JPH11254106A - Method for continuously casting blank for pc steel bar and continuous casting apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method of a blank for PC steel bar, with which relating to a duplex layer cast billet as the blank for PC steel bar having the high strength and the delayed fracture resistance without deteriorating at spot weldability, the duplex layer cast billet having a low carbon layer having the suitable thickness on the surface layer part and a high carbon layer having sufficient strength of the core part can be obtd. at low cost. SOLUTION: In the case of continuously casting the steel, molten low carbon steel is supplied into a mold 3 from a tundish 1 through a nozzle 2. Further, a wiry additive (wire 5) inserting anyone among C, C+Si, C+Mn and C+Si+ Mn into a metal-made outer is continuously supplied into the molten steel in the mold and at the position which the solidified layer in the cast billet has a prescribed thickness, the molten steel is electromagnetically stirred in the horizontal direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously casting a material for a PC steel bar.

[0002]

2. Description of the Related Art PC steel rods are steel rods used as tension members for prestressed concrete. Depending on the manufacturing method, there are three types: drawn steel rods, rolled steel rods, and heat-treated steel rods, with a minimum of 80 kgf / mm.
It is required to have a strength in a range from ² to a maximum of 145 kgf / mm ² and a delayed fracture resistance that does not deteriorate particularly by spot welding.

[0003] As a material production method satisfying these characteristics, for example, as disclosed in Japanese Patent Application Laid-Open No. Sho 62-267420, steel of a predetermined component is directly rolled and directly quenched, thereby decarburizing the surface layer and removing the interior. Has a martensite structure or a martensite + ferrite structure. However, according to this method, an extremely long decarburization treatment is indispensable to form a sufficiently thick decarburized layer on the surface layer so that the heat effect of the spot welding does not reach the inside. However, there was a drawback that remarkably increased.

On the other hand, as a raw material for a PC steel rod, a multi-layer cast slab having a low carbon surface layer by pouring and solidifying a low carbon molten steel around a core in a casting step in advance in a casting mold. Is also available. However, this method is not practical because it requires special equipment to completely join the core material and the molten steel at the mutual interface, and furthermore, low carbon steel is used to reduce the carbon content of the surface layer to the level of the decarburized layer. There is a drawback that the cost must be increased due to the necessity of melting.

On the other hand, Japanese Patent Application Laid-Open No. 6-57367 discloses a method of continuously inserting a steel cylinder filled with C or another alloy element into the center of molten steel in a continuous casting mold. Have been. According to this, special equipment for completely joining the core material and the molten steel at the mutual interface as described above is not required, and it is not necessary to melt the ultra-low carbon steel. But,
According to this method, the melting position of the steel cylinder is not fixed at a predetermined position and is unstable. Therefore, there is a disadvantage that it is difficult to stably form a low-concentration layer having an appropriate thickness on the surface layer.

[0006]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the prior art, the present invention provides a multilayer slab as a material for a PC steel bar having high strength and delayed fracture resistance which does not deteriorate due to spot welding. The present invention provides a continuous casting method for a PC steel rod material, which has a low-carbon layer of an appropriate thickness on a surface layer and a high-strength carbon layer of a sufficient strength on a core material to obtain a low-cost multilayer slab. The purpose is to do.

[0007]

According to the present invention, a low carbon molten steel is supplied from a tundish into a mold by a nozzle in continuous casting of steel, and C, C + Si, C + Mn, C
+ Si + Mn is continuously supplied to the molten steel in the mold with a wire-shaped additive material stored in a metal jacket, and the molten steel is electromagnetically displaced horizontally at a position where the solidified layer of the slab becomes a predetermined thickness. This is a continuous casting method for a raw material for a PC steel bar, which is characterized by stirring.

[0008] In the above invention, it is preferable to adjust the supply speed so that the wire-shaped additive is melted at the position of the molten steel under electromagnetic stirring. Further, in the present invention, the composition of the slab central part is C: 0.25-0.40% by weight%,
i: It is preferably 1.5 to 3.0%. Further, the present invention provides a continuous casting apparatus suitable for carrying out the above-described method for continuously casting a material for a PC steel bar, in a continuous casting apparatus for steel provided with a tundish stopper, wherein the tundish stopper is formed as a hollow body. A wire feeder that supplies a wire-like additive to the unsolidified molten steel in or below the mold through the hollow part of the tundish stopper, and electromagnetic stirring that can horizontally stir the unsolidified molten steel in or below the mold Another object of the present invention is to provide a steel continuous casting apparatus provided with the apparatus.

[0009]

FIG. 1 is a schematic view showing an example of a continuous casting facility suitable for carrying out the present invention. As shown,
In the present invention, when continuously casting steel, low carbon molten steel is supplied from a tundish 1 into a mold 3 by a nozzle 2 and one of C, C + Si, C + Mn, and C + Si + Mn is housed in a metal jacket such as steel. A wire-shaped additive (hereinafter simply referred to as a wire) 5 is continuously supplied to a preferably central portion of molten steel 6 in a mold, and the molten steel is electromagnetically moved horizontally at a position where a solidified layer has a predetermined thickness. Stir. In addition, 4 is a wire supply device for supplying a wire, and 7 is an electromagnetic coil for applying an electromagnetic force to molten steel.

In addition, in the example shown in FIG. 1, a tundish stopper is formed as a hollow body, and the wire is supplied from above to the center of the molten steel via the stopper and the immersion nozzle. This is preferable because the wire can be supplied without involving the mold powder on the molten steel bath surface in the mold. The wire may be added to the molten steel by another method as long as the same effect can be obtained.

The supply speed of the wire 5 is set so that the wire 5 is melted at a position where the above-mentioned solidified layer having a predetermined thickness is obtained. At this predetermined position, the electromagnetic force is applied to the molten steel 6 by the electromagnetic coil 7 to stir the molten steel 6 in the horizontal direction. This horizontal stirring has two important significances. one is,
The purpose is to ensure that additional elements such as C eluted in the molten steel due to the melting of the wire are uniformly distributed in the unsolidified molten steel that will later become a core material. Another is that, due to the flow of the molten steel in the horizontal direction, the low-concentration molten steel above it is separated from the high-concentration molten steel below it, and mixing with each other is less likely to occur. Thus, mixing of the molten steel above and below the predetermined position is minimized. The predetermined thickness of the solidified layer is desirably about 10 to 12 mm.

By effectively utilizing the electromagnetic stirring, a portion solidified above the predetermined position becomes a low carbon surface layer portion, and a portion solidified below the predetermined position uniformly concentrates the added element by the wire 5. Since the high carbon core material becomes a high carbon core material, a multilayer slab having a low component layer of an appropriate thickness in the surface layer and a core material having sufficient strength inside can be obtained. Preferably, C is graphite, Si is ferrosilicon, and Mn is ferromanganese powder or granular material.

[0013] The PC steel rod according to the present invention has a sufficiently low carbon in the surface layer, has low delayed fracture susceptibility, and suppresses the generation and propagation of cracks from the surface layer, and thus has excellent delayed fracture resistance. I have. Since the thickness of the low carbon surface layer portion is accurately controlled within an appropriate range, even if spot welding is performed, the heat affected zone does not reach the high carbon core portion and the delayed fracture resistance does not deteriorate. According to the knowledge of the present inventors, the appropriate thickness range of the low carbon surface layer portion of the PC steel rod is approximately 1.0 to 1.5 mm.
Since the carbon content of the core material is sufficiently high, the high strength required value as a PC steel bar is sufficiently satisfied as a whole cross section.

According to the present invention, the surface layer can be made low carbon in the casting stage, so that it is not necessary to perform a long-time decarburization treatment on the cast slab after solidification, and it is simple to use in a conventional continuous casting facility. Since the present invention can be implemented only by adding a wire supply device having a simple structure and an electromagnetic coil, there is no increase in cost. In the present invention, in order to provide more excellent performance as a PC steel rod, it is preferable that the components of the core part of the multilayer slab be as follows.

If the C of the core material portion is less than 0.25%, it is difficult to obtain sufficient strength, so that it is 0.25% or more, and if it exceeds 0.40%, quenching cracks easily occur, so it is preferable to limit it to 0.40% or less. If the Si content of the core material is less than 1.5%, sufficient ductility is difficult to obtain, so the content is set to 1.5% or more. If it exceeds 3.0%, the improvement in ductility reaches a plateau and the hot workability deteriorates.
% Is preferable. The Mn of the core part is 0.1
If it is less than 0.1%, it is difficult to obtain sufficient strength.
If it exceeds 5%, burning cracks are likely to occur, so it is preferable to limit the content to 1.5% or less.

[0016]

EXAMPLES (Example 1) Using the continuous casting equipment shown in FIG. 1, a multilayer slab as a material for a PC steel rod was manufactured. C in the hollow part of steel wire 5 with outer diameter 12.0mm and inner diameter 10.0mm
(Graphite), melted to a composition indicated by symbols 1A to 1E in Table 1, and cast from a tundish 1 through a nozzle 2 into a mold 3
The molten steel 6 poured into the inside of the mold is continuously supplied to the central part in the mold, and the wire is formed at a position where the solidified layer thickness becomes 10 to 12 mm (in this experimental condition, a position of 0.32 to 0.46 m below the surface of the molten steel in the mold). Example 5 was performed in such a manner that the molten steel 5 was melted and the molten steel was cast by the electromagnetic coil 7 disposed so as to sandwich the melting position while horizontally stirring the molten steel.

On the other hand, for the molten steel having the composition indicated by the symbols 1F to 1J in Table 1, the wire 5 is supplied to the center of the molten steel in the mold in the same manner as in Example 1, except that the electromagnetic coil 7 is not used and the casting is performed. Was carried out to obtain Comparative Example 1. The multilayer slab obtained by this casting was formed into a wire having a diameter of 13 mm by hot rolling, and the wire was subjected to a heat treatment of quenching and tempering under the conditions shown in Table 2 to produce a PC steel rod. Was measured to evaluate the surface layer thickness, and the mechanical properties were investigated. Further, using the PC steel rod, a PC pole reinforcing body was assembled by spot welding, and the PC steel rod having a heat-affected zone by the spot welding was put into a 20 kg NH ₄ NO ₃ aqueous solution of 95 kg at 100 kgf / m 2.
The time until fracture was measured by applying a stress of m ² , and the delayed fracture resistance was evaluated. In addition, the composition of the core part and the surface part of this PC steel rod was chemically analyzed in order to judge the quality of the slab multilayering.
In this chemical analysis, in order to exclude the influence of decarburization in a process downstream from the casting process, a range from 0.2 mm to 1 mm in depth from the surface of the wire for the surface layer was used for the chemical analysis sample. With respect to the core portion, a range from the center of the wire to a radius of 5 mm was provided to the chemical analysis sample, and the uniformity of components in the cross section was examined by EPMA.

Tables 2 and 3 summarize the results of these investigations. From Table 2, it can be seen that in Example 1 (1A to 1E), the surface layer thickness was in the appropriate range of 1.0 to 1.3 mm as compared with Comparative Example 1 (1F to 1J), and the proof stress, tensile strength and delayed fracture resistance were obtained. Both excellent PC steel bars were obtained. Further, as shown in Table 3, in Example 1, the C content of the core portion was higher than that of Comparative Example 1 and was evenly distributed.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

[Table 3]

Example 2 Using the continuous casting equipment shown in FIG. 1, a multilayer slab as a material for a PC steel rod was manufactured.
C in the hollow part of steel wire 5 with outer diameter 12.0mm and inner diameter 10.0mm
Filled with (graphite) and Si and continuously supplied to the central part of the molten steel 6 melted into a composition indicated by symbols 2A to 2E in Table 4 and poured into the mold 3 from the tundish 1 through the nozzle 2 Then, the wire 5 is melted at a position with a solidified layer thickness of 10 to 12 mm (in this test condition, at a position of 0.32 to 0.46 m below the surface of the molten steel in the mold), and an electromagnetic coil disposed so as to sandwich the melting position Example 7 was carried out by performing casting while horizontally stirring the molten steel according to Example 7.

On the other hand, for molten steel having the composition indicated by symbols 2F to 2J in Table 4, the wire 5 was supplied to the center of the molten steel in the mold in the same manner as in Example 2, except that the electromagnetic coil 7 was not used and the steel was cast. To obtain Comparative Example 2. The multilayer slab obtained by this casting was formed into a wire rod having a diameter of 13 mm by hot rolling, and the wire rod was subjected to a heat treatment of quenching and tempering under the conditions shown in Table 5 to produce a PC steel rod. Was measured to evaluate the surface layer thickness, and the mechanical properties were investigated. Further, using the PC steel rod, a PC pole reinforcing body was assembled by spot welding, and the PC steel rod having a heat-affected zone by the spot welding was put into a 20 kg NH ₄ NO ₃ aqueous solution of 95 kg at 100 kgf / m 2.
The time until fracture was measured by applying a stress of m ² , and the delayed fracture resistance was evaluated. In addition, the composition of the core part and the surface part of this PC steel rod was chemically analyzed in order to judge the quality of the slab multilayering.
In this chemical analysis, in order to exclude the influence of decarburization in a process downstream from the casting process, a range from 0.2 mm to 1 mm in depth from the surface of the wire for the surface layer was used for the chemical analysis sample. With respect to the core portion, a range from the center of the wire to a radius of 5 mm was provided to the chemical analysis sample, and the uniformity of components in the cross section was examined by EPMA.

Tables 5 and 6 summarize the results of these investigations. From Table 5, it can be seen that in Example 2 (2A to 2E), the surface layer thickness was in the appropriate range of 1.0 to 1.3 mm as compared with Comparative Example 2 (2F to 2H), and the proof stress, tensile strength, uniform elongation, A PC steel rod excellent in both delayed fracture resistance has been obtained. Note that, in Comparative Example 2, 2I and 2J caused quenching cracks, so that the mechanical properties and the delayed fracture resistance were not examined. Also, from Table 6, in Example 2, the C content of the core portion was higher and more evenly distributed than in Comparative Example 2 (2F to 2H) in which no cracking occurred. In addition, the comparative example 2 (2
In (I, 2J), Si was excessively concentrated in the core portion exceeding 3.0%.

[0025]

[Table 4]

[0026]

[Table 5]

[0027]

[Table 6]

As is apparent from the above embodiment, according to the present invention in which the addition of the component elements by the wire to the central portion of the molten steel in the mold and the appropriate electromagnetic stirring at a predetermined position are combined,
A more excellent multilayer slab can be obtained as a material for C steel bars.

[0029]

As described above, according to the present invention, a core material having a low component surface layer having a thickness sufficient to secure excellent delayed fracture resistance and having a component concentrated enough to ensure sufficient strength and ductility. It has an excellent effect that a multi-layer cast piece as a material for a PC steel bar having a portion can be manufactured at low cost.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an example of a continuous casting facility suitable for carrying out the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Tundish 2 Nozzle 3 Mold 4 Wire supply device 5 Wire 6 Molten steel 7 Electromagnetic coil 8 Tundish stopper

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/04 C22C 38/04

Claims (4)

[Claims] In a continuous casting of steel, a low-carbon molten steel is supplied from a tundish into a mold by a nozzle using a nozzle, and any one of C, C + Si, C + Mn, and C + Si + Mn is housed in a metal jacket. The additive material is continuously supplied to the molten steel in the mold, and the molten steel is electromagnetically stirred horizontally at a position where the solidified layer of the slab has a predetermined thickness.
Continuous casting method for C steel rod material. 2. The continuous casting method for a raw material for a PC steel rod according to claim 1, wherein the supply speed is adjusted so that the wire-shaped additive material is melted at a molten steel position that is subjected to electromagnetic stirring. 3. The composition of the slab center portion is C: 0.25 to% by weight.
3. The method for continuously casting a material for a PC steel bar according to claim 1, wherein the content is 0.40% and Si is 1.5 to 3.0%. 4. A continuous casting apparatus for steel provided with a tundish stopper, wherein the tundish stopper is formed as a hollow body, and a wire-like addition is made to the unsolidified molten steel in or below the mold through the hollow portion of the tundish stopper. A continuous casting apparatus for steel, comprising: a wire supply device for supplying a material; and an electromagnetic stirring device capable of horizontally stirring unsolidified molten steel in or below the mold.