JP7529980B2 - Method for manufacturing steel ingots for rolls and steel ingots for rolls - Google Patents

Description

This invention relates to a method for manufacturing steel ingots for rolls and steel ingots for rolls.

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.

Publication No. 62-203258

This ESR melting method makes it possible to suppress segregation to a certain extent. However, it has not been easy to completely remove freckle segregation. If a rolling roll is manufactured from a steel ingot for rolls that still has freckle segregation, there is a risk that partial cracks will occur in the rolling roll, starting from the location of the freckle segregation, when a steel plate is rolled using this rolling roll. If such cracks appear on the surface of the rolling roll, they will be transferred to the surface of the rolled steel plate, forming a pattern, which may impair the quality of the rolled steel plate.

The present invention was made in consideration of the above circumstances, and aims to provide a method for manufacturing steel ingots for rolls that can eliminate freckle segregation. Another aim is to provide a steel ingot for rolls that is free of freckle segregation.

In order to solve the above problems and achieve the object, the present invention employs the following means: (1) A method for producing a steel ingot for a roll according to one aspect of the present invention is a method for producing a steel ingot for a roll while melting a consumable electrode by an electroslag melting process to form a molten pool, wherein, where a pool interface gradient when viewed in vertical cross section of the molten pool is G, a melting rate of the consumable electrode is Vc (mm/min), a frequency of a current passed through the consumable electrode is f (Hz), and a mold diameter of the steel ingot for a roll is D (mm), the pool interface gradient G calculated by the following formula 1 satisfies 0.65 or less:
G=0.349×Vc+0.0036×f+0.0014×D-1.62...(Formula 1)
According to the method for producing a steel ingot for rolls described above in (1), the combination of the melting rate Vc, the frequency f, and the mold diameter D in actual operation can be adjusted so that the pool interface gradient G calculated from Equation 1 is 0.65 or less, thereby making it possible to keep the pool interface gradient small.

(2) In the method for producing a steel ingot for rolls described in (1) above, the frequency f may be within a range of 0.5 Hz to 10 Hz.
In the case of the method for producing a steel ingot for rolls described in (2) above, by restricting the upper limit of the frequency f to 10 Hz or less, it is possible to more reliably eliminate freckle segregation while widening the range of combinations of the melting rate Vc and the mold diameter D.

(3) In the method for producing a steel ingot for rolls according to (1) or (2) above, the melting rate Vc is set within a range of 1.0 mm/min to 6.0 mm/min.
In the case of the method for producing a steel ingot for rolls described in (3) above, by restricting the upper limit of the melting rate Vc to 6.0 mm/min, the depth of the melt pool can be made shallower and the gradient of the pool interface can be made smaller, thereby more reliably preventing the occurrence of freckle segregation.

(4) In the method for producing a steel ingot for rolls according to any one of (1) to (3) above, the mold diameter D may be 500 mm to 1000 mm.
In the case of the method for producing a steel ingot for rolls described above in (4), it is possible to obtain a steel ingot for rolls having an outer diameter suitable for producing, for example, a rolling roll.

According to the above-mentioned aspects of the present invention, a method for manufacturing a steel ingot for rolls that can eliminate freckle segregation can be provided. It is also possible to provide a steel ingot for rolls in which freckle segregation is eliminated.

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. 3A and 3B are longitudinal cross-sectional views each showing a schematic diagram of the method for producing a steel ingot for rolls according to the embodiment, in which FIG. 3A shows the state before melting, and FIG. 3B shows the state during melting. FIG. 2 is an enlarged longitudinal sectional view of a molten interface portion including position Pa in FIG. 1 , showing the occurrence or absence of freckle segregation determined by flow analysis for the method for producing a steel ingot for rolls of the present embodiment and a conventional method for producing a steel ingot for rolls. FIG. 4 is an enlarged view of the matrix of FIG. 3 in which normal melting and a high-frequency power source are combined. 1 is a graph in which the pool interface gradient obtained by flow analysis is plotted on the horizontal axis and the pool interface gradient obtained by Equation 2 is plotted on the vertical axis. 1 is a graph showing that freckle segregation can be eliminated at a certain pool interface gradient G regardless of the melting rate (casting rate), where the horizontal axis represents the melting rate and the vertical axis represents the pool interface gradient G. 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. 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.

The method for producing a steel ingot for rolls and one embodiment of the steel ingot for rolls of the present invention will be described below with reference to Figures 1 and 2. Note that Figure 1 is a vertical cross-sectional view that shows a schematic diagram of an apparatus for producing a steel ingot for rolls according to this embodiment. Also, Figure 2 is a vertical cross-sectional view that shows a schematic diagram of the method for producing a steel ingot for rolls according to this embodiment, where (a) shows the ingot before melting and (b) shows the ingot during melting.

As shown in FIG. 1 , the apparatus for manufacturing a steel ingot for rolls of 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 CL 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.

The chamber 2 is a cylindrical body with a bottom, and houses the mold 1 coaxially inside. 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 are connected to 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.

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. The cooling water supply mechanism then 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 this 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.

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 coaxially welded 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 steel composition of the consumable electrode 3 is as follows, expressed in mass %:
C: 0.6% to 1.5%
Si: 0.1% to 1.5%
Mn: 0.05% to 1.5%
Cr: 2.0% to 15.0%
Mo: 0.1% to 3.0%
Remainder: Fe and impurities

In response to an instruction from the control device 6, the AC power supply 5 supplies an AC current having a predetermined frequency and a predetermined current value between the stub 4 and the chamber 2. The stub 4 and the consumable electrode 3 are electrically connected, and the chamber 2 and the mold 1 are also electrically connected.
As shown in FIG. 2(a), molten slag SL is held in the mold 1 before current is applied.
Next, as shown in FIG. 2(b), the consumable electrode 3 is lowered and immersed in the molten slag SL, and an alternating current is applied from an AC power source 5. As a result, the molten slag SL generates heat due to Joule heat. The consumable electrode 3 melts from the bottom end 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 from the bottom end to the top end, and the molten metal solidifies in the mold 1, thereby obtaining a steel ingot for rolls.

According to this method for manufacturing steel ingots for rolls using the ESR (Electro Slag Remelting) melting method, the molten metal stored in the mold 1 solidifies while forming a relatively shallow molten pool P. During this process, the droplets of the molten consumable electrode 3 are removed of inclusions such as oxides as they sink through the molten slag SL.

The frequency f (Hz) of the AC current passed through the consumable electrode 3 and the mold diameter D (mm) of the steel ingot for rolls are input to the control device 6 shown in Figure 1 by a user as production conditions. The control device 6 calculates the pool interface gradient G of the melting pool P as viewed in vertical section as shown in Figure 1 by the following equation 2. When a steel ingot for rolls is produced while melting the consumable electrode 3, the melting rate Vc (mm/min) of the consumable electrode 3 is controlled so that the pool interface gradient G is 0.65 or less. The value of the melting rate Vc may be calculated by the control device 6 as described above, or the user may manually calculate the value using equation 2 and input it to the control device 6.
G=0.349×Vc+0.0036×f+0.0014×D-1.62...(Formula 2)

Here, as shown in FIG. 1, the pool interface gradient G is the inclination of the pool interface b in the range of r/4 (mm) to 3r/4 (mm) radially outward from the central axis CL of the mold 1 in a vertical section including the central axis CL (i.e., the central axis of the melt pool P), where r (mm) is the radius of the space inside the mold 1. More specifically, the pool interface position y in the above range is approximated by a linear function y = Gr, and the coefficient G is found by the least squares method. Here, the pool interface position y represents the coordinate in the central axis direction with the bottom end of the mold as the origin.

In determining the above formula 2, the numerical ranges of each variable were as follows.
・Mold diameter D: 500mmφ to 1000mmφ
・Dissolution rate Vc: 1.0mm/min to 6.0mm/min
Frequency f: 0.5Hz to 60Hz

The pool interface b is a surface formed by the boundary between the molten pool P and the solidified portion S (the portion where the molten pool P has cooled and solidified), as shown in FIG. 4 described later. The pool interface b is a concave curved surface that is deepest at the position of the central axis CL and becomes shallower as it moves radially outward from the central axis CL.

Here, the occurrence of freckle segregation can be eliminated by adjusting the combination of the melting rate Vc, frequency f, and mold diameter D so that the pool interface gradient G in formula 2 is 0.65 or less. This point will be explained below with reference to Figures 3 to 6. Note that Figure 3 is a diagram showing the occurrence or nonoccurrence of freckle segregation determined by flow analysis for the method for manufacturing a steel ingot for rolls of this embodiment and the conventional method for manufacturing a steel ingot for rolls, and is an enlarged vertical cross-sectional view of the pool interface portion including position Pa in Figure 1. Figure 4 is an enlarged view of the matrix in Figure 3, showing the combination of normal melting and a high-frequency power source. Figure 5 is a graph with the pool interface gradient determined by flow analysis on the horizontal axis and the pool interface gradient determined by formula 2 on the vertical axis. Figure 6 is a graph showing that freckle segregation can be eliminated at a constant pool interface gradient G regardless of the melting rate.

Figure 3 shows the inclination of the pool interface b and the occurrence of freckle segregation for four combinations of two different melting rates and two different power supply frequencies. The mold diameter D is the same, 840 mmφ, for all cases. The steel composition is also the same, containing 0.9 mass% C, 1.0 mass% Mn, 4.0 mass% Cr, and 0.6 mass% Mo. The shading in each figure in Figure 3 indicates the concentration distribution of Mo.

As shown in the upper left of Figure 3, with the combination of low-speed melting (7 kg/min = 1.6 mm/min) and low-frequency power supply (1 to 5 Hz), the pool interface b is shallow and the pool interface gradient G is a small value of 0.65 or less. Therefore, freckle segregation does not occur.
As shown in the upper right of Figure 3, with the combination of medium-rate melting (11 kg/min = 2.5 mm/min) and low-frequency power supply (1 to 5 Hz), the pool interface b is deep because the melting rate Vc is high. However, by keeping the frequency f low, the pool interface gradient G is kept below 0.65, and freckle segregation does not occur.
As shown in the lower left of FIG. 3, in the combination of low-rate melting (7 kg/min = 1.6 mm/min) and high-frequency power supply (60 Hz), although the frequency f is high, the melting rate Vc is kept low, so that the pool interface gradient G is kept to 0.65 or less, and no freckle segregation occurs.

On the other hand, as shown in the lower right of Figure 3, in the combination of medium-speed melting (11 kg/min = 2.5 mm/min) and high-frequency power source (60 Hz), the pool interface b is deeper because the melting rate Vc is high. In addition, the frequency f is also high, so the pool interface gradient G exceeds 0.65.
That is, as shown in Figure 4, the slope of the pool interface b at position Pa becomes locally large and disturbed, and the occurrence of freckle segregation is observed as indicated by the rectangular dashed line frame. Here, vortexes of the molten metal as indicated by numerous arrows are observed in the vicinity of the position where freckle segregation occurs in the melt pool P. Such disturbances in the melt pool P have not been observed in any other form. As described above, in the conventional method for manufacturing steel ingots for rolls, as the melting rate Vc increases, the pool interface b becomes deeper and the slope becomes larger, and in addition, because the frequency f is high, disturbances occur in the melt pool P, causing disturbances at the pool interface b as well. As a result, the pool interface gradient G is affected, and freckle segregation is likely to occur.

Based on the above findings, the inventors performed flow analysis multiple times while changing the variables of dissolution rate Vc, frequency f, and mold diameter D, and determined the pool interface gradient G for each case. The results of the flow analysis are shown in Tables 1 to 3.

Then, by performing multiple analyses using the results of Tables 1 to 3, we arrived at the above formula 2, which calculates the pool interface gradient G using the dissolution rate Vc, frequency f, and mold diameter D as variables. Figure 5 shows a graph in which the calculation results of the pool interface gradient calculated using formula 2 are plotted on the vertical axis, and the pool interface gradient obtained as a result of the flow analysis is plotted on the horizontal axis. As can be seen from Figure 5, the pool interface gradient can be calculated easily and with high accuracy using formula 2.

Furthermore, the inventors investigated whether freckle segregation occurred or not for each case in the flow analysis. Then, by investigating the relationship between each case and the pool interface gradient G, they found that the occurrence or non-occurrence of freckle segregation is determined at the boundary of pool interface gradient G = 0.65. As an example, FIG. 6 shows the results of the flow analysis plotted with the dissolution rate Vc on the horizontal axis and the pool interface gradient G on the vertical axis. In the figure, cases where freckle segregation occurred are indicated by cross marks, and cases where freckle segregation did not occur are indicated by circles. In both the cross marks and circles, the thick line indicates the case where the mold diameter D = 840 mmφ, and the thin line indicates the case where the mold diameter D = 650 mmφ. As can be seen from the figure, freckle segregation can be eliminated by suppressing the pool interface gradient G to 0.65 or less.

As described above, the method for producing a steel ingot for a roll according to the present embodiment is a method for producing a steel ingot for a roll while melting a consumable electrode 3 by an electroslag melting process to form a molten pool P, in which the pool interface gradient G calculated by equation 2 above is controlled to be 0.65 or less, where G is the pool interface gradient when viewed in vertical cross section of the molten pool P, Vc (mm/min) is the melting rate of the consumable electrode 3, f (Hz) is the frequency of the current passed through the consumable electrode 3, and D (mm) is the mold diameter of the steel ingot for a roll.
According to this method for producing a steel ingot for rolls, it is possible to accurately reduce the actual pool interface gradient G to 0.65 or less by adjusting the combination of the melting rate Vc, frequency f, and mold diameter D in the above formula 2. As a result, it is possible to produce a steel ingot for rolls in which the occurrence of freckle segregation is eliminated.

A steel ingot for rolls (Figure 7) was produced by the method for producing a steel ingot for rolls of the present embodiment, and a steel ingot for rolls (Figure 8) was produced by a conventional method for producing a steel ingot for rolls. Both Figures 7 and 8 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 this embodiment was produced under the following production conditions. As a result, the pool interface gradient G was 0.57 , which was not more than 0.65, and therefore a steel ingot for rolls of excellent quality free from freckle segregation could be obtained.
・Dissolution rate Vc=2.84mm/min
Frequency f = 5Hz
・Mold diameter D = 840 mm

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: As a result, the pool interface gradient G was 0.76 , which exceeded 0.65, and freckle segregation was confirmed at many positions indicated by circles in the photograph.
・Dissolution rate Vc=2.84mm/min
Frequency f = 60Hz
・Mold diameter D = 840 mm

The above results confirm that by setting the pool interface gradient G obtained by the above formula 2 to 0.65 or less, a high-quality steel ingot for rolls in which freckle segregation is eliminated can be obtained. Therefore, when this steel ingot for rolls is processed to manufacture, for example, a rolling mill roll, cracks due to freckle segregation do not occur on the surface, making it possible to obtain a product of excellent quality without damaging the surface of the rolled steel plate.

3 Consumable electrode P Dissolution pool

Claims (4)

A method for producing a steel ingot for rolls by melting a consumable electrode by an electroslag melting method to form a molten pool, comprising the steps of:
a pool interface gradient G calculated by the following formula 1 satisfies 0.65 or less, where G is a pool interface gradient when viewed in vertical cross section of the melting pool, Vc (mm/min) is a melting rate of the consumable electrode, f (Hz) is a frequency of a current passed through the consumable electrode, and D (mm) is a mold diameter of the steel ingot for rolls:
G=0.349×Vc+0.0036×f+0.0014×D-1.62...(Formula 1) 2. The method for producing a steel ingot for rolls according to claim 1, wherein the frequency f is within a range of 0.5 Hz to 10 Hz. 3. The method for producing a steel ingot for rolls according to claim 1, wherein the melting rate Vc is set within a range of 1.0 mm/min to 6.0 mm/min. 4. The method for producing a steel ingot for rolls according to claim 1, wherein the mold diameter D is 500 mm to 1000 mm.

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