JP2000045027A - Manufacture of steel plate excellent in surface characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inhibit the occurrence of defects in scale occurring at the time of hot rolling of an extra low carbon steel slab and to manufacture a steel plate excellent in surface characteristic. SOLUTION: A steel, which has a composition containing, by weight, <0.010% C, <=0.05% Si, 0.1-2.5% Mn, <=0.1% P, <=0.03% S, 0.01-0.1% sol.Al, and <=0.01% N and also containing, if necessary, at least one kind among <=0.20% Ti, <=0.10% Nb, <=0.10% V, and <=0.005% B, is cast into a slab by continuous casting. The resultant cast slab is heated in a heating furnace under the condition that, when P (%), T ( deg.C), and L (mm) represent partial pressure of oxygen on the outlet side of the heating furnace, slab surface temperature directly after the slab comes out of the heating furnace, and scale thickness, respectively, the value of Z determined by equation Z=2.56*A-0.32+2.98*1010*(A/D)-1.57 [where A=8.34*104*L-1*exp(-3.26*104/(T+273))*P1/2 and D=0.16*exp(-3.16*104/T+273)) are satisfied] becomes <=100 within the slab surface temperature (T) between 1,170 and 1,300 deg.C. Subsequently, the slab is hot rolled and then finish rolled at rolling finishing temperature not lower than Ar3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet used as a base material of an ultra-low carbon steel sheet for use in automobiles, electric equipment and the like, particularly, a hot rolled steel sheet, a cold rolled steel sheet, a zinc sheet, etc. having excellent surface properties. The present invention relates to a method for producing a plated steel sheet.

[0002]

2. Description of the Related Art Ultra-low carbon steel is used as a steel plate used for an outer plate of an automobile, an electric device, or the like, since high workability is required. With respect to the surface properties of such an ultra-low carbon steel sheet, defects caused by steelmaking, such as blowholes and slivers, which are relatively poor, have become a problem. For this purpose, various studies have been made on steelmaking conditions, and for example, Nippon Steel Technical Report 351 (199)
3), p59 reports a technique for preventing defects by using high-viscosity powder during continuous casting.

[0003] On the other hand, it has been known that a hot ductility defect causes a scale defect due to heating a slab at a high temperature in a heating furnace. Iron and Steel ”67 (1981), pS1
128 report that fact.

[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 Ar ₃
Since the above need arises, it is extremely difficult to secure the finish rolling end temperature in the case of extremely low carbon steel having a high Ar ₃ .

[0005] Regarding the reason for the occurrence of scale defects in a heating furnace, it is reported in "Iron and Steel" 67 (1981), pS1128, that it is caused by grain boundary oxidation on the surface layer of the slab. There are many unclear points about.

[0006] With respect to ultra-low carbon steel, Japanese Patent Application Laid-Open No. 8-41587.
In the gazette, the composition of the grain boundary oxidation is fayalite (Fe ₂ SiO
₄ ) Therefore, a method (hereinafter referred to as prior art) has been disclosed in which the amount of Si in steel is regulated to greatly reduce the depth of grain boundary oxidation. In the above prior art, since the internal oxidation also exhibits the same form as the grain boundary oxidation when connected to the scale of the surface layer, the amount of O in the steel is also specified, and the grain boundary oxidation is generated or the internal oxidation is reduced to the surface layer. It is described that when connected to a scale of No. 3, the scale peeling property is reduced at the time of descaling, and a scale defect occurs.

[0007]

However, according to the above prior art, the occurrence of scale defects cannot be completely suppressed, and a drastic solution has not been achieved. As described above, at the present time, there is no clue for effective measures for reducing defects caused by scales generated when re-heating ultra-low carbon steel at a high temperature and hot rolling.

Accordingly, an object of the present invention is to solve the above-mentioned problems, to suppress the occurrence of scale defects that occur when hot rolling a very low carbon steel slab, and to produce a steel sheet having excellent surface properties. Is to provide.

[0009]

In order to solve the above-mentioned problems and to develop a method for producing a steel sheet having excellent surface properties without scale defects, the present inventors first put a slab in a heating furnace. The cause of the scale defect caused by high-temperature heating was investigated. As a result, the cause of the scale defect is not due to the conventionally considered poor descaling caused by the grain boundary oxidation of the slab surface layer.
Cracking occurred during hot rolling due to the granular oxide existing deeper from the slab surface layer than the grain boundary oxidation, that is, due to the granular oxide on the surface layer, scale was generated and generated there The mechanism by which the scale becomes defective due to further bite was revealed.

The present inventors have conducted research based on the above, and as a result, it has been found that the generation of surface defects can be suppressed by controlling the depth of the granular oxide from the surface layer of the slab. The depth of the granular oxide from the surface of the slab is determined by the atmosphere (including oxygen partial pressure) in the furnace, the surface temperature of the slab, and the residence time at each temperature. It is very difficult to measure.

Therefore, the slab surface temperature in the heating furnace is:
Although it must be estimated based on the ambient temperature in the heating furnace, even if the ambient temperature is the same, the surface temperature of the slab differs depending on the length of the slab and the arrangement of the slabs in the heating furnace. It is difficult to accurately predict the surface temperature. Further research on this point has shown, however, that by defining the surface temperature and scale thickness of the slab immediately after leaving the heating furnace, the granular oxide depth from the slab surface layer can be suppressed. Was.

The present invention has been made based on the above findings, and the invention according to claim 1 has a C: 0.01 value.
Less than 0 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: A steel containing 0.01 wt.% Or less is melted, and the steel is cast into a slab by continuous casting.
In the heating furnace, the oxygen partial pressure on the exit side of the heating furnace is P (%), and the slab surface temperature immediately after the slab has left the heating furnace is T.
(° C.) and the thickness of the scale is L (mm), the slab surface temperature (T) is in the range of 1170 to 1300 ° C., and the Ζ (zeta) value obtained by the following equation is 10:
It is characterized in that it is heated under conditions of 0 or less, then hot-rolled, and finish-rolled at a rolling end temperature of Ar ₃ or more.

[0013] ^{Ζ = 2.56 * A -0.32 +2.98 *} 10
^{10 *} (A / D) ^-1.57, where 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 the invention described in claim 2, the steel described in claim 1 further includes Ti: 0.20 wt.%. Hereinafter, it is characterized in that it has a chemical component composition containing one or more of Nb: 0.10 wt.% Or less, V: 0.10 wt.% Or less, and B: 0.005 wt.% Or less. Have

According to a third aspect of the present invention, when the slab is heated in the heating furnace, the scale thickness and the temperature of the slab are measured immediately after the slab comes out of the heating furnace. Things.

According to a fourth aspect of the present invention, when the slab is hot-rolled, the rough-rolled coarse bar is heated to raise its surface by 30 to 150 ° C.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The reason for limiting the chemical composition of steel in the method of the present invention as described above will be described below.

C: C forms cementite and deteriorates workability. Further, when used as a cold rolled material, deep drawability is significantly deteriorated. Therefore, by adding at least one of Ti, Nb, and V to steel to form C as a precipitate, the above-described deterioration effect can be reduced. However, when the C content is 0.010 wt.% Or more, the amount of carbides increases and the workability deteriorates significantly. Therefore, the C content should be limited to less than 0.010 wt.%.

Si: Si has the effect of solid-solution strengthening ferrite without deteriorating the workability and increasing the balance between the strength of the steel sheet and the workability. Therefore, an amount corresponding to the required steel sheet strength level is contained as a solid solution strengthening element. However, when the Si content exceeds 0.05 wt.%, Toughness and weldability are deteriorated. Therefore, S
The i content should be limited to 0.05 wt.% or less.

Mn: Mn is an element effective for increasing the strength of a steel sheet. Therefore, an amount corresponding to the required steel sheet strength level is contained as a solid solution strengthening element. Further, by fixing S in the steel as MnS, it has an effect of preventing slab cracks at the time of hot rolling caused by the grain boundary embrittlement of S. However, when the Mn content is 0.1.
If the amount is less than 1 wt.%, Desired effects cannot be obtained in the above-described operation. On the other hand, if the Mn content exceeds 2.5 wt.%, The weldability deteriorates. Therefore, the Mn content is 0.1 to 2.5 w
It should be limited to the range of t.%.

P: P is an element effective for enhancing the strength and corrosion resistance 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. P
The lower limit of the content is not particularly limited, but is preferably 0.01 wt.% Or more from the viewpoint of production cost. However, if the P content exceeds 0.1 wt.%, It segregates at the grain boundaries and deteriorates the secondary workability. Therefore, the P content is 0.1.
It should be limited to 1 wt.% Or less.

S: S may segregate at the grain boundaries during hot rolling to cause slab cracking and accelerate the generation of surface flaws. Therefore, by adding Mn, S
Is fixed as MnS, but excessive MnS becomes a starting point of a void at the time of processing, so that the ductility is reduced.
When Ti is added, Ti-based sulfide precipitates, but this precipitate is not only coarse and does not contribute to an increase in strength, but also serves as a starting point of voids during processing, so that the ductility of the steel sheet decreases. Invite. Therefore, the smaller the content of S, the better. From this viewpoint, the content of S should be limited to 0.03 wt.% Or less.

Sol. Al: sol. Al has a function of reducing inclusions in steel as a deoxidizing element. However, sol. If the Al content is less than 0.01 wt.%, Desired effects cannot be obtained in the above-described operation. On the other hand,
l. If the Al content exceeds 0.1 wt.%, Alumina-based inclusions increase and ductility decreases. Therefore, sol. The Al content should be limited to the range of 0.01-0.1 wt.%.

N: If the N content exceeds 0.01 wt.%, Slab cracks are caused during hot rolling, and surface flaws may occur. Therefore, the N content should be limited to 0.01 wt.% Or less. Although the lower limit of the N content is not particularly limited, from the viewpoint of manufacturing cost, the lower limit is 0.001 w.
It is preferably at least t.%.

In the present invention, one or more of the following elements may be contained, if necessary, in addition to the above-mentioned elements. Ti: Ti forms fine Ti-based carbonitrides, refines the structure, and has the effect of improving the strength of the steel sheet by strengthening its precipitation. When used as a cold-rolled material, deep drawability can be improved by fixing solid solution C in steel as carbide, and in this case, it is necessary to add C amount or more in atomic ratio. is there. However, if the content of Ti exceeds 0.2 wt.%, The effect is saturated, so the upper limit is limited to 0.2 wt.%.

Nb: Like Nb, Nb is also an effective element for refining the structure. In order to impart high strength without impairing workability, it is effective to make the structure finer. Furthermore, N
The formation of the b-based carbonitride has an effect of improving the strength of the steel sheet. When used as a cold-rolled material, deep drawability can be improved by fixing solid solution C in steel as carbide, and in this case, it is necessary to add C amount or more in atomic ratio. is there. However, the effect is saturated even if Nb is contained in excess of 0.10 wt.%, So the upper limit is limited to 0.10 wt.%.

V: V forms fine V-based carbonitrides,
It has the effect of refining the structure and improving the strength of the steel sheet by its precipitation strengthening. Therefore, a necessary amount is contained according to the required strength level of the steel sheet. However, the effect is saturated even if V is contained in excess of 0.10 wt.%, So the upper limit is limited to 0.10 wt.%.

B: Since B has the effect 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 the secondary workability. Has the effect of improving However, B
If the content exceeds 0.005 wt.%, Not only the grain refining effect is saturated, but also the roll load is increased due to the accumulation of strain during hot rolling, making the rolling extremely difficult. Therefore, the upper limit of the B content is limited to 0.005 wt.%.

Next, the reason why the heating and rolling conditions of the continuously cast slab in the present invention are limited as described above will be described below. When a continuously cast slab is heated in a heating furnace, high-temperature heating produces granular oxide deep in the surface of the slab, starting at which cracks occur during hot rolling and forming scale at the cracked portions. However, the generated scale further bites into the steel sheet, causing surface defects.

When the slab is heated so that the surface temperature immediately after leaving the heating furnace exceeds 1300 ° C., the penetration depth of the particulate oxide becomes large, and not only the surface defects are generated, but also the surface defects are generated. An increase in the scale-off amount causes a decrease in the yield, and also increases the energy cost. Therefore, the upper limit of the surface temperature of the slab immediately after leaving the heating furnace is 1300 ° C.
And Further, from the viewpoint of the material, since the final temperature of the finish rolling needs to be Ar ₃ or more, the lower limit of the slab heating temperature is set to 1170 ° C.

The depth of the particulate oxide from the surface of the slab during heating was investigated by a slab oxidation experiment described below. As a test steel, a test piece No. 1 obtained by melting a steel having a chemical composition shown in Table 1 in a laboratory was used, and a test piece of 25 × 50 ×
A 50-mm dice-shaped block was prepared.

[0031]

[Table 1]

The oxidation experiment for this block was performed using a heating device.
The test was performed in a chamber having an apparatus. N_TwoIn the atmosphere
In, the sample was heated to 1200 to 1290 ° C.
Then, the atmosphere in which the oxygen partial pressure was variously changed in the range of 1 to 10%.
Replace with gaseous gas and soak for 30 to 90 minutes to generate scale
I let it. The atmosphere gas is CO 2 in addition to the above oxygen. _Two
7-8% partial pressure, H_TwoO partial pressure is 14-18% and the rest is
N_TwoAnd

After the sample after the scale formation was taken out of the furnace, it was immediately cooled with water to freeze the state of formation of the particulate oxide immediately after the soaking. The cross-section of the sample after water cooling was examined, and the depth of the particulate oxide was measured. Furthermore, the amount of reduction in the thickness of the local iron (the amount of loss from burning) due to scale formation was also measured.

FIG. 1 shows the influence of the heating temperature T ₁ (° C.), the oxygen partial pressure P ₁ (%) and the soaking time t ₁ (s) on the burnout amount X ₁ (mm) of the base iron. Baked reduction rate X ₁ is found to be regression equation (1) below.

X ₁ = 3.05 ^* 10 ^{2 *} exp (-1.63 ^* 10 ⁴ / (T ₁ +273)) ^* P ₁ ^{1/4 *} t ₁ ^1/2 (1) The depth of the granular oxide was determined by the above. 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.

Therefore, the depth of the grain boundary oxidation is d ₁ (μm), and the depth of the granular oxide generated deeper than the grain boundary oxidation from the tip of the grain boundary oxidation is ζ ₁ (μm). particulate oxides from depth Zeta ₁ a ([mu] m) was determined by the following equation (2).

Ζ ₁ = d ₁ + ζ ₁ (2) Here, the analysis shows that the grain boundary oxides are mainly FeO and Fe ₂ SiO ₄
And the particulate oxide was mainly MnO 2. 3, baked losing rate of the base steel on grain boundary oxidation depth (d ₁₎ (∂X ₁ /
影響 t ₁ ). Where ∂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) is there. It can be seen that the grain boundary oxidation depth decreases as the burning rate of the base iron increases, and is regressed by the following equation (4).

D ₁ = 2.56 ^* (∂X ₁ / ∂t ₁ ) ^-0.32 (4) FIG. 4 shows the ^effect of the ground on the depth of the granular oxide (ζ ₁ ) from the tip of the grain boundary oxidation. grilled reduced rate of iron (∂X ₁ / ∂t ₁₎ and steel in M
3 shows the influence of the diffusion coefficient D _Mn (cm ² / s) on n. Where Mn
The diffusion coefficient D _Mn, determined by the following equation (5).

D _Mn = 0.16 ^* exp (-3.16 ^* 10 ⁴ / (T ₁ +273)) (5) The depth of the particulate oxide is such that the burning rate of the ground iron is large and Mn is Mn. It can be seen that the smaller the diffusion coefficient is, the smaller the diffusion coefficient is, and the regression is performed by the following equation (6).

Ζ ₁ = 2.98 ^* 10 ^{10 *} ((∂X ₁ / ∂t ₁ ) / D _Mn ) ^-1.57 ... (6) Accordingly, the depth of the granular oxide from the surface layer of the base iron Ζ ₁ ( Zeta)
Is expressed by the following equation (7) by substituting the equations (4) and (6) into the equation (2). Ζ ₁ = 2.56 ^* (∂X ₁ / ∂t ₁ ) ^-0.32 +2.98 ^* 10 ^{10 *} ((∂X ₁ / ∂t ₁ ) / D _Mn ) ^-1.57 ... (7) where (1) from the ^{_{equation, t 1 = 3.28 * 10 -3}} * X 1 exp (1.63 * 10 4 / (T 1 +273)) * P 1 -1/4) 2 ······ (8) ( 8) Substituting equation (3) 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, assuming that the scale thickness when the amount of burnout of the ground iron is X ₁ (mm) is L ₁ (mm), L ₁ can be calculated by the following equation (10).

X ₁ = L ₁ (ρ _FeO / ρ _Fe ) ^* (M _Fe / M _FeO ) (10) Here, ρ _Fe : specific gravity of Fe (≒ 7.8), ρ _FeO : FeO
X ₁ = 0.56 ^* L ₁ (11) (11) From the specific gravity of M _Fe : Fe atomic weight (≒ 56) and M _FeO : FeO atomic weight (≒ 72) Substituting the equation 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 equations (5) and (12) into equation (7), 、 ₁ = 2.56 ^* (8.34 ^* 10 ^{4 *} L ₁ ^{-1 *} exp (-3.26 ^* 10 ⁴ / (T ₁ +273)) ^* P ₁ ^1/2 ) ^-0.32 +2.98 ^* 10 ^{10 *} ((8.34 ^* 10 ^{4 *} L ₁ ^{-1 *} exp (-3.26 ^* 10 ⁴ / (T ₁ +273)) ^* P ₁ ^1/2 ) / (0.16 ^* exp (-3.16 ^* 10 ⁴ / (T ₁ +273)))) ^-1.57・ ・ ・ ・ ・ ・ ・ (13) Granular oxide depth from
₁ (μm) is scale thickness L ₁ (mm), heating temperature T ₁ (° C)
And the oxygen partial pressure P ₁ (%).

By using such a prediction formula of the depth of the granular oxide, the depth of the granular oxide generated in the heating furnace is predicted, and the temperature, the oxygen partial pressure, and the furnace time of the heating furnace are controlled. In addition, generation of surface defects can be suppressed.

That is, in the heating furnace, the oxygen partial pressure on the exit side of the heating furnace is P (%), 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 Ζ (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, where 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 scale thickness immediately after the slab comes out of the heating furnace is
The scale may be mechanically peeled off from the slab surface and the scale thickness may be measured off-line, or may be measured on-line using a laser or the like. Also, the temperature immediately after the slab has left the heating furnace may be measured online using a radiation thermometer.

During the hot rolling of the slab, the coarse bar after the completion of the rough rolling is heated by a heating device to increase the scale-off amount of the ground iron, so that the particulate oxide on the surface layer of the rough bar can be removed. Thereby, generation of surface defects of the steel sheet can be further suppressed.

The heating temperature of the surface of the coarse bar is preferably in the range of 30 to 150 ° C.
If the rise temperature of the rough bar surface is less than 30 ° C., the desired effect cannot be obtained. On the other hand, if the rise temperature of the rough bar surface exceeds 150 ° C., not only the effect is saturated, but also the yield is reduced, which is disadvantageous. become.

The finish rolling end temperature must be Ar ₃ or higher. When the finish rolling end temperature is lower than Ar ₃ ,
There is a problem that the surface layer of the steel sheet becomes coarse and the workability is significantly deteriorated.

[0048]

Next, the present invention will be described with reference to examples and comparative examples. Example 1 As shown in Table 2, steel No. 1 having a chemical composition within the range of the present invention was obtained. 2 to 21 were melted in a converter, and then cast into slabs by continuous casting.

[0049]

[Table 2]

After the slab was heated under the conditions shown in Table 3, hot rolling was performed. 2-21 were prepared. Specimen No. For each of 2 to 21, it counts the number of surface defects per 1 m ^2, lists the results also shown in Table 3.

[0051]

[Table 3]

Specimen No. Nos. 2 to 5 are hot-rolled pickled steel sheets; Nos. 6 to 16 are cold-rolled alloyed hot-dip galvanized steel sheets. 17 and 18 are cold-rolled hot-dip galvanized steel sheets; 19 is a cold-rolled galvanized steel sheet;
20 is a hot-rolled hot-dip galvanized steel sheet; 21 is a cold rolled steel plate.

Heating and hot rolling conditions are within the scope of the present invention.
Inventive specimen No. 2-4, 6, No. 8-10, 13
Nos. 1 to 17 and No. 19 to 21
/ M ^TwoIt was below.

On the other hand, the slab surface temperature immediately after leaving the heating furnace was 1320 ° C., which was higher than the range of the present invention. No. 11 has a large number of surface defects of 9 / m ² , and the slab surface temperature immediately after leaving the heating furnace is 1150 ° C., which is out of the range of the present invention. In No. 12, the final temperature of hot rolling was 865 ° C., which was lower than the austenite single phase region.

FIG. 5 shows the relationship between the Ζ (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 Ζ (zeta) value is 100 or less and within the range of the present invention, the number of surface defects is 1 / m ²
It is as follows. On the other hand, when the Ζ (zeta) value exceeds 100, the number of surface defects increases to 7 / m ² or more.

[Example 2] Steel Nos. 22 to 25 having a chemical composition within the range 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]

The slab was heated under the conditions shown in Table 5 and then subjected to hot rolling. 2
2 to 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 test sample No. 6 of the present invention in which the coarse bar was not reheated.

[0059]

[Table 5]

The test piece of the present invention No. 22 to 25 are cold-rolled alloyed hot-dip galvanized steel sheets. As is clear from Table 5, after the slab was roughly rolled, the obtained coarse bar was reheated, and the surface temperature of the specimen No. 22 of the present invention No. 22 whose surface temperature was increased by 30 to 150 ° C. was 1 piece. / M ² , and the number of surface defects in each of the specimens Nos. 23 to 25 of the present invention was 0 / m ² .

[0061]

As described above, according to the present invention,
It is possible to suppress the occurrence of scale defects that occur when hot rolling an ultra-low carbon steel slab, to manufacture a steel sheet having excellent surface properties, and to obtain an industrially useful effect.

[Brief description of the drawings]

[Fig. 1] Heating temperature on the amount of burnout X ₁ (mm) of ground iron
FIG. 3 is a graph showing the influence of T ₁ (° C.), oxygen partial pressure P ₁ (%), and soaking time t ₁ (s).

FIG. 2 is a view schematically showing a surface layer portion of a sample section after annealing.

Fig. 3 Grain boundary oxidation depth when annealed in the laboratory (d ₁ )
FIG. 3 is a diagram showing the effect of the rate of burnout of the local iron (∂X ₁ / 地 t ₁ ) on the temperature.

[Fig. 4] The rate of burnout of ground iron on the depth of granular oxide (ζ ₁ ) from the grain boundary oxidation tip when annealed in the laboratory
FIG. 3 is a graph showing the influence of (∂X ₁ / ∂t ₁ ) and the diffusion coefficient D _Mn (cm ² / s) of Mn in steel.

FIG. 5: Slab surface temperature immediately after leaving the heating furnace is 1170-
FIG. 3 is a diagram showing the relationship between the Ζ (zeta) value and the number of surface defects at 1300 ° C.

────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] October 12, 1998 (1998.10.
12)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claim 1

[Correction method] Change

[Correction contents]

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] Claim 2

[Correction method] Change

[Correction contents]

[Procedure amendment 3]

[Document name to be amended] Statement

[Correction target item name] 0013

[Correction method] Change

[Correction contents]

[0013] ^{Ζ = 2.56 * A -0.32 +2.98 *} 10
¹⁰ * (A / D) ^-1.57 where 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 the invention described in claim 2, the steel described in claim 1 further includes Ti: 0.20 wt.%. Hereinafter, it is characterized in that it has a chemical component composition containing one or more of Nb: 0.10 wt.% Or less, V: 0.10 wt.% Or less, and B: 0.005 wt.% Or less. Have

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0035

[Correction method] Change

[Correction contents]

X ₁ = 3.05 * 10 ² * exp (-1.63 * 10 ⁴ / (T ₁ +27
3)) * P ₁ ^1/4 * t ₁ ^1/2 (1) Next, the depth of the granular oxide was determined by a sectional microscope. 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.

[Procedure amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0037

[Correction method] Change

[Correction contents]

Ζ ₁ = d ₁ + ζ ₁ ...
(2) Here, the analysis shows that the grain boundary oxides are mainly FeO and Fe ₂ SiO ₄
And the particulate oxide was mainly MnO 2. 3, baked losing rate of the base steel on grain boundary oxidation depth (d ₁₎ (∂X ₁ /
影響 t ₁ ). Where ∂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 as the burning rate of the base iron increases, and is regressed by the following equation (4).

[Procedure amendment 6]

[Document name to be amended] Statement

[Correction target item name] 0038

[Correction method] Change

[Correction contents]

D ₁ = 2.56 * (∂X ₁ / ∂t ₁ ) ^-0.32 ...
... (4) Fig. 4 shows the rate of burn-off (地 X ₁ / ∂t ₁ ) and M in steel on the depth of granular oxide (ζ ₁ ) from the grain boundary oxidation tip.
3 shows the influence of the diffusion coefficient D _Mn (cm ² / s) on n. Where Mn
The diffusion coefficient D _Mn, determined by the following equation (5).

[Procedure amendment 7]

[Document name to be amended] Statement

[Correction target item name] 0039

[Correction method] Change

[Correction contents]

D _Mn = 0.16 * exp (-3.16 * 10 ⁴ / (T ₁ +273))
(5) It can be seen that the depth of the granular oxide becomes smaller as the burning rate of the base iron is larger and the diffusion coefficient of Mn is smaller, and the depth is regressed by the following equation (6).

[Procedure amendment 8]

[Document name to be amended] Statement

[Correction target item name] 0040

[Correction method] Change

[Correction contents]

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 of the base iron can be obtained by substituting equations (4) and (6) into equation (2), ) Expression. Ζ ₁ = 2.56 * (∂X ₁ / ∂t ₁ ) ^-0.32 +2.98 * 10 ¹⁰ * ((∂X ₁ / ∂t ₁ ) / D _Mn ) ^-1.57・ ・ ・ ・ ・ (7) (1) from the _{equation, t 1 = (3.28 * 10} -3 * X 1 exp (1.63 * 10 4 / (T 1 +273)) * P 1 -1/4) 2 ····· (8) ( 8) Substituting equation (3) 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, assuming that the scale thickness when the amount of burnout of the ground iron is X ₁ (mm) is L ₁ (mm), L ₁ can be calculated by the following equation (10).

[Procedure amendment 9]

[Document name to be amended] Statement

[Correction target item name] 0041

[Correction method] Change

[Correction contents]

X ₁ = L ₁ (ρ _FeO / ρ _Fe ) * (M _Fe / M
_FeO ) ··· (10) where ρ _Fe : specific gravity of Fe (≒ 7.
8), ρ _FeO : Specific gravity of FeO (≒ 5.57) M _Fe : From the atomic weight of Fe (≒ 56) and M _FeO : the 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 equations (5) and (12) into equation (7), 、 ₁ = 2.56 * (8.34 * 10 ⁴ * L ₁ ^-1 * exp (-3.26 * 10 ⁴ / (T ₁ +273)) * P ₁ ^1/2 ) ^-0.32 +2.98 * 10 ¹⁰ * ((8.34 * 10 ⁴ * L ₁ ^-1 * exp (-3.26 * 10 ⁴ / (T ₁ +273)) * P ₁ ^1/2 ) / (0.16 * exp (-3.16 * 10 ⁴ / (T ₁ +273)))) ^-1.57 From equation (13), the depth of the granular oxide from the surface of the ground ironΖ
₁ (μm) is scale thickness L ₁ (mm), heating temperature T ₁ (° C)
And the oxygen partial pressure P ₁ (%).

[Procedure amendment 10]

[Document name to be amended] Statement

[Correction target item name] 0044

[Correction method] Change

[Correction contents]

[0044] ^{Ζ = 2.56 * A -0.32 +2.98 *} 10
¹⁰ * (A / D) ^-1.57 where 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 scale thickness immediately after the slab comes out of the heating furnace is
The scale may be mechanically peeled off from the slab surface and the scale thickness may be measured off-line, or may be measured on-line using a laser or the like. Also, the temperature immediately after the slab has left the heating furnace may be measured online using a radiation thermometer.

[Procedure amendment 11]

[Document name to be amended] Statement

[Correction target item name] 0057

[Correction method] Change

[Correction contents]

[0057]

[Table 4]

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/06 C22C 38/06 38/14 38/14 (72) Inventor Yoshihide Ishii Marunouchi, Chiyoda-ku, Tokyo 1-2, Nihon Kokan Co., Ltd. (72) Inventor Jun Taniai 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Isao Mori 1-1, Marunouchi, Chiyoda-ku, Tokyo No. 2 Nihon Kokan Co., Ltd. F term (reference) 4E002 AA05 AA07 AD02 AD04 BD02 BD08 CB03

Claims (4)

[Claims] 1. C: less than 0.010 wt.%, Si: 0.0
5 wt.% Or less, Mn: 0.1 to 2.5 wt.%, P: 0.1 w
t.% or less, S: 0.03 wt.% or less, sol. Al: 0.
Steel containing 0.1 to 0.1 wt.% And N: 0.01 wt.% Or less is melted, and the steel is cast into a slab by continuous casting, and the obtained slab is heated in a heating furnace. Outgoing oxygen partial pressure is P
(%), When the slab surface temperature immediately after the slab comes out of the heating furnace is T (° C.) and the scale thickness is L (mm), the slab surface temperature (T) is in the range of 1170 to 1300 ° C. Is heated under the condition that the Ζ (zeta) value obtained by the following formula is 100 or less, then hot-rolled, and Ar
_A method for producing a steel sheet having excellent surface properties, comprising finish rolling at a rolling end temperature of ₃ or more. ^{Ζ = 2.56 * A -0.32 +2.98 *} 10 10 * (A /
D) ^-1.57 where 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)) 2. C: less than 0.010 wt.%, Si: 0.0
5 wt.% Or less, Mn: 0.1 to 2.5 wt.%, P: 0.1 w
t.% or less, S: 0.03 wt.% or less, sol. Al: 0.
01 to 0.1 wt.%, N: 0.01 wt.% Or less, and T
i: 0.20 wt.% or less, Nb: 0.10 wt.% or less, V:
A steel containing one or more kinds of 0.10 wt.% Or less and B: 0.005 wt.% Or less is melted, and the steel is cast into a slab by continuous casting. In the above, when the oxygen partial pressure on the exit side of the heating furnace is P (%), the slab surface temperature immediately after the slab exits the heating furnace is T (° C.), and the scale thickness is L (mm), When (T) is in the range of 1170 to 1300 ° C., the material is heated under the condition that the Ζ (zeta) value obtained by the following equation is 100 or less, then hot-rolled, and finish-rolled at a rolling end temperature of Ar ₃ or more. A method for producing a steel sheet having excellent surface properties. ^{Ζ = 2.56 * A -0.32 +2.98 *} 10 10 * (A /
D) ^-1.57 where 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)) 3. A scale thickness and a temperature of the slab are measured immediately after the slab has left the heating furnace when the slab is heated in the heating furnace.
3. The method for producing a steel sheet having excellent surface properties according to item 1. 4. The excellent surface texture according to claim 1, wherein, during hot rolling of the slab, the coarsely-rolled coarse bar is heated to raise the surface thereof by 30 to 150 ° C. Steel sheet manufacturing method.