JPH06190507A - Stainless steel cast slab having excellent surface characteristic and continuous casting method thereof - Google Patents

Abstract

PURPOSE:To enable non-cleaning rolling for a stainless steel cast slab by selecting mold cooling, mold material, powder characteristic, etc., in order to uniformize the solidifying speed of the top part and the bottom part of an oscillation mark on the cast slab. CONSTITUTION:In the cast slab cast by continuous casting, the ratio (S B)/(S T) of a secondary dendritic arm interval (S B) at the bottom part of the oscillation to a secondary dendritic arm interval (S T) at the top part of the oscillation below the surface shell of this cast slab at 150-250mum is made to be <1.4. In the case this ratio (S B)/(S T) becomes >=1.4, the breakage of solidified shell is caused, and the larger the ratio of these secondary arm intervals is, the deeper a segregation zone is. As to the solidifying speed at the top part and the bottom part, that at the bottom part is ordinarily lower than at the top part, but by making this ratio almost >=0.5, the segregation zone can be prevented. By this method, the non-cleaning rolling can be attained, and by saving the cleaning to the cast slab, the cost can be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a steel strip which has been hot-rolled, pickled and cold-rolled from a slab which is left unmaintained on the surface or slightly maintained (hereinafter referred to as "non-maintenance"). The present invention relates to an austenitic stainless steel slab having a good surface property in which uneven glossy patterns and bald defects are hardly generated, and a continuous casting method thereof.

[0002]

2. Description of the Related Art Since stainless steel has a small amount of scale formation during heating in hot rolling, when hot rolling is performed without casting a slab in which surface defects such as component segregation and cracks occur in casting, These surface defects remain on the surface of the hot-rolled steel strip without being scaled off. Further, when this hot-rolled steel strip is cold-rolled without being subjected to a flaw removal treatment while being pickled, the surface defects remain on the surface of the cold-rolled steel strip. Therefore, in order to roll stainless steel continuously cast slabs without maintenance, it is essential to obtain slabs with no surface defects or very few surface defects (hereinafter referred to as defect-free).

However, in continuous casting of austenitic stainless steel typified by SUS304, when the mold is oscillated in a casting direction with a sine wave or a non-sinusoidal wave under normal conditions, the surface of the slab shown in FIG. Defects such as a segregation zone in which an element such as Ni is concentrated, and entrainment and cracking of powder occur in the valley portion of the oscillation mark. For this reason, when hot rolling the slab without maintenance, there was a problem that uneven hot spots, bald spots, etc. were generated on the surface of the hot rolled steel strip and remained on the cold rolled steel strip. .

As a method of reducing the above-mentioned defects in the valley portion of the oscillation mark of the continuously cast slab, Japanese Patent Laid-Open No. 57-13 / 1987 has been proposed.
No. 0741 discloses that the viscosity of the powder is lowered and the maximum descending speed of the mold is made smaller than the slab drawing speed, or when the maximum descending speed of the mold is not less than the slab drawing speed, a high cycle oscillating There is disclosed a method for improving the surface property of a slab by performing continuous casting as a ration condition. Further, Japanese Patent Application Laid-Open No. 2-30357 discloses a method of reducing the surface defects of a cast piece by selecting the viscosity and the solidification temperature of the powder.

[0005]

However, the conditions for forming the segregation zone, which is the cause of the defects, also vary depending on the cooling conditions of the mold and the like. For this reason, the conventional method has a problem in that the appropriate range varies depending on other factors such as the cooling conditions of the mold, and a sufficient effect cannot be stably obtained. The present invention, in the continuous casting of austenitic stainless steel, clarifying the segregation zone formation prevention condition considering the cooling conditions of the mold by the investigation of the segregation zone formation mechanism of the oscillation mark valley part, to the level at which maintenance-free rolling is possible. An object of the present invention is to provide a continuous casting means capable of stably casting a slab with reduced defects.

[0006]

Means for Solving the Problems The gist of the present invention for solving the above problems is to provide an oscillation mark for the secondary dendrite arm interval (S.ltoreq.T) in the oscillating mark ridge of 150 to 250 .mu.m on the surface of a cast piece. Ratio of secondary dendrite arm spacing (S | B) / (S | B) / (S
It is a stainless steel slab with excellent surface properties characterized by a ‖T) of less than 1.4.

Further, in continuously casting a stainless steel slab while oscillating the casting mold in the casting direction,
A continuous casting method for a stainless steel slab having excellent surface properties, characterized in that continuous casting is carried out under conditions satisfying the following formula (1). λl + α ₀ (dl-dosm) ≧ 0 ……………… (1) where λl: thermal conductivity (cal) of liquid phase powder at 1300 ℃
/ cm ・ s ・ ℃) α ₀ : Overall heat transfer coefficient between mold cooling water and solid powder (c
al / cm ² · s · ° C) dl: Thickness of liquid phase powder (cm) dosm: Depth of initial oscillation mark (cm).

[0008]

In order to solve the above problems, the present inventor conducted a casting experiment in which the mold cooling conditions, mold materials, mold vibration conditions, powders, etc. were changed, and the oscillation mark trough
As a result of research on the segregation zone formation mechanism of (Fig. 3), the following findings were obtained. (A) The segregation zone in the valley portion of the oscillation mark is formed because the molten steel enriched between the dendrite trees inside oozes due to the fracture of the solidified shell.

(B) The breakage of the solidified shell is caused by the secondary dendrite arm interval (S.ltoreq.T) at the peak of the oscillation mark.
), The ratio of the secondary dendrite arm interval (S | B) / (S | B) / (S | T) in the trough of the oscillation mark (subcutaneous segregation zone if there is a segregation zone) is 1.4 or more. When the ratio of the secondary dendrite arm spacing increases, that is, as the solidification rate of the oscillation mark expressed by the following equation (2) becomes more uneven between the valley and the peak, segregation occurs. The belt becomes deep. Further, it was clarified that the solidification cooling rate of the valley and the peak is usually smaller than that of the valley, but if the ratio is set to about 0.5 or more, the segregation zone can be prevented. CR = (S // 111.3) ^-2.22 formula (2) where CR: Oscillation mark Cooling speed of peak and valley (° C / s) S /: Secondary dendrite arm Interval (μm).

The secondary dendrite arm spacing in the ridges and valleys of the oscillation marks (subcutaneous segregation zones, if segregation zones are present) is 150-250 μm subcutaneously on each surface.
Positions in the range of m were measured.

FIG. 1 shows the secondary dendrite arm spacing (S.ltoreq.T) at the peak of the oscillation mark and the secondary dendrite arm spacing (S) at the valley of the oscillation mark.
The ratio of (‖B) (S‖B) / (S‖T) and the slab surface defects, that is, segregation zone, powder entrainment and cracking defects are 1 (good).
It is a figure which shows the relationship with what was indexed to 5 (defective).
By setting the ratio of (S / B) / (S / T) to less than 1.4, it was possible to reduce surface defects to a level at which maintenance-free rolling is possible.

Further, for the purpose of elucidating the conditions for obtaining the above-mentioned result, the heat transfer from the surface of the slab to the cooling water for the mold was particularly focused and studied. The heat transfer coefficient (αw) between the mold cooling water and the mold copper plate can be generally obtained from the following equation (3). αw = 0.023 · (D · u · ρ / μ) ^0.8 · (Cp · μ / λ) ^0.4 · λ / D = 0.023 · Re ^0.8 · Pr ^0.4 · λ / D ………… (3) Formula (However, Re = 10000 to 20000, Pr = 0.7 to 12
0, l / D ≧ 60) where D: Equivalent diameter of mold cooling water channel (cm) l: Length of mold cooling water channel (cm) ρ: Density of cooling water (g / cm ³ ) λ: Cooling water Thermal conductivity (cal / cm ・ s ・ ℃) Cp: Constant pressure specific heat of cooling water (cal / g ・ ℃) μ: Cooling water viscosity (poise) u: Cooling water flow rate (cm / s) Re: Reynolds number Pr: Prandtl number.

The overall heat transfer coefficient (α ₀ ) between the mold cooling water and the solid phase powder can be obtained from the following equation (4). (1 / α ₀ ) = (1 / αw) + (dCu / λCu) + (dNi / λNi) + (ds / λs) (4) where αw is heat between the mold cooling water and the mold copper plate Transfer coefficient (cal / m
²・ s ・ ℃) λCu: Thermal conductivity of mold copper plate (cal / cm ・ s ・ ℃) dCu: Thickness of mold copper plate (cm) λNi: Thermal conductivity of Ni plating on mold copper plate surface (cal / cm)
・ S ・ ° C) dNi: Ni plating thickness (cm) λs: Thermal conductivity of solid phase powder (cal / cm ・ s ・ ° C) ds: Solid phase powder thickness (cm)

The overall heat transfer coefficient (α ₁ ) between the mold cooling water and the ridge portion of the oscillation mark can be obtained from the following equation (5). (1 / α ₁ ) = (1 / αw) + (dCu / λCu) + (dNi / λNi) + (ds / λs) + (dl / λl) = (1 / α ₀ ) + (dl / λl) ... …………………… (5) where, λl: thermal conductivity of liquid powder (cal / cm ・ s ・ ° C) dl: thickness of liquid powder (cm).

The thickness (dl) of the liquid phase powder was calculated from the amount of powder consumed and the specific gravity of the molten powder.
The thickness (ds) of the solid phase powder is separated into the liquid phase powder and the solid phase powder by using a powder to which a raw material other than the powder component such as SrO ₂ is added during or after the casting. In this state, the thickness of the solid phase powder collected from the wall surface of the mold was measured.

The overall heat transfer coefficient (α ₂ ) between the mold cooling water and the valley of the oscillation mark can be obtained from the following equation (6). (1 / α ₂ ) = (1 / αw) + (dCu / λCu) + (dNi / λNi) + (ds / λs) + ([dl + dosm] / λl) = (1 / α ₀ ) + ([dl + dosm] / Λl) (6) where dosm is the depth (cm) of the initial oscillation mark, that is, if a segregation zone occurs, the depth of the oscillation mark including the segregation zone.

The conditions for obtaining the above slab, the ratio α ₂ / α ₁ of alpha ₂ for overall heat transfer coefficient alpha ₁ was found to be as long as 0.5 or more. That is, (α ₂ / α ₁ ) = (α ₀ λl / {λl + α ₀ [dl + dosm]}) / (α ₀ λl / {λl + α ₀ dl}) ≧ 0.5 = (λl + α ₀ dl) / (λl + α ₀ [dl + dosm]) ≧ 0.5 (7) Expression From the above Expression (7), the following Expression (8) is obtained. λ1 + α ₀ (dl-dosm) ≧ 0 ………………………………………………………………………………………………………… obtain. (8) Equation (8) is a non-uniform thermal conductivity of the peaks and troughs of the oscillation mark so that surface defects of the slab do not occur. It is considered to indicate the critical value once.

FIG. 2 is considered to be an index showing the non-uniformity of the thermal conductivity of the peaks and valleys of the oscillation mark (8).
1 on the left side of the formula, λl + α ₀ (dl-dosm), and the slab surface defects, that is, segregation zone, powder entrainment and cracking defects.
It is a figure which shows the relationship with what was indexed to (good) -5 (bad). By performing the casting under the casting conditions satisfying the relational expression (8), the secondary dendrite arm interval (S‖T) of the oscillation mark peak portion (usually the valley portion is The secondary dendrite arm spacing is large (S ‖B)> (S ‖T) due to the low solidification cooling speed, and the secondary dendrites in the valley of the oscillation mark (subcutaneous segregation zone if there is a segregation zone) It is possible to manufacture a slab having an arm spacing (S | B) of less than 1.4 times, that is, a ratio of the solidification cool speeds of the crests and the troughs of the oscillation mark is about 0.5 or more.

Since the slab has more uniform cooling of the peaks and valleys of the oscillation mark, the tensile stress generated in the valleys can be reduced. as a result,
Since it is possible to prevent the solidified shell from breaking at the valley portion due to excessive tensile stress, it is possible to prevent the formation of a segregation zone due to the seepage of the concentrated molten steel inside. Further, since it is possible to prevent the formation of the segregation zone, it is possible to prevent the powder entrapment and the cracking defects which are concentrated in the segregation zone. Further, even if a segregation zone is generated, the depth thereof is shallow, so that it can be suppressed below the level at which maintenance-free rolling is possible.

The method for carrying out the present invention includes, for example, conditions such as temperature and flow rate of mold cooling water, mold and plating conditions, mold vibration conditions, powder physical properties and oscillation mark depth, and equation (8). Λl, α ₀ , dl, do
The sm relationship and the above relational expression are input to a computer, and the control factor is online controlled so as to satisfy this relational expression.

[0021]

EXAMPLES SUS304 stainless steel was continuously cast under various casting conditions shown in Table 1 using a mold having dimensions of 168 mm × 1290 mm. Then, a cross-section sample in a direction parallel to the casting direction was taken from the obtained slab, and the state of occurrence of defects such as segregation zones, entrainment of powder, and cracks was investigated. In addition, maintenance-free rolling is performed to a thickness of 3
A hot rolled steel strip of mm was obtained. After pickling this steel strip, the occurrence rate of uneven glossy pattern and bald spots on the surface was investigated. The results are also shown in Table 1.

[0022]

[Table 1]

In Table 1, the "gloss unevenness pattern" on the surface of the steel strip is displayed in four levels of "strong", "medium", "weak" and "absent", and the "health defect rate" is Width 1 across the length of the steel strip
The surface was classified into 245 mm × 1000 mm length and displayed by the ratio of the surface where the bald defects were generated.

The stainless steel slab continuously cast according to the method of the present invention has reduced surface defects, and it is possible to suppress the occurrence of uneven gloss patterns and bald spots to a low level even after performing careless rolling. there were. On the other hand, when the slab continuously cast under the condition out of the range specified in the method of the present invention is subjected to maintenance-free rolling, gloss unevenness patterns or bald spots frequently occur and maintenance-free rolling is impossible. I found out.

[0025]

According to the method of the present invention, it becomes possible to perform maintenance free rolling of a stainless steel slab, which has been difficult so far, and it is possible to significantly reduce the cost by omitting the slab servicing step.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the ratio of the secondary dendrite arm interval (S | B) in the valley to the secondary dendrite arm interval (S || T) in the peak of the oscillation mark and the slab surface defect index. is there.

FIG. 2 is a diagram showing a relationship between a cast surface defect index and an index indicating a non-uniformity of thermal conductivity of a crest portion and a trough portion of an oscillation mark.

FIG. 3 is a photograph of a segregation zone of oscillation marks and a metallographic structure of secondary dendrites in peaks and valleys.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ken Nakano 3434 Shimada, Hikari City, Yamaguchi Prefecture Nippon Steel Corporation Hikari Steel Works Co., Ltd.

Claims (2)

[Claims] 1. A slab cast by continuous casting, wherein the slab has a secondary dendrite arm spacing (S || T
), The ratio of the secondary dendrite arm spacing (S | B) in the valley of the oscillation mark (S | B) / (S | T) is
A stainless steel slab having excellent surface properties characterized by being less than 1.4. 2. When continuously casting a stainless steel slab while oscillating the casting mold in the casting direction,
A continuous casting method for a stainless steel slab having excellent surface properties, which is characterized in that continuous casting is carried out under conditions satisfying the expression (1). λl + α ₀ (dl-dosm) ≧ 0 ……………… (1) However, λl: thermal conductivity of liquid phase powder at 1300 ° C (cal /
cm ・ s ・ ℃) α ₀ : Overall heat transfer coefficient between mold cooling water and solid powder (ca
l / cm ²・ s ・ ° C) dl: Liquid phase powder thickness (cm) dosm: Initial oscillation mark depth (cm)