JPH0447005B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for continuously spheroidizing a steel bar cut to a predetermined length, and to manufacturing the same.

Conventionally, steel bars that require cold forgeability are subjected to spheroidization treatment, but this treatment includes, for example,
It took a long processing time, about 20 hours.
In response to this, a new method for spheroidizing a steel wire rod has been proposed in JP-A-57-23026. If this method is used, spheroidization can be achieved in an extremely short processing time of, for example, 20 minutes. That is, as shown in Fig. 1, the material is heated to Ac ₁ +30℃~
Rapid heating to maximum heating temperature T ₁ between Ac + 150℃, then cooling to Ar ₁ transformation point, then
The material can be sufficiently softened by maintaining it at a temperature T ₂ below the Ar ₁ transformation point or by slowly cooling it.

The present invention relates to a specific method for applying this principle in the above-mentioned prior invention to the softening treatment of steel bars, particularly carbon steel for mechanical structures and alloy steel. In this case, a particular problem is that temperature unevenness occurs at the center of the steel bar in the axial direction, the surface, and the center and tip in the longitudinal direction.

In addition, the optimal maximum heating temperature T ₁ for steel bars is
Although it depends on the steel type and heating rate, it is necessary to suppress the variation in T ₁ to within 30°C in order to obtain sufficient cold forgeability, especially in carbon steels for mechanical structures and alloy steels. Figure 2 shows SCr440 steel bar (10mmφ)
After heat-treating under the conditions shown in the figure, a test piece of 8φ x 12H (mm) was prepared and a cold forging test was performed. Maximum heating temperature T ₁ and cold forging crack occurrence limit compression ratio (%) This is a graph showing the relationship, but T ₁ is 790
It shows particularly good cold forging properties at temperatures between ℃ and 820℃.
Figure 3 is a graph showing a similar relationship when the same test as that shown in Figure 2 is conducted on S40C steel, and in this case, it is particularly good when T ₁ is between 750 and 780°C. It shows cold forging property.

According to the above facts, in both steel types,
It can be seen that the allowable deviation range for T ₁ is 30°C.

Therefore, in order to rapidly spheroidize a steel bar,
It is necessary to keep the variation in T ₁ within the steel bar within 30°C.

FIG. 4 is a graph showing the temperature rise characteristics at the surface and center of a steel bar (36 mmφ) when heated with a gas burner. According to this, there is a very large temperature difference between the two, and in this case, the difference between the maximum heating temperature T ₁ between the surface and the center is 30
It can be seen that it is impossible to keep the temperature within ℃.

On the other hand, Fig. 5 is a graph showing the temperature rise characteristics at a similar location when a steel bar made of the same material as in Fig. 4 is rapidly heated in an induction heating furnace. It can be seen that the temperature difference between the surface and the center is very small, and therefore it is possible to suppress the difference in maximum heating temperature T ₁ between the two to within 30°C.

In this way, the temperature difference between the center and the surface of the steel bar can be made sufficiently small by employing induction heating. However, it is conventionally known that when induction heating is used, the temperature at both ends of the steel bar becomes lower than the temperature at the center. The problem with this point is
This problem can be solved by increasing the frequency of induction heating, but increasing the frequency increases the so-called skin effect and increases the temperature difference between the surface and center of the steel bar.

Further, as described with reference to FIG. 5, when induction heating is used to sufficiently reduce the temperature difference between the surface and the center, there is still an optimum frequency depending on the diameter of the steel bar.

However, when the present inventor conducted an experiment, it was found that at that frequency, the temperature at both ends of the steel bar was lower than that at the center, and temperature unevenness occurred in the longitudinal direction.

In order to solve this temperature unevenness problem, we conducted further experiments and discovered the following fact.

First, by feeding the steel bar into the induction heating furnace intermittently at a constant feed length, the positions of both ends of the steel bar in the heating furnace are always kept constant, and the number of turns of the induction coil is adjusted. By increasing the amount of heat applied to both ends by performing the following procedure, it was found that the steel bar could be heated to a relatively uniform temperature over its entire length.

However, with only the above method, the variation width of the maximum heating temperature T ₁ at each position along the longitudinal direction of the steel bar can be reduced by 30
It was not possible to keep the temperature below ℃. That is,
Figure 6 is a graph showing the temperature rise characteristics of both ends 1 and 3 and the center part 2 when a 36φ x 200L steel bar is heated by intermittent feeding in an induction heating furnace. , the heating time is about 80 seconds, and the heating temperature is
When the temperature exceeds 750℃, the temperatures at positions 1, 2, and 3 are almost the same, but when the heating time exceeds 140 seconds, only the temperature at the tip 1 decreases.
The temperature of the central part 2 and rear end part 3 continues to rise, resulting in 1
The difference in maximum heating temperature between 3 and 3 reached as much as 70°C. This phenomenon occurs because the steel bar is gradually taken out from within the induction coil, so only the tip of the steel bar comes out of the induction coil and heating stops, while other parts are still heated. In order to solve this uneven heating, we tried variously changing the number of turns of the induction coil and the position of the steel bar within the coil, but as long as the steel bar was gradually removed from the heating furnace, the above problem could not be solved. I understand. Therefore, an experiment was conducted in which the steel bars were intermittently fed into the induction heating furnace and the steel bars were taken out of the heating furnace all at once.

In other words, Fig. 7 is the same as Fig. 6 when the same steel material as in Fig. 6 is intermittently fed into an induction heating furnace and heated, and then taken out from the heating furnace all at once. 2 is a graph showing the temperature rise characteristics at each position of a steel bar. As is clear from this, the maximum heating temperature in the longitudinal direction of the steel bar
The variation in T ₁ is very small. Therefore, if an intermittent feeding and one-shot extraction method is adopted for induction heating, the difference in T ₁ between the center and surface as well as between both ends of the steel bar should be maintained in the longitudinal direction.
It was found that the temperature could be kept within 30℃.

The present invention was invented based on such knowledge, and it becomes possible for the first time to uniformly heat the entire steel bar.

In the present invention, a steel bar cut to a predetermined length is intermittently fed into an induction heating furnace at a constant feed length, and after rapidly heating the steel bar to a maximum heating temperature of Ac ₁ +30°C to Ac ₁ +80°C, The steel bar is removed from the induction heating furnace at once, thereby reducing the difference in maximum heating temperature within the bar by 30%.
It is characterized in that it is kept at a temperature of 650°C to 710°C and then sequentially fed into a furnace maintained at a temperature of 650°C to 710°C.

Figure 8 shows a 20〓
To compare the cold forgeability of a 200L steel bar (SCr40), the same steel bar (SCr40) was heated by intermittent feeding with the heat pattern shown in Figure 6, but the same steel bar was spheroidized without being taken out of the furnace all at once. It is a graph shown in comparison with the cold forgeability of steel materials. According to this, the steel bars treated by the method of the present invention all exhibited high cracking critical compression ratios (over 75%) in all test specimens taken from the tip, center, and rear end. ing.

On the other hand, the critical compression ratio for cracking of the test piece taken from the rear end of the steel bar treated by the comparative method was 62.5%, which is clearly poor. Therefore, it is clear that good cold forgeability can be obtained over the entire length of the steel bar by combining the three conditions of induction heating, intermittent feeding, and all-at-once withdrawal as in the present invention.

The maximum heating temperature T ₁ is the temperature at which the pearlite in the previous structure becomes austenite and a structure in which the core of spheroidized carbide remains, is Ac ₁ +30°C to _{Ac 1} +80°C. To explain in more detail, this range differs depending on the steel type, and for carbon steel bars for mechanical structures, Ac ₁ +
30°C to _{Ac 1} +60°C, and for machine structural alloy steel bars, Ac ₁ +40°C to _{Ac 1} +80°C.

If T ₁ is lower than these temperatures, the pearlite of the previous structure will remain until after the treatment, and cold forgeability will not be improved, and if it is higher, recycled pearlite will precipitate and cold forgeability will deteriorate.

In the holding furnace, the temperature is maintained or slowly cooled just below the Ar ₁ transformation point, which can be achieved by setting the holding furnace temperature to 650 to 710°C.

An example of an apparatus for implementing the present invention will be described below.

FIG. 9 shows a layout diagram of a steel processing apparatus that implements the present invention. The steel bar 1 placed on the supply chute 2 is
It is intermittently fed into the high frequency induction heating furnace 4 by the push piston 3. After being heated to a predetermined maximum temperature, the steel bar is taken out at once at the outlet of the heating furnace 4 by a mechanism 5 (consisting of a chain and a pressure roll).
It is taken out at once by the pusher piston 6, and then fed into the constant temperature holding furnace 7 by the extrusion piston 6. Inside the holding furnace 7, two rails are laid in parallel with a slope, and the fed steel bar rolls on these rails and advances toward the exit of the furnace. In addition, a steel bar removal mechanism is provided at the outlet of the holding furnace 7.
Steel bars are taken out one by one at regular intervals.

Table 1 shows S40℃ and SCr40 using the above equipment.
This figure shows a comparison of the mechanical properties and time required for spheroidization of each 36φ x 200L steel bar after spheroidization treatment using the method of the present invention and the conventional method. . From Table 1, it is clear that according to the invention mechanical properties comparable to those of conventionally treated steel bars can be obtained in a very short treatment time of 20 minutes.

[Brief explanation of the drawing]

FIG. 1 is a diagram schematically showing a heat pattern in rapid spheroidization treatment. Figure 2 is a chart showing the relationship between the maximum heating temperature and the critical compressibility for cracking in a cold forging test for SCr440 steel bars that have undergone rapid spheroidization treatment, and Figure 3 shows the same relationship for S40C steel bars. Figure 4 is a diagram showing the temperature rise characteristics at the surface and center of a 36φ steel bar when heated by a gas burner, and Figure 5 is a diagram showing the temperature increase characteristics when the same steel bar is heated by induction heating. , Figure 6 is a chart showing the temperature rise characteristics at each position in the longitudinal direction when a 36φ x 200L steel bar is intermittently fed into an induction heating furnace and heated, and Figure 7 is a diagram showing the temperature rise characteristics at each position in the longitudinal direction when the same steel bar is fed intermittently and taken out at once. Chart showing the temperature rise characteristics at each position in the longitudinal direction when heated in an induction heating furnace, No. 8
The figure is a chart showing the critical compressibility for cracking at each position in the longitudinal direction when a 20φ x 200L steel bar of SCr440 is treated by the method of the present invention and the comparative method. FIG. 2 is a schematic layout diagram of the spheroidization processing device used. 1... Steel bar, 2... Steel bar supply chute, 3... Pushing piston, 4... High frequency induction heating furnace, 5...
...One-shot take-out mechanism, 6... Push-out piston,
7... Constant temperature holding furnace, 8... Inverter, 9... Load matching panel, 10... Operation panel.

Claims (1)

[Claims] 1 A steel bar cut to a predetermined length is intermittently fed into an induction heating furnace at a constant feeding length, and the bar is heated to Ac ₁
After being rapidly heated to a maximum heating temperature of +30°C to Ac ₁ +80°C, it is removed from the furnace at once, thereby suppressing the difference in maximum heating temperature within the bar to within 30°C, and then heated to a temperature of 650°C to 710°C. A continuous spheroidizing heat treatment method for a steel bar, which is characterized by sequentially feeding a steel bar into a furnace held at a temperature.