り量Ｘ₁(mm) におよぼす加熱温度Ｔ₁(℃) 、酸素分圧Ｐ₁(％) および均熱時間ｔ₁ （ｓ）の影響を示す。焼減り量Ｘ₁ は、下記(1) 式で回帰されることがわかる。
【００３５】
Ｘ₁ ＝3.05* 10 ² *exp(-1.63 * 10⁴/(T₁+273)) * P₁ ^1/4 *t₁ ^1/2 ・・・(1)
次に、断面検鏡によって粒状酸化物の深さを求めた。図２に、サンプルの断面表層部を模式的に示す。図２から、粒状酸化物が粒界酸化よりも深く生成している状態が確認された。
【００３６】
そこで、粒界酸化深さをｄ₁(μｍ) 、粒界酸化より更に深く生成している粒状酸化物の粒界酸化先端からの深さをζ₁(μｍ) として、地鉄表層からの粒状酸化物深さΖ₁(μｍ) を下記（２）式によって求めた。
【００３７】
Ζ₁ ＝ｄ₁ ＋ζ₁ ・・・・・・・・・(2)
ここで、分析により粒界酸化物は、主に FeOとFe₂SiO₄ であり、粒状酸化物は主にMnO であった。図３に、粒界酸化深さ (ｄ₁)におよぼす地鉄の焼き減り速度（∂X₁／∂t₁）の影響を示す。ここで、
∂X₁／∂t₁＝1.53* 10 ² *exp(-1.63 * 10⁴/(T₁+273)) * P₁ ^1/4 *t₁ ^{-1 /2}・・・(3)
である。粒界酸化深さは、地鉄の焼き減り速度が大きいほど小さくなり、下記(4) 式で回帰されることがわかる。
【００３８】
ｄ₁ ＝2.56 *（∂X₁／∂t₁）^-0.32 ・・・・・・(4)
図４に、粒界酸化先端からの粒状酸化物深さ (ζ₁)におよぼす地鉄の焼き減り速度（∂X₁／∂t₁）および鋼中Ｍｎの拡散係数Ｄ_Mn(cm²/s) の影響を示す。ここで、Ｍｎの拡散係数Ｄ_Mnは、下記(5) 式によって求められる。
【００３９】
Ｄ_Mn＝0.16* exp(-3.16 * 10⁴/(T₁+273)) ・・・・・・(5) 粒状酸化物の深さは、地鉄の焼き減り速度が大きく且つＭｎの拡散係数が小さいほど小さくなり、下記（６）式で回帰されることがわかる。
【００４０】
ζ₁ ＝2.98* 10¹⁰ * ((∂X₁／∂t₁）／Ｄ_Mn）^-1.57 ・・(6) 従って、地鉄表層からの粒状酸化物深さΖ₁(ゼータ）は、(4) 、(6) 式を(2) 式に代入して、下記(7) 式によって表される。
Ζ₁ ＝2.56* ( ∂X₁/ ∂t₁）^-0.32 ＋2.98* 10¹⁰ * ((∂X₁／∂t₁)/Ｄ_Mn) ^-1.57 ・・・・・(7)
ここで、(1) 式より、
ｔ₁ ＝（3.28* 10^-3 * X₁exp(1.63 * 10⁴/(T₁+273))* P₁ ^-1/4)²・・・・・(8)
(8) 式を(3) 式に代入して、
∂X₁／∂t₁＝4.67* 10 ⁴ *X₁ ^-1exp(-3.26 * 10⁴/(T₁+273)) * P₁ ^1/2 ・・・(9)
更に、地鉄の焼き減り量がＸ₁(mm) のときのスケール厚さをＬ₁(mm) とすると、Ｌ₁ は下記(10)式により計算できる。
【００４１】
Ｘ₁ ＝Ｌ₁(ρ_FeO / ρ_Fe) * ( Ｍ_Fe/ Ｍ_FeO ) ・・・・(10)ここで、ρ_Fe：Ｆｅの比重（≒7.8)、 ρ_FeO ：ＦｅＯの比重（≒5.57）
Ｍ_Fe：Ｆｅの原子量（≒56) 、Ｍ_FeO ：ＦｅＯの原子量（≒72）より、Ｘ₁ ＝０．５６* Ｌ₁ ・・・・・・(11)
(11)式を(9) 式に代入して、
∂X₁／∂t₁＝8.34* 10 ⁴ *L₁ ^-1 * exp(-3.26 * 10⁴/(T₁+273)) * P₁ ^1/2 ・・(12)
さらに、(5) 式および(12)式を(7) 式に代入して、

従って、(13)式より地鉄表層からの粒状酸化物深さΖ₁(μｍ) は、スケール厚さＬ₁(mm) 、加熱温度Ｔ₁(℃）および酸素分圧Ｐ₁(％）より予想することができる。
【００４２】
このような、粒状酸化物深さの予想式を用いて、加熱炉で生成する粒状酸化物深さを予想し、加熱炉の温度、酸素分圧、在炉時間を制御することによって、表面欠陥の発生を抑制することができる。
【００４３】
即ち、加熱炉内において、加熱炉出側の酸素分圧をＰ（％）、スラブが加熱炉から出た直後のスラブ表面温度をＴ（℃）、スケール厚さをＬ（ｍｍ）としたとき、下式によって求められるΖ（ゼータ）値の上限を１００に規定する。
【００４４】
Ζ＝２．５６* Ａ^-0.32 ＋２．９８* １０¹⁰ * （Ａ／Ｄ）^-1.57
但し、Ａ＝8.34* 10 ⁴ * L^-1 * exp(- 3.26* 10⁴/(T+273))* P^1/2
Ｄ＝0.16* exp(- 3.16* 10⁴/(T+273))
なお、スラブが加熱炉から出た直後のスケール厚さは、スラブ表面から機械的にスケールを剥離し、オフラインでスケール厚さを測定してもよいし、レーザ等を使用しオンラインで測定してもよい。また、スラブが加熱炉から出た直後の温度も放射温度計を使用しオンラインで測定してもよい。
【００４５】
スラブの熱間圧延時において、粗圧延終了後の粗バーを加熱装置で加熱し、地鉄のスケールオフ量を大にすれば、粗バー表層の粒状酸化物を除去することができ、これによって、鋼板の表面欠陥の発生を一段と抑制することができる。
【００４６】
上記粗バーの加熱に際し、その表面の上昇温度は３０〜１５０℃の範囲内とすることが好ましい。粗バー表面の上昇温度が３０℃未満では、所望の効果が得られず、一方、粗バー表面の上昇温度が１５０℃を超えると、その効果が飽和するだけでなく、歩留りの低下を招き不利になる。
【００４７】
仕上圧延終了温度は、Ａｒ₃ 以上とすることが必要である。仕上圧延終了温度がＡｒ₃ 未満では、鋼板の表層が粗大粒になり、加工性が著しく劣化する問題が生ずる。
【００４８】
【実施例】
次に、この発明を実施例により比較例と対比しながら説明する。
〔実施例１〕
表２に示す、本発明の範囲内の化学成分組成を有する鋼Ｎｏ．２〜２１を転炉にて溶製し、次いで、連続鋳造することによってスラブに鋳造した。
【００４９】
【表２】

【００５０】
このスラブを、表３に示す条件で加熱した後、熱間圧延を行い、供試体Ｎｏ．２〜２１を調製した。供試体Ｎｏ．２〜２１の各々について、１ｍ² 当たりの表面欠陥数をカウントし、その結果を同じく表３に示した。
【００５１】
【表３】

【００５２】
供試体Ｎｏ．２〜５は熱延酸洗鋼板、供試体Ｎｏ．６〜１６は冷延合金化溶融亜鉛めっき鋼板、供試体Ｎｏ．１７，１８は冷延溶融亜鉛めっき鋼板、供試体Ｎｏ．１９は冷延電気亜鉛めっき鋼板、供試体Ｎｏ．２０は熱延溶融亜鉛めっき鋼板、供試体Ｎｏ．２１は冷延鋼板である。
【００５３】
加熱および熱延条件が本発明の範囲内である本発明供試体Ｎｏ．２〜４、６、No. ８〜１０、１３〜１７、No. １９〜２１の表面欠陥個数はいづれも１個／ｍ² 以下であった。
【００５４】
これに対し、加熱炉から出た直後のスラブ表面温度が１３２０℃で本発明範囲を超えて高い比較供試体Ｎｏ．１１の表面欠陥個数は９個／ｍ² で多く、加熱炉から出た直後のスラブ表面温度が１１５０℃で本発明範囲を外れて低い比較供試体Ｎｏ．１２は、熱延最終温度が８６５℃でオーステナイト単相域を下回った。
【００５５】
図５に、加熱炉を出た直後のスラブ表面温度が１１７０〜１３００℃のときのΖ（ゼータ）値と表面欠陥個数との関係を示す。Ζ（ゼータ）値が１００以下で本発明の範囲内のときの表面欠陥個数は１個／ｍ² 以下である。これに対し、Ζ（ゼータ）値が１００を超えると、表面欠陥個数は７個／ｍ² 以上に増加する。
【００５６】
〔実施例２〕
表４に示す、本発明の範囲内の化学成分組成を有する鋼No. ２２〜２５を転炉にて溶製し、次いで、連続鋳造することによってスラブに鋳造した。
【００５７】
【表４】

【００５８】
このスラブに対し、表５に示す条件によって加熱した後、熱間圧延を行い、本発明供試体Ｎｏ．２２〜２５を調製した。供試体Ｎｏ．２２〜２５の各々について、１ｍ² 当たりの表面欠陥数をカウントし、その結果を、粗バーを再加熱しない本発明供試体No. ６と共に、同じく表５に示した。
【００５９】
【表５】

【００６０】
本発明供試体Ｎｏ．２２〜２５は、冷延合金化溶融亜鉛めっき鋼板である。表５から明らかなように、スラブを粗圧延した後、得られた粗バーを再加熱し、その表面温度を３０〜１５０℃上昇させた本発明供試体No. ２２の表面欠陥個数は１個／ｍ² であり、本発明供試体No. ２３〜２５の表面欠陥個数はいづれも０個／ｍ² であった。
【００６１】
【発明の効果】
以上述べたように、この発明によれば、極低炭素鋼スラブを熱間圧延する際に生ずるスケール性欠陥の発生を抑制し、表面性状の優れた鋼板を製造することができ、工業上有用な効果がもたらされる。
【図面の簡単な説明】
【図１】地鉄の焼き減り量Ｘ₁(mm) におよぼす加熱温度 T₁(℃) 、酸素分圧P₁(%) および均熱時間t₁(s) の影響を示した図である。
【図２】焼鈍後のサンプル断面表層部を模式的に示した図である。
【図３】実験室で焼鈍したときの、粒界酸化深さ（d₁) におよぼす地鉄の焼き減り速度( ∂X₁/ ∂t₁) の影響を示した図である。
【図４】実験室で焼鈍したときの、粒界酸化先端からの粒状酸化物深さ (ζ₁)におよぼす地鉄の焼き減り速度 (∂X₁/ ∂t₁) および鋼中Mnの拡散係数Ｄ_Mn(cm²/s) の影響を示した図である。
【図５】加熱炉から出た直後のスラブ表面温度が1170〜1300℃のときのΖ（ゼータ）値と表面欠陥個数との関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
TECHNICAL FIELD The present invention relates to a method for producing a steel plate using an ultra-low carbon steel plate as a base material used in automobiles, electrical equipment, etc., in particular, a hot-rolled steel plate having excellent surface properties, a cold-rolled steel plate, and a plated steel plate such as zinc. It is.
[0002]
[Prior art]
As a steel plate used for an outer plate of an automobile, an electric device or the like, extremely low carbon steel is used because high workability is required. With respect to the surface properties of such an ultra-low carbon steel sheet, defects caused by steel making, such as blowholes and sliver, have become a problem. For this reason, various studies have been made regarding steelmaking conditions. For example, “Nippon Steel Technical Report” 351 (1993), p59 includes a technique for preventing defects by using high-viscosity powder during continuous casting. It has been reported.
[0003]
On the other hand, with regard to hot ductility defects, it has been known that scale defects are generated due to the fact that slabs are heated at high temperatures in a heating furnace. 67 (1981), pS1128.
[0004]
Thus, by heating the slab at a low temperature, it is possible to suppress the occurrence of scale defects in the heating furnace, from the viewpoint of material, the finish rolling end temperature for the need arises to Ar ₃ or more, the Ar ₃ In the case of high ultra-low carbon steel, it is extremely difficult to ensure the finish rolling finish temperature.
[0005]
Regarding the reason for the occurrence of scale defects in the heating furnace, “Iron and Steel” 67 (1981), pS1128 reports that it is caused by grain boundary oxidation in the slab surface layer, but the mechanism of the defect generation is unknown. There are many points.
[0006]
Regarding ultra-low carbon steel, JP-A-8-41587 discloses that the composition of grain boundary oxidation is firelite (Fe ₂ SiO ₄ ). Therefore, by defining the amount of Si in the steel, the grain boundary oxidation depth is determined. A method (hereinafter, referred to as “prior art”) for greatly reducing the above is disclosed. In the above prior art, when the internal oxidation is connected to the scale of the surface layer, it exhibits the same form as the grain boundary oxidation. Therefore, the amount of O in the steel is also defined, and the grain boundary oxidation is generated or the internal oxidation is the surface layer. It is described that when it is connected to the scale, scale peelability is reduced at the time of descaling, and scale defects are generated.
[0007]
[Problems to be solved by the invention]
However, according to the above-described prior art, the occurrence of scale defects cannot be completely suppressed, and no radical solution has been achieved. Thus, the present condition is that the clue of the effective reduction countermeasure of the defect resulting from the scale produced when carrying out hot rolling by reheating extremely low carbon steel at high temperature cannot be found.
[0008]
Accordingly, an object of the present invention is to solve the above-mentioned problems and to provide a method for producing a steel sheet having excellent surface properties by suppressing the occurrence of scale defects that occur when hot rolling an extremely low carbon steel slab. There is.
[0009]
[Means for Solving the Problems]
In order to solve the above-mentioned problems and to develop a method for producing a steel sheet having excellent surface properties free of scale property defects, the present inventors firstly developed a scale property defect associated with high-temperature heating of a slab in a heating furnace. We investigated the cause of the occurrence. As a result, the cause of the generation of scale defects is not due to the descaling failure caused by the grain boundary oxidation of the slab surface layer, which has been considered in the past, but the granular oxide existing deeper from the slab surface layer than the grain boundary oxidation. That is, the mechanism that cracks occur during hot rolling due to the granular oxide on the surface layer, scales are generated there, and the generated scales become defects due to further biting. It was.
[0010]
As a result of advancing research based on the above, the present inventors have found that the generation of surface defects can be suppressed by controlling the depth of the granular oxide from the slab surface layer. The depth of the granular oxide from the slab surface depends on the atmosphere in the furnace (including oxygen partial pressure), the slab surface temperature, and the residence time at each temperature. It is very difficult to measure.
[0011]
Therefore, the slab surface temperature in the heating furnace must be estimated by the atmospheric temperature in the heating furnace, but even if the atmospheric temperature is the same, the slab surface temperature depends on the length of the slab and the arrangement of the slabs in the heating furnace. Since the surface temperatures of the slabs are different, it is difficult to accurately predict the surface temperature of the slab during heating. As a result of further research on this point, however, it was found that the granular oxide depth from the surface layer of the slab can be suppressed by defining the surface temperature and scale thickness of the slab immediately after leaving the heating furnace. It was.
[0012]
The present invention has been made on the basis of the above findings. The invention according to claim 1 is based on C: less than 0.010 wt.%, Si: 0.05 wt.% Or less, Mn: 0.1-2. 5 wt.%, P: 0.1 wt.% Or less, S: 0.03 wt.% Or less, sol. A steel having a composition containing Al: 0.01 to 0.1 wt.% And N: 0.01 wt.% Or less, with the balance being Fe and inevitable impurities is melted, and the steel is slabd by continuous casting. In the heating furnace, the oxygen partial pressure on the outlet side of the heating furnace is P (%), the surface temperature of the slab immediately after the slab has left the heating furnace is T (° C.), and the scale thickness is When L (mm) is set, the slab surface temperature (T) is within a range of 1170 to 1300 ° C., and heating is performed under conditions where the zeta value obtained by the following formula is 100 or less, and then hot rolling. It is characterized by finish rolling at a rolling finish temperature of Ar ₃ or higher.
[0013]
Ζ = 2.56 * A ^-0.32 +2.98 * 10 ¹⁰ * (A / D) ^-1.57
However, A = 8.34 * 10 ⁴ * L ^-1 * exp (-3.26 * 10 ⁴ / (T + 273)) * P ^1/2
D = 0.16 * exp (-3.16 * 10 ⁴ / (T + 273))
According to a second aspect of the present invention, the steel according to the first aspect further comprises Ti: 0.20 wt.% Or less, Nb: 0.10 wt.% Or less, V: 0.10 wt.% Or less, B: 0 It is characterized by having a chemical component composition containing one or more of 0.005 wt.% Or less.
[0014]
The invention according to claim 3 is characterized in that when the slab is heated in the heating furnace, the scale thickness and temperature of the slab are measured immediately after the slab comes out of the heating furnace. .
[0015]
The invention described in claim 4 is characterized in that when the slab is hot-rolled, the rough-rolled rough bar is heated to raise its surface by 30 to 150 ° C.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
The reason why the chemical composition of steel is limited as described above in the method of the present invention will be described below.
[0017]
C: C forms cementite and deteriorates workability. Further, when used as a cold rolled material, the deep drawability is significantly deteriorated. Therefore, the deterioration effect can be reduced by adding at least one of Ti, Nb, and V to steel to make C a precipitate. However, when the C content is 0.010 wt.% Or more, the amount of carbide increases, and the workability is remarkably deteriorated. Therefore, the C content should be limited to less than 0.010 wt.%.
[0018]
Si: Si has the effect of solid-solution strengthening ferrite without degrading workability and increasing the balance between strength and workability of the steel sheet. Therefore, an amount corresponding to the required strength level of the steel sheet is contained as a solid solution strengthening element. However, if the Si content exceeds 0.05 wt.%, The toughness and weldability are deteriorated. Therefore, the Si content should be limited to 0.05 wt.% Or less.
[0019]
Mn: Mn is an element effective for increasing the strength of a steel sheet. Therefore, an amount corresponding to the required strength level of the steel sheet is contained as a solid solution strengthening element. Furthermore, by fixing S in the steel as MnS, it has the effect of preventing slab cracking during hot rolling that occurs due to the grain boundary embrittlement of S. However, if the Mn content is less than 0.1 wt.%, A desired effect cannot be obtained in the above-described action. On the other hand, if the Mn content exceeds 2.5 wt.%, The weldability is deteriorated. Therefore, the Mn content should be limited to the range of 0.1 to 2.5 wt.%.
[0020]
P: P is an element effective for increasing the strength and corrosion resistance of a steel sheet. Therefore, the solid solution strengthening element contains an amount corresponding to the required strength level of the steel sheet. The lower limit of the P content is not particularly specified, but is preferably 0.01 wt.% Or more from the viewpoint of production cost. However, when the P content exceeds 0.1 wt.%, Segregation occurs at the grain boundaries and the secondary workability is deteriorated. Therefore, the P content should be limited to 0.1 wt.% Or less.
[0021]
S: S segregates at the grain boundary during hot rolling to cause slab cracking and may promote the generation of surface defects. Therefore, by adding Mn, S is fixed as MnS. However, excessive MnS serves as a starting point of the void during processing, thereby causing a reduction in ductility. In addition, when Ti is added, Ti-based sulfides are precipitated, but these precipitates are not only coarse and do not contribute to the increase in strength, but also serve as starting points for voids during processing. Invite. Therefore, the smaller the S content, the better. From this viewpoint, the S content should be limited to 0.03 wt.% Or less.
[0022]
sol. Al: sol. Al has an action of reducing inclusions in the steel as a deoxidizing element. However, sol. If the Al content is less than 0.01 wt.%, A desired effect cannot be obtained in the above-described action. On the other hand, sol. If the Al content exceeds 0.1 wt.%, Alumina inclusions increase and ductility decreases. Therefore, sol. The Al content should be limited to the range of 0.01 to 0.1 wt.
[0023]
If the N: N content exceeds 0.01 wt.%, Slab cracking may occur during hot rolling, and surface defects may occur. Therefore, the N content should be limited to 0.01 wt. The lower limit of the N content is not particularly defined, but is preferably 0.001 wt.% Or more from the viewpoint of manufacturing cost.
[0024]
In the present invention, in addition to the elements described above, one or more of the following elements may be contained as necessary.
Ti: Ti forms fine Ti-based carbonitrides, refines the structure, and has the effect of improving the strength of the steel sheet by precipitation strengthening. When used as a cold-rolled material, deep drawability can be improved by fixing solute C in steel as a carbide. In this case, addition of C amount or more in atomic ratio is necessary. is there. However, since the effect is saturated even if Ti is contained in excess of 0.2 wt.%, The upper limit is limited to 0.2 wt.%.
[0025]
Nb: Nb is also an element effective for refinement of the structure like Ti. In order to impart high strength without impairing workability, it is effective to refine the structure. Furthermore, it has the effect | action which improves the intensity | strength of a steel plate by formation of Nb type carbonitride. When used as a cold-rolled material, deep drawability can be improved by fixing solute C in steel as a carbide. In this case, addition of C amount or more in atomic ratio is necessary. is there. However, since the effect is saturated even if Nb is contained in excess of 0.10 wt.%, The upper limit is limited to 0.10 wt.%.
[0026]
V: V forms fine V-based carbonitrides, refines the structure, and has the effect of improving the strength of the steel sheet by precipitation strengthening. Therefore, the necessary amount is included according to the required strength level of the steel sheet. However, since the effect is saturated even if V is contained in excess of 0.10 wt.%, The upper limit is limited to 0.10 wt.%.
[0027]
B: Since B has an action of suppressing the release of strain during hot working, it refines the structure and improves the strength of the steel sheet, and segregates at the grain boundaries to improve secondary workability. Has an effect. However, even if B is contained in an amount exceeding 0.005 wt.%, Not only the fine graining effect is saturated, but also the roll load is increased due to accumulation of strain during hot rolling, making rolling extremely difficult. Therefore, the upper limit of the B content is limited to 0.005 wt.%.
[0028]
Next, the reason for limiting the heating and rolling conditions of the continuously cast slab as described above in the present invention will be described below.
When a continuously cast slab is heated in a heating furnace, if granular oxide is generated deep in the surface layer of the slab due to heating at a high temperature, cracks occur during hot rolling starting from that and a scale is generated in the cracked part. In addition, the generated scale further bites into the steel plate to cause surface defects.
[0029]
When the slab is heated so that the surface temperature immediately after coming out of the heating furnace exceeds 1300 ° C., the penetration depth of the granular oxide becomes large and not only surface defects occur, but also the scale-off amount. This increases the yield and decreases the energy cost. Therefore, the upper limit of the surface temperature of the slab immediately after leaving the heating furnace is set to 1300 ° C. Furthermore, from the viewpoint of the material, the final temperature of the finish rolling needs to be Ar ₃ or higher, so the lower limit of the slab heating temperature is defined as 1170 ° C.
[0030]
The depth of the granular oxide from the surface layer of the slab during heating was investigated by a slab oxidation experiment described below.
As a test steel, a specimen No. 1 in which a steel having a chemical composition shown in Table 1 was melted in a laboratory was used to prepare a 25 × 50 × 50 (mm) dice block.
[0031]
[Table 1]

[0032]
Oxidation experiments on this block were performed in a chamber with a heating device. After heating the sample to 1200 to 1290 ° C. in an N ₂ atmosphere, the oxygen partial pressure was replaced with an atmospheric gas with various changes in the range of 1 to 10%, and the temperature was soaked for 30 to 90 minutes to generate a scale. . In addition to the above oxygen, the atmospheric gas was a CO ₂ partial pressure of 7 to 8%, an H ₂ O partial pressure of 14 to 18%, and the remainder was N ₂ .
[0033]
The sample after the scale generation was taken out of the furnace and immediately cooled with water to freeze the granular oxide immediately after soaking. The sample after water cooling was subjected to cross-sectional microscopy, and the depth of the granular oxide was measured. Furthermore, the amount of metal thinning (burn-out amount) associated with scale generation was also measured.
[0034]
FIG. 1 shows the effects of the heating temperature T ₁ (° C.), the oxygen partial pressure P ₁ (%), and the soaking time t ₁ (s) on the burn-out amount X ₁ (mm) of the ground iron. It can be seen that the burn-out amount X ₁ is regressed by the following equation (1).
[0035]
X ₁ = 3.05 * 10 ² * exp (-1.63 * 10 ⁴ / (T ₁ +273)) * P ₁ ^1/4 * t ₁ ^1/2 (1)
Next, the depth of the granular oxide was determined by cross-sectional microscopy. FIG. 2 schematically shows a cross-sectional surface layer portion of the sample. From FIG. 2, it was confirmed that the granular oxide was generated deeper than the grain boundary oxidation.
[0036]
Therefore, the grain boundary oxidation depth is d ₁ (μm), and the depth from the grain boundary oxidation tip of the granular oxide generated deeper than the grain boundary oxidation is ζ ₁ (μm). The oxide depth Ζ ₁ (μm) was determined by the following equation (2).
[0037]
Ζ ₁ = d ₁ + ζ ₁ (2)
Here, according to the analysis, the grain boundary oxides were mainly FeO and Fe ₂ SiO ₄ , and the granular oxides were mainly MnO 2. FIG. 3 shows the influence of the burn-out rate (∂X ₁ / ∂t ₁ ) of the base iron on the grain boundary oxidation depth (d ₁ ). here,
_{∂X 1 / ∂t 1 = 1.53 *} 10 2 * exp (-1.63 * 10 4 / (T 1 +273)) * P 1 1/4 * t 1 -1 / 2 ··· (3)
It is. It can be seen that the grain boundary oxidation depth decreases with increasing burn-in rate of the steel, and is regressed by the following equation (4).
[0038]
d ₁ = 2.56 * (∂X ₁ / ∂t ₁ ) ^-0.32 (4)
Fig. 4 shows the rate of quenching of the iron base (∂X ₁ / 1t ₁ ) and the diffusion coefficient D _Mn (cm ² / s) of Mn in the steel, which affects the granular oxide depth (ζ ₁ ) from the grain boundary oxidation tip. ). Here, the diffusion coefficient D _Mn of _Mn is obtained by the following equation (5).
[0039]
D _Mn = 0.16 * exp (-3.16 * 10 ⁴ / (T ₁ +273)) (5) The depth of the granular oxide is high in the rate of burn-out of the steel and the diffusion coefficient of Mn It can be seen that the smaller the value is, the smaller it is, and the regression is made by the following equation (6).
[0040]
ζ ₁ = 2.98 * 10 ¹⁰ * ((∂X ₁ / ∂t ₁ ) / D _Mn ) ^-1.57・ ・ (6) Therefore, the granular oxide depth Ζ ₁ (zeta) from the surface layer is (4 ) And (6) are substituted into the equation (2) and expressed by the following equation (7).
Ζ ₁ = 2.56 * (∂X ₁ / ∂t ₁ ) ^-0.32 +2.98 * 10 ¹⁰ * ((∂X ₁ / ∂t ₁ ) / D _Mn ) ^-1.57 (7)
Here, from equation (1)
^{_{t 1 = (3.28 * 10 -3}} * X 1 exp (1.63 * 10 4 / (T 1 +273)) * P 1 -1/4) 2 ····· (8)
Substituting equation (8) into equation (3),
_{∂X 1 / ∂t 1 = 4.67 *} 10 4 * X 1 -1 exp (-3.26 * 10 4 / (T 1 +273)) * P 1 1/2 ··· (9)
Further, when the scale thickness when the burn-out amount of the ground iron is X ₁ (mm) is L ₁ (mm), L ₁ can be calculated by the following equation (10).
[0041]
_{X 1 = L 1 (ρ FeO} / ρ Fe) * Here _{(M Fe / M FeO) ····} (10), ρ Fe: Fe of specific gravity _{(≒ 7.8), ρ FeO:} FeO of specific gravity (≒ 5.57 )
From M _Fe : atomic weight of Fe (≈56) and M _FeO : atomic weight of FeO (≈72), X ₁ = 0.56 * L ₁ (11)
Substituting equation (11) into equation (9),
_{∂X 1 / ∂t 1 = 8.34 *} 10 4 * L 1 -1 * exp (-3.26 * 10 4 / (T 1 +273)) * P 1 1/2 ·· (12)
Furthermore, substituting Equation (5) and Equation (12) into Equation (7),

Therefore, from equation (13), the granular oxide depth Ζ ₁ (μm) from the surface layer of the steel is derived from the scale thickness L ₁ (mm), the heating temperature T ₁ (° C.) and the oxygen partial pressure P ₁ (%). Can be expected.
[0042]
By using the prediction formula of the granular oxide depth, the granular oxide depth generated in the heating furnace is predicted, and the surface defects are controlled by controlling the heating furnace temperature, oxygen partial pressure, and in-furnace time. Can be suppressed.
[0043]
That is, when the oxygen partial pressure on the outlet side of the heating furnace is P (%) in the heating furnace, the slab surface temperature immediately after the slab has left the heating furnace is T (° C.), and the scale thickness is L (mm). The upper limit of the zeta value obtained by the following equation is defined as 100.
[0044]
Ζ = 2.56 * A ^-0.32 +2.98 * 10 ¹⁰ * (A / D) ^-1.57
However, A = 8.34 * 10 ⁴ * L ^-1 * exp (-3.26 * 10 ⁴ / (T + 273)) * P ^1/2
D = 0.16 * exp (-3.16 * 10 ⁴ / (T + 273))
Note that the scale thickness immediately after the slab comes out of the heating furnace may be measured off-line by mechanically peeling the scale from the slab surface, or measured online using a laser or the like. Also good. Further, the temperature immediately after the slab comes out of the heating furnace may be measured online using a radiation thermometer.
[0045]
At the time of hot rolling of the slab, if the rough bar after the rough rolling is heated with a heating device and the scale-off amount of the base iron is increased, the granular oxide on the surface of the rough bar can be removed, thereby Moreover, generation | occurrence | production of the surface defect of a steel plate can be suppressed further.
[0046]
When heating the coarse bar, the rising temperature of the surface is preferably in the range of 30 to 150 ° C. If the rising temperature of the rough bar surface is less than 30 ° C., the desired effect cannot be obtained. On the other hand, if the rising temperature of the rough bar surface exceeds 150 ° C., the effect is not only saturated but also the yield is lowered. become.
[0047]
The finish rolling end temperature needs to be Ar ₃ or more. If the finish rolling finish temperature is less than Ar ₃ , the surface layer of the steel sheet becomes coarse and the workability deteriorates significantly.
[0048]
【Example】
Next, the present invention will be described by way of comparison with comparative examples.
[Example 1]
As shown in Table 2, steel Nos. Having chemical composition within the scope of the present invention. 2 to 21 were melted in a converter and then cast into a slab by continuous casting.
[0049]
[Table 2]

[0050]
After this slab was heated under the conditions shown in Table 3, it was hot-rolled to obtain a specimen no. 2-21 were prepared. Specimen No. The number of surface defects per 1 m ² was counted for each of 2 to 21, and the results are also shown in Table 3.
[0051]
[Table 3]

[0052]
Specimen No. Nos. 2 to 5 are hot-rolled pickled steel sheets, specimen Nos. 6 to 16 are cold-rolled alloyed hot-dip galvanized steel sheets, specimen Nos. 17 and 18 are cold-rolled hot-dip galvanized steel sheets; 19 is a cold-rolled electrogalvanized steel sheet, specimen No. No. 20 is a hot-rolled hot-dip galvanized steel sheet. 21 is a cold-rolled steel plate.
[0053]
Heating and hot rolling conditions are within the scope of the present invention. The number of surface defects of Nos. 2 to 4, 6, Nos. 8 to 10, 13 to 17, and Nos. 19 to 21 was 1 / m ² or less.
[0054]
On the other hand, the slab surface temperature immediately after coming out of the heating furnace is 1320 ° C., which exceeds the scope of the present invention, and is a high comparative specimen No. No. 11 has a large number of surface defects of 9 / m ² , and the slab surface temperature immediately after coming out of the heating furnace is 1150 ° C., which is out of the scope of the present invention and is a low comparative specimen No. No. 12 had a final hot rolling temperature of 865 ° C. and was below the austenite single phase region.
[0055]
FIG. 5 shows the relationship between the zeta value and the number of surface defects when the slab surface temperature immediately after leaving the heating furnace is 1170 to 1300 ° C. When the zeta value is 100 or less and is within the range of the present invention, the number of surface defects is 1 / m ² or less. On the other hand, when the zeta value exceeds 100, the number of surface defects increases to 7 / m ² or more.
[0056]
[Example 2]
Steel Nos. 22 to 25 having chemical composition within the scope of the present invention shown in Table 4 were melted in a converter and then cast into a slab by continuous casting.
[0057]
[Table 4]

[0058]
The slab was heated under the conditions shown in Table 5 and then hot-rolled. 22-25 were prepared. Specimen No. For each of Nos. 22 to 25, the number of surface defects per 1 m ² was counted, and the results are shown in Table 5 together with the specimen No. 6 of the present invention in which the coarse bar was not reheated.
[0059]
[Table 5]

[0060]
Specimen No. of the present invention. 22 to 25 are cold-rolled alloyed hot-dip galvanized steel sheets. As is apparent from Table 5, after rough rolling the slab, the obtained rough bar was reheated, and the surface temperature of the specimen No. 22 of the present invention in which the surface temperature was increased by 30 to 150 ° C. was one. / m ^2, the surface number of defects present invention specimen No. 23 to 25 are Izure was also 0 / m ^2.
[0061]
【The invention's effect】
As described above, according to the present invention, it is possible to suppress the generation of scale defects that occur when hot rolling an extremely low carbon steel slab, and to produce a steel sheet with excellent surface properties, which is industrially useful. Effect.
[Brief description of the drawings]
FIG. 1 is a graph showing the effects of heating temperature T ₁ (° C.), oxygen partial pressure P ₁ (%) and soaking time t ₁ (s) on the amount of burn-out X ₁ (mm) .
FIG. 2 is a diagram schematically showing a sample cross-sectional surface layer portion after annealing.
FIG. 3 is a graph showing the influence of the rate of quenching of the base iron (∂X ₁ / ∂t ₁ ) on the grain boundary oxidation depth (d ₁ ) when annealed in the laboratory.
[Fig.4] The rate of quenching of the iron base (∂ X ₁ / ∂t ₁ ) and the diffusion of Mn in steel on the granular oxide depth (ζ ₁ ) from the grain boundary oxidation tip when annealed in the laboratory It is the figure which showed the influence of coefficient D _Mn (cm < ² > / s).
FIG. 5 is a diagram showing the relationship between the zeta value and the number of surface defects when the slab surface temperature immediately after exiting the heating furnace is 1170 to 1300 ° C.

Claims (4)

C: Less than 0.010 wt.%, Si: 0.05 wt.% Or less, Mn: 0.1 to 2.5 wt.%, P: 0.1 wt.% Or less, S: 0.03 wt.% Or less, sol. Al: 0.01 to 0.1 wt.%, And N: 0.01 wt.% Or less, the steel of the composition consisting of Fe and unavoidable impurities as the balance ,
The steel is cast into a slab by continuous casting, and the obtained slab is heated in a heating furnace with an oxygen partial pressure P (%), and the slab surface temperature immediately after the slab exits the heating furnace is T ( ℃), when the scale thickness is L (mm), the slab surface temperature (T) is within the range of 1170 to 1300 ℃, and heating is performed under the condition that the zeta value obtained by the following formula is 100 or less. Then, a method for producing a steel sheet having excellent surface properties, which is hot-rolled and finish-rolled at a rolling finish temperature of Ar ₃ or higher.
Ζ = 2.56 * A ^-0.32 + 2.98 * 10 ¹⁰ * (A / D) ^-1.57
However, A = 8.34 * 10 ⁴ * L ^-1 * exp (-3.26 * 10 ⁴ / (T + 273)) * P ^1/2
D = 0.16 * exp (-3.16 * 10 ⁴ / (T + 273)) C: Less than 0.010 wt.%, Si: 0.05 wt.% Or less, Mn: 0.1 to 2.5 wt.%, P: 0.1 wt.% Or less, S: 0.03 wt.% Or less, sol. Al: 0.01 to 0.1 wt.%, N: 0.01 wt.% Or less, and Ti: 0.20 wt.% Or less, Nb: 0.10 wt.% Or less, V: 0.10 wt.% Or less, B: Melting steel having a composition of not more than 0.005 wt.% Or less, with the balance being Fe and inevitable impurities ,
The steel was cast into a slab by continuous casting, and the obtained slab was heated in a heating furnace with an oxygen partial pressure of P (%), and the slab surface temperature immediately after the slab exited the heating furnace was T ( ℃), when the scale thickness is L (mm), the slab surface temperature (T) is within the range of 1170 to 1300 ℃, and heating is performed under the condition that the zeta value obtained by the following formula is 100 or less. Then, a method for producing a steel sheet having excellent surface properties, which is hot-rolled and finish-rolled at a rolling finish temperature of Ar ₃ or higher.
Ζ = 2.56 * A ^-0.32 + 2.98 * 10 ¹⁰ * (A / D) ^-1.57
However, A = 8.34 * 10 ⁴ * L ^-1 * exp (-3.26 * 10 ⁴ / (T + 273)) * P ^1/2
D = 0.16 * exp (-3.16 * 10 ⁴ / (T + 273)) The steel sheet with excellent surface properties according to claim 1 or 2, wherein when the slab is heated in the heating furnace, the scale thickness and temperature of the slab are measured immediately after the slab comes out of the heating furnace. Method. The method for producing a steel sheet having excellent surface properties according to any one of claims 1 to 3, wherein when the slab is hot-rolled, the rough-rolled rough bar is heated and the surface thereof is raised by 30 to 150 ° C. .

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