JPS6016486B2 - Localized quenching of steel using resistance heating - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for efficiently locally hardening steel materials using resistance heating.

In various products using steel materials, depending on the way they are used, certain parts of the parts require high hardness in terms of strength, wear resistance, etc., while other parts require high toughness. , high elongation is often required from the viewpoint of workability.

In such cases, if you use a method of partially tempering only the parts that require hardness, you will go through a complicated process of first hardening the entire part and then tempering only the parts that do not require hardness. Since this is not necessary, the stability of the tissue is improved and thermal energy is also saved. Therefore, various local hardening methods have been developed, and flame hardening methods, induction hardening methods, electron beam methods, etc. have already been put into practical use. However, flame quenching uses 4.5 mm of thermal energy, so it takes a long time to heat and is not efficient, and induction quenching is
Although it can be hardened in a short time, it is difficult to harden parts accurately. Furthermore, although electron beam hardening is capable of hardening even minute portions, it is practically difficult to apply it to thin steel materials because thermal distortion tends to occur. Such conventional local quenching methods have advantages and disadvantages, and there is no quenching method that can be applied to thin-walled steel materials. Therefore, as a result of repeated research, the present inventors have found that it is possible to quench locally in a short period of time, efficiently, and without causing thermal strain even when applied to thin steel materials. successfully developed the law. That is, the steel material is pressed firmly with at least one pair of electrodes (hereinafter referred to as the first step), and then the steel material is heated locally by resistive heating to a temperature higher than the austenitizing temperature of the steel material (hereinafter this is referred to as the first step). 2
(hereinafter referred to as the 8th step), and after stopping the current, the heated steel material is rapidly cooled by continuously pressing the electrode onto the steel material (hereinafter referred to as the 8th step). There is a hardening method. When steel is quenched using this method, heating utilizes internal heat generation due to the inherent resistance of the steel, so it is more efficient and takes less time than external heating.

In addition, ``cooling is also performed through solid contact with the electrode, so it can be done quickly and completely.Therefore, quenching can be done in a short overall processing time and with high thermal efficiency.
The resulting steel material has no thermal distortion and has high quenched hardness. In the method of the present invention, any type of steel can be used as the material to be treated, as long as it has the property of being hardened by quenching. The method of the present invention is particularly useful for steel types with low carbon content, such as SPCC materials used for automobile outer panels, which are difficult to harden sufficiently due to the slow cooling rate using other ordinary hardening methods. Useful. When applying the method of the present invention, the shape of the steel material to be treated may be circular, square, or plate-like.

However, hardening of a material with a large wall thickness requires a very large amount of current, which is practically difficult due to the capacity of the power supply. On the other hand, for thin steel materials that are difficult to harden due to thermal strain caused by conventional methods, there is no problem with the amount of current applied, and the method of the present invention is highly effective. As shown in Figure 1, electrodes 1 and 2 used in the first step
must be at least a pair of electrodes facing each other in order to clamp the material 3 to be treated and conduct electricity between them.

However, if two or more pairs of electrodes are used, it is possible to treat one or more locations at the same time, which is more efficient. The material of the electrode is preferably a material that has high thermal and electrical conductivity and high mechanical strength, particularly high temperature.

For example, copper, which is used as an electrode material for spot welding, or a chromium steel alloy is suitable. The pair of electrodes do not necessarily need to be made of the same material. If both are made of the same material, the quench hardened portions 31 are formed at equal distances inward from both surfaces of the steel material, as shown in FIG. 2a. On the other hand, if the two electrodes are made of different materials and have different electrical conductivities, the quench hardened portion 81 is formed biased towards the electrode 1 side, which has a higher resistance, as shown in FIG. 2b. The shape of the electrode has a close relationship with the current distribution during energization, the cooling effect during cooling, and the deformation of the electrode at high temperatures.

If the contact surface between the tip of the electrode and the steel material is small, the current density at the contact surface increases, the temperature of the contact surface becomes high, and the steel material and the electrode are likely to be welded together, which is undesirable. At the same time, the heat removal effect of the steel material by the tip portion of the electrode decreases, making it impossible to rapidly cool the steel material. Therefore, it is desirable that the electrode have a shape that has as much contact surface as possible with the steel material, that is, a so-called flat shape or a truncated cone shape as shown in FIGS. 3a and 3b. In addition, when only the periphery of the opening of a steel material having an opening is to be heated, hardening can be performed with good thermal efficiency by using an electrode having the same shape as the shape of the hardened portion of the steel material. In order to enhance the heat removal effect of the steel material by the electrodes, a water cooling mechanism as shown in FIG. 4 having cooling water flow passages 11, 21 and cooling water outflow passages 12, 22 can be provided. This mechanism also serves to prevent the tip of the electrode from softening due to high temperatures and to prevent deformation of the tip. The clamping and pressing by the electrodes needs to have at least a pressing force that can ensure that the steel material and the electrode are in contact with each other to the extent that electricity can be passed over the entire surface thereof.

If the contact is weak, the electrical contact resistance at the contact surface will increase,
There is a risk of welding between the electrode and the steel material. However, if the pressure applied by the electrodes is too strong, it is not preferable because the electrodes may cause indentations on the surface of the steel material, which has been softened by the high temperature. The second step is to energize the electrode that presses the steel material,
This is a process of heating the steel material to a temperature higher than the austenitizing temperature by locally generating resistance heat generation. The current value required to heat each steel material to its austenitizing temperature is determined by the relationship between the amount of thermal energy required to heat each steel material to its austenitizing temperature and the fixed resistance of each steel material, which has already been determined. can be specifically determined. Resistance heating is
Usually occurs on the surface of steel materials with high contact electrical resistance.

However, even if heat is generated, the temperature is not raised because the heat is removed by the electrodes, and the temperature rise due to heating ends up occurring in the center of the steel material. FIG. 2a also shows the portion where the temperature rise occurs within the steel material. However, the location where resistance heats up can also be changed by changing the material of one of the electrodes and making the electrical resistance values different from each other, as described above. Furthermore, even if the current density or contact resistance at the contact surfaces between the electrodes and the steel material differs, resistance heat generation is biased towards the contact surfaces where the current density is high or the contact resistance is large. The eighth step is a step of rapidly cooling the steel material by continuously pressing the electrode with the ball after stopping the current.

When the electricity is stopped while the steel is heated above the austenitizing temperature, the heat generated in the steel is rapidly removed through air and electrodes that have higher thermal conductivity than the surrounding steel.

However, in order for the electrode to have a good heat removal effect from the steel material, it is necessary to press the electrode and bring it into complete contact with the steel material. Furthermore, in order to enhance the heat removal effect of the electrode, it is preferable to provide the electrode with a water cooling mechanism. By bringing the electrode into contact with the steel material, the method of the present invention enables rapid cooling, so it is possible to obtain a sufficient hardening effect even for low carbon steel, which conventionally did not have a sufficient hardening effect due to slow cooling. Now I can do it. In addition, according to this method, heat is removed through the electrode before it is conducted to the surrounding steel materials other than the quenched part, which prevents the occurrence of thermal distortion that would otherwise occur due to heat conduction to the surrounding area. can do. Note that 1 second is usually sufficient for the pressing time for cooling. By quenching a steel material using the above method, the quenching can be carried out in a short time and with good thermal efficiency, and the obtained steel material has no thermal distortion and has a high quenched hardness.

Hereinafter, an experimental example will be shown in which the quenching hardness of steel materials to which the method of the present invention is applied and through-quenching conditions such as current value and time for each steel type were determined.

Experimental Example 1 For each steel type having the composition shown in Table 1, an experiment was conducted to determine the hardness obtained by ice water quenching and hardening according to the method of the present invention.

In addition, in order to compare each steel type under the same conditions excluding the influence of pretreatment, two Akatsuki semi-standard materials were created for each steel type through the following steps. That is, test materials of each steel type were rolled to a thickness of approximately 1.5 willows, held at 950° C. for 30 minutes in a salt bath, and then cooled in air. Before conducting the experiment, the surface was polished to a thickness of 1 sail. Then, the average hardness of these two Akatsuki materials was measured. The results are shown in Table 2 a. Then, one of the pieces was heated at 950°C for 3 whiskers, then quenched in ice water, and the hardness was measured. The results are shown in Table 2, b. In addition, the other one was made using a copper electrode with a diameter of 13 ribs at the tip. After rapid exposure for 1/3 second with Uta A (Kiranbaa) and heating, the current was stopped and held for 1 second to cool down, and the method of the present invention was applied to perform quenching. The results are shown in Table 2, c. Table 1 Table 2 The quenching method of the present invention has a Hv5 lower than that of ice water quenching, which is generally said to provide excellent quenching hardness.
0 indicates excellent hardness.

Moreover, when looking at SPCC materials with a small amount of carbon, it was previously thought that ice-water quenching would only increase the hardness by about twice as much, but it has been possible to increase the hardness to more than three times as much. As described above, it can be seen that the method of the present invention has excellent hardenability equivalent to or better than ice-water hardening, and is particularly effective for hardening low carbon steel, which was previously thought to be difficult to harden sufficiently. Experimental Example 2 Experiments were conducted on the current values necessary to obtain the desired quenching hardness when the method of the present invention was applied to each of the steel types shown in Table 1.

Using a copper electrode, the current value was varied from a fine A to 2 A, and after energizing at a rate of 0.9 sand, it was held for 1 second to cool down.

The results are shown in Table 3. If the quenching hardness is equivalent to the hardness obtained by ice water quenching, mark ○, and mark H because the current is too low.
If the hardness remains smaller than v200, mark it with an x.
The case where the electrode and the steel material were welded together due to the steel material becoming so hot was marked with a △ mark. Table 3 shows that the higher the carbon content of the steel, the more appropriate quenching hardness can be obtained even at a low current value.

This is because, as is generally known, the higher the carbon steel, the lower the austenitizing temperature, but as the carbon content increases, the fixed resistance of the steel material increases.
This is because the steel material can be heated to the austenitizing temperature even with resistance heat generation due to a low current. Experimental Example 3 The relationship between current application time and hardening hardness in hardening according to the method of the present invention was investigated.

SPCC material test piece (vertical 50 columns x width 2 broadside x thickness 0.7
Using an adjustment electrode with a diameter of 20 ribs on the tip of the tip, apply a current of 17 points A from 1/6 seconds to 1/2
Electricity was applied for up to 2 seconds, and then solid cooling was performed using electrodes. The repellent hardened portion was formed as shown in FIG. The hardness of the scaled hardened part was measured from the center of the scaled part as the origin toward both ends. The results are shown in FIG. In addition,
In the diagram, ○ mark is 1/current @, △ mark is 1/Kagami-suna and × mark is 1
/a indicates the value when acid current is applied. As is clear from FIG. 4, even if the current application time is changed from 1/6 seconds to 1/2 seconds, the hardness hardly changes.

After all, as shown in Table 2, it is thought that the quenched hardness is mainly influenced by the carbon content.
As is clear from the above experimental examples, the method of the present invention, which utilizes resistance heat generation in steel materials, can achieve a hardening depth equal to or greater than that of ice-water hardening, which is conventionally considered to have an extremely excellent hardening effect, with short-time energization and It is obtained by cooling.

Example: In order to harden the periphery of the closing part of a steel washer (inner diameter 6 sides x outer diameter 13 ribs x thickness 1 skin) equivalent to JIS standard SIOC which has a seal opening in the center, an electrode with a cooling mechanism of a spot welding machine was used. A pair of copper electrode tips having a diameter of 13 and having the same shape as the desired hardened portion of the washer were attached to the tip portion.

3. Sandwich both sides of the washer with the electrode tip.
A current of 1.1 chick A was applied to the electrode for 0.9 stages while applying a pressing force of 0k9/fence. Then, after the current supply was stopped, pressure was continuously applied for 1 second to cause rapid cooling. The maximum hardness of the day-hardened part of the washer after quenching was Hv425.
However, the hardness of this washer before quenching is Hv12.
5, the effect of hardening by this method is fully evident. For reference, the same washer was heated to 950℃.
After heating it for 30 minutes and pouring it into ice water, I measured its hardness and found it to be Hv325.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing an example of the method of the present invention, and FIG.
FIG. 3 is an explanatory diagram showing the shape of an electrode that can be used in the method of the present invention. FIG. 4 is an explanatory diagram of an electrode having a water cooling mechanism. , FIG. 5 is a diagram showing the relationship between energization time and rice bran hardness. 1, 2... Electrode, 11, 21... Electrode cooling water inflow passage, 12, 22... Electrode cooling water outflow passage. Figure 1 Figure 3 Figure 2 Figure 4 Figure 5

Claims (1)

[Claims] 1. Clamp and press the steel material with at least one pair of electrodes, then apply electricity to locally generate resistance heat to the steel material to a temperature higher than the austenitizing temperature of the steel material, and then stop the current and continue clamping the electrodes. A method of localized quenching of steel materials using resistance heating, which is characterized by rapidly cooling the steel material that has been heated by pressing.