Method for manufacturing steel ingots for rolls

Abstract

To provide a method for manufacturing a steel ingot for a roll that can eliminate freckled segregation, and to provide a steel ingot for a roll from which freckled segregation is eliminated.SOLUTION: A method for manufacturing a steel ingot for a roll manufactures a steel ingot for a roll while forming a melting pool by melting a consumable electrode with an electroslag melting method, where a frequency f of an AC current to be applied to the consumable electrode 3 is restricted to 10 Hz or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to Manufacturing method of steel ingots for rolls . [Background technology]

[0002] For example, rolls such as work rolls and backup rolls are used for rolling cold-rolled steel sheets, etc. Rolls are made from cylindrical steel ingots, which are forged to reduce the diameter, mainly at both ends, and then subjected to heat treatment and various types of machining. This type of roll requires high wear resistance as a basic performance. Therefore, by increasing C to about 1% and Cr to about 3% to 10%, wear resistance is improved by precipitation hardening of carbides. In addition, about 1% of Mo is also added to increase the strength of the rolling roll. Since these elements have different specific gravities, segregation is likely to occur during the production of steel ingots for rolls. In order to prevent segregation, steel ingots for rolls are produced by an ESR (Electro Slag Remelting) melting method as disclosed in Patent Document 1 below, for example. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Utility Model Application Publication No. 62-203258 Summary of the Invention [Problem to be solved by the invention]

[0004] In the electroslag melting apparatus disclosed in Patent Document 1, a polarity converter is provided to mechanically reverse the polarity of the power supply. This polarity change changes the flow direction of the molten metal in the melting pool, resulting in a homogeneous steel ingot. However, according to the results of the inventors' studies, when a high-frequency current of, for example, about 50 to 60 Hz is applied to the consumable electrode, the Lorentz force acts mainly on the surface layer of the melting pool and may not reach the inside of the melting pool sufficiently. In this case, the lack of Lorentz force prevents the molten metal from flowing uniformly inside the melting pool, and freckle segregation is likely to occur. If a rolling roll is manufactured from a steel ingot for rolls that still has freckle segregation, when this rolling roll is used to roll a steel plate, there is a risk that partial cracks will occur in the rolling roll starting from the position of the freckle segregation. If such cracks appear on the surface of the rolling roll, they may be transferred to the surface of the rolled steel plate and become a pattern, which may impair the quality of the rolled steel plate.

[0005] The present invention has been made in view of the above circumstances, and has an object to provide a method for producing steel ingots for rolls that can eliminate freckle segregation. Let us assume that. [Means for solving the problem]

[0006] In order to solve the above problems and achieve the above objects, the present invention employs the following means. (1) A method for producing a steel ingot for rolls according to one aspect of the present invention is a method for producing a steel ingot for rolls by an electroslag melting method. One A method for producing a steel ingot for rolls while melting a consumable electrode to form a molten pool, wherein the frequency f of an AC current applied to the consumable electrode is limited to 10 Hz or less. According to the method for producing a steel ingot for rolls described in (1) above, it is possible to produce a steel ingot for rolls free of freckle segregation. In other words, when the frequency of the AC current passed through the consumable electrode is limited to 10 Hz or less, the Lorentz force acts uniformly inside the melting pool. This makes the flow of the molten metal in the melting pool uniform, suppressing segregation of steel components. In addition, since there is no need to newly install a polarity conversion device or the like as in Patent Document 1, it is possible to produce a steel ingot for rolls at low cost.

[0007] (2) In the method for producing a steel ingot for rolls described in (1) above, the steel ingot for rolls may have an outer diameter D of 450 mm to 1200 mm. In the case of the method for producing a steel ingot for rolls described above in (2), it is possible to obtain a steel ingot for rolls having an outer diameter suitable for producing, for example, a rolling roll. Effect of the Invention

[0009] According to the above-mentioned aspects of the present invention, a method for producing a steel ingot for rolls capable of eliminating freckle segregation is provided. can. [Brief description of the drawings]

[0010] [Figure 1] 1 is a vertical sectional view showing a schematic diagram of an apparatus for manufacturing a steel ingot for rolls according to one embodiment of the present invention. FIG. [Diagram 2] FIG. 2 is a longitudinal sectional view showing a schematic diagram of the method for producing a steel ingot for rolls according to the embodiment, in which (a) shows the state before melting and (b) shows the state during melting. [Diagram 3] FIG. 1 is a photograph of a cross section perpendicular to the central axis of a steel ingot for rolls produced by the method for producing a steel ingot for rolls of the present embodiment. [Figure 4] FIG. 1 is a photograph of a cross section perpendicular to the central axis of a steel ingot for rolls produced by a conventional method for producing a steel ingot for rolls. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] An embodiment of a method for producing a steel ingot for rolls and a steel ingot for rolls according to the present invention will be described below with reference to Figures 1 and 2. Figure 1 is a vertical cross-sectional view that typically shows an apparatus for producing a steel ingot for rolls according to the present embodiment. Figure 2 is a vertical cross-sectional view that typically shows the method for producing a steel ingot for rolls according to the present embodiment, where (a) shows the state before melting and (b) shows the state during melting.

[0012] As shown in FIG. 1, the apparatus for manufacturing a steel ingot for rolls in this embodiment includes a mold 1, a chamber 2, a consumable electrode 3, a stub 4, an electrode lifting mechanism (not shown), an AC power source 5, a cooling water supply mechanism (not shown), and a control device 6. The mold (water-cooled copper mold) 1 is a cylindrical body with a bottom, and is disposed so that its opening 1a faces upward and its central axis is aligned vertically. The inner diameter dimension (mold diameter) of the mold 1 is the same as the outer diameter dimension of the steel ingot for rolls to be produced.

[0013] The chamber 2 is a cylindrical body with a bottom, and houses the mold 1 coaxially therein. A flow path 2a is formed between the outer peripheral surface and outer bottom surface of the mold 1 and the inner peripheral surface and inner bottom surface of the chamber 2. A cooling water inlet 2b and a cooling water outlet 2c are formed in the upper part of the chamber 2. Both the cooling water inlet 2b and the cooling water outlet 2c communicate with the flow path 2a. The cooling water W introduced from the cooling water inlet 2b cools the mold 1 from its surroundings as it passes through the flow path 2a, and is then discharged from the cooling water outlet 2c.

[0014] The cooling water supply mechanism introduces the cooling water W discharged from the cooling water outlet 2c and lowers the temperature of the water to a predetermined temperature. Then, the cooling water supply mechanism sends the cooling water W toward the cooling water inlet 2b. In this manner, the cooling water supply mechanism cools the molten metal in the mold 1 while circulating the cooling water W. Note that, although the present embodiment describes a case where the mold 1 is water-cooled, it is not limited to water cooling, and air cooling may also be used.

[0015] The consumable electrode 3 is the base material of the steel ingot for the roll, and has a cylindrical shape with a central axis along the vertical direction. A stub 4 is welded and fixed coaxially to the upper end of the consumable electrode 3. The stub 4 is raised and lowered in the vertical direction by the electrode lifting mechanism. Thus, the consumable electrode 3 is raised and lowered by the electrode lifting mechanism while being supported within the mold 1 by the stub 4. The stub 4 and the consumable electrode 3 are electrically connected, and the chamber 2 and the mold 1 are also electrically connected.

[0016] The AC power source 5 receives an instruction from the control device 6 and supplies an AC current having a predetermined frequency f and a predetermined current value between the stub 4 and the chamber 2. The upper limit of the frequency f is limited to 10 Hz. A more preferable upper limit of the frequency f is 5 Hz. The lower limit of the frequency f is 0.5 Hz, and more preferably 1 Hz.

[0017] As shown in FIG. 2(a), molten slag SL is held in the mold 1 before current is passed through it. Next, as shown in FIG. 2(b), the consumable electrode 3 is lowered and immersed in the molten slag SL, and an AC current is applied from an AC power source 5. This causes the molten slag SL to heat up due to Joule heat. The consumable electrode 3 melts gradually from the bottom end due to the heat generated by the molten slag SL. The melted consumable electrode 3 becomes molten droplets, sinks in the molten slag SL, and solidifies in layers while being stored in the mold 1. In this way, the consumable electrode 3 melts gradually from the bottom end to the top end, and the molten metal solidifies gradually in the mold 1, thereby obtaining a steel ingot for rolls.

[0018] According to the method for producing steel ingots for rolls using the ESR (Electro Slag Remelting) melting method, the molten metal stored in the mold 1 solidifies while forming a molten pool P. During this process, the molten droplets of the molten consumable electrode 3 are removed of inclusions such as oxides as they sink in the molten slag SL. At this time, since the frequency of the AC current passed through the consumable electrode 3 is limited to 10 Hz or less, the Lorentz force acts uniformly inside the molten pool P. As a result, the flow of the molten metal in the molten pool P becomes uniform, and segregation of the steel components is suppressed. This makes it possible to produce a steel ingot for rolls that is free of freckle segregation. Since the steel ingot for rolls obtained in this manner is free of freckle segregation, when this steel ingot for rolls is processed to produce, for example, a rolling roll, no cracks due to freckle segregation will occur on the surface. This makes it possible to obtain a rolled steel sheet of excellent quality that is free of surface scratches.

[0019] The lower the frequency of the alternating current applied to the consumable electrode 3, the more preferable it is. However, the lower limit is set to the above-mentioned 0.5 Hz for the reason that this can be easily obtained from the viewpoint of equipment. Furthermore, by setting the mold diameter D (mm) of the steel ingot for rolls to be produced to 450 mm to 1200 mm, it is possible to obtain a steel ingot for rolls having an outer diameter suitable for producing mill rolls.

[0020] As described above, the method for producing a steel ingot for rolls of this embodiment is a method for producing a steel ingot for rolls while melting a consumable electrode 3 by an electroslag melting method to form a molten pool P, and the frequency f of the alternating current passed through the consumable electrode 3 is limited to 10 Hz or less. According to this method for producing a steel ingot for rolls, it is possible to reliably eliminate freckle segregation and produce a steel ingot for rolls having excellent quality. EXAMPLES

[0021] A steel ingot for rolls (Fig. 3) was produced by the method for producing a steel ingot for rolls of the present embodiment, and a steel ingot for rolls (Fig. 4) was produced by a conventional method for producing a steel ingot for rolls. Both Fig. 3 and Fig. 4 are photographs of a cross section perpendicular to the central axis of the steel ingot for rolls. The steel ingot for rolls produced by the method for producing a steel ingot for rolls of the present embodiment was produced under the following production conditions. As a result, a steel ingot for rolls of excellent quality free of freckle segregation could be obtained. ·Dissolution rate Vc=3.3mm / min Frequency f=5Hz Mold diameter D=830mm

[0022] On the other hand, in the case of a steel ingot for rolls produced by a conventional method for producing steel ingots for rolls, the production conditions were as follows: Freckle segregation was confirmed at many positions indicated by circles in the photograph. ·Dissolution rate Vc=3.3mm / min ·Frequency f=60Hz Mold diameter D=830mm

[0023] From the above results, it was confirmed that by limiting the frequency f of the AC current applied to the consumable electrode 3 to 10 Hz or less, a high-quality steel ingot for rolls in which freckle segregation is eliminated can be obtained. [Explanation of symbols]

[0024] 3 Consumable electrode P soluble pool

Claims

1. A method for producing a steel ingot for rolls by melting one consumable electrode by an electroslag melting process to form a molten pool, comprising the steps of: The frequency f of the AC current applied to the consumable electrode is limited to 10 Hz or less.

1. A method for producing steel ingots for rolls, comprising the steps of:

2. The outer diameter D of the steel ingot for rolls is 450 mm to 1,200 mm.

2. The method for producing a steel ingot for rolls according to claim 1.

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