JPH08246040A - Rapid continuous spheroidizing annealing treatment for steel material - Google Patents

Abstract

PURPOSE: To attain revolutionary rapid spheroidizing annealing capable of shortening spheroidizing annealing treatment time from conventional 25 to 30hr or more to <=1hr. CONSTITUTION: In the course of temp. rise for a steel material, the following element processes are combinedly performed once or two or more times: element process (1), the steel material is subjected to temp. rise and heating up to a temp. in the region between the Ae1 point and (Ae1 point +150K) at >=1K/sec temp. rise rate, held in the temp. region for 0-<600sec, and then cooled through the temp. region between (Ae1 point +50K) and (Ae1 point -150K) at <=5K/sec cooling rate or held at a temp. in this temp. region; element process (2), the steel material is subjected to temp. rise and heating up to a temp. in the region between (Ae1 point +80K) and (Ae1 point +270K) at >=1K/sec temp. rise rate, held in the temp. region for 0-<120sec, and then cooled through the temp. region between the Ae1 point and (Ae1 point -150K) at <=5K/sec cooling rate or held at a temp. in this temp. region.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rapid continuous spheroidizing annealing method for steel materials such as wire rods, steel bars, strip steels, steel plates (hereinafter simply referred to as steel materials).

[0002]

2. Description of the Related Art Conventional spheroidizing annealing for bearing steel and various cold forging steel materials is performed by charging the steel material into a pot annealing furnace in a state of a large weight coil, or by giving a required heat history, or It has been carried out by continuously moving in the furnace of the annealing furnace in the state of such a coil and giving a required heat history during that time.

FIG. 1 is an explanatory view showing a heat pattern of spheroidizing annealing of a coil in a batch furnace in such a conventional method. In the figure, spheroidization is achieved through steps of temperature rising → soaking → slow cooling. .

In the annealing of cold-rolled steel sheets, there is a so-called continuous rapid annealing treatment method for a steel sheet, in which a coil of the steel sheet is unwound and continuously run in an annealing furnace for treatment. This rapid continuous annealing is Ae ₁ The ferrite is softened and annealed by heating at a temperature below the transformation point, and has a completely different metallurgical significance from the spheroidized annealing for spheroidizing the carbide which is the object of the present invention.

As such a conventional technique, for example, Japanese Patent Laid-Open No. 59-110736 can be cited. However, such a conventional spheroidizing annealing method has the following drawbacks. (1) Difficulty in further shortening the spheroidizing annealing time: With the conventional spheroidizing annealing method, the cementite is sufficiently decomposed and dissolved in austenite by heating during annealing,
A method of reprecipitating and growing cementite into a spherical shape during the cooling of annealing is adopted by using slightly remaining carbides as nuclei.

Therefore, in the cooling process in the spheroidizing annealing, it is necessary to slowly cool the cementite for a sufficient time so that the cementite precipitates and grows in a spherical shape. If the cementite is not gradually cooled for several hours to several tens of hours, the cementite becomes rod-shaped or It deposits in layers. Therefore, it is common knowledge in the art that spheroidizing of cementite requires an essentially long time.

Since a thermal history of heating / cooling is applied in a heavy coil state, the thermal inertia is extremely large, and it takes a very long time to raise / cool the temperature. Therefore, the productivity is extremely low and the heat treatment cost is high. For example, in the case of SUJ2 bearing steel, the temperature rise is 8 hours, soaking is 8 hours, and slow cooling is, for example, 14 hours.
It takes 30 hours in total, and in the case of a medium carbon low alloy steel for cold forging, it takes 8 hours for temperature rise, 8 hours for soaking, 8 hours for slow cooling, for a total of 24 hours.

On the other hand, Japanese Patent Publication No. 61-15930 and Japanese Patent Publication No. 61-57891 propose a method of continuously and rapidly spheroidizing a wire. The former invention aims at shortening the time of spheroidizing annealing by stopping the treatment of the wire rod in the coil state and rewinding the coil and continuously heating and cooling it by passing one coil.

The latter invention is a continuous descaling process,
In addition, while cold drawing is applied, heating and cooling are continuously performed to shorten the time for spheroidizing annealing, thereby substantially streamlining the manufacturing process of annealed wire rods.

FIG. 2 is an explanatory view of a heat pattern of spheroidizing annealing by such a single-pass repeating method in the conventional method. In the figure, as an example, spheroidizing is performed through each process showing heating temperature and time. Is planned.

However, in these conventional techniques, the method of efficiently imparting a heat history for spheroidization is the main technique, and the spheroidization of cementite is based on the conventional spheroidizing mechanism described in It is not an innovative technology that has reformed the spheroidizing mechanism of cementite from the essence.

In order to further shorten the spheroidizing time with respect to these prior arts, the limit is reached only by improving and improving the efficiency of such a treatment method, and without technological innovation from the essence of the spheroidizing mechanism of cementite. Is in a situation where it is difficult to further speed up and shorten spheroidizing annealing.

(2) The size of the spheroidized cementite particles is fine: In the conventional spheroidizing annealing for a long time, the cementite particles formed are sufficiently large because they are gradually cooled at an extremely slow cooling rate. Can grow to size. On the contrary,
In the rapid spheroidizing annealing shown as the prior art, there is little time margin for the growth of cementite, so even if cementite can be spheroidized, its size is often very small.
The structure after annealing, hardness and ductility tend to be locally non-uniform.

The reason why the fine cementite is unevenly dispersed is considered to be due to the following circumstances: a) The cementite is decomposed into a solid solution in austenite by heating during annealing, and a small amount of remaining carbides and the like are nucleated. Since the conventional spheroidization method of re-precipitating and growing cementite into a spherical shape during cooling during annealing is adopted, in order to precipitate and grow to a large size of cementite, C atoms and Fe, Cr, etc. which compose cementite, etc. It is necessary that the alloy atoms of (3) diffuse and coagulate sufficiently.

B) However, the prior art methods do not make any improvement to the above-described growth mechanism of the cementite particles, so that the cementite particles become finer as the cooling rate during slow cooling increases, that is, rapidly. The tendency that the cementite particles become finer as the spheroidizing annealing is attempted becomes unavoidable.

[0016]

Thus, the general object of the present invention is to provide a spheroidizing annealing treatment for steel which has conventionally required a treatment time of about 20 hours (especially for alloy steels such as bearing steels having a grain size of 25- (It required a long-time treatment of 30 hours or more)
The objective is to develop a technology that realizes an innovative rapid spheroidizing annealing method capable of treating C. for a very short time of 1 hour or less (preferably 30 minutes or less).

A more specific object of the present invention is to establish a new annealing heat treatment heat pattern for accelerating cementite spheroidization, and at the same time, to reduce the spheroidizing tendency of spheroidal carbide, which is unavoidable when spheroidizing annealing is accelerated. It is to establish a new thermo-mechanical treatment method to prevent this.

[0018]

As already mentioned, it is essentially difficult for the conventional annealing technology to simultaneously realize the further rapid spheroidization and the enlargement of the cementite particles. Therefore, the present inventors re-examined the process of morphological change of cementite in detail, and started the research by reexamining the mechanism of spheroidization of cementite from the basics, and started a new rapid study based on a new technical idea and technical method. We aimed to develop a spheroidizing technology.

Here, the spheroidizing phenomenon of the carbide is due to Fe and Cr.
Is a phenomenon that progresses by diffusion and reprecipitation of alloying elements such as C and austenite in C, and is a process that is essentially rate-controlled by diffusion and grain growth controlled by temperature and time. Therefore, it is extremely difficult in principle to simultaneously realize both spheroidizing annealing of cementite and spheroidizing annealing in a short time, and at the same time, enlargement of cementite particles.

Here, the inventors of the present invention used the conventional spheroidizing annealing method, "Cementite was dissolved and dissolved in austenite by heating during annealing, and a small amount of carbide remained as a core. From the method of reprecipitating and growing cementite into spheres during cooling by annealing, and `` I want to eliminate solid solution by dissolving only non-spheroidized cementite particles and fine cementite particles around the austenite and want to eliminate solid solution '' By promptly dissolving and dissolving the cementite particles in the surrounding austenite, the undesired cementite is promptly dissolved into solid solution and the C atoms constituting the cementite are diffused in a wide range.
It also facilitates spherical precipitation of cementite during subsequent cooling, "and" only fine cementite particles are preferentially decomposed and dissolved in austenite during the heating stage during annealing, and large cementite particles are undecomposed and unsolidified. " In the non-equilibrium state, the fine cementite particles are decomposed and disappeared to remain in a molten state, and the solid-dissolved C atoms are re-precipitated and grow around the remaining undecomposed / undissolved cementite particles. Based on the idea of utilizing the reaction, the decomposition and solid solution behavior of cementite during the heating process in spheroidizing annealing were investigated in detail, and the following new findings were obtained.

(I) The order of decomposition and solid solution of cementite is as follows.
(1) layered eutectoid pearlite, (2) rod-shaped cementite,
(3) The order is lumpy or spherical cementite, and the more non-spherical cementite that requires spheroidizing, the faster and preferentially decomposed and solid-dissolved.

(II) The finer cementite particles are decomposed and solid-dissolved faster and disappear. (III) Both the heating temperature and the heating time affect the disappearance of these solid solutions of cementite, but the higher the heating temperature and the shorter the heating time, that is, the higher temperature rapid heat treatment,
The tendencies of (I) and (II) above are strengthened.

(IV) The phenomena of (I) and (II) above are non-equilibrium transient phenomena, and the equilibrium solid solution disappearance rapidly observed in the conventional annealing heat treatment in a time of several seconds to several minutes. Reach the state.

Further, according to the knowledge of the present inventors, it has been found that the effect of shortening the spheroidization is further promoted by adding plastic deformation during heating as follows. (V) When C atoms solid-dissolved in austenite precipitate on cementite again during cooling, it becomes very difficult to agglomerate and precipitate on large spherical cementite particles if C atoms are distributed in a wide range in a dilute manner. It is extremely effective that the solid-dissolved C atoms are not distributed over a very wide range and are concentrated around the undissolved cementite particles because the cementite is largely spherically precipitated even during rapid cooling.

Therefore, when a plastic strain immediately before the transformation from ferrite to austenite is applied, dislocations due to processing are densely accumulated around the cementite particles, and the ferrite around the cementite particles becomes austenite very rapidly. Therefore, if processing is performed before transformation during the temperature rise of rapid heating, the cementite particles are finely austenized everywhere only around the cementite, and the cementite particles in contact therewith are rapidly decomposed to form a solid solution.

At this time, since the austenite is remarkably different from the one appearing by ordinary heating and is formed by surrounding the carbide very thinly, the C atom is concentrated and dissolved in such austenite.

As described above, the conventional annealing treatment techniques, including the accelerated technique of the prior art, are all techniques based on the phenomenon in the state of solid solution disappearance. On the other hand, the technique of the present invention was not possible in all annealing treatment techniques including the accelerated technique of the prior example, and the heating (soaking) time was from 0 seconds to several minutes, which was an extremely short time, and was still in a transitional stage. By utilizing a certain non-equilibrium phenomenon, and by adding processing just before the austenite transformation, it is possible to take advantage of the fact that these transient non-equilibrium states occur extremely locally in the very vicinity of cementite. However, these points are the points that decisively differ from the prior art.

With the help of processing strain during the above rapid temperature rise, a very large number of non-equilibrium / transient phenomena are locally generated, thereby eliminating only inconvenient cementite and spherically expanding the cementite particles. Will be possible. Furthermore, in order to take advantage of these non-equilibrium and transient phenomena, rapid processing, which was impossible in the prior art, is required, which is exactly the purpose of the present invention.

The decomposition and solid solution process of cementite in the heating process in the rapid thermal treatment of the present invention was further divided into the following two elementary processes. These elementary processes can be selectively used according to the cementite morphology of the steel material before annealing, and the respective advantages can be utilized while complementing the disadvantages.

In the case of a non-spheroidized structure in which a large amount of large pearlite or rod-shaped cementite is present, the heating process is set to be slightly low and the heating time is set to be slightly long, so that these inconvenient cementites are completely eliminated. Take advantage of.

If necessary, a processing strain is applied just before the Ac ₁ transformation point at this time, and the plastic strain causes the Ac ₁ only around the cementite particles before the dislocation density accumulated around the cementite particles decreases due to recovery or the like. It is preferable to cause the transformation.

In the case of a fine structure in which many fine cementite particles are present and the spheroidization is relatively good but the hardness is high, the heating temperature is set high and the heating time is set to a very short time so that these inconvenient fine particles are formed. Try to completely eliminate only the cementite particles. Also Ac ₁ a processing strain added immediately before transformation in this elementary process, very possible to cause only the Ac ₁ transformation near the cementite particles while the dislocation density integrated around the cementite particles by the plastic strain is not reduced by such recovery It is valid.

By combining or repeating these elementary processes, the shape and size distribution of cementite can be made close to a desired one. Particularly, in the case of the elementary process, fine spherical cementite is easily re-precipitated, but they can be eliminated by the elementary process, and the two elementary processes have an effective complementary relationship with each other.

As described above, the conventional annealing treatment techniques, including the accelerated technique of the prior art, are all appropriate repetitions of the technique based on the phenomenon in the state of solid solution disappearance in equilibrium. On the other hand, the present invention, including the accelerated technology of the prior art, has a heating (soaking) time of 0 seconds to several minutes, which is not possible with all conventional annealing treatment technologies, and is still in a transient stage. By using the two types of non-equilibrium phenomena described in 1., these two types of elementary processes are compounded according to the pre-structure of the material to be heat treated and the desired spheroidized structure, and the shape of cementite is formed while achieving an effective complementary relationship with each other. And size distribution close to the desired one.

The gist of the present invention is as follows. (1) A rapid continuous spheroidizing annealing method for a steel material, which is characterized by performing the following elementary processes individually or in combination once or twice or more on a steel material.

Elementary process: Ae ₁ point or more, Ae ₁ point + 150K or less in a temperature range at a heating rate of 1 K / sec or more, and kept in the temperature range for 0 second or more and less than 600 seconds. After, Ae ₁
It holds the temperature range of the point + 50K～Ae ₁ point -150K to or the temperature range of the temperature is cooled below the cooling rate of 5K / sec; and fundamental processes: Ae ₁ point + 80K or more, Ae ₁ point + Ae ₁ point to Ae after heating up in the temperature range of 300K or less at a heating rate of 1K / sec or more and holding in the temperature range for 0 second or more and less than 120 seconds
₁ point-Cool within the temperature range of -150K at a cooling rate of 5K / sec or less, or keep the temperature within the temperature range.

(2) The method according to (1) above, in which the steel material is subjected to plastic working with a strain of 7% or more within a temperature range of less than Ac ₁ point and 623 K or more during the temperature rise in each of the elementary steps and each of the steps. (3) The method according to (1) or (2) above, which is performed after the steel material is subjected to plastic working of 15% or more in warm or cold with Ae less than ₁ point.

(4) In the continuous spheroidizing annealing treatment method for steel products comprising repeated heating / cooling cycles, one or more cycles of the repeating heating / cooling cycles are subjected to the elementary process in (1) or (2) above and A rapid continuous spheroidizing annealing method for steel products, which comprises replacing with a combination of elementary processes.

[0039]

Next, the reason why the processing conditions are limited in the present invention as described above and the function and effect thereof will be described in detail. In addition,
The `` steel material '' targeted in the present invention is not only hot rolled steel material, but also steel material that has been subjected to various heat treatments in advance (e.g. normalization, quenching and tempering, annealing, etc.) or various plastic working (e.g. cold working). Steel material that has been subjected to working, warm working, etc.).

The object is expressed only as steel, and refers to all steel types to which spheroidizing annealing is applied. Therefore, numerical values such as the composition of the target steel are not limited. FIG. 3 is an explanatory view showing a case where the heat pattern of the continuous spheroidizing annealing process according to the present invention is continuously performed in the elementary steps and, basically, in each step (heating) →
It consists of the steps of (heating) → (uniform heating) → (cooling) → (gradual cooling). The heating means in the heat treatment according to the present invention is not particularly limited as long as the desired heating conditions can be realized, but in practice, there are induction heating, electric heating, etc., and electric heating is particularly preferable as used in Examples described later.

(1) Elementary process: (i) Heating temperature: Ae ₁ or higher, Ae ₁ + 150K or lower In the elementary process of the present invention, coarse spheroidized coarse cementite particles are once decomposed and solid-dissolved in austenite. Therefore, the heating temperature must be Ae ₁ point or higher.

When the heating temperature exceeds Ae ₁ + 150K, not only the cementite of the eutectoid pearlite structure to be decomposed and solid-dissolved but also the spherical cementite particles to be left to be decomposed and solid-dissolved, the upper limit temperature is set to Ae ₁ + 150K Furthermore, if the heating temperature exceeds this upper limit, the austenite region limited around the cementite particles induced by plastic working grows and grows at once, so the effect of imparting plastic working in the temperature rising process disappears completely. Will end up. The lower limit of the heating temperature is preferably Ae ₁ + 25K. Ae ₁ ~ Ae ₁
This is because the cementite of eutectoid pearlite, which is desired to be decomposed and dissolved, may not be sufficiently decomposed and dissolved between + 25K.

(Ii) Rate of temperature rise: 1 K / s or more The rate of temperature rise during heating in the elementary process is also an important factor. If the heating rate is a slow heating rate as in ordinary heat treatment, the cementite will decompose and form a solid solution during the heating process until it reaches an equilibrium state. The intention of melting and disappearing will not be realized, and it will be the same as if the heating temperature was too high. In addition, the dislocations introduced by the bending process are recovered, and the dislocations are reduced and disappeared by recrystallization, so that it becomes impossible to cause the Ac ₁ transformation only around the cementite particles. For this reason, the temperature rising rate needs to be 1 K / s or more. No matter how high the heating rate is, there seems to be no inconvenience, but in reality, if the heating rate is too high, temperature control becomes difficult, and the reached heating temperature becomes too high or too low and the above problems occur. Be careful because it is likely to happen.

(Iii) Soaking time: 0 s or more and 600 s or less (preferably 10 s or more and 300 s or less) The soaking time is the most important factor together with the heating temperature. In the elementary process, one of the missions is to completely dissolve the non-sphericalized cementite remaining in the pre-structure into a solid solution, and the soaking time is very important to fulfill this mission. Therefore, the length of the soaking time must be properly selected according to the way of the pre-structure. Generally, the shorter the better, the better, but for the purpose of decomposing and solidifying the incompletely spheroidized cementite in the elementary process, it is often preferable to soak for at least 10 seconds. on the other hand,
When the soaking time is up to 600 seconds, the amount of decomposed solid solution of cementite in which the austenite generated by work-induced growth around the cementite has grown and coarsened increases, and the diffusion of C atoms progresses and the concentration of C atoms becomes diluted. On the contrary, since the fine cementite re-precipitates on subsequent cooling and the physical condition deteriorates, the upper limit was set to 600 seconds.

(Iv) Cooling: Ae ₁ + 50K to Ae ₁ -150K (Ae ₁ + 50 ° C
~ Ae ₁ -150 ℃) within 5K / sec (300 ℃ / min)
[Preferably 5K / sec (300 ° C / min) or less, 0.1K / sec (6 ° C / min)
Min) or above] or keep the temperature within the temperature range. The slow cooling temperature range was to Ae ₁ + 50K~Ae ₁ -150K, this range is because it is mainly temperature range in which precipitation of cementite from austenite occur.

The slow cooling rate is 5 K / sec (300 ° C./min) or less, preferably 5 K / sec (300 ° C./min) or less and 0.1 K / sec (6 ° C./min) or more because precipitation occurs. This is because the slow cooling rate needs to be 5 K / sec or less in order for the cementite to be spherical, and preferably 0.1 K / sec or more in order to realize the short-time rapid annealing which is the object of the present invention. is there. Since the austenitization can be locally generated around each cementite particle by applying plastic working in the temperature rising process, the cooling rate in the present invention can be further increased as compared with the prior art. .

(V) In the elementary process, if necessary, within the temperature range of 623K (350 ° C) or more below the Ac ₁ point during the temperature rise.
% Or more strain plastic working may be added: The plastic working in the elementary process in the present invention is extremely effective. The reason is as described above. Plastic strain is given to concentrate dislocations intensively on the ferrite material that is about to transform into austenite around the carbide to be agglomerated and spheroidized, and naturally it must be less than Ac ₁ point. is there. Furthermore, since the plastic working temperature only needs to be able to concentrate dislocations in the ferrite around the carbides, the dislocations introduced by the working process are recovered, and if the dislocations are reduced and disappeared by recrystallization or the like, the effect is lost. However, it is considered effective to process in a low temperature range below the recrystallization temperature, but if the processing temperature is below 623K (350 ° C), the dislocations recover and regenerate during the subsequent rapid temperature rise to Ae ₁ point or higher. Since it decreases and disappears due to crystals, etc., 623K (350
(° C) was set as the lower limit temperature. The larger the amount of plastic strain to be added, the better, but the upper limit value is naturally determined by the deformability in the processing of steel materials, so there is no particular limitation. The smaller the amount of plastic strain added, the easier the facility and the operation, but in order to effectively promote the precipitation of spherical cementite and the size enlargement, at least 7% or more of plastic strain is necessary. 7% was set as the lower limit of the amount of plastic strain.

(2) Elementary process: (i) Heating temperature: Ae ₁ + 80K or more, Ae ₁ + 300K or less (preferably Ae ₁ + 80K or more, Ae ₁ + 270K or less) Heating temperature during heating in the elementary process Is too low at less than Ac ₁ point + 80 ° C, and even if it is very fine cementite, it does not decompose and remains in large quantity and does not fulfill the purpose of the present invention of increasing the average size of spherical cementite, so the lower limit value is Ae. ₁ + 80K.

The heating temperature during heating in the elementary process is 1273.
If it exceeds K (1000 ° C), it is too high and most of the cementite is decomposed to form a solid solution and disappears, which is a fatal situation in the method of the present invention. When this happens, the spheroidizing of re-precipitated cementite can no longer be realized by the technique of the present invention, and the recovery means can only be achieved by waiting for the cementite to precipitate spherically while slowly cooling it over a very long time as in the conventional method. The upper limit is Ae ₁ + 300K, preferably Ae ₁ + 270K
I made it.

(Ii) Temperature rising rate: 1 K / s or more The temperature rising rate during heating in the elementary process is an extremely important factor. If the heating rate is a slow heating rate as in ordinary heat treatment, the cementite will decompose and form a solid solution during the heating process until it reaches an equilibrium state. The intention of melting and disappearing will not be realized, and it will be the same as if the heating temperature was too high. In the elementary process, the heating temperature is raised and the heating time is shortened in order to eliminate solid solution of only fine cementite in the non-equilibrium process during the heating process. It falls into a situation. Therefore, the temperature rising rate needs to be 1 K / s or more. No matter how high the heating rate is, there seems to be no inconvenience, but in reality, if the heating rate is too high, temperature control becomes difficult, and the above-mentioned troubles occur because the reached heating temperature becomes too high or too low. Be careful because it will be easier.

(Iii) Soaking time: 0 s or more and 120 s or less (preferably 0 s or more and 10 s or less) The soaking time is the most important factor together with the heating temperature. In the elementary process, the mission is to dissolve only the very fine spheroidized cementite remaining in the pre-structure and to increase the average cementite particle size, but the soaking time is considered to affect this mission. is important. The length of soaking time must be properly selected according to the method of the previous structure. Generally, the shorter the better, the better.
At the end of the second, even large spherical cementite decomposes and forms a solid solution, and fine cementite re-precipitates on subsequent cooling, which is extremely unsatisfactory, so the upper limit was set to 120 seconds. The smaller the amount of fine cementite particles, the longer the proper value of this time can be taken, but it is desirable to process in 10 seconds or less for general structures.

(Iv) Cooling: Ae _{1 to} Ae ₁ -150K (Ae _{1 to} Ae ₁ -150
℃) within 5K / sec (300 ℃ / min) or less [preferably 5
K / sec (300 ° C / min) or less, 0.5K / sec (30 ° C / min) or more]
The cooling is performed at the cooling rate of, or the temperature is maintained within the temperature range.

[0053] of the slow cooling temperature range was to Ae ₁ ~Ae ₁ -150K is,
This is because this range is the temperature at which the precipitation of cementite from austenite occurs. The slow cooling rate is 5K / sec (300
0 ° C / min) or less, preferably 5 K / sec (300 ° C / min) or less.
The rate of 1 K / sec (6 ° C / min) or more is set because the rate of slow cooling must be 5 K / sec or less in order for the precipitated cementite to be spherical, and preferably 0.1 K / s or more. This is because the short-time rapid annealing that is the object of the present invention is realized. Since the austenitization can be locally generated around each cementite particle by applying plastic working in the temperature rising process, the cooling rate in the present invention can be further increased as compared with the prior art. .

(V) Also in the elementary process, if desired, within the temperature range of less than Ac ₁ point and 623 K (350 ° C.) or more during the temperature rise,
It is preferable to add plastic working with strain of not less than%. The plastic working in the elementary process of the present invention is extremely effective. The reason is as described above. Plastic strain is given to concentrate dislocations intensively on the ferrite material that is about to transform into austenite around the carbide to be agglomerated and spheroidized, and naturally it must be less than Ac ₁ point. is there.

Further, since the plastic working temperature only needs to be able to concentrate dislocations in the ferrite around the carbides, dislocations introduced by working carefully are recovered and recrystallized, and the like, and disappear if they disappear. Because there is no
It is considered effective to process in a low temperature range below the recrystallization temperature, but if the processing temperature is below 623 K (350 ° C), dislocations recover and recrystallize during the subsequent rapid temperature rise to Ae ₁ point or higher. Since it decreases and disappears due to such factors, the lower limit temperature was 623K (350 ° C). The larger the amount of added plastic strain, the better, but its upper limit is not particularly limited because it is naturally determined by the transformation capacity in the processing of steel materials. The smaller the amount of plastic strain added, the easier the facility and the operation, but in order to effectively promote the precipitation of spherical cementite and the size enlargement, at least 7% or more of plastic strain is necessary. Since 7% or more of plastic strain is required, 7% was set as the lower limit of the amount of plastic strain.

(3) Each elementary process is applied once or in combination two or more times: The objectives and effects are different from those of the elementary process, and either one may be used, but if both effects are used together, a large effect is achieved. Rapid spheroidizing annealing in which cementite particles are distributed is possible. Therefore, in the present invention, it is desirable to combine these two elementary processes. Furthermore, the effect is further enhanced by repeating each elementary process as necessary.

(4) Ae is subjected to plastic working with a cross-sectional area reduction of 15% or more in a warm temperature region of less than ₁ point or in a cold state: the steel material is cooled before the rapid annealing method of the present invention is applied. If the inter-working is added, the average size of the cementite particles after annealing further increases, and the annealing process can be accelerated. The effect of cold working is that dislocations are accumulated at a high density in the matrix around the cementite particles before annealing, and austenitization occurs around the individual cementite particles during rapid temperature rise.

That is, in the processed steel material, only the periphery of each cementite particle becomes austenite and the decomposition and solid solution of cementite begins to proceed, and most of the matrix outside thereof is not austenitized. Phenomenon occurs. When cooled from this state, the reprecipitated cementite grows into spherical cementite by precipitation even at a very high cooling rate.

Since the effect of such working becomes remarkable when the working ratio exceeds 15%, the lower limit of the working ratio is set to 15%. The larger the workability is, the greater the effect tends to be, but there is a limit to the workability.

The processing performed before the annealing treatment of the present invention may be cold or warm. After all, 15% or more is effective. Cementite decomposition and pearlite regeneration begin when the temperature at which processing is applied exceeds Ae ₁ point, so adding more than ₁ point Ae has no effect. Next, the function and effect of the present invention will be described more specifically with reference to Examples.

[0061]

【Example】

Example 1 A commercially available wire coil made of a wire having a diameter of 18 mm produced by hot rolling a steel having the steel composition shown in Table 1 was used. These wire rod coils, those obtained by cold drawing of these coil, and those obtained by warm rolling were used as test materials and subjected to conventional annealing under the respective conditions shown in Tables 2 to 10 and rapid annealing according to the present invention. gave.

Table 2 shows the spheroidizing annealing conditions by the batch furnace shown in FIG. 1, and Table 3 shows the spheroidizing annealing conditions by the one-pass, repeating method shown in FIG. Further, in each table, the description of “comparative method” is referred to as “comparative method” to indicate comparison with the method of the present invention described in each claim.

A pair of wire rod rolling mills electrically insulated from each other between roll stands for rapid annealing are used as current-carrying electrodes.
The sample material is rolled through these as necessary, and the wire to be rolled is electrically heated from the power source connected between the roll stands, and the temperature of the sample material is preset at the exit roller outlet by measuring the temperature of the sample material with an infrared radiation thermometer. The energizing energy was automatically adjusted to give various heat histories according to the temperature control program.

The results are summarized in Table 11. From Table 11, the characteristics of the conventional method and the characteristics of the method of the present invention and the comparative example of the conditions thereof are as follows. (i) Experiment Nos. 1 and 2 where the conventional coil was annealed in the batch furnace
In the case of cementite, the particle size is large and the hardness is low.
With spheroidizing annealing in (4), the grain size of cementite is small and the hardness cannot be lowered sufficiently.

(Ii) On the other hand, in the experiment numbers (36) to (41) of the present invention, the average particle size of cementite becomes very large, and in the experiment numbers (1) and (2) of the conventional method, Approaching the size of. The hardness is also sufficiently low, and it is not much different from the value of the conventional spheroidized material.

(Iii) Whereas the conventional annealing in a batch furnace requires a treatment time of about 25 hours, the one-pass repeating method is surprisingly shortened to just under 1 hour. However, in the prior art, shortening the annealing time has reached the limit, and even if cementite can be spheroidized, it is no longer desirable to enlarge cementite grains and further reduce hardness.

In the present invention, a cementite particle size distribution control method utilizing a non-equilibrium process newly introduced to eliminate only fine particles of cementite and to cause aggregation and enlargement into large particles is further used for 60 minutes to We have succeeded in shortening the spheroidizing annealing time to about 30 minutes, and the structure and hardness after annealing are far superior to those of the conventional one-pass repeating method. Approaching infinitely.

(Iv) The results in these experimental examples are truly revolutionary ones that completely overturn the conventional concept for the processing time of spheroidizing annealing, and the technical and industrial significance and effects of the present invention are immeasurable. Not a big one. Next, let us examine the effect of heating conditions on each elementary process. First, when the elementary process is examined in Experimental Examples 5 to 24, the following points are found.

(I) As in Experiment No. (5), the heating temperature during heating in the elementary process was too low at less than Ae ₁ point (710 ° C.), and cementite did not decompose and many long pearlite-like substances remained. And do not serve the purpose of spheroidization.

(Ii) The heating temperature at the time of heating in the elementary process is too high at 900 ° C. as seen in Experiment No. (6), and most of cementite is decomposed and dissolved to disappear. Is deadly. In this case, the spheroidizing annealing can no longer be realized by the technique of the present invention, and the only remedy is to wait for the cementite to be spherically precipitated while slowly cooling over a very long time as in the conventional method.

(Iii) The rate of temperature rise during heating in the elementary process is also an important factor. If the heating rate is a slow heating rate as in ordinary heat treatment as in Experiment No. (7), cementite will decompose and form a solid solution during the heating process to the equilibrium state. The intention to preferentially dissolve only cementite into solid solution disappears, and the heating temperature is too high. No matter how high the heating rate is, there seems to be no inconvenience, but in reality, if the heating rate is too high, temperature control becomes difficult, and the above-mentioned troubles occur because the reached heating temperature becomes too high or too low. Be careful because it will be easier.

(Iv) The soaking time is the most important factor together with the heating temperature. In the elementary process, it is one of the missions to completely dissolve the non-sphericalized cementite remaining in the pre-structure into a solid solution, and the soaking time is important for fulfilling this mission. Therefore, the length of the soaking time must be properly selected according to the way of the pre-structure. Generally, the shorter the length, the better.Experiment number (8)
As described above, when 600 seconds is reached, the amount of decomposed solid solution of cementite increases, and the fine cementite re-precipitates on subsequent cooling, which deteriorates the condition.

(V) Even if the end temperature of slow cooling is lowered to 500 ° C.
(Experimental Example 9) Since there is no difference between the case of 580 ° C and (Experimental Example 21), the annealing time is merely wasted. Next, when the elementary process is examined in Experimental Examples 25 to 43, the following points are found.

(I) As in Experimental Example 25, the heating temperature during heating in the elementary process was too low at less than Ae ₁ point + 80 ° C. (800 ° C.), and much fine cementite remained without being decomposed. It does not serve the purpose of the invention to increase the average size of spherical cementite.

(Ii) The heating temperature at the time of heating in the elementary process is too high when it exceeds 1000 ° C. as seen in Experimental Example 26, and most of cementite decomposes and dissolves and disappears. Fatal. In this case, the spheroidizing of re-precipitated cementite can no longer be realized by the technique of the present invention, and it is necessary to wait for the cementite to be spherically precipitated while slowly cooling over a very long time as in the conventional method. Absent.

(Iii) The rate of temperature rise during heating in the elementary process is an extremely important factor. When the heating rate is a slow heating rate as in the case of the normal heat treatment as in Experimental Example 27, the cementite is decomposed and solid-dissolved to the equilibrium state during the heating process, and thus the fine cementite which is the aim of the present invention The intention to preferentially dissolve only the solid solution disappears and the heating temperature becomes too high. In the elementary process, the heating temperature is raised and the heating time is shortened in order to eliminate the solid solution of only fine cementite in the non-equilibrium process during the temperature rise. Will fall into the target situation. No matter how high the heating rate is, there seems to be no inconvenience, but in reality, if the heating rate is too high, temperature control becomes difficult, and the above-mentioned troubles occur because the reached heating temperature becomes too high or too low. Be careful because it will be easier.

(Iv) Soaking time is the most important factor together with the heating temperature. In the elementary process, the mission is to dissolve only the very fine spheroidized cementite remaining in the pre-structure into solid solution, and the soaking time is important as it influences this mission. The length of soaking time must be properly selected according to the method of the previous structure. Generally, the shorter the length, the better.
By the second, even large spherical cementite decomposes and forms a solid solution, and fine cementite re-precipitates on subsequent cooling, which is very unsatisfactory.

(V) Even if the end temperature of slow cooling is lowered to 500 ° C.
(Experimental Example 29) Since there is no difference from the case of 585 ° C. (Experimental Example 40), only the annealing time is wasted. The repetition of the elementary process in the present invention is effective, but the larger the number of repetitions, the longer the annealing time, and therefore the balance with the increase in the effect due to the repetition is a practical problem. Regarding this, the following points can be seen.

Compared with the case of only once in Experimental Example 10,
The structure / characteristic becomes better as the number of times of the experimental example 44 becomes twice and the number of times of the experimental example 45 becomes three, but there is a tendency that even if the number of times is increased from the experimental example 44 to the experimental example 45, the improvement range becomes small. Therefore, it can be said that the number of repetitions is practically 2 to 3 times. of course,
Whether or not it is practical depends on how time and cost are evaluated, so it will not be absolute, but may be repeated as appropriate for the purpose.

Further, in the present invention, it is very effective to perform cold working or warm working in advance and then perform the elementary process and the annealing treatment of the elementary process. For example, Experimental Example 48
And 49, the degree of pre-processing in cold or warm
At 10%, there is no big difference from Example 10 in which no processing is performed in advance, but the characteristics are remarkably improved when the degree of processing reaches 30% as in Experimental Examples 50 and 51.

(Example 2) In this example, Example 1 was repeated except that processing was performed in the course of temperature increase in the elementary process.

For the processing during the temperature rise, among the pair of roll stand groups of the first embodiment, the number of roll stands of the energizing portion corresponding to the temperature raising stage of each elementary process is 3 or 4, and in the case of 3 rolls, the middle stand is used. While heating the steel material between the front and rear stands by heating it by heating, the second two groups are pressed down, and in the case of four groups, the middle two pairs are pressed down. It was carried out between the base stand and the third stand and the fourth stand.

The operating conditions and results are shown in Tables 12, 13 and 14
Are shown together. Compared to the case of Example 1, the grain size of the carbide was larger, the hardness after annealing was lower, and the spheroidizing annealing effect was further improved. With regard to the action and effect of the implementation conditions, almost the same results as in Example 1 were obtained.

[0084]

[Table 1]

[0085]

[Table 2]

[0086]

[Table 3]

[0087]

[Table 4]

[0088]

[Table 5]

[0089]

[Table 6]

[0090]

[Table 7]

[0091]

[Table 8]

[0092]

[Table 9]

[0093]

[Table 10]

[0094]

[Table 11]

[0095]

[Table 12]

[0096]

[Table 13]

[0097]

[Table 14]

[0098]

[Table 15]

[0099]

[Table 16]

[0100]

[Table 17]

[0101]

[Table 18]

[0102]

[Table 19]

[0103]

As described above, according to the present invention, the spheroidizing annealing treatment of steel, which conventionally required 20 hours or more, can be achieved in a short time of about 1 hour, and the effect is amazing. ,
From the peculiarity of the metallurgical phenomenon behind it, the present invention makes a great contribution to the development of this field.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a heat pattern of a conventional method.

FIG. 2 is an explanatory diagram of another conventional heat pattern.

FIG. 3 is a schematic explanatory diagram of a heat pattern of a spheroidizing annealing process according to the present invention.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C21D 9/52 103 C21D 9/52 103B

Claims (4)

[Claims] 1. A rapid continuous spheroidizing annealing method for a steel material, which comprises subjecting the steel material to each of the following elementary steps and one or more times once or in combination. Elementary process: Ae ₁ point or more, Ae ₁ point + 150K or less temperature range,
The temperature is raised at a heating rate of 1 K / sec or more and heated to 0 within the temperature range.
After holding 600 less seconds or more seconds, Ae ₁ point + 50K~Ae ₁
Cooling within the temperature range of -150K at a cooling rate of 5K / sec or less, or maintaining the temperature within the temperature range; and elementary process: within the temperature range of Ae ₁ point + 80K or more and Ae ₁ point + 300K or less. Ae ₁ point to Ae after heating at a heating rate of 1 K / sec or more and holding for 0 second to less than 120 seconds within the temperature range
₁ point-Cool within the temperature range of -150K at a cooling rate of 5K / sec or less, or keep the temperature within the temperature range. 2. The method according to claim 1, wherein the steel material is subjected to plastic working with a strain of 7% or more in a temperature range of less than Ac ₁ point and 623 K or more during the temperature rising in each of the elementary steps and in each of the steps. 3. A steel material is warmed or coldened with Ae less than ₁ point 15
The method according to claim 1 or 2, which is carried out after the plastic working of not less than%. 4. A continuous spheroidizing annealing method for a steel product comprising repeated heating and cooling cycles, wherein one or more cycles of the repeated heating and cooling cycles are carried out. A rapid continuous spheroidizing annealing method for steel materials, which comprises replacing with.