JPS6252011B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for continuous rapid annealing of steel strips or steel wires having high hardness and high toughness through quenching and tempering. Traditionally, medium-high carbon steel or medium-high carbon low alloy steel strips or steel wires manufactured by steel manufacturers are hot- and cold-rolled, then reticulated or layered carbides are spheroidized.
That is, it is common to carry out the box annealing method and sell it as a steel material. Consumers usually process and cut this steel into products, and then use quenching and tempering to create a highly hard tempered martensitic structure to impart properties to the product that suit the conditions of use. At this time, due to the high hardness, the toughness deteriorates. That is, a tempered martensitic structure with high hardness is very advantageous in terms of wear resistance, but is disadvantageous in terms of impact resistance. Therefore, if impact resistance is important, it is necessary to raise the tempering temperature and lower the hardness.
This creates a vicious cycle in which wear resistance deteriorates. Therefore, first of all, it is necessary to manufacture a steel material with high hardness and high toughness. Secondly, steel manufacturers incorporate a carbide spheroidizing annealing process to enable processing and cutting of steel materials. This process requires stacking steel strips or steel wires and subjecting them to long-term heat treatment (usually over 40 hours) using a box annealing method in a controlled atmosphere, resulting in extremely low production efficiency and high fuel consumption. There is. That is, the conventional spheroidizing annealing of reticulated and layered carbides is a box annealing method, in which a steel strip or steel wire wound in a coiled shape is stacked and a bell called an inner cover is placed on top of the steel strip or steel wire. By sending non-oxidizing atmosphere gas into the
Heating at a temperature of 750-780℃, soaking, then approx.
It is slowly cooled to 700°C, held at this temperature for several hours, and then cooled to below 200°C before being taken out. For this reason, it is common that a heat treatment process of two or more days is required to put the steel material into a furnace and take it out. The present invention has been made as a result of intensive study and research in view of the above points, and its gist is that medium-high carbon steel strips or steel wires produced by hot and cold rolling are heated, soaked, and _A3
Soak for 30 seconds to 5 minutes at a temperature in the fully austenite range that is 50℃ to 200℃ higher than the Acm line and Acm line, and then soak for 10℃ to 200℃ to 200℃ or less in the primary quenching zone.
After rapid cooling at a rate of °C/sec, cycle rapid heating/
A medium-high carbon steel strip or steel characterized in that fine spheroidal carbides are precipitated by repeating a cycle of rapid heating and cooling in a temperature range of 780 to 800°C and 100°C or less 2 to 6 times in a cooling zone. Continuous rapid spheroidizing annealing method for wire. The present invention will be explained in detail below. The steel strip or steel wire that is the object of the present invention is produced by conventional steel manufacturing methods such as electric furnaces or converters, vacuum degassing, continuous casting, etc., and then undergoes the steps of ingot making, blooming, or continuous casting. Next, it is subjected to the steps of soaking, hot rolling, pickling, and cold rolling, and any conditions can be adopted for each of these steps. Regarding the content of the steel, medium-high carbon steel having a C content of 0.45 to 1.5, which is within the range in which spheroidizing annealing is possible, can fully exhibit the effects of the present invention. Alternatively, Cr may be added as an alloying element within the range of 0.2 to 3.0%.
Even if Mo, Ni, W, etc. are contained, similar effects can be achieved. In the continuous rapid spheroidizing annealing method of the present invention, the steel strip or steel wire is unwound from the coil and fed into the annealing furnace in the form of a single plate or wire, so the heat capacity of the material to be treated is small, and the heat capacity of the material to be treated is small. Even with indirect or direct heating, it is possible to bring the material to be treated to a soaking temperature of around 950°C within one minute. Then, by rapid cooling, a martensitic structure can be obtained. Subsequently, a cycle process of rapid heating and cooling is performed in a temperature range of 780 to 800°C and 100°C or less in a cycle rapid heating and cooling zone. However, the process of rapid heating and rapid cooling of the lever is repeated 2 to 6 times. Therefore, it is possible to spheroidize carbide annealing continuously in a short period of time, which significantly improves production efficiency and saves energy.Also, the rapid heating and cooling cycle treatment allows carbide to be finely precipitated. By quenching and tempering, it is possible to manufacture products with excellent hardness and toughness. Regarding the soaking time, the temperature was maintained at 780° C. for 1 minute in up to 6 cycles of heat treatment, but experimental results showed that 30 seconds was sufficient. Also 5
The reason why it is set as 5 minutes is because in the examples, a heat treatment at 660° C. for 5 minutes was performed after the cycle heat treatment to perform the softening treatment. For the above reasons, the soaking time was limited to 30 seconds to 5 minutes. Regarding the quenching rate, in the case of static oil cooling, it is 10℃/
The rapid cooling rate was limited to 10 to 200°C/second because the cooling rate was 200°C/second in the case of water cooling with a stutter. The reason why the cycle heat treatment was performed from 2 times to 6 times is because the m value indicating superplasticity is 0.3 or more even after 2 times. Further, the desired purpose can be sufficiently achieved by cycle heat treatment up to six times. The present invention will be explained below with reference to Examples. Example 1 C: 0.85%, Si: 0.22%, blown in a converter
Mn: 0.41%, P: 0.021%, S: 0.005%, Cr:
After the killed steel ingot consisting of 0.12%, residual Fe and unavoidable impurities is made into a slab by the usual method, 1200
After soaking for 5 hours at ℃, it is hot rolled to a thickness of 4.5 mm to a final finishing temperature of 850 ℃, annealed to remove processing distortion after pickling, and then cold rolled to a thickness of 3.0 mm. A steel strip finished to the required thickness is manufactured using the thermal cycle model and line configuration of the continuous rapid spheroidizing annealing method of the present invention shown in FIG. To explain this, first the above steel material is heated to 950°C.
After heating with high frequency for 1 minute, quenching is performed in an oil bath. The steel material having a martensitic structure produced by quenching is continuously fed to the next cycle rapid heating/cooling zone, and the cycle heat treatment promotes the formation of fine spherules of carbides. That is, in the cycle rapid heating and cooling zone, the steel material is rapidly heated to 780°C by the high-frequency heating device 5, immediately cooled to 80°C by the water jet nozzle 6, and then heated to 780°C by the high-frequency heating device 7.
Rapidly heat to 780℃ and immediately spray with water injection nozzle 8.
Cooled to 80°C, then rapidly heated to 780°C using the same high-frequency heating device 9, and immediately turned on to the water injection nozzle 1.
The film was cooled to 80° C. at 0° C., and then the rapid heating and cooling treatment was repeated as described above, and the cycle was repeated 6 times, and the film was wound onto a tension reel 4. The properties of this steel material are as follows. FIG. 2 is a photograph of the structure taken by a microscope, and b shows a treated material obtained according to an example of the present invention, and a shows a conventional example annealed to form a spheroid in a conventional box annealing furnace. In the case of the present invention, it is clear that the carbides are refined as shown in b. In order to confirm this more precisely, a thin film was prepared by electrolytic polishing, and a photograph of this film examined under a transmission electron microscope is shown in Figure 3. The grain size and crystal grain of the carbide were measured from these photographs, and the size and standard deviation were determined and are shown in the table below.

[Table] According to this, the average grain size of carbides in conventional materials is 0.8
μm, the standard deviation is 0.35 μm, and the average grain size of the carbide of the 6-cycle treated material in this example is 0.39 μm.
The standard deviation was 0.27 μm. That is, it is clear that according to the continuous rapid annealing method of the present invention, the size of the spherical carbides is approximately halved and the variation is also reduced. Further, the crystal grain size of the conventional material was 5.7 μm, and that of the 6-cycle-treated material of this example was 2.0 μm, and it is clear that the crystal grains were considerably refined. As the crystal grains become finer in this way, 650
When a tensile test is performed at different strain rates at temperatures of 710°C and 710°C, it exhibits a superplastic phenomenon. The strain rate sensitivity index (m value) was experimentally determined as a measure of this superplastic phenomenon, which shows an elongation of several hundred percent. It is generally said that an m value of 0.3 or more indicates a superplastic phenomenon. FIG. 4 shows the relationship between the number of cycle rapid heating and cooling treatments and the m value. It can be seen that when the rapid heating and cooling process is repeated two or more times, the m value becomes 0.3 or more.
Therefore, by employing the annealing method of the present invention, it is also possible to produce superplastic materials. FIG. 5 shows how the hardness of the material changes when spheroidizing fine carbide is annealed by the annealing method of the present invention. This shows that the hardness becomes nearly constant after 3 cycles or more. From the point of view of annealing hardness, three or more cycle treatments are considered to be industrially necessary. The fine spheroidized carbide steel produced by the annealing method of the present invention is used by customers after processing, cutting, quenching, and tempering. The advantages of this heat treatment will be explained in detail. Figure 6 shows the relationship between the quenching temperature and hardness of the conventional material and the treated material according to the present invention.It can be said that the conventional material undergoes quenching only when the quenching temperature reaches 800°C or higher when the holding time is 5 minutes. . This is because the spheroidized carbide is relatively large and takes time to dissolve into austenite. The 6-cycle-treated material according to this example can be sufficiently hardened even at 750°C for 5 minutes. This is thought to be due to the fact that the spheroidized carbide is so fine that it quickly forms a solid solution in the austenite, and the treated material of the present invention can ensure sufficient hardness at a low quenching temperature. When conventional materials are quenched industrially, a quenching temperature of 840°C is required to account for quenching variations, but the 6-cycle-treated material of this example requires a quenching temperature of 780°C.
It can be hardened in large quantities at a temperature of , and the variation in hardness is stable. Because of this difference in quenching temperature of about 60°C, the steel produced by the continuous rapid spheroidizing annealing method of the present invention is very advantageous in terms of energy saving and production efficiency for commercialization. Hardened steel materials are generally tempered at a temperature of 300°C or less. This is because a hardened, hard martensitic structure alone is extremely lacking in toughness. In order to evaluate this toughness, we used an instrumented Sharpie impact tester to record the dynamic fracture behavior on a recorder as changes in the load-time axis, and measured the absorbed energy up to the crack occurrence and the propagation of the crack. Toughness was evaluated using three values: energy and brittle fracture surface ratio of the fracture surface. 7th
The figure shows the relationship between tempering temperature, hardness, and absorbed energy until cracking occurs. As is clear from this, the hardness decreases as the tempering temperature increases, but the conventional material and the material treated in this example show the same tendency, and the tempering resistance is the same, but the energy absorbed until cracking is At each tempering temperature, the treated material of this example is better than the conventional material. For example, assuming that the absorbed energy value is the same, the tempering temperature of the treated material of this example is 200℃, while the tempering temperature of the conventional material is 250℃.
℃, the temperature difference is about 50℃, and the hardness is about 50℃.
The difference will be Rc3. From the point of view of wear resistance, the difference in Rc3 makes a considerable difference at high hardness levels. FIG. 8 shows the relationship between tempering temperature, crack propagation energy and brittle fracture ratio. The crack propagation energy tends to improve as the tempering temperature increases. At any tempering temperature, the treated material of this example shows better values than the conventional material. Furthermore, when looking at the brittle fracture rate, the brittle fracture rate of the material treated in this example is 20% lower than that of the conventional material, which means that the ductile fracture rate is 20% higher. The result is shown below. From the above, it is clear that the treated material of this example shows very good results in toughness compared to the conventional material. Example 2 Bearing steel SUJ2 (0.98%, C, 0.21%, Si, 0.28
%, Mn, 0.014%, P, 0.008%, S, 1.4%,
Cr, 0.08%, Cu, 0.01%Mo) steel wire (diameter 3
An example is shown below regarding mm). This steel wire was subjected to cycle heat treatment as shown in FIG. That is, preheat to 650°C for 10 minutes, raise the temperature to 1000°C, hold for 30 minutes, and then cool down to 850°C. It is oil quenched at a temperature of 850°C to create a complete martensitic structure with no residual carbide. Optimal cycle heat treatment is performed on this martensitic structure material. That is, the material was rapidly heated (approximately 50°C/sec) to 780°C, held at this temperature for 1 minute, and cooled with oil (the quenching degree H value of the cooling rate was 0.2). After repeating the cycle 6 times, heat at 660°C for 5 minutes and cool in oil. When such cycle heat treatment is performed, the average ferrite grain size of the conventional material is 16.34 μm, but it is reduced to an average of 1.73 μm.
The carbide grain size is refined to an average of 0.19 μm from the average 0.39 μm of conventional material. Therefore, as shown in FIG. 10, as the ferrite grains and carbide grains become finer, that is, as the number of cycles becomes two or more, the m value becomes 0.3 or more. It is said that a value of 0.3 or more indicates a superplastic phenomenon. The elongation at this time showed a value of 400% to 500% (m
The values were determined by the strain conversion method at a temperature of 710°C. ) A bearing ball is manufactured from the wire of this 6-cycle heat-treated material, and after this ball is oil quenched by heating at 850°C for 5 minutes, it is immediately subjected to sub-zero treatment at -70°C. After that, tempering treatment was performed at 200°C for 30 minutes. This heat-treated ball was ground. Life test of this ball using a rolling fatigue tester (Soda type)
I did this. The results are shown in Figure 11. The 50% cumulative frequency on the vertical axis of this figure is the average lifespan value. For the conventional material, the value was 7.16×10, and for the 6-cycle material, the value was 18.84×10. In other words, rolling fatigue life was improved by about 2.5 times by 6-cycle heat treatment. As described above, the steel products employing the continuous rapid spheroidizing annealing method of the present invention have a remarkable effect on improving the quality of products for customers.

[Brief explanation of the drawing]

FIGS. 1a and 1b are diagrams showing a thermal cycle model of the annealing method of the present invention and the configuration of its annealing line. FIGS. 2a and 2b are microscopic microscopic micrographs of the spheroidized carbides of the conventional material and the 6-cycle-treated material of the present invention.
FIG. 3 is a microstructure photograph taken with a transmission electron microscope of a thin film of the 6-cycle treated material of the present invention. Figure 4 shows
A graph showing the relationship between the strain rate sensitivity index (m value) and the number of cycles of rapid heating and cooling. FIG. 5 is a graph showing the relationship between hardness and the number of cycles of rapid heating and cooling when annealed to form fine spherules. FIG. 6 is a graph showing the relationship between quenching temperature and quenching hardness. Figure 7 shows
A graph showing the relationship between tempering temperature, hardness, and absorbed energy until cracking occurs. Figure 8 is a graph showing the relationship between tempering temperature, crack transfer energy, and brittle fracture ratio. Figure 9 is a schematic diagram of the optimum cycle heat treatment method for bearing steel SUJ2. Figure 10 is a graph showing the relationship between the number of cycles, ductility and FIG. 11 is a diagram showing the relationship between the maximum M value and the Weibull distribution of the rolling fatigue life of the conventional bearing steel and the 6-cycle heat-treated material. 1... Beio four reel, 2, 5, 7, 9...
High frequency heating device, 3... oil tank or water tank, 4...
Tension reel, 6, 8, 10...Water injection nozzle, a...Heating soaking zone, b...First rapid cooling zone, c
...Cycle rapid heating and cooling zone.

Claims (1)

[Claims] 1 Medium-high carbon steel strips or steel wires manufactured by hot and cold rolling are continuously passed through a continuous annealing furnace having heating/soaking, primary quenching, and cycle rapid heating/cooling zones in series. In the heating soaking zone, soaking is performed for 30 seconds to 5 minutes at a temperature in the complete austenite range that is 50℃ to 200℃ higher than the _A3 line and Acm line, and then in the primary quenching zone, the temperature is 10℃ to 200℃ to 200℃ or less. After quenching at a rate of ℃/sec, 780-800℃ and 100℃ in the cycle rapid heating/cooling zone.
A continuous rapid spheroidizing annealing method for medium-high carbon steel strip or steel wire, characterized in that fine spheroidized carbides are precipitated by repeating a cycle of rapid heating and cooling 2 to 6 times in a temperature range of 0.degree. C. or lower.