JPH1177269A - Continuous casting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method, by which crack on a continuously cast slab is not developed and not only V segregation and reverse V segregation but also spotting segregation developed in a liquid phase space part remained in the solidified end stage part can be improved. SOLUTION: In a continuous casting method for drawing out the continuously cast steel slab while executing the rolling reduction between at least one pair faced rolls, in the zone from the position where solid phase ratio at the center part of the cast slab becomes 0.1-0.4, to the arbitrary position where the ratio becomes in the range of 0.8-0.9, the rolling reduction is executed to the cast slab so as to compensate the total solidified shrinkage quantity in this zone. In the zone after coming to the above arbitrary position till completing the solidification, the rolling reduction is executed so that the rolling reduction gradient (%/m) showing the rolling reduction ratio (%) to the thickness of the cast slab per the drawing-out length (unit: m) of the cast slab satisfies the inequality. The inequality (0.5-0.38×[C]) <= the rolling reduction gradient (%/m)<=(1.58-1.72×[c]). Wherein, [C] is carbon content in the cast slab (mass %).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method capable of minimizing segregation occurring at the center of a slab continuous cast slab thickness, and more particularly to a method for continuously casting C, Mn, Si,
The present invention relates to a continuous casting method capable of preventing alloy elements such as P and S from segregating at the center in the thickness direction of a slab and producing a homogeneous steel.

[0002]

2. Description of the Related Art In a continuous casting method, one of the important issues is how to reduce segregation and center porosity generated in the center of a slab. Of these, segregation prevention is represented by the application of electromagnetic stirring technology, low-temperature casting, or the addition of heterogeneous nucleation promoting substances.
Practical use of segregation and dispersion technology due to the generation of a large amount of equiaxed crystals, and introduction of high-purification technology to reduce the concentration of impurity elements (especially P, S, etc.) in molten steel, or prevention of bulging by using dense rolls Technology has been introduced, and each has achieved considerable results.

However, focusing on the late stage of solidification, a sufficient solution has been established for the segregation caused by the flow of molten steel accompanying the solidification shrinkage at the end of solidification or the formation of center porosity which is a direct result of the solidification shrinkage. The fact is that they are not.

In recent continuous casting techniques, a plurality of rolling rolls are provided in the final stage of the slab drawing process, and the final solidified slab having an unsolidified portion left at the center is reduced at a low reduction ratio. It has been proposed. When the reduction at such a low reduction rate is applied, the flow of the molten steel can be suppressed to contribute to the prevention of segregation, and at the same time, the compensation for solidification shrinkage is performed to prevent the generation of center porosity, and the casting defect can be prevented. It is possible to provide a continuous casting product without any.

[0005] As a technique for performing the reduction at such a low reduction rate, Japanese Patent Publication No. Sho 59-16682 and Japanese Patent Publication No. Hei 3-6855 are disclosed.
No. 3-8863, No. 3-8864, No. 4-20
Nos. 696, 4-22664, and 5-30548 are known. These known techniques are referred to as a section in which low-rate reduction is performed (meaning a section from the start to the end of low-rate reduction in the final stage of the drawing process in consideration of the unsolidified state of the slab central part, hereinafter the same). The reduction is started from the solid fraction of 0.1 to 0.3 based on the solid fraction of the central part in the step (1), and when the solid fraction of the central part becomes high in the latter half of the drawing process, for example, the central solid fraction is 0.8 Recognizing that the point at which it reaches 0.9 is the point at which the flow limit is reached even if unsolidified molten steel remains, it is common that the reduction is stopped or only a slight reduction is performed thereafter doing.

However, when the reduction is stopped until the center solid phase ratio reaches 0.8 to 0.9, when an equiaxed crystal is observed at the center of the slab, for example, as in the case of continuous continuous blooming, The liquid phase space (spot-like segregation) in which impurity elements such as phosphorus and sulfur remaining dispersed between the equiaxed crystals are concentrated becomes relatively large, reaching 3 to 5 mm. If such spot-like segregation remains, high-carbon steel wire rods such as steel for steel cord and spring may cause breakage such as disconnection from the spot-like segregation part as a starting point during cooling in the post-process. become.

[0007] The present inventors have been studying the reduction of spot-like segregation caused by the liquid space left behind at the end of solidification, and as a part of the research, have previously described a technique for improving spot-like segregation. A proposal has been made (Japanese Patent Application No. 8-80214). In this technology, the center solid phase ratio is 0.8 from the temperature position corresponding to the solid phase ratio 0.2 at the center of the slab.
After rolling down the slab to compensate for the total solidification shrinkage in the solidification time range up to the point of reaching ~ 0.9, the area from then on until solidification is completed is the length of the slab in the drawing direction (unit: m) )
The rolling gradient (% / m), which indicates the ratio (%) of the rolling reduction to the slab thickness per unit, is 0.08% / m or more and 1.5% / m or less. With the development of such a technique, a blooming cast slab exhibiting so-called equiaxed solidification exhibited a remarkable improvement effect on the reduction of the spot-like segregation as described above.

[0008]

However, when the above-mentioned invention is applied to a continuous slab slab exhibiting a columnar crystal, although the effect is somewhat improved, the above-mentioned invention is required without cracking inside the slab. To reduce the spot-like segregation to a certain level, and the C _max / C ₀ value (C
For _max and C ₀ are not able to 1.1 or less which will be described later) was circumstances. If the spot-like segregation remains, there is a problem that it is necessary to perform a treatment such as breakdown or soaking diffusion in a post-process in the steel for a thick plate.

The present invention has been made with the above circumstances in mind, and does not cause cracks in the continuous slab slab, and V segregation, reverse V segregation, as well as left behind at the end of solidification. An object of the present invention is to provide a continuous casting method which can improve spot-like segregation caused by a liquid phase space.

[0010]

SUMMARY OF THE INVENTION The present invention, which has solved the above-mentioned problems, is directed to a continuous casting method for drawing a continuous cast piece of steel slab while reducing the pressure between at least one pair of opposing rolls. In the region from the position where the solid phase ratio of the part is 0.1 to 0.4 to an arbitrary position within the range of 0.8 to 0.9, the slab is reduced to compensate for the total solidification shrinkage in the region, From the position until the solidification is completed, the reduction gradient (% / m) indicating the ratio (%) of the reduction amount to the slab thickness per the drawing direction length (unit: m) of the slab is as follows: This is a continuous casting method having a gist in reducing the rolling so as to satisfy the expression (1). (0.5-0.38 x [C]) ≤ rolling gradient (% / m) ≤ (1.58-0.72 x [C]) (1) where [C] is the carbon content (% by mass) of the slab.

The reduction in the region from the position where the solid fraction of the center is 0.1 to 0.4 to an arbitrary position within the range of 0.8 to 0.9 is as follows. ,
While in each area of (B), the reduction gradient (% / m)
It is preferable that the pressures satisfy the following expressions (2) and (3), respectively. 0.1 to 0.4 ≦ central solid phase ratio ≦ 0.65
In the region (A) of ~ 0.75, (0.12-0.12 x [C]) ≤ rolling gradient (% / m) ≤ (0.59-0.36 x [C]) (2) where [C] is a cast slab. In the region (B) where the carbon content (mass%) is 0.65 to 0.75 ≦ the central solid phase ratio ≦ 0.8 to 0.9, (0.13-0.12 × [C]) ≦ the rolling gradient (% / m) ≦ (0.69-0.36 × [C]) ... (3) where [C] is the carbon content (% by mass) of the slab

That is, the region is divided into at least two regions corresponding to the increase in the ratio of the central solid phase accompanying the growth of solidification, and the optimum rolling conditions expressed by the above formula (2) or (3) are applied accordingly. It performs continuous casting. In a region (AB) where the region (A) and the region (B) intersect at a center solid phase ratio of 0.65 to 0.75, the following expression (4) is satisfied, and (0.12-0.12 × [C]) ≤ rolling gradient (% / m) ≤ (0.69-0.36 x [C]) ... (4) Same or smaller than rolling gradient (% / m) selected in region (A), selected in region (B) The present invention also includes a reduction with a reduction gradient (% / m) which is equal to or greater than the reduced gradient (% / m). In this case, since the rolling gradient is divided into two or three arbitrary regions, and the rolling gradient is changed so as to satisfy the equation (2) → (4) → (3), continuous casting is performed. is there.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, a section for rolling down a slab slab is roughly divided into two sections. The first section is 0.8 to 0.8 from the position where the solid fraction of the slab center is 0.1 to 0.4.
This is a reduction in an area reaching an arbitrary position within the range of 0.9. In this section, the slab is reduced so as to compensate for the total solidification shrinkage in the area. The next section is the reduction in the area following the first section until solidification is completed. In this section, the amount of reduction relative to the slab thickness per drawing direction length (unit: m) of the slab Is continuously reduced so that the rolling gradient (% / m) indicating the ratio (%) of the formula (1) satisfies the following formula (1). (0.5-0.38 x [C]) ≤ rolling gradient (% / m) ≤ (1.58-0.72 x [C]) (1) where [C] is the carbon content (% by mass) of the slab.

Here, the solid fraction at the center is defined as a finite element method, a difference method, or the like, using a solid phase fraction-temperature relationship in consideration of microsegregation analysis determined according to the method described in the following document. It can be obtained by performing unsteady heat transfer solidification analysis by computer simulation based on. Iron and Steel 78 (1992) No. 275-281

In the present invention, the first section is started from the position where the central solid fraction thus obtained is 0.1 to 0.4 (in other words, the position where the solid fraction is 0.1 to 0.4 at the center of the slab). In addition, reduction is started (hereinafter, the solid phase ratio at this time may be referred to as “rolling start solid phase ratio”). The reduction in this first section is performed to compensate for the total coagulation contraction in the area. 0.1% to 0.4
The reason for this is that it is preferable to perform rolling down from the position where the solid phase ratio is 0.1 so as to compensate for the total coagulation shrinkage in the area, but since the rolling down area becomes long, many roll stands for rolling down are required. Therefore, in order to prevent equipment costs from increasing, if the roll stand for rolling is minimized, the degree of center segregation will increase even if the solid phase ratio at the start of rolling is increased to a maximum of 0.4, but this will not be a problem. be able to.

The conditions for the reduction are not particularly limited as long as this condition is maintained. However, since the central solid fraction gradually increases on the downstream side of the slab drawing process, it is preferable that the central solid fraction be set at the lower stage. The reduction is continued while selecting the optimal reduction gradient that is selected so as to decrease stepwise in response to the target increase, and the reduction is defined as the first section of the present invention until the central solid phase ratio reaches 0.8 to 0.9. Do. The preferable rolling condition in the first section will be further described later.

Next, a description will be given of the rolling conditions for improving the reduction after the center solid fraction of 0.8 to 0.9 (0.80 to 0.90 when more strictly defined), ie, spot-like segregation. First, the rolling conditions are defined by the above formula (1) in relation to [C] [the carbon content (mass%) in the slab] as described above. The reason is as follows.

In general, the solidification state in steel changes as follows depending on [C]. That is, as [C] increases, the position of the crater end where the center of the slab is completely solidified shifts further downstream from the meniscus position, which is the surface of molten steel in the casting. Accordingly, when [C] increases at a position where the slab center has the same solid phase ratio, the solidification thickness from the slab surface decreases. Therefore, since the amount of reduction applied from the surface more effectively affects the deformation of the unsolidified region inside the slab, the amount of contraction of the molten steel can be compensated with a smaller amount of reduction as compared with the case where [C] is low. This is advantageous from the viewpoint of preventing occurrence of cracks. That is, when [C] is high, cracking sensitivity is increased, so that cracks in the slab are likely to occur. However, it is possible to compensate for the shrinkage of molten steel with a smaller rolling reduction. The present invention has been made by focusing on the difference in the coagulation state due to [C].

At the end of solidification when the center solid phase ratio is 0.8 to 0.9 or later, a liquid phase space is left as shown in FIG. 1. However, if the drawing is completed without applying any reduction in this state, the liquid The molten steel in the phase space absorbs the molten steel in which the amount corresponding to the solidification shrinkage is concentrated between the dendrite resins in the periphery, and remains as spot-like segregation. At this time, a portion corresponding to the coagulation shrinkage remains as porosity.

Therefore, it is expected that spot-like segregation and porosity can be reduced if an amount corresponding to the solidification shrinkage can be applied from the outside. However, if excessive reduction is applied, cracks will be generated in the continuous slab slab exhibiting columnar crystal solidification.

The present inventors investigated the effect of [C] by casting at two different steel types, [C] = 0.16% and 0.55%:
The occurrence of segregation was investigated with a mold size of 1.2 m / min and a mold size of 2100 × 280 (mm). At this time, as an index of segregation,
The C _max / C ₀ value was used. Here, C _max is a depth of 5 mmφ using a drill of the center of the cross section of the slab.
The maximum value of the carbon analysis values obtained by sampling a sample up to 10 mm at a pitch of 20 mm, and C ₀ is the carbon analysis value obtained by sampling the sample from a position of 1 / 4t in the thickness direction, and the ratio of these C _max / C _{When the 0} value is 1.1, it can be determined that the spot-like segregation as described above has not been generated.

First, in order to investigate the optimum conditions in an area from an arbitrary position where the central solid phase ratio is within the range of 0.8 to 0.9 and until solidification is completed (the central solid phase ratio is up to 1.0), the center of the slab is examined. Then, rolling is performed with a low rolling gradient of 0.2% / m so that cracks do not occur in the region from the solid fraction of 0.1 to the above-mentioned arbitrary position, and then the rolling gradient after the central solid fraction exceeds 0.8 to 0.9 is changed. Therefore, various relations with the center segregation were examined. The results are shown in FIG. 2 (when [C] = 0.16%) and FIG. 3 ([C]
= 0.55%).

As apparent from these results, C _max /
It can be seen that there is an optimum range for the rolling gradient in order to keep the C ₀ value at 1.1 or less and not to cause cracks at the same time. It can also be seen that as the value of [C] increases, the appropriate range of the rolling gradient shifts to the lower side.

Next, the rolling gradient in the region where the solid fraction in the center of the slab is 0.9 to 1.0 is 0.6% / m, which is a value within the above-mentioned optimum range, while the solid fraction is 0.7 to 1.0. The rolling gradient in the region of 0.9 was set to 0.2% / m, and the rolling gradient in the region of the solid fraction of 0.1 to 0.7 was changed. The results are shown in FIG. 4 (when [C] = 0.16%) and FIG. 5 (when [C] = 0.55%), respectively.

As is apparent from these results, C _max /
It can be seen that there is an optimum range for the rolling gradient in order to keep the C ₀ value at 1.1 or less and not to cause cracks at the same time. It can also be seen that as the value of [C] increases, the appropriate range of the rolling gradient shifts to the lower side.

Next, in order to grasp the optimum range of the reduction gradient in the region where the solid phase ratio in the center of the slab is 0.7 to 0.9, the reduction gradient in the region where the solid phase ratio in the center of the slab is 0.9 to 1.0 is described. Is 0.6% / m, which is a value within the obtained optimum range, while the solid fraction is 0.1 to
The rolling gradient in the region of 0.7 is 0.2% / m, and the solid fraction is 0.7 to 0.9.
The rolling gradient in the region was changed. The result is shown in FIG.
(When [C] = 0.16%) and FIG. 7 (when [C] = 0.55%), respectively.

As is apparent from these results, C _max /
It can be seen that there is an optimum range for the rolling gradient in order to keep the C ₀ value at 1.1 or less and not to cause cracks at the same time. It can also be seen that as the value of [C] increases, the appropriate range of the rolling gradient shifts to the lower side.

Next, the rolling gradient in the region where the solid fraction at the center of the slab is 0.9 to 1.0 is set to a value within the optimum range of 0.6% / m, and the solid fraction at the center of the slab is 0.7 to 0.9. 0.2% reduction in area
/ and m, the change in C _max / C ₀ value when the solid phase ratio of the slab center has a rolling start solid fraction varied from 0.1 to 0.6 reduction gradient of 0.7 smaller area as 0.2% / m FIG. 8 ([C] = 0.
16%) and FIG. 9 (when [C] = 0.55%). As is evident from these results, in order to keep the value of C _max / C ₀ at 1.1 or less, the solid phase ratio at the start of rolling must be at most 0.1%.
Can take up to 4.

In the same manner as described above, for various steel types of 0.1% ≦ [C] <0.6%, the slab size is 1770 × 2100 to 230 × 2100 (mm)
FIGS. 10 to 12 show the results when the casting speed was in the range of 1.1 to 1.4 using the continuous slab caster of FIG. FIG. 10
11 shows that when the solid fraction is 0.9 to 1.0, FIG.
FIG. 12 shows the optimum ranges of the respective reductions when the solid phase ratio is 0.7 to 0.9 when the solid phase ratio is 0.7 to 0.9.

The present inventors have examined and arranged these results, and found that in the region where the central solid phase ratio is 0.9 to 1.0, the C _max / C ₀ value is set to 1.1 or less, and at the same time, cracks do not occur. It was found that the rolling gradient (% / m) should satisfy the equation (1).

In the region where the central solid fraction reaches 0.9 to 1.0, 0.1 to 0.4 ≦ central solid fraction ≦ 0.65 to 0.75
(A) and 0.65 to 0.75 ≦ central solid fraction ≦ 0.8 to 0.9
(B), and it was found that the rolling gradient (% / m) in each area should satisfy the above-mentioned equations (2) and (3).

In the above-mentioned regions (A) and (B), when the center solid fraction is around (0.65 to 0.75), the molten steel melt flowability changes depending on the composition of the steel. It is appropriate to divide the area with high flexibility. In the present invention, from this point of view, when dividing the area, the degrees of freedom were given as indicated by the upper limit of the area (A) and the lower limit of the area (B).
As shown in B), it is permissible to give a wider degree of freedom to the division of the region itself. The point is that the purpose of the present invention is to provide an appropriate rolling gradient in accordance with the divided areas. Therefore, in each area, the above-mentioned formulas (2) and (3) can be used under the conditions not contrary to this purpose. The rolling gradient should be selected and changed so as to satisfy the equations (4) and (4).

The rolling gradient is a numerical value indicating the reduction rate (%) at which the rolling reduction (%) is performed in the thickness direction of the slab per length (unit: m) in the drawing direction of the slab. And are given in units of% / m. The rolling roll used in the present invention is not particularly limited, and a general-purpose flat roll or medium-sized roll can be used in the present invention.

[0034]

EXAMPLES Casting was performed at a casting speed of 1.1 m / min using a continuous slab casting machine having a slab size of 2100 × 280 (mm) for steel types having the chemical composition shown in Table 1 below. . The rolling conditions at this time were three types shown in Table 2 below.

[0035]

[Table 1]

[0036]

[Table 2]

With respect to the obtained slab slab, the above C _max
The / C ₀ value was measured and the presence or absence of cracks was investigated. The results are shown in Table 3 below. From these results, the following can be considered. First, condition 1 corresponds to the above (1) to
It is found that all of the rolling conditions defined by the equation (3) are satisfied, and the obtained slab slab has a C _max / C ₀ value of 1.1 or less and no cracks are generated. On the other hand, the condition 2 is such that although the rolling conditions in the range of 0.1 ≦ central solid phase ratio ≦ 0.7 are out of the range defined by the above equation (2), no crack is generated in the slab slab, but C _max
/ C ₀ value exceeds 1.1. The condition 3 is that the rolling condition in the range of 0.7 <center solid fraction ≦ 0.9 is out of the range defined by the above formula (3), so that although the C _max / C ₀ value is 1.1 or less, the slab slab is cracked. Has occurred.

[0038]

[Table 3]

[0039]

The present invention is constructed as described above, and in the region where the central solid phase ratio is in the range of 0.1 to 0.8 to 0.9, a reduction corresponding to the total coagulation shrinkage is performed, and the central solid phase ratio in the final stage of solidification is 0.8.
In the region from 0.9 to 0.9, the reduction was performed with an appropriate reduction gradient according to the carbon concentration of the slab, so not only V segregation, reverse V segregation and center porosity, but also no spot-like segregation in the axial center part It has become possible to manufacture slab slabs.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a situation in which a liquid phase space left at the end of solidification changes.

FIG. 2 is a graph showing the relationship between the reduction gradient and the C _max / C ₀ value in a slab having a carbon content of 0.16% when the solid fraction at the center is in the range of 0.9 to 1.0.

Fig. 3 Central solid fraction in slab with 0.55% carbon content
It is a graph which shows the relationship between a rolling-down gradient in the range of 0.9-1.0 and _Cmax / _C0 value.

Fig. 4 Center solid fraction of cast slab with 0.16% carbon content
It is a graph which shows the relationship between the rolling-down gradient in the range of 0.1-0.7 and _Cmax / _C0 value.

FIG. 5: Central solid fraction in a slab with a carbon content of 0.55%
It is a graph which shows the relationship between the rolling-down gradient in the range of 0.1-0.7 and _Cmax / _C0 value.

FIG. 6: Central solid fraction in cast slab with carbon content of 0.16%
It is a graph which shows the relationship between the rolling gradient in the range of 0.7-0.9 and _Cmax / _C0 value.

FIG. 7: Central solid fraction in a slab with a carbon content of 0.55%
It is a graph which shows the relationship between the rolling gradient in the range of 0.7-0.9 and _Cmax / _C0 value.

FIG. 8 is a graph showing a relationship between a solid content ratio at the start of rolling and a C _max / C ₀ value in a slab having a carbon content of 0.16%.

FIG. 9 is a graph showing the relationship between the solid phase ratio at the start of rolling and the C _max / C ₀ value in a slab having a carbon content of 0.55%.

FIG. 10 is a graph showing the influence of the carbon content and the rolling gradient in the range of 0.9 to 1.0 of the solid fraction at the center of the slab on the occurrence of cracks and segregation.

FIG. 11 is a graph showing the effect of the carbon content and the rolling gradient in the range of 0.4 to 0.7 of the solid fraction at the center of the slab on the occurrence of cracks and segregation.

FIG. 12 is a graph showing the effect of the carbon content and the rolling gradient in the range of the solid fraction of 0.7 to 0.9 at the center of the slab on the occurrence of cracks and segregation.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yoshinori Onoe 1 Kanazawa-cho, Kakogawa-shi, Hyogo Inside Kobe Steel Works Kakogawa Works (72) Inventor Masahiko Kokita 1 Kanazawa-cho, Kakogawa-shi, Hyogo Kami Co., Ltd. Inside the Kakogawa Works of Toko Steel Works (72) Inventor Ken Inoue 1 Kanazawacho, Kakogawa City, Hyogo Prefecture Inside of Kakogawa Works, Kobe Steel Works, Ltd.

Claims (3)

[Claims] 1. A continuous casting method for drawing a continuous cast piece of steel slab while reducing the pressure between at least one pair of opposing rolls, wherein 0.8 to 0.9 is set from a position where the solid phase ratio at the center of the cast piece is 0.1 to 0.4. In the area reaching any position within the range of,
The slab is lowered so as to compensate for the total solidification shrinkage in the area, and the area from the above-mentioned arbitrary position to the completion of solidification is the slab per the length (unit: m) in the drawing direction of the slab. The reduction gradient (% / m) indicating the ratio (%) of the reduction amount to the thickness is as follows (1)
A continuous casting method characterized by rolling down to satisfy the formula. (0.5-0.38 x [C]) ≤ rolling gradient (% / m) ≤ (1.58-0.72 x [C]) (1) where [C] is the carbon content (% by mass) of the slab. 2. The reduction in the area from the position where the solid fraction of the center is 0.1 to 0.4 to an arbitrary position within the range of 0.8 to 0.9 is as follows. ),
While in each area of (B), the reduction gradient (% / m)
The continuous casting method according to claim 1, wherein the pressure is reduced so as to satisfy the following equations (2) and (3), respectively. In the region (A) of 0.1 to 0.4 ≦ center solid fraction ≦ 0.65 to 0.75, (0.12-0.12 × [C]) ≦ downgradient (% / m) ≦ (0.59-0.36 × [C]) (2) However, [C]: the carbon content (% by mass) of the slab In the region (B) of 0.65 to 0.75 ≦ central solid fraction ≦ 0.8 to 0.9, (0.13-0.12 × [C]) ≦ down gradient ( % / m) ≦ (0.69-0.36 × [C])… (3) where [C] is the carbon content (mass%) of the slab 3. The continuous casting method according to claim 2, wherein the ratio of the central solid phase at which the region (A) intersects with the region (B) = 0.
In the range (AB) of 65 to 0.75, the following equation (4) is satisfied, and (0.12-0.12 × [C]) ≦ downhill gradient (% / m) ≦ (0.69-0.36 × [C]) … (4) Reduction gradient (% / m) equal to or smaller than the reduction gradient (% / m) selected in region (A), and equal to or greater than the reduction gradient (% / m) selected in region (B) % / m).