JPH04231156A - Continuous casting - Google Patents

Abstract

PURPOSE:To reduce stage load and to obtain a homogeneous billet by deciding that the state of segregation of a billet is deteriorated when the solidus rate in the center of the billet on the inlet side of a rolling reduction roll area and the average passing speed exceed a prescribed range. CONSTITUTION:The billet 3a is extracted by plural pairs of rolls as it is rolled down at the end of solidification to execute continuous casting of molten metal. Then, when the solidus rate in the center of the billet 3a on the inlet side of the rolling reduction roll area and the average passing speed exceed a prescribed range, the state of segregation of the billet 3a is decided to be deteriorated. In this way, the state of segregation of the billet 3a can be decided easily.

Description

Detailed Description of the Invention [0001] [Industrial Application Field] The present invention is a method for determining the segregation state of carbon, sulfur, phosphorus, manganese, etc. found in the center of the thickness of a slab manufactured by continuous casting. It also relates to a continuous casting method that aims to obtain a homogeneous product by changing the heat treatment conditions in the heating and rolling process of the slab, depending on the judgment results. BACKGROUND OF THE INVENTION In continuous casting, it is desirable to maintain a constant casting speed, but changing the ladle or changing other conditions forces the casting speed to vary. [0003] Conventionally, there has been no effective means for dealing with fluctuations in casting speed, and manufacturing process conditions have been set that take into account variations in quality of slabs that include such fluctuations. [0004] The present invention can reduce the process load and produce homogeneous slabs by determining and stratifying center segregation without directly inspecting it, and feeding it forward to the heat treatment conditions in the slab heating and rolling process. Provides continuous casting method. [Means for Solving the Problems] The present invention advantageously solves the above-mentioned problems, and provides a continuous casting method for molten metal in which a slab is drawn while being compressed by at least two pairs of rolls at the final stage of solidification. A continuous casting method characterized in that the segregation state of the slab is determined to have deteriorated when the solid fraction at the center of the slab on the entrance side of the rolling roll and the average passing speed of the rolling roll band exceed a certain predetermined range. It is. [Operations] The present invention will be explained in detail below with reference to the accompanying drawings. FIG. 1 is a diagram showing the position of the rolling strip of a slab 3a in continuous casting equipment. The distance from the meniscus position 1 in the mold 3 to the slab rolling start position 2 is l1, and the slab rolling range is l2.
shall be. FIG. 2 is a diagram showing the relationship between the casting speed at the meniscus position and the casting length in continuous casting equipment, and the relationship between the slab segregation index and the casting length. The slab segregation index shown here was determined by cutting the slab at the center of its width in the casting direction and averaging the sizes of the segregated grains observed in the cross section. [0010] As shown in Fig. 5, the slab segregation index is obtained by taking the etch print 5 of the cross section 4 in the longitudinal direction at the center of the width of the slab, and determining the area by using the dark part of the etch print as the segregated grain size. , find the diameter of the same area. As a result, the slab segregation index shall be determined according to the classification shown in Table 1. [0011] [Table 1] [0012] In Fig. 2, the casting speed at the meniscus position is an example of an unavoidable drop in casting speed, and even when the molten steel temperature is high or low, it is necessary to avoid operational accidents. It is common practice to change the casting speed. In FIG. 2, when the casting speed at the meniscus position decreases, the segregation index of the slab (in the range l2) located in the slab reduction roll zone at that time deteriorates significantly. Furthermore, when the casting speed at the meniscus position decreases, the segregation index of the slab (in the range l1) that has already been cast and is located in front of the slab reduction roll band is unstable. The reason why segregation is bad will be explained. The reason why segregation worsens when the casting speed decreases is that, although it is preferable that the slab placed in the slab reduction roll zone be rolled down commensurate with the amount of solidification shrinkage between each roll, due to the decrease in passing speed of the reduction roll zone, Since the amount of solidification shrinkage between the rolls becomes larger than the amount of reduction, molten steel flow occurs and segregation worsens. [0017] The slab located in front of the slab reduction roll band is
Since the time required from the meniscus to the input side of the reduction roll is extended, solidification progresses and the temperature of the slab decreases. If the same rolling force is applied to each roll, the rolling reduction will be smaller when the slab temperature is low, causing molten steel flow and worsening segregation. [0018] The slab located in this slab reduction roll zone is
The line speed when passing through the roll-down roll band is decreasing, and the time it takes for the slab located in front of the slab roll-down roll band to reach the entrance side of the slab roll-down roll band from the meniscus area is longer. By the time the slab reaches the inlet of the reduction roll zone, solidification has progressed and the solid fraction at the center of the slab is high. FIG. 3 is a diagram showing the relationship between the solid fraction at the center of the slab on the entry side of the reduction roll band, the average passing speed of the reduction roll band, and the slab segregation index in continuous casting equipment. In FIG. 3, if the slab segregation index is 2 or less, it is marked as ○.
, 2 was given an × for those exceeding 2. From FIG. 3, it can be confirmed that the region where the slab segregation index is 2 or less is determined by the solid fraction at the center of the slab on the entrance side of the roll roll and the average passing speed of the roll roll. FIG. 4 is a diagram showing the solid fraction at the center of the slab on the entrance side of the reduction roll shown in FIG. 3 and the average passing speed of the reduction roll at each casting length, and the meniscus position at this time. The casting speed is an example in which casting is temporarily stopped and then returned to a normal state. In addition, the same figure also shows the control range and determination results for the solid fraction at the center of the slab on the rolling roll band entry side and the average passing speed of the rolling roll band. The control range shown here can be determined by the segregation level corresponding to the steel grade. [Equation 1] [Equation 2] [Example] For example, meniscus ~ machine end
38.965m meniscus ~ Rolling roll band entry side
22.565m meniscus ~ Reduction roll band exit side 34
．． 165m rolling strip length 11.6
Casting length at the start of casting speed variation at the meniscus position when m; L1 Average passing speed of the reduction roll band
〃 ;L2, then
L1 - L2 = 34.165m Casting length at the end of the casting speed fluctuation at the meniscus position; L3 Average passing speed of the reduction roll band 〃
; If L4, then L3 - L4 = 22
．． 565m (L4 - L2) = (L3 - L1) +
It will be 11.6m. The performance of the average passing speed of the reduction roll band is determined by the casting speed pattern at the meniscus position. The calculation method is obtained using the following equation 3. [Equation 3] Average passing speed of the rolling roll band (Vs) l2 ; Length of the rolling roll band (11.6 m) t1 ; Time t0 on the rolling roll band exit side of the relevant part; Rolling roll belt entry side of the relevant part [0027] Fluctuation range of the solid fraction at the center of the slab on the entrance side of the reduction roll band L1 - L5 = 22.565 m L3 - L6 = 22.565 m L3 = L7 [0028] Actual average passing speed of the reduction roll band The calculation method is as follows. [Equation 4] TLL: Liquidus temperature (°C) TSL: Solidus temperature (°C) TC: Center temperature of slab (°C) TC is determined by the two-dimensional difference method shown in Equation 5. [Equation 5] fs=EXPA, A=-62.6521 + 23
．． 5098x+ 4.06918x2 - 1.275
34x3
- 0.47515x4 +
0.106007x5 x=lnt t; Required time (minutes) from the meniscus to the input side of the rolling roll band [0031] Table 2 is a table showing the results of the segregation score of the wire rod in the slab portion determined in Fig. 4. . The wire rod segregation score used here was determined by corroding the cross section of the wire coil with hydrochloric acid and visually observing it, and the average value was 0.
A score of 2 or less is considered passing. [Table 2] [0033] In Table 2, short-time heating of a slab refers to the minimum time necessary for rolling the slab, and long-term heating of a slab refers to the heating time set to diffuse segregation. This is an extended process. [0034] In Table 2, only when the solid fraction at the center of the slab on the entry side of the reduction roll band and the average passing speed of the reduction roll band satisfied the control values, could the wire rod segregation rating standard value be achieved with short-time processing of the slab. . In addition, if one or both of the control values for the solid phase ratio at the center of the slab on the entrance side of the reduction roll band and/or the passing speed of the reduction roll band cannot be satisfied, the wire rod segregation score standard value will be achieved only for the long-term heating of the slab. did it. [0035] As explained above, according to the present continuous casting method, the determination layer is adjusted so that the solid fraction at the center of the slab on the entry side of the reduction roll band and the average passing speed of the reduction roll band satisfy the control values. By separating, the state of segregation can be easily determined. Based on this result, the heat treatment conditions in the heating and rolling process of the slab can be changed and feedforward can be carried out to reduce the process load and create a homogeneous slab. Obtainable.

[Brief explanation of the drawing]

FIG. 1 is an apparatus diagram illustrating the position of slab reduction rolls in continuous casting equipment.

FIG. 2 is a diagram showing the relationship between casting speed at the meniscus position, slab segregation index, and casting length.

FIG. 3 is a diagram showing the relationship between the average passing speed of the reduction roll band and the solid fraction at the center of the slab on the entrance side of the reduction roll band.

[Figure 4] Casting speed at meniscus position at each casting length,
FIG. 3 is a diagram showing the relationship between the average passing speed of the reduction roll band and the solid phase ratio at the center of the slab thickness on the entrance side of the reduction roll band.

FIG. 5 is a diagram showing an etch print collection procedure.

[Explanation of symbols]

1 Meniscus position 2 Slab rolling start position 3 Mold 3a Slab 4 Cross section 5 Etch print

Claims (1)

[Claims] Claim 1: In a continuous casting method for molten metal, in which a slab is pulled down while being rolled down by at least two pairs of rolls at the final stage of solidification, the solid fraction at the center of the slab on the roll-in side of the roll-down roll and the average passing speed of the roll-down roll band are , a continuous casting method characterized by determining that the segregation state of the slab has deteriorated when it exceeds a certain predetermined range.