JPS59166626A - Continuous spheroidizing heat treatment of rod steel - Google Patents

Abstract

PURPOSE:To impart good cold forging property to rod steel over the entire length thereof, by a method wherein a rod steel having a predetermined length is intermittently fed into an induction heating furnace in a constant feed length to be rapidly heated to a max. heating temp. and the heated rod steel is taken out therefrom at a stroke and successively fed in a holding furnace to be subjected to the titled treatment. CONSTITUTION:Rod steel cut in a predetermined length is intermittently sent into an induction heating furnace in a constant feed length and rapidly heated to a max. heating temp. of Ac1+30 deg.C-Ac1+80 deg.C. Subsequently, the heated rod steel is taken out at a stroke from the aforementioned furnace to suppress the difference of the max. heating temp. in the rod steel in 30 deg.C or less. In the next step, the treated rod steel is successively sent into a thermostatic holding furnace held to 650-710 deg.C. By this method, continuous spheroidizing heat treatment is applied to the rod steel and good cold forging property is imparted to the rod steel over the entire length thereof.

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 have been subjected to spherical 1H treatment, but this treatment required a long treatment time, for example, 20 hours. In response to this, a new method for treating the spherical ratio of steel wire rods has been proposed in JP-A-57-23026. Using this method, a spherical ratio can be achieved in a very short processing time, for example 20 minutes. That is, as shown in FIG.
c, maximum heating temperature between +30℃ and AC+150℃ (T
It is possible to soften the material sufficiently by rapidly heating it to 1), then cooling it to the Ar1 transformation point, and then maintaining it at a temperature (T2) below the Ar1 transformation point, or 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 between the center in the axial direction, the surface, and the center and tip in the longitudinal direction.

In addition, the optimal maximum heating temperature (T1) for steel bars is determined by the steel type and heating rate, but in order to obtain sufficient cold forgeability, especially for carbon steels and alloy steels for machine structures, s Tt
It is necessary to suppress the variation in temperature to within 30°C. Figure 2 shows the maximum heating temperature (T1) when a 5Or440 steel bar (10mmφ) was heat-treated under the conditions shown, and a cold forging test was conducted using a test bench of 8φ×12H (mm). This is a graph showing the relationship between cold forging crack occurrence limit compression ratio (%).
"I shows particularly good cold forging properties when I is between 790°C and 820°C. Also, Figure 3 shows that the cold forging properties of 8400 steel
This is a graph showing a similar relationship when the same test as that shown in the figure was conducted, and in this case, T1 was 750°C to 780°C.
It shows particularly good cold forging properties when the temperature is ℃.

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

Therefore, in order to subject a steel bar to rapid spherical heat treatment, it is necessary to keep the variation in T within the steel bar within 30°C.

Therefore, FIG. 4 is a graph showing the temperature rise characteristics at the surface and center of a steel bar (36 nmφ) when heated with a full gas burner. According to this, a very large temperature difference occurs between the two, and in this case, it is impossible to suppress the difference in the maximum heating temperature (T□) between the surface and the center to within 30°C. I understand that.

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. , the temperature difference between the surface and the center is very small, so the difference in maximum heating temperature (T8) between the two is 30°C.
It can be seen that it is possible to keep it within the range.

In this way, the temperature difference between the center and the surface of the steel bar can be made sufficiently small by fully employing induction heating. However, it is conventionally known that when induction heating is used, the temperature at both ends of the steel bar is lower than the temperature at the center. 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 experiments, 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.

Therefore, we conducted further experiments in order to completely solve the problem of temperature unevenness, and the following fact was discovered.

First, by intermittently feeding the steel bar into the induction heating furnace at a constant feed length, the position of both ends of the steel bar in the heating furnace was always constant, and the number of turns of the induction coil was adjusted. By performing all of the above operations to increase the heat tit' applied to both ends, it was found that the steel bar could be heated to a relatively uniform temperature over its entire length.

However, using only the above method, it was not possible to suppress the variation width of the maximum heating temperature T1 at each position in the longitudinal direction of the steel bar to within 30°C. That is, Fig. 6 shows 36φX
Both ends (1) of a 200L steel bar heated by intermittent feeding in an induction heating furnace.

This is a graph showing a custom-made temperature increase for (3) and the center part (2). By adopting the intermittent feed method, the heating time is 80 seconds, and when the heating temperature exceeds 750'C, (1 ),
The temperatures at each position (2) and (a) are almost the same, but when the heating time exceeds 140 seconds, only the temperature at the tip (1) decreases, and the temperature at the center (2) and rear end decreases. Part (3)
The temperature continued to rise, and as a result, the difference in maximum heating temperature between (1) to (3) reached 70°C. This phenomenon occurs because the entire 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 eliminate 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 all the bars were 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 removed from the heating furnace all at once.

In other words, Fig. 7 shows an experiment in which the same steel material as in Fig. 6 was intermittently fed into an induction heating furnace and heated, and then removed from the heating furnace all at once ((
6 is a graph showing a custom temperature increase at each position of the steel bar, similar to FIG. 6 in FIG.

As is clear from this, the variation in the maximum heating temperature T in the longitudinal direction of the steel bars is very small. Therefore, if the intermittent feeding and all-at-once methods are adopted for induction heating, the difference in T1 between the center and the surface of the steel bar in the longitudinal direction, as well as between both ends, can be suppressed to within 30°C. I understand.

The present invention was invented based on such knowledge,
The present invention makes it 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 feeding length, and the steel bar is heated to Ac1 +
After rapidly heating to a maximum heating temperature of 30°C to AcI +80°C, the steel bar is taken out at once from the induction heating furnace, thereby keeping the difference in maximum heating temperature within the steel bar within 30°C, and then maintained at a temperature of 650°C to 710°C. The main feature is that the materials are sequentially fed into the furnace.

Figure 8 is a comparison method of the cold forgeability of a 20φ x 20OL steel bar (SOr40) that has been subjected to spherical ratio treatment according to the method of the present invention with the heat pattern as shown in Figure 7, and the heat pattern as shown in Figure 6. Heating is performed with intermittent feed, but
This is a graph showing a comparison of the cold forgeability of the same steel material which was subjected to spherical treatment without being taken out from the furnace all at once. This shows the critical compression ratio (exceeding 75 inches) of rods processed by the method of the present invention.

The shrinkage rate 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 air extraction as in the present invention.

The maximum heating temperature T1 is the temperature at which the pearlite in the previous structure becomes austenite and becomes a structure in which the core of spheroidized carbon remains. In other words, this range differs depending on the steel type, and for carbon steel bars for machine structures, it is Ac, +30℃~A
c1 + 60℃, and for machine structural alloy steel bars, Ac
, +40°C to AC1 +80°C.

If T+e is lower than these temperatures, the tsukurite of the previous structure will remain until after the treatment, and cold forgeability will not be improved; if it is higher than these temperatures, recycled pearlite will precipitate, resulting in poor cold forgeability.

In the holding furnace, Ar is used to maintain or slowly cool the temperature just below the 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 pushed into the pushing piston 3
It is intermittently fed into the high frequency induction heating furnace 4 by. 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 take-out mechanism 5 (consisting of a chain and a pressure roll), and then transferred to a constant temperature holding furnace by an extrusion piston 6. Sent to 7. 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 take-out mechanism is installed at the outlet of the holding furnace 7, and one steel bar is removed at regular intervals.
Books are taken out one by one.

Table 1 shows the mechanical properties of the 8400 and 5Or40 steel bars of 36φ x 20OL after spheroidization using the method of the present invention and the conventional method using the above-mentioned apparatus. This figure shows a comparison of the time required for spheroidization and the time required for spheroidization.

From Table 1, it can be seen that according to the present invention, the mechanical properties of the steel bar treated by the conventional method are comparable to those of the steel bar treated by the conventional method.
It is clear that this can be obtained in an extremely short processing time of 0 minutes.

[Brief explanation of the drawing]

FIG. 1 is a diagram schematically showing a turn in a heat nozzle in a rapid spherical rhi process. 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 a 5Or440 steel bar that has been fully subjected to rapid spherical treatment.
A chart showing the same relationship for a 00 steel bar, Figure 4 is a chart showing the custom temperature rise of the surface and center of the bar when heated by a full gas burner for a 36φ bar, and Figure 5 is a chart showing the custom temperature rise of the bar surface and center when heated by a full gas burner for the same bar. A chart showing the temperature rise characteristics mentioned above when heated. Fig. 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. Fig. 7 Figure 8 is a chart showing the temperature rise characteristics at each position in the longitudinal force direction when the same steel bar is heated in an induction heating furnace using intermittent feeding and sudden withdrawal. When processed by the invention method and comparative method, respectively,
FIG. 9 is a diagram showing the critical compressibility for cracking at each position in the longitudinal force direction. FIG. 9 is a schematic layout diagram of the spherical ratio processing device used in the embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Steel bar, 2... Steel bar supply chute, 3... Pushing piston, 4... High frequency induction heating furnace, 5...
- Air extraction mechanism, 6... Pushing piston, 7...
- Constant temperature holding furnace, 8... Inverter, 9... Load matching panel, 10... Operation panel. Agent Koji Suzuki Figure 1 Time (min'. Figure 2 Maximum force n Heat temperature T1 ("C) Figure 2 Maximum force n Heat temperature T1 ('C) Figure 4 Maximum power n Heat temperature T1 ('C) Figure 4 Power mouth Time fL"] (, 5ec) Garden 5 power mouth heat time (sea> bribe interval C5ec) 5th 艮LjV li 4 Songη) (I Ding place

Claims (1)

[Claims] A steel bar cut to a predetermined length is intermittently fed into an induction heating furnace at a constant feeding length, and the steel bar is heated to AC, +30℃~A.
c. After rapidly heating to a maximum heating temperature of +80°C, take it out from the furnace at once, thereby suppressing the difference in maximum heating temperature within the steel bar to within 30°C, and then heating from 650°C to
A continuous spherical steel bar [heat treatment method] characterized by sequentially feeding the steel bar into a furnace maintained at a temperature of 710°C.