JPH0616922B2 - Casting method for wire slab with excellent wire drawability - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for casting a wire slab, which is a material for wire drawing and is mainly used for producing a steel wire.

(Prior Art) Wire drawing is an inexpensive and generally performed method for metal working of wire rods, but here, a case where wire drawing of steel rods is performed will be described as an example.

Generally, wire drawing is performed by continuously passing a wire rod through a tool called a die having a cross-sectional area smaller than that of a base metal (wire rod) to form the die according to the hole size. Since this wire drawing is performed by plastically deforming with a die tool, the wire is subjected to tensile, compression and shear deformations. At this time, if the wire drawing conditions are unsuitable, or if there is a defect in the wire material to be drawn out, wire breakage or chevron cracks occur, resulting in operational troubles and quality defects. The disconnection is called a cuppy disconnection because the fracture surface often has a conical shape. A chevron crack is one that does not lead to a cuppy disconnection but has internal cracks.

These problems are extremely serious problems because they are forced to interrupt the wire drawing work, hinder productivity, and significantly reduce product quality. Therefore, it is industrially important to solve these problems.

Conventionally, the following methods have been performed as means for solving the above problems. That is, the causes of wire breaks and cracks are considered separately as inappropriate drawing work or defects in the drawing material.

Defects during wire drawing work include the fact that stress generated during working, particularly tensile stress, acts on the wire, and segregation of elements such as carbon and P called center segregation in the solidification process causes solidification of carbides such as Fe ₃ C. Or phosphide, such as Fe ₃ P, is generated, and the central part of the wire is brittle under the influence of the compound.

In general, it is known that cuppy disconnection and chevron cracks are more likely to occur as the die angle increases and the total processing rate increases. It is also caused by wear of the die due to wire drawing and collapse of the angle. As a result of experimentally elucidating the conditions under which the cuppy disconnection occurred, the die angle was 1
It is said that it is desirable to make the total processing rate as small as possible at about 0 °.

Further, in the case of material defects such as decarburization, segregation and initial cracks, it is necessary to prevent their generation. Since decarburization occurs due to heat treatment before wire drawing,
The aim is to select a heat treatment process that prevents decarburization. For example, it is reported that cracking and disconnection due to decarburization occur rather than performing complete annealing, so decarburization can be prevented by methods such as spheroidizing annealing and lead patenting, and wire drawing can be performed satisfactorily. There is.

Further, in order to prevent segregation and cracks, it was aimed to prevent the center segregation of the slab, which is a cause in the wire rod manufacturing process, especially in the casting process. Central segregation is generated by macrosegregation generated by the distribution of solute between solid and liquid that occurs when transforming from a liquid to a solid in the solidification process.To prevent this, the solidification structure is changed to a columnar structure in the casting process. It is known that it is effective to change the structure from equiaxed crystal structure to suppress dispersion of component segregation during solidification. Low temperature casting, electromagnetic stirring, etc. are effective for making the solidified structure equiaxed. On the other hand, a method called light reduction at the end of solidification has been developed as a means for preventing the concentrated molten steel from remaining in the slab, and center segregation is controlled to take quality measures capable of withstanding wire drawing.

Regarding cracks, quality control in the casting process is performed by optimizing secondary cooling, controlling the casting bus line of the continuous casting machine, suppressing bulging by densifying the slab guide rolls, optimizing the straightening temperature, and generating cracks. To prevent.

Further, as means for solving these problems, a small-section billet continuous casting method has recently been disclosed in Japanese Patent Application Laid-Open Nos. 61-195755 and 61-195756, and is drawing attention.

(Problems to be solved by the invention) However, the above quality measures require facility measures such as electromagnetic stirring, light pressure reduction, and increase of cast strip guide rolls in the continuous casting machine, which increase the manufacturing cost of the wire rod. It is necessary to establish an industrial technology that can be manufactured at low cost without using it.

In the manufacturing process of the wire rod, although some improvement can be seen by the above-mentioned quality measures, it is physically and chemically impossible to completely prevent segregation due to solidification. Solute elements such as P are concentrated during the solidification process. This segregation produces carbides and phosphide,
Embrittlement of the wire cannot be completely avoided. In order to prevent the disconnection of the cuppy and the chevron crack, it is necessary to establish a manufacturing method of the wire rod in consideration of the tensile stress distribution acting on the wire drawing.

(Means for Solving the Problem) The present invention is, when casting using an endless groove type ring mold or a groove type ingot mold with an open upper part, a circle equivalent radius R of a wire rod cross section to be a product, The eccentricity e (e = r / R), which is the ratio of the distance r from the centroid of the wire to the final solidification position, is 0.
Excellent wire drawability characterized by eccentric solidification of the slab by making the cooling rates of the three surfaces supported by the mold different from the open upper surface so as to satisfy the relationship of 15 ≦ e ≦ 0.9. The method of casting a slab for a wire rod and an endless groove type ring mold with an open upper part or a grooved ingot mold are used for three-sided cooling, and the position corresponding to the final solidification in the cross section of the wire member is different from the centroid of the wire member. When eccentrically solidifying so that
Casting of wire rod slabs with excellent wire drawability, which is characterized by eccentricizing the final solidification equivalent position by controlling the heating, heat retention or cooling of the upper surface of the mold to make the cooling rate different from the other three surfaces. Is the way. It is preferable to make the upper open surface a non-oxidizing atmosphere. As for the form of casting, both a continuous casting method and a general ingot making method are possible.

Note that FIG. 8 shows the relationship between the circle equivalent radius R of the cross section of the wire, the distance r from the centroid (center portion) G of the wire to the final solidification position S, and the eccentricity e. This figure shows the case of a circular wire rod, and in the case of a deformed wire rod such as a square wire, the centroid G is the centroid of the cross-sectional shape, and the wire rod equivalent radius R is the equivalent area circle conversion centered on the centroid. Is the radius of time.

(Working) Defects that occur during wire drawing are the maximum tensile stress in the center of the material caused by drawing both of the chevron crack and the disconnection of the cuppy. Due to. In addition, it has been empirically found that the center of the wire, that is, the centroid, is almost coincident with the centroid of the slab, and that the centroid is the final solidification position in general continuous casting.

Therefore, there is a fundamental problem in that the material defects in the center of the wire and the points of maximum tensile working stress inevitably coincide with each other due to the uniform cooling of the four surfaces (that is, the eccentricity e = 0 of the present invention). It is an effective means to prevent disconnection or internal defect that the final solidification position is eccentric so that the eccentricity is at least e> 0 by changing the cooling degree of one surface from the other four surfaces from uniform cooling of four surfaces. It is believed that there is.

When an endless groove type ring mold made of copper or a copper alloy with an open upper part is used, the cooling surface is mainly 3 surfaces,
The contact speed between the slab and the mold is higher than that of cooling using radiation of the open surface or gas such as air. Therefore, the solidification form is concave, and the final solidification position is inevitably eccentric upward from the centroid of the wire rod of the cast slab. Further, the solidification amount (solidification rate) can be easily changed by arbitrarily changing the cooling or heat transfer state on the upper surface side, and the final solidification position of the cast piece can be controlled.

That is, control of the final solidification position of the slab can be achieved by controlling the heating, heat retention, or cooling of the open mold upper surface, for example, electrical means such as induction heating, plasma, electric heating, highly adiabatic fire resistance. It is possible to apply a heat-retaining means using a conductive material. Of course, a sufficient effect can be obtained by introducing a gas such as an inert gas such as Ar and cooling it.

The final solidification position can be easily determined by knowing the processing conditions in the wire drawing, that is, the stress state.

On the other hand, the cross-sectional shape of the conventional continuously cast slab for wire rods is usually such that the long / short side ratio of the slab is a value of about 1 to 2, and the slab is called a bloom or billet. Since the four surfaces of the continuous casting mold for bloom or billet are cooled to almost the same level, and the slab shape ratio (long / short side ratio) is close to 1,
Even if the cooling after the mold is adjusted, it is practically difficult to decenter the final solidification position from the centroid of the wire rod of the slab. Therefore, since the concentration of components in the center inevitably occurs, as described above, it is possible to concentrate the components by electromagnetic stirring in the solidification process, light pressure at the end of solidification or casting with the molten steel temperature lowered as much as possible. I had no choice but to reduce casting.

Next, the reason for defining the eccentricity e to be 0.15 to 0.9 is that if it is smaller than 0.1, the effect of eccentric solidification does not appear, and as the value approaches 0.9, the final solidified portion approaches the surface and surface cracking occurs during wire drawing. Occurrence is seen, and when it is larger than 0.9, it becomes more remarkable. Also, the chevron crack has an eccentricity e
It is improved by securing 0.15 or more, and the range of 0.25 to 0.6 is particularly desirable.

(Example) The details of the present invention will be further described with reference to Examples and Comparative Examples.

FIG. 1 is a diagram showing an outline of a horizontal rotary continuous casting apparatus using an endless groove type ring mold 4, FIG. 2 is a diagram showing a cross section taken along the line AA of FIG. 1, and FIG. 3 is a line taken along the line B- of FIG. It is a figure which shows B cross section.

Molten metal composed of components to be used for wire drawing is poured from a ladle 1 into an endless groove type ring mold 4 made of copper or a copper alloy through a pouring amount control device 2, a sand dish 14 and a pouring nozzle 3. A molten metal backflow prevention weir 5 is provided in the mold 4, and the molten metal is cast only in the direction of arrow 6.
If necessary, a mold upper surface solidification amount control means 7 and / or a non-oxidizing atmosphere holding means 8 are provided between the molten metal backflow prevention dam 5 and the slab withdrawal correction means 10 to control the amount of solidification on the upper surface side, Alternatively, oxidation of the molten metal is prevented to control the final solidification position to prevent quality deterioration. The mold upper surface solidification amount control means 7 comprises a heat insulating agent, a heating device or a cooling device, or a combination thereof. The solidified slab 9 is taken out from the mold 4 by the slab withdrawal correction means 10 and is passed through a cutting machine 11 to obtain a slab that can be rolled. The mold 4 is driven by the electric motor 12, and the injection amount control device 2 is tilted by the electric motor 13. The endless groove type ring mold 4 is cooled and sealed by a cooling water pipe 15 and a non-oxidizing gas 16.

On the other hand, when the groove-type ingot casting mold 17 shown in FIGS. 4 and 5 is used, the same ingot can be obtained by substituting the endless groove-type ring mold 4 for casting.

FIG. 6 (a) is a diagram schematically showing the solidification state of a slab obtained by the endless groove type ring mold 4 or the groove type ingot mold 17, and the intersection point S is the final solidification point. The intersection S is eccentric from the centroid of the wire due to the difference in the amount of heat removed from the mold surface. In the casting process, since the back surfaces of the molds 4 and 17 are forcibly cooled with a cooling medium such as water, the slab cooling capacity of the three surfaces can be made higher than the open top surface, so that the solidification proceeds while showing a concave shape. Also, the final solidification position is eccentric to the upper surface side from the center of the mold center by natural cooling on the upper surface side or the mold upper surface solidification amount control means 7. Further, in the endless groove type ring mold 4, since the solidification shrinks or the heat shrinks as the solidification progresses, the mold closely adheres to the inner peripheral surface of the slab, and strictly speaking, the final solidification point S is above the core of the transverse cross-sectional view of the mold and in the mold. It is diagonally above the axis of rotation.

6 (b) and 6 (c) are schematic views of typical solidification states found in ordinary bloom and billet casting. Same figure
(b) shows a case where the whole cast piece shows a solidification form called columnar crystal, and the shape is such that the centroid of the wire rod of the cast piece and the final solidification position T substantially coincide with each other. Further, FIG. 7C shows the case where the transition between the columnar crystal and the equiaxed crystal is accompanied during the solidification, but the final solidification position T is the center of the slab. When the degree of superheat of molten steel is lowered or electromagnetic stirring is performed during casting to give molten steel flow to the solidification interface and solidify, the solidification morphology of FIG. Conventionally, in order to improve the center segregation, it has been aimed to change the structure of FIG. 6B to the structure of FIG.

When the slab cast by the method of the present invention is subjected to hot rolling, particularly wire rod rolling, the final solidification position of the slab remains eccentric from the center of the wire rod. Therefore, the segregation portion no longer exists at the center of the wire, and the material is a material that avoids embrittlement at the center. That is, the embrittlement region due to carbide or the like inevitably generated in the final solidified portion can be separated from the central portion which is a problem in wire drawing, and has sufficient deformability even when tensile stress acts in wire drawing, It is possible to significantly reduce defects such as disconnection of cuppies and chevron cracks.

No. 1 for molten steel components and manufacturing conditions cast as wire drawing materials
The results are shown in Table 2 and Table 2.

The examples were based on continuous casting by the continuous casting machine shown in FIGS. 1 to 3 and the ingot making method using the ingot making molds shown in FIGS. 4 and 5. On the other hand, the comparative example is carried out by a normal continuous casting method in which molten steel is continuously poured into a water-cooled mold to produce a blame, and the continuous casting machine type has a slab cross-sectional size of 247 × 300 mm,
Continuous casting machine This is an arc type continuous casting machine with a radius of 12 mR.

Φ by wire rod rolling from a slab manufactured under the above casting conditions
A 5.5 mm wire rod was manufactured. Next, by wire drawing, φ1.7mm
I drew up to. The die angle used for processing was set to 30 ° in comparison with the usual 10 °, and the test was conducted under strict conditions where a chevron crack or a disconnection of the cuppy was apt to occur.

The wire rod cast by the method of the present invention did not cause the cuppy disconnection due to the defect in the center portion of the wire rod. However, due to severe processing conditions, some chevron cracks occurred.

On the other hand, in the comparative example, the disconnection of the cuppy or the chevron crack was easily generated. When the rate of occurrence of cracks in the example is evaluated with the rate of occurrence of chevron cracks (including cuppy disconnection) of the comparative example being 100, the eccentricity defined by the ratio of the eccentricity amount from the center of the wire to the final solidification position and the radius of the wire is calculated. A strong dependence was observed.

FIG. 7 shows the crack generation rate. When the eccentricity is 10%, the occurrence rate is 80%, 20% is 50%, 25% is 15%, 30
% Was 3%.

As described above, when manufacturing a wire drawing wire, by making the center position of the wire material different from the final solidification position during casting, it is possible to significantly reduce material defects such as chevron cracks during wire drawing. Is possible.

(Effects of the Invention) According to the present invention, a slab for a wire rod in which a central defect unavoidably generated in the wire rod is eccentric from the maximum stress generation position during the wire rod processing is obtained, and it is possible to obtain a cuppy disconnection or a chevron crac during the wire drawing processing. Occurrence of defects can be prevented.

[Brief description of drawings]

FIG. 1 is a view showing an outline of a horizontal rotary continuous casting apparatus using an endless groove type ring mold, FIG. 2 is a view showing a cross section taken along the line AA of FIG. 1, and FIG. 3 is a line taken along the line BB of FIG. Sectional drawing, FIG. 4 and FIG. 5 are drawings showing a groove-type ingot casting mold, FIG. 6 (a) to (c) are drawings schematically showing the solidified state of the slab, and FIG. 7 is a crack. It is a figure which shows an incidence. FIG. 8 is a diagram showing an example of a relationship between a circle equivalent radius R of a cross section of a wire rod, a distance r from the centroid of the wire rod to a final solidification position, and an eccentricity e. 1 ... Ladle, 2 ... Injection amount control device, 3 ... Injection nozzle, 4 ... Endless groove type ring mold, 5 ... Molten metal backflow prevention weir, 6 ... Casting direction, 7 ... Mold upper surface solidification Quantity control means, 8 ... Non-oxidizing atmosphere holding means, 9 ... Cast slab, 10
...... Slab withdrawal correction means, 11 …… Cutting machine, 12, 13
...... Electric motor, 14 …… Tundish, 15 …… Cooling water piping, 16 …… Non-oxidizing gas, 17 …… Groove type ingot mold.

Claims (3)

[Claims] 1. When casting using an endless groove type ring mold or a groove type ingot mold with an open upper part, the circle equivalent radius R of the cross section of the wire rod to be the product and from the centroid of the wire rod to the final solidification position Of the three surfaces supported by the mold and the open upper surface so that the eccentricity e (e = r / R), which is the ratio with the distance r of 0.15 ≦ e ≦ 0.9, is satisfied. A method for casting a slab for wire rods having excellent wire drawability, which comprises eccentrically solidifying the slab by making the speeds different. 2. An endless groove type ring mold having an open upper part or a groove type ingot mold is used for three-side cooling, and eccentricity is made so that the final solidification equivalent position in the cross section of the wire is different from the centroid of the wire. When solidifying, by controlling the heating, heat retention or cooling of the upper surface of the mold, the cooling rate is made different from the other three surfaces to make the final solidification equivalent position eccentric. Casting method for casting slabs. 3. The method for casting a slab for wire rod having excellent wire drawability according to claim 1 or 2, wherein the upper open surface is a non-oxidizing atmosphere.