JPH07116503B2 - Method for manufacturing ultrafine structure steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for stably producing a steel material having an ultrafine uniform structure on an industrial scale.

<Prior art and its problems> It has been widely known that various properties of steel materials (for example, low temperature toughness, various ductility, yield strength, corrosion resistance, superplasticity, etc. improve as the structure becomes finer). However, for this reason, various techniques for producing a microstructured steel have been developed so far, for example, by suppressing the crystal grain growth coarsening of steel by adjusting the composition of components. In addition, Fe-13-18 wt% Cr-8
~ 12 wt% Ni austenitic stainless steel is cold worked at room temperature to transform the austenite to martensite, then heated to a stable austenite region and annealed to reverse transform the martensite to austenite. It has been announced that a fine austenite grain structure can be obtained ["Iron and Steel" No. 6, 1974 (1988) No. 6, No.
1052-1057]. This technique involves cold rolling the material obtained by hot rolling or subjecting it to sub-zero rolling at room temperature or lower, and then heating it in the austenite region to anneal it. Is the one It is interesting to obtain an ultra-fine structure even with such heat treatment, but such an effect is that the composition of the highly Cr-high Ni stainless steel having a transformation point of austenitization of reverse transformation at 500 to 600 ° C is even more limited. This is achieved because of the steel of which the heat treatment is generally less than 15 μm, and even 10 μm.
It was not possible to realize the following austenite grain size structure. Therefore, in recent years, a technique for refining the structure of hot-rolled steel by controlling the rolling conditions during hot rolling (so-called "controlled rolling technique") has made remarkable progress, and many proposals relating to the technique have been made. It is well known that it has made great achievements in the production and supply of high-quality steel materials.

However, even with these technologies of controlled rolling, which brought about a great effect on the refinement of the structure of hot-rolled steel,
For example, it is extremely difficult to obtain a uniform fine structure having a ferrite grain size of 10 μm or less, and it is actually impossible to obtain a uniform ultrafine structure having a ferrite grain size of 5 μm or less.

Therefore, there is a technique (so-called “accelerated cooling technique”) for adjusting the cooling rate after controlled rolling so as to increase the nucleation number of transformation grains from austenite, for example, the number of nucleation such as ferrite crystal grains, to further refine the structure. It came to be developed.

However, even with the technique of combining controlled rolling with accelerated cooling, the austenite structure itself before transformation is only refined by controlled rolling and is not affected by accelerated cooling. There is still a limit to the final grain size of austenite, and it was impossible to obtain a uniform ultrafine austenite structure that breaks this limit. Therefore, it was difficult to naturally cause the refinement limit to occur in the steel material structure after cooling formed based on this structure. That is, the austenite grains that are the pre-stage structure are abandoned for further refinement, and even if accelerated cooling is attempted to promote the refinement of the transformation structure generated from them at least, the original austenite grains themselves are generated because they are large. It was difficult to refine the structure of ferrite etc. as desired.

Moreover, if the cooling is strengthened in order to enhance the effect of accelerated cooling, a fatal problem that only a semi-quenched structure composed of ferrite and martensite, which is unintentionally obtained, can be obtained even when aiming at the ferrite structure, for example. was there.

Although it is a technology that also combines controlled rolling and accelerated cooling, recently, fine ferrite generated when austenite structure was strongly worked at a temperature above the transformation point of a low carbon steel was used, and austenite recrystallization by this was used. By the refining effect of bainite and martensite produced by accelerated cooling and the subsequent accelerated cooling, the content of "1 to less than 50% of ferrite crystal grains having an average grain size of about 5 μm is contained, and the balance is burnt of martensite or bentonite. Proposal to obtain a "hot-rolled steel material with a special structure" (Japanese Patent Publication No. 62-
No. 42021) was also made, but even in this case there is still a limit to the refinement of the austenite grains before transformation, so the existing limit on the microstructuring and homogenization of the hot-worked steel material obtained is overcome. It could not be the technology to do.

In other words, the problems seen in these conventional technologies are derived from the limit of the conventional controlled rolling technology that "the austenite grains produced by hot working cannot be further refined when they become fine to some extent". Therefore, even if an attempt is made to forcibly form fine ferrite from an austenite structure that does not achieve sufficient refinement by accelerated cooling, a satisfactory and uniform ultrafine structure cannot be obtained at all. Therefore, unless the austenitic structure itself in hot rolling is made to be an ultrafine structure by special means, it is necessary to make the microstructure of hot-worked steel
It was considered that the above limit for homogenization cannot be fundamentally eliminated.

On the other hand, for example, a cord wire used for a tire of an automobile is formed by twisting wire-strengthened steel wires. For such applications, wire-strengthened steel wire (usually called “filament”) is used. Therefore, a high ultimate strength (tensile strength obtained after the final cold drawing step) as well as excellent ductility capable of withstanding stranded wire processing are required for the “filament” hereinafter.

This filament is made by drawing a high carbon steel wire rod into a wire rod with a diameter of about 2 mm, then applying patenting and then cold drawing to reduce the wire diameter to a fraction of a few tenths to a few tenths. At the same time, the tensile strength is increased by work hardening. Therefore, it is necessary for the wire rod for wire drawing to "endure large cold working", that is, "have good wire drawability".

By the way, the wire drawability of the above wire is a patenting structure,
Specifically, it is said to be influenced by the lamellar spacing of the pearlite structure and the block size (grain size). this house,
Since the relationship between the pearlite lamella spacing and the wire drawability has been noticed and studied in the past, some proposals regarding a specific method for controlling the pearlite lamella spacing can be found (the applicant of the present invention. Japanese Patent Laid-Open No. 61-186118, etc.), the pearlite block size has not been studied in detail, and the effect on the wire drawability has not been clarified so much. Therefore, conventionally, as a measure for ensuring the wire drawability from the viewpoint of the pearlite block size, avoid high temperature heating during wire manufacturing,
Only the method of preventing austenite coarsening by rolling in a low temperature region has been attempted, but such measures alone cannot be expected to significantly improve wire drawability and ultimate strength and ductility. It was Therefore, the ultimate strength of the conventional “filament with a diameter of about 0.2 mm” used for cord wires, etc. is limited to 320 kgf / mm ^{2 at} best, as can be seen from the test examples described below, and its ductility is also quite low. there were.

That is, FIGS. 1 to 3 are graphs showing the results of investigations regarding the ultimate strength and ductility of conventional filaments. In this survey, C: 0.82% (hereinafter,% indicating the component ratio is% by weight), Si: 0.31%, Mn: 0.41%, P: 0.014%, S: 0.010%, balance: Fe and unavoidable High carbon steel wire made of impurities with wire diameter 2.1
The wire was drawn up to mm, patented, and then drawn to obtain a filament obtained by drawing "tensile strength", "twist value" and "180 degree bending failure probability". .

Now, Fig. 1 shows the relationship between the wire drawing workability (ε) and the drawing and tensile strength. From Fig. 1, the workability is 3.
It can be seen that the drawing value sharply decreases from around 3 and the tensile strength (reached strength) reaches approximately 320 kgf / mm ² . The processing degree (ε) is “ln
(A ₀ / A _n ) ”and is a logarithmic value of a value obtained by dividing the cross-sectional area (A ₀ ) of the wire material by the cross-sectional area (A _n ) after n-pass drawing.

Although FIG. 2 shows the relationship between the wire drawing workability and the twist value,
From FIG. 2, it can be seen that the peak of the twist value is around the workability: 1.5, and when it exceeds that, it gradually decreases and rapidly decreases from around 3.0. That is, FIG. 2 shows that the ductility is improved while the workability is low, but is suddenly deteriorated when the workability is increased.

Furthermore, Fig. 3 shows the relationship between the wire drawing workability and the fracture probability by the 180-degree bending test. From this Fig. 3, it is clear that when the workability exceeds 3.3, the fracture probability increases rapidly and the ductility increases. It turns out that is worse.

As described above, the conventional wire rod for wire-strengthened steel wire has a low workability capable of maintaining ductility, so the wire drawing limit is low, and therefore the strength of the obtained filament is limited to around 320 kgf / mm ^2. However, "If it is possible to control the block size (grain size) of the pearlite structure to make it uniform and fine, it is possible to improve the wire drawability and further improve ultimate strength and ductility. With the expectation that "maybe", the development of means for miniaturizing the pearlite block size has been gradually attracting attention.

However, in terms of miniaturization of the pearlite block size, in reality, it is considered impossible to realize unless the austenite structure before pearlite transformation itself is made even finer as described above. In the present situation, it was not possible to sufficiently confirm whether or not the drastic miniaturization of the pearlite block size would lead to a remarkable improvement in the wire drawability and the ultimate strength and ductility could be further improved.

From the above, the present invention aims to realize a uniform ultrafine austenite structure that could not be realized in the conventional technology by a new means which is completely different from the conventional controlled rolling technology. It is to provide a means capable of forming a uniform and ultrafine transformation structure formed in, further, high ultimate strength suitable for the filament wire material of the cord wire (for example, 380 kgf / mm ² or more). Tensile strength) and excellent ductility (eg 20
The aim was also to make it possible to realize a wire rod for wire-strengthened reinforced steel wire that has both a twisting value of more than one turn and a probability of 180 degree bending failure of 5% or less).

<Means for Solving the Problems> Therefore, in order to achieve the above-mentioned object, the inventors of the present invention firstly performed a hot working capable of making the “austenite crystal grains before transformation by cooling” finer than the conventional technique. We have earnestly studied to find means.

What has been particularly noted here is, "however the existing austenite grains are processed, as long as new austenite grains are generated by recrystallization in hot working, the ultrafine austenite grains targeted by the present invention are An organization cannot be realized ”. That is, since the size of the crystal grains that can be generated by the most suitable controlled rolling to obtain fine grains eventually depends on the size of the austenite grains before rolling, by some means, the austenite before being processed is processed. The reason why the research was advanced from the viewpoint that "the limit of austenite grain refinement included in the prior art" can be overcome only by taking measures to generate grains in a completely fine state. Is.

As a result, the present inventors have first obtained the knowledge as shown in the following items (a) to (e). That is, (a) in the case of hot working of steel, it undergoes a heat history or a working history as in known hot working in the pre-processing stage,
Then, once the temperature and the structure are controlled so that at least a part of the steel structure exhibits a ferrite structure, the temperature is raised to exceed the transformation point while applying plastic working as the final stage of processing, and the ferrite structure is By reverse-transforming into an austenite structure, an ultrafine austenite structure that cannot be obtained by conventional controlled rolling or the like can be realized.

(B) Further, as described above, the ultrafine austenite structure produced by the reverse transformation has a pre-structure (structure mainly composed of ferrite) for the reverse transformation once during the processing before the hot working reaches the final stage. It can be realized by placing the steel material under the temperature condition so that the above temperature can be obtained, and performing the process of raising the temperature and exceeding the transformation point while applying plastic working to this ferrite structure at the final stage of the subsequent processing. A pre-structure (structure mainly composed of ferrite) for preparing an austenite structure by reverse transformation is prepared from the first stage, and first, processing is performed in the cold temperature range or the warm temperature range, and then the processing is performed. It is also realized by performing a process of "increasing the temperature and increasing the temperature to exceed the transformation point while applying plastic working" at the final stage of.

(C) As described above, when plastic transformation is applied to the ferrite structure to raise the temperature to exceed the transformation point and undergo reverse transformation to the austenite structure, in order to sufficiently complete the reverse transformation, while performing plastic working After completion of the temperature rise process to be carried out, the A ₁ transformation point in perfect equilibrium, that is, Ae ₁
It is preferable to maintain the temperature above the spot for a certain period of time.

(D) The hot-worked steel material having an ultrafine-grained austenite structure thus obtained is then subjected to various cooling means (eg, standing cooling, gradual cooling) that have been conventionally applied to impart desired properties to the product. , Heat retention, acceleration, cooling, cooling while applying work, quenching, or a combination thereof), a uniform and ultrafine transformation structure cannot be obtained by conventional techniques.

(E) Furthermore, when using such hot working means,
Hot-worked steel materials undergo a phase transformation of "ferrite → austenite → ferrite", so carbides and nitrides precipitated during processing (these are often used for strengthening steel).
Eliminates the matching of the crystal lattice with the matrix, and the strengthening mechanism of the steel by the carbide or nitride changes from "coherent precipitation strengthening" to "non-coherent precipitation strengthening". Therefore, a steel material aiming at precipitation strengthening brings about an extremely preferable effect that it is strengthened without embrittlement.

And, the present inventors who obtained the new knowledge as described above,
Further research was conducted to make use of this knowledge in the production of high-carbon steel wire for wire drawing, and the following knowledge was also obtained.

First, the inventors of the present invention intend to further improve the ultimate strength and ductility of filaments used as element wires of cord wires and the like, and therefore, there are various reasons for the low drawability (limit workability) of conventional patenting materials. A re-examination was conducted by making full use of a devised method, and as shown in Fig. 4, the limit machining value improves as the pearlite block size becomes finer, but as the refinement progresses, the limit machining value becomes It was confirmed that it would continue to rise even more significantly.

Therefore, the findings shown in the above (a) to (e) that "austenite grains can be made ultrafine if appropriate plastic working is applied while raising the temperature in an appropriate temperature range even for high carbon steel wire rods" In addition, we conducted a further study on the miniaturization of the pearlite block size, and said, `` If the ultrafine austenite grains are adjusted and cooled to cause the pearlite transformation as described above, an extremely fine pearlite block that has never been produced is generated. The wire drawability is remarkably improved, and the ultimate strength and ductility of the filament obtained by wire drawing are significantly improved without patenting the wire material. "

The present invention has been made on the basis of the above-mentioned findings and the like. “The C content is 2.5% or less, and at least a part of ferrite is added to the strain term while adding plastic working of a strain amount of 20% or more. Microstructure consisting of the ferrite, by raising the temperature to the Ac ₁ point or higher (preferably Ac ₃ point or higher), or by holding the temperature in the Ae ₁ point for a time not exceeding 1 hour after the temperature rise. Part or all of the alloy is transformed into austenite (reverse transformation) and then cooled to enable stable production of ultra-fine structure hot-worked steel material. ” Steel with a C content of 0.70-0.90% and a structure in the temperature range of 600 ° C-Ae ₁ point, at least a part of which is ferrite, strain rate: 20% : 30
While increasing the temperature at ℃ / sec or more, the temperature is raised to a temperature range of Ac ₃ point or higher, or after this temperature rising, the structure made of ferrite is once transformed into austenite, and then controlled cooling is performed. It is possible to stably produce an ultrafine structure steel material that can realize a wire-strengthened steel wire having good wire drawability and excellent ultimate strength and ductility by causing the pearlite transformation. To do.

By the way, the term "ferrite structure" as used herein means a structure composed of a ferrite phase in contrast to an austenite phase, and not only isotropic ferrite structure but also acicular ferrite structure, pearlite structure, bainite structure and martensite structure. It means any form of a ferrite structure having a ferrite phase as a constituent element, such as a structure and a tempered martensite structure.

Further, the steel used as a raw material in the method according to the present invention is "C
As long as the content is 2.5% or less and the steel has a structure in which at least part of it is ferrite, it does not matter about the other constituent components or composition, and it does not matter whether it is carbon steel or alloy steel. . It should be noted that "steel having a structure in which at least a part is made of ferrite" means not only "steel having a whole ferrite structure" but also "ferrite and carbide, nitride,
"Mixed structure steel composed of one or more intermetallic compounds", "Mixed structure steel composed of ferrite and austenite" or "Mixed structure steel composed of ferrite, austenite and carbide, nitride, one or more of intermetallic compounds", etc. It goes without saying that it also means.

Here, the reason why the C content in the steel is particularly limited to 2.5% or less is as follows. That is, C is the most basic alloying element of steel, and Fe-based alloys, that is, steels, always contain a proper amount of C according to the purpose. The amount is essentially determined from the viewpoint of the alloy system design of steel. However, if the C content exceeds 2.5%, a huge eutectic cementite or graphite appears and it becomes impossible to uniformly refine the structure. Therefore, in the present invention, the upper limit of the C content in steel is set to 2.5%. Further, according to the present invention, an ultrafine structure can be obtained from "commercial low carbon steel" to "pure iron for which it was very difficult to obtain an ultrafine grain structure in the prior art". It is not necessary to limit the lower limit of the amount.

In addition, according to the means of the present invention, not only carbon steel but also various alloy steels, a novel toughness hot work material having a significantly refined structure without being significantly affected by components other than C is provided. As described above, the structural range of the components other than C in the carbon steel or the alloy steel is not particularly limited because it can be realized, and if the C content and the ferrite grain size in the structure are taken into consideration, the hot working intended by the present invention is performed. It is clear that the steel material can be fully specified.

However, regarding the pearlite structure for strengthening the wire drawing, if the C content is less than 0.70%, it is difficult to increase the ultimate strength of the filament to the target of 380 kgf / mm ² , while the C content exceeds 0.90%. If C is included, cementite precipitates and rather lowers the tensile strength, so the C content is 0.70.
〜0.90%, and Si content other than C is 0.20〜0.30
%, Mn content is preferably 0.45 to 0.55%.

Further, as a processing method for obtaining a desired ultrafine structure hot-worked steel material, known plate rolling machines, various rolling machines for seamless steel pipes, piercing machines, hole-type rolling machines for bar steel and wire rods, drawing machines, etc. In addition to the above, any well-known hammer, swager, stretch reducer, stretcher, twisting machine, etc. can be used as long as it is a method capable of processing the required degree of processing in the required temperature range. , Is not particularly limited.

As described above, in the present invention, in hot working of steel, at least before the final stage of hot working, a structure containing ferrite is exposed in the steel material, and the temperature is increased while the composition working is applied to this structure. A new uniform ultra-fine structure hot-worked steel material (e.g., average ferrite grain size of 10 μm or less, preferably 5 μm, which could not be realized in the past by adding a step of reversely transforming the above ferrite from austenite to ferrite) The main point is to realize the following steel materials and steel materials having an extremely fine pearlite block size, etc., but the various conditions for manufacturing the hot-worked steel material according to the present invention are limited to the above as described above. Explain the reason.

<Operation> A) Reason for using a structure having at least a part of ferrite as the prestructure In the method for manufacturing a hot-worked steel material according to the present invention, the ferrite single structure or the mixed structure centering on ferrite is used as the prestructure. This is because, as described above, the present invention has a main requirement to cause reverse transformation from a ferrite phase to an austenite phase while applying plastic working to steel,
This is because unprecedented fine austenite grains are generated, and a uniform and ultrafine transformation structure is generated from the fine austenite grains by subsequent cooling.

Although the effect of the present invention is greater when the amount of ferrite is greater,
Depending on the steel type, it may be difficult to realize a structure that "100% of ferrite" or "100% of ferrite and carbide (depending on the steel type, it may be nitride or other precipitates)" during hot working. Yes, and depending on the product, "ferrite and austenite" or "ferrite, austenite and carbide (in some cases it may be called nitride, or other compounds or precipitates)" Also in this case, the volume ratio of ferrite is 20% or more, preferably 50% or more.

By the way, regarding the pearlite structure for strengthening the wire drawing, "a steel material having a structure having at least a part of ferrite as the front structure" was defined as "the _{one in} the temperature range of 600 ° C to Ae ₁ point", This is because if the processing start temperature is less than 600 ° C, it takes too much time to reach the processing end temperature (Ac ₃ points or more), and it becomes difficult to refine the austenite grains. This is because it was considered that it is difficult to reach the target Ac ₁ point or higher at the processing end temperature because the temperature rise range until reaching is too large. Therefore, if it is possible to reach the processing end temperature Ac ₁ point or higher by improving some rolling technology or using auxiliary heating means, set the processing start temperature to any temperature lower than 600 ° C. You can Further, that the working start temperature exceeds Ae ₁ point (around 720 ° C. in the steel having a pearlite structure for strengthening wire drawing) means that austenite tends to appear in a part of the steel material structure. In this case, since the effect of refining austenite becomes small, the processing is started in the temperature range below Ae ₁ .

B) Strain Amount of Composition Processing at the Time of Reverse Transformation from Ferrite Phase to Austenite Phase The strain amount of plastic working at this time is important in that the following three actions are caused. One is the action of forming extremely fine austenite crystal grains from the work-hardened ferrite by processing, and the second is the effect that the ferrite is transformed to austenite. The third is the action of generating heat generated by working to raise the temperature of the worked material.The third is work hardening of the generated fine austenite crystals, and the work-induced transformation of finer ferrite grains during subsequent ferrite formation. It is an action to generate.

However, when the strain amount of the plastic working is less than 20%, even if the ferrite is transformed to austenite, the "induced formation" of fine austenite grains is insufficient, and the austenite crystal grains to be produced are targeted. 15 μm
It becomes difficult to do the following. Also, the strain amount of plastic working is 20
If it is less than%, the heat generated by working is small, so that there arises a disadvantage that some auxiliary working means for raising the temperature of the material to be processed to cause reverse transformation during working is indispensable.

In other words, a uniform fine austenite structure of 15 μm or less can be realized relatively easily for the first time by setting the strain amount of plastic working to 20% or more. Therefore, the plasticity added during the reverse transformation from the ferrite phase to the austenite phase can be achieved. The distortion amount of processing was set to 20% or more. However, if the strain amount of this plastic working is 50% or more, depending on the working shape and working speed, the desired action and effect can be completely obtained without using auxiliary heating means. It is desirable that the strain amount of plastic working applied when the phase is reversely transformed to the austenite phase is 50% or more if possible.

By the way, FIG. 5 shows that after heating the high carbon steel wire while plastic working under the conditions that the working start temperature is 650 ° C., the working end temperature is 900 ° C., and the heating rate in this section is 100 ° C./sec. A graph showing the results of investigating the relationship between the plastic workability (percentage of the value obtained by dividing the cross-sectional area of the wire after processing by the cross-sectional area of the steel wire before processing) and the pearlite block size for the pearlite transformed material. is there.

From this FIG. 5, it can be seen that the pearlite block size suddenly coarsens as the plastic working degree decreases. This is because the effect of refining the austenite grains before transformation into pearlite diminishes as the degree of plastic working decreases. Further, as is clear from FIG. 5, when the plastic workability is less than 20%, the pearlite block size is very large and is on the same level as that of the normal material.

The plastic working of the wire to obtain such a fine pearlite structure can be performed by any processing machine such as rolling by a wire mill, wire drawing by a drawing die, or roll bender.

C) Temperature rising temperature at the time of reverse transformation from ferrite phase to austenite phase It is essential that the temperature rising temperature of the steel material to be processed rises to the temperature at which ferrite reversely transforms to austenite, that is, Ac ₁ point or higher. Of course, even if it is in the temperature range of Ac ₁ point or more, if the temperature is less than Ac ₃ point, a two-phase mixed structure of ferrite and austenite will be obtained, but in the method according to the present invention, processing is performed while increasing the temperature, Even in the temperature range below the Ac ₃ point, the crystal grains are sufficiently refined by processing and recrystallization. However, “By processing ferrite, very fine austenite crystal grains are generated from the work-hardened ferrite by processing and are generated.”
In order to fully exhibit the characteristic actions and effects of the method according to the present invention, it is desirable to raise the temperature to the Ac ₃ point or higher if possible. However, some products need to have a two-phase structure of ferrite and austenite, and it is necessary to keep the temperature rise for these products within the temperature range below the Ac ₃ point. Needless to say.

However, in the case of a pearlite structure for strengthening the wire drawing, it is necessary to obtain a complete austenite phase before the pearlite transformation in order to obtain a uniform pearlite structure. Must be Ac ₃ or higher.

For a pearlite structure for wire drawing strength, the rate of temperature increase is preferably 30 ° C / sec. That is, FIG. 6 shows that for a steel wire rod having a wire diameter of 6 mm, starting from 650 ° C. and finishing at 900 ° C. (plastic working degree: 50%), the pearlite transformation was performed on the wire. The results of an investigation of the relationship between the heating rate and the pearlite block size are shown. From FIG. 6, the heating rate is
It can be seen that the pearlite block size significantly increases when the temperature is less than 30 ° C / sec. Considering that the pearlite block size of the conventional patenting material was about 20 μm, it is clear that a finer pearlite block size than the conventional material can be secured by setting the heating rate to 30 ° C / sec or more. Is. In order to secure a more stable fine pearlite block size, the temperature rising rate is preferably 100 ° C./sec or more if possible.

D) Reason for raising the temperature while performing the reverse transformation from the ferrite phase to the austenite phase The reason for raising the temperature while applying plastic working during the reverse transformation from the ferrite phase to the austenite phase is as described above. "Ferrite grain refinement by processing in the ferrite region", "Work-induced formation of fine austenite grains from work-hardened ferrite grains" and "Fine grain refinement by processing austenite grains", and "Fine ferrite from work-hardened austenite grains" The purpose of this is to promote "strain-induced transformation of grains", and in the method according to the present invention, these various actions and the effects resulting therefrom are continuously condensed in the technique of "increasing temperature while processing". Is.

When the "pre-structure" before reverse transformation contains carbides, the above-mentioned processing causes the carbides to be mechanically crushed and finely dispersed, and in addition, the carbides are transformed from ferrite to austenite. It becomes a nucleus and promotes finer reverse transformation austenite organization. In addition, in this case, the effect of promoting decomposition and solid solution of carbides by the processing is also enhanced.

Furthermore, the step of increasing the temperature while performing plastic working to reversely transform the ferrite into austenite also brings about the following actions and effects. That is, in conventional controlled rolling, "fine carbides and nitrides strengthen the steel by strain-induced precipitation during rolling."
Although this effect is also utilized, this precipitation strengthening effect also causes embrittlement of the steel. And the embrittlement is
This is because the matrix around the precipitate has an elastic strain field because the precipitate is crystallographically compatible with the matrix, and is a phenomenon that is always associated with precipitation hardening. On the other hand, according to the method of the present invention, the matrix having the consistency with the precipitate once undergoes reverse transformation to austenite, and further transforms to ferrite, thus completely losing the compatibility with the precipitate. Therefore, embrittlement does not occur.

E) While processing, raise the temperature to the temperature range of Ac ₁ point or higher, then Ae ₁
Reasons for holding in the temperature range of ₁ point or higher For some time, it is necessary to raise the temperature of the steel while processing it to the temperature range of ₁ point or higher of Ac and then hold it in the temperature range of ₁ point or higher of Ae for an appropriate period of time.
Since it is an extremely important factor for obtaining a uniform fine austenite structure, it is a useful means to be adopted as necessary.

That is, when the steel is heated and reversely transformed to austenite according to the method of the present invention, the working speed tends to be rapid and the temperature rises rapidly, so that the reverse transformation to austenite actually progresses. There is a concern that there is not enough time. Therefore, if the material to be rolled is cooled immediately after the hot working is completed, the worked ferrite grains may be cooled before the transformation into austenite is completed, and the large ferrite remains as it is without undergoing reverse transformation. It is possible that it remains. In this case, the above-described actions and effects aimed by the present invention cannot be sufficiently obtained, which leads to the fact that the object of the present invention cannot be sufficiently achieved. Therefore, in order to eliminate such problems, after finishing the reverse transformation process under the required conditions, in order to allow a time margin for the ferrite grains with built-in processing strain to reverse transform to austenite, Ae ₁ It is extremely effective to maintain the temperature range above the point. Since the holding temperature ferrite falls below _a point Ae can not cause transformation to thermodynamically longer austenitic, the lower limit of the holding temperature is naturally a temperature of inevitable Ae ₁ point.

Also, the required holding time in the temperature range of Ae ₁ point or more is significantly different depending on the rolling conditions and steel types, and in the case of high-purity iron, almost instantaneous seconds can be practically sufficient. Some need enough. Therefore, the holding time is set to a time that can sufficiently cover these and is also acceptable in terms of workability, and the upper limit value and the lower limit value of the time are not particularly limited.

By the way, in the method of the present invention, the cooling condition after reverse transformation is not particularly limited, but it goes without saying that for a pearlite structure for strengthening wire drawing, adjusted cooling is required to obtain the structure. . As the means for this adjustment cooling, for example, a cooling method by immersion in a lead bath or a wind blast can be adopted, whereby pearlite transformation is generated from the reverse transformed austenite and fine pearlite blocks are generated. Since the wire rod produced in this way has a markedly improved drawability, a filament having high ultimate strength and excellent ductility can be produced using this as a raw material.

Next, the present invention will be described more specifically based on examples.

<Example> Example 1 First, each steel shown in Table 1 was melted in the air in an induction heating melting furnace and then cast into a 3-ton steel ingot, which was then soaked and slab-rolled to obtain a cross section of 130 mm × Without a 130 mm billet, 100 kg
The pieces cut into pieces were hot forged into 50 mm x 30 mm square bars. After that, the eight steel types from Steel A to Steel H were heat-normalized at 950 ° C, and Steel I and Steel J were heated at 1150 ° C and then cooled in the furnace, and the thickness was 9mm, 10mm, 12mm, 15mm, 20mm, 25mm and width. Is 30mm
Steel strips from steel A to steel H are 950
After normalizing at ℃, Steel I and Steel J are cooled at 1150 ℃
Used as a material for rolling experiments.

Test Example i Steels A to K shown in Table 1 were used rolled materials having a cross section of 20 mm × 30 mm, and each was heated to a temperature as shown in Table 2 by an induction heating furnace, and then 7.5 m at a stretch by a planetary mill.
Rolled to m thickness. Therefore, as shown in Table 2, the structure of each steel material before processing is either ferrite single phase or ferrite. It had an austenite mixed structure or a mixed structure of these with carbides, intermetallic compounds and the like. Although it cannot be confirmed by an optical microscope, it is considered that various nitrides are mixed from the viewpoint of steel composition. Further, the temperature of the material to be rolled at the exit of the rolling mill increased due to the heat generated by the working due to the large reduction rolling in the planetary mill, and reached the temperature shown in Table 2 as the "rolling end temperature". It was confirmed that the temperature can be changed and controlled by changing the rolling speed.

For the steel materials after rolling, the structures of eight steel types from steel A to steel H were investigated. Ferrite grain size was measured in the cold-rolled material and tempered in the water-cooled sample after rolling, and then the prior austenite grain boundary was preferentially corroded to measure the former austenite grain size.

In Steel A, the sample that was water-quenched after rolling could not have a quenched structure, and the austenite grain size before ferrite transformation was estimated and measured while vague from the generation state of pro-eutectoid ferrite in the sample that had been water-cooled.

On the other hand, for comparison, steel A and steel E with a cross section of 20 mm x 30 mm are heated to 950 ° C and then heated at 850 to 825 ° C in a plate rolling experimental mill
The so-called "controlled rolling method" of pass rolling and cooling, and the prototype of "controlled rolling / work cooling method" in which the same rolling was performed, followed by rapid cooling to 650 ° C by spraying water and then cooling went. The austenite grain size of these samples was measured by a structure obtained by quenching in salt water immediately after controlled rolling and tempering it.

The results of these measurements are also shown in Table 2.

The following can be seen from the results shown in Table 2. That is,
Conventionally, the microstructure of steel obtained by applying the “controlled rolling / accelerated cooling method”, which was said to be the most effective for refining the microstructure, is austenite grain as shown in the column of the conventional method in Table 2. Diameter 20.2 to 24.7 μm, ferrite grain size 10.2 to 13.1
However, if the method according to the present invention is applied to this, a steel material with an austenite grain size of 3.7 to 7.2 μm and a ferrite grain size of 1.9 to 5.9 μm has been realized, and it has never been possible to achieve a uniform fineness. It is clear that a steel material having a very fine grain structure can be obtained.

It was also confirmed that the effect of uniformly refining the steel structure by applying the method according to the present invention can be widely realized from ultra low carbon ferrite single phase steel to high carbon high alloy tool steel.

Test Example ii Steel G shown in Table 1 has plate thicknesses of 9 mm, 10 mm, 12 mm, 15 mm, 20 mm,
A trial experiment of hot-rolled steel was performed using six types of 25 mm rolled materials and varying the degree of rolling.

Here, the rolling of the rolling material having a thickness of 9 mm and a pressure of 10 mm is performed in one pass in the same manner as in Test Example i using a planetary mill.
Rolled to 7.5 mm. Since the temperature of the rolled material immediately after rolling rises only to 765 ° C and 790 ° C, respectively, the temperature was rapidly raised to 905 ° C by the induction heating coil provided at the exit of the rolling mill. Then, some of the rolled and induction-heated samples were kept at 905 ° C. for 5 seconds and then water-cooled, and the rest were immediately cooled without holding the temperature.

On the other hand, a rolled material with a thickness of 12 mm to 20 mm was also rolled using the planetary mill in the same manner as in Test Example vii, but in this case, the temperature of the rolled material on the exit side of the planetary mill was 905 ° C. As a result, some of them are left to cool immediately after rolling, and the rest are cooled by two means, that is, they are kept at the above temperature for 5 seconds after rolling in an induction heating furnace provided at the exit of the rolling mill and then water cooled. did.

Furthermore, for a rolled material with a thickness of 25 mm, a laboratory-scale plate rolling machine and an induction heating furnace are used, and heating is performed by an induction heating furnace between each pass to increase the temperature of the material to be rolled by 50 ° C. 5m
Four-pass rolling under m rolling was continuously performed to obtain a hot rolled steel material. The rolling mill used at this time had a roll (2) installed in an induction heating furnace (1) as shown in FIG. 7 and was configured to be rolled in the heating furnace. After heating the material to be rolled (3) welded to the dummy strip steel before and after the line in the infrared heating furnace (4), the temperature is raised and adjusted by the induction heating coil (5) installed between the rolling rolls. While doing. Then, the rolled material was wound by heat retention by the heat retention furnace (7), cooling by water, and water cooling by the water cooling nozzle (8) from the final roll exit to the winding coiler (6).

The results are shown in Table 3 together with the processing conditions. In Table 3, the test numbers with odd numbers are those which were left to cool after rolling and the ferrite grain size after cooling was measured, and those with even test numbers were 905 after rolling.
The sample was kept isothermal at 5 ° C. for 5 seconds and then water-quenched, and the austenite grain size was measured immediately after the sample was kept isothermal for 5 seconds.

The results shown in Table 3 show the following. That is,
20 μm for hot rolled steel sheets according to test numbers 2-1 and 2-2
The ferrite particles are large as described above, and the situation is almost the same as that of the case where rolling is performed after normal "austenization by heating".

In the case of test numbers 2-3 and 2-4, the work-induced reverse transformation began to occur in earnest, and the austenite grains became considerably finer, but all were not only fine austenite grains due to the work-induced reverse transformation, but also ordinary austenite. It has a mixed structure in which large austenite grains generated in the process remain. Because of this, cold The ferrite grains after rejection also have a mixed grain structure, with some fine grains of 5 μm or less and large grains of 15 μm or more mixed, and the average austenite grain size is 20.6 μm.

After the test number 2-5, all the austenite grains are fine austenite grains having a grain size of 5 μm or less due to the work-induced reverse transformation. Therefore, in order to obtain an ultrafine austenite grain structure by reverse transformation heat treatment, it is necessary to set the rolling reduction amount to 20% or more, but preferably 30% or less.
It turns out that it is good to set it to% or more.

The results of Test Nos. 2-3 to 2-6 indicate that, after rolling and isothermal holding at 905 ° C. for 5 seconds, reverse transformation proceeds and the mixed grain structure decreases. Therefore, in order to obtain a uniform fine austenite structure by reverse transformation heat treatment, it is preferable to hold isothermally for a suitable time after rolling and then rapidly cool depending on the rolling conditions.

From the results of test Nos. 2-11 and 2-12, fine austenite grains of 10 μm or less can be realized even by conventional multi-pass rolling if devised so that rolling can be performed while raising the temperature by using auxiliary means. It can be confirmed that a fine ferrite grain structure of 5 μm or less can be obtained using this as a starting structure.

Test Example iii A rolling test similar to that of Test Example i was carried out using a rolled material having a steel thickness of 20 mm shown in Table 1 and having a thickness of 20 mm. in this case,
Since the rolling is performed under high pressure, the temperature of the material to be rolled on the delivery side of the rolling mill rises due to heat generated by processing, but the temperature changes depending on the rolling speed in the planetary mill. Therefore, the temperature of the material to be rolled at the end of rolling was variously adjusted by adjusting the rolling speed. Then, the rolled material after rolling was subjected to two kinds of treatments, that is, a treatment of immediately cooling with water using different samples and a treatment of holding the same temperature at the end of rolling for 1 minute by induction heating and then cooling with water. .

As for the pre-structure of the material to be rolled, Steel A had a ferrite single phase, Steel G had a ferrite-pearlite-bentenite structure and a martensite-quenched structure, and Steel H had a pearlite structure.

These results are shown in Table 4 together with the treatment conditions. Test numbers 3-1 to 3-10 are examples in which the structure before rolling is a ferrite single phase and the rolling end temperature is changed to Ac ₃ or higher. Test Nos. 3-11 to 3-22 are related to medium carbon low alloy steels, and the previous structure is ferrite / pearlite /
From the bainite mixed structure where the rolling end temperature is less than Ac ₁ point, A
c This is an example when the number of points changes by ₃ or more. And, in Table 4, those marked with "Austenite area ratio" in the column of "Pre-cooling structure" in Test Nos. 3-7 to 3-10 are austenite grains in which a quenched structure cannot be obtained. Since the field could not be clearly discriminated, the results were estimated from the ferrite structure after cooling. In addition, although the ferrite grain size in the same column is also marked with *, this indicates that the appearance of ferrite before cooling was estimated from the ferrite structure after cooling because the austenite structure is not clear as described above. It is a thing. The ferrite structure before rolling and before cooling has a grain size of 5 μm or less that is produced by reverse transformation. On the other hand, since the ferrite grains remaining untransformed were stretched and the grain size was 5 μm or more, it was possible to sufficiently discriminate and estimate the two.

The results shown in Table 4 show the following. That is,
When the preceding structure is a ferrite single phase, it can be confirmed that all structures become reverse transformed austenite when the rolling end temperature exceeds 870 ° C, as seen from the results of test number 3-7 and later.

In the case of medium carbon steel, the reverse transformation austenite structure appears only when the pressure rolling end temperature reaches 740 ° C (see Test No. 3-13). And, when it reaches 820 ℃, 1 after rolling
It can be seen that a 100% reverse transformation austenite structure is obtained by maintaining the temperature isothermally for a minute (see Test No. 3-18). Note that the austenite grain size tends to increase as the isothermal holding time becomes longer, and from both viewpoints of increasing the generation ratio of fine reverse transformation austenite grains and preventing growth coarsening of the austenite grains, the rolling end temperature and rolling Later retention time must be considered.

Further, when the pre-structure is martensite, the reverse transformation austenite grains are more effectively refined (test number 3).
-23 to 3-25), and even in the case of eutectoid steel in which the anterior structure is all pearlite, it changes to a very fine structure when the rolling end temperature exceeds the Ac ₁ point (test numbers 3-26 and 3-27). See also).

Needless to say, if the austenite grains before cooling are sufficiently fine, the microstructure of the steel material after cooling is also extremely fine.

Test Example iv A rolled material having a plate thickness of 20 mm of steel D shown in Table 1 was used, which was heated to 740 ° C, 780 ° C, and 850 ° C and the austenite / ferrite ratio was changed to be a starting material. Test example i
A rolling test similar to the above was performed. The rolling end temperature was adjusted to about 810 ° C by adjusting the rolling speed. In addition, the structure before rolling was investigated with the material that was quenched after heating and without rolling,
Immediately after rolling, water-quenched or left-cooled ones and those subjected to isothermal holding for 1 minute after rolling as in Test Nos. 4-7 and 4-8 were examined.

The results are shown in Table 5.

The results shown in Table 5 show the following. That is,
As in Test Nos. 4-9 to 4-12, when there is no ferrite in the structure before rolling, the austenite grain size after rolling is about 30 μm, which is not much different from the conventional controlled rolling method. In the case where the ferrite structure is present before rolling like 4-1 to 4-8, the austenite grains after rolling are remarkably refined.

As described above, if the austenite grains are remarkably refined, a sufficiently refined structure can be obtained after cooling.

Test Example v A rolled material having a steel G plate thickness of 20 mm shown in Table 1 was used. The material was heated to 875 ° C in an infrared heating furnace and then allowed to cool before rolling. When each temperature of 650 ° C., 625 ° C. and 600 ° C. was reached, rolling was carried out in the planetary mill in the same manner as in Test Example i. At this time, the rolling speed was adjusted so that the rolling finish temperature of the material to be rolled was approximately 850 ° C. Also, in order to confirm the steel structure before rolling, the same material was used
After heating to ℃, it was left to cool to each temperature of 675 to 600 ℃, quenched and tempered without rolling, and the structure was observed to estimate the structure before rolling.

Further, the steel G having a plate thickness of 20 mm shown in Table 1 was patented in a salt bath to form a "painite structure".
The same material was oil-quenched and then tempered at 200 ° C. to obtain a “tempered martensite structure” as a material for rolling. These materials were also subjected to rolling and post-treatment under the same conditions as above, and the structure was observed.

Table 6 shows the results when the reverse transformation rolling was performed on the pre-structure in which the proportion of the ferrite structure was changed by heating the structure to the completely austenite structure under the above conditions.

From the examples of test numbers 5-7 and after in Table 6, it can be seen that the structure before cooling is austenite grains of 15 μm or less. Therefore, it was confirmed that by preparing 20% or more of ferrite structure during hot rolling and then performing reverse transformation rolling, it is possible to manufacture ultrafine structure steel material by reverse transformation work heat treatment on hot rolling line. it can.

From Test Nos. 5-13 and 5-14, it is clear that the pre-structure prepared prior to reverse transformation rolling may be bainite or martensite.

Test Example vi Using a square bar of steel I having a cross section of 50 mm × 30 mm shown in Table 1, this was heated to 200 ° C. and then forged into a square bar of 20 mm × 30 mm cross section in the temperature range of 1050 to 700 ° C. with an air hammer. . Then, this
The structure was maintained in a furnace at 700 ° C for 5 minutes to 2 hours to form a microstructure of austenite, spheroidal carbide and nitride, ferrite and pearlite. Then, the forged material taken out from the 700 ° C. furnace was immediately rolled in the same manner as in Test Example i and allowed to cool. Then, the rolled material was allowed to cool to room temperature and then immediately tempered to measure the austenite grain size.

The results are shown in Table 7 together with the processing conditions.

As is clear from the results shown in Table 7, in the test numbers 6-1 and 6-2, since the structure of the ferrite phase (here, pearlite) is only 10%, rolling The austenite grain size before post-cooling is 20 μm or more,
Test No. 6-3 held at 700 ° C for 20 minutes after hot forging
In 6-5, since pearlite is increased, the austenite grain size after reverse transformation rolling becomes 15 μm or less, and especially in the case of test number 6-4 where 13% of ferrite phase structure is observed, the austenite grain size is 5 μm or less. It can be seen that it has become even finer.

Therefore, it can be confirmed that a sufficiently fine and fine structure can be secured even after cooling.

Example 2 Each of the steels shown in Table 1 was melted in the induction heating melting furnace in the air and then cast into a 3 ton steel ingot, which was then soaked and slab-rolled to form a steel piece having a cross section of 130 m × 130 mm. Then, it was cut into 100kg pieces and hot-forged into 50mm x 30mm square pieces.
After that, the eight steel types from Steel A to Steel H were heated at 950 ° C and standardized, Steel K was heated at 1050 ° C and standardized, and Steel I and Steel J were heated at 1150 ° C and then cooled in a furnace. 9mm, 10mm, 12m
A strip steel sheet rolled into a plate shape of m, 15 mm, 20 mm, and 25 mm and a width of 30 mm was obtained. After that, steels A to H were heat-normalized again at 950 ° C, and steel K was heat-normalized at 1050 ° C. Steel J was used as a material for rolling experiments after being cooled in a heating furnace at 1150 ° C.

Test Example vii Steels A to K shown in Table 1 were used rolled materials having a cross section of 20 mm x 30 mm, and each was heated to a temperature as shown in Table 8 by an induction heating furnace, and then 7.5 m at a stretch by a planetary mill.
Rolled to m thickness. Therefore, the structure of each steel material before processing was a ferrite single phase or a mixed structure of ferrite and austenite, or a mixed structure of these with carbides, nitrides, intermetallic compounds, etc., as shown in Table 8. . Further, the temperature of the material to be rolled at the exit of the rolling mill was increased by the heat generated by processing by the large reduction rolling in the planetary mill, and reached the temperature shown in Table 8 as the "rolling end temperature".
It was confirmed that the temperature can be changed and controlled by changing the rolling speed.

The steel material after rolling was kept at the rolling end temperature for various times up to 1 hour and then cooled with water. Then, first, the grain size of the ferrite grains existing in the as-quenched structure was observed and measured, and the grain size of the austenite grains immediately before the rapid cooling fixed by the quenching was measured.

On the other hand, for comparison, steel A and steel E with a cross section of 20 mm x 30 mm are heated to 950 ° C and then heated at 850 to 825 ° C in a plate rolling experimental mill for 3
We also made prototypes of the so-called "controlled rolling method" in which the material was pass-rolled and allowed to cool, and the sample subjected to the "controlled rolling / accelerated cooling method" in which it was rapidly cooled to 650 ° C by spraying water after the same rolling went. The austenite grain size of these samples was measured by a structure obtained by quenching in salt water immediately after controlled rolling and tempering it.

The results of these measurements are also shown in Table 8.

The following can be seen from the results shown in Table 8. That is,
Conventionally, the structure of steel obtained by applying the “controlled rolling / accelerated cooling method”, which was said to be most effective for the refinement of the structure, has the austenite grain size as shown in the comparison method column in Table 8. 17.7 to 23.4 μm, ferrite grain size 9.6 to 12.0 μm
However, when the method according to the present invention is applied, the austenite grain size is 2.0 to 9.6 μm even after the rolling and before the cooling, and the ferrite grain size that has not been reverse transformed yet remains slightly. Has achieved steel materials of 2.0 to 4.9 μm. The untransformed ferrite grains can be almost transformed into austenite by appropriately maintaining the holding time after rolling.

The appropriate holding time greatly differs depending on the steel type, and if it becomes too long, the austenite grains generated by reverse transformation grow and become coarse, so holding for an excessively long time must be avoided. That is, in order to be able to both increase the generation ratio of fine reverse transformation austenite grains and prevent growth coarsening of the reverse transformation austenite grains, an appropriate rolling end temperature and holding temperature / time after rolling depending on the target steel type. Need to consider. It is clear that under such appropriate conditions, the method of the present invention makes it possible to obtain an unprecedented uniform fine ultrafine grain steel.

Further, by applying the method according to the present invention, the uniform refining effect of the reverse transformation processing heat treatment of the steel material structure can be widely realized from ultra-low carbon ferrite single phase steel to high carbon high alloy tool steel. I can confirm.

Test Example viii Steel G shown in Table 1 has plate thicknesses of 9 mm, 10 mm, 12 mm, 15 mm, 20 mm,
A trial experiment of hot-rolled steel was performed using six types of 25 mm rolled materials and varying the degree of rolling.

Here, for the rolling of the 9 mm thick and 10 mm thick rolling materials, a planetary mill was used to roll to 7.5 mm in one pass in the same manner as in Test Example vii. Since the temperature of the rolled material immediately after rolling rises only to 765 ° C and 790 ° C, respectively, the temperature was rapidly raised to 900 ° C by the induction heating coil provided at the exit of the rolling mill. Some of the rolled and induction-heated samples were heated to 900 ° C and left to cool immediately, and the rest were heated to 900 ° C at the exit of the rolling mill, and then heated to the previous temperature for 5 seconds, 30 seconds, and 1 minute, respectively. After holding, it was cooled by two means of water cooling.

On the other hand, a rolled material having a thickness of 12 mm to 20 mm was also rolled in the same manner as in Test Example vii using a planetary mill, but in this case, the temperature of the rolled material on the exit side of the planetary mill was 845 ° C. Therefore, some of them are left to cool immediately after rolling, and some of them are kept at the above temperature for 5 seconds, 30 seconds and 1 minute after rolling in an induction heating furnace installed at the exit of the rolling mill, and then water cooled. It was cooled by two means.

Furthermore, for the rolled material with a thickness of 25 mm, a laboratory-scale plate rolling machine and an induction heating furnace are used, and heating is performed by an induction heating furnace between each pass to increase the temperature of the material to be rolled by 60 ° C in each pass. 5m
Four-pass rolling under m rolling was continuously performed to obtain a hot rolled steel material. The rolling mill used at this time had a roll (2) installed in an induction heating furnace (1) as shown in FIG. 7 and was configured to be rolled in the heating furnace. After heating the rolled material (3) welded to the dummy strip steel before and after the line in the infrared heating furnace (4), raise the temperature with the induction heating coil (5) installed between the rolling rolls. It was carried out while adjusting. Then, the rolled material was wound by heat retention by the heat retention furnace (7), cooling by water, and water cooling by the water cooling nozzle (8) from the final roll exit to the winding coiler (6).

The results are shown in Table 9 together with the processing conditions. In Table 9, test numbers 8-1, 8-5, 8-9, 8-13,
8-17 and 8-21 were left to cool immediately after rolling, and the ferrite grain size after cooling was measured.
2-8-4,8-6-8-8,8-10-8-12,8-14-8-1
6,8-18 to 8-20,8-22 to 8-24 are 5 at 900 ℃ after rolling.
The temperature is kept isothermal for 1 second to 1 minute and then water-cooled and quenched, and the austenite grain size is measured immediately after the isothermal holding.

The results shown in Table 9 show the following. That is,
The hot-rolled steel sheet according to Test No. 8-1 has large ferrite grains of 20 μm or more, which is almost the same as that obtained by rolling after the usual “austenization by heating”.

In test numbers 8-2 to 8-4, the austenite grain size was 50.
It is around μm, and miniaturization has been fully achieved. It has not been.

In the case of test numbers 8-5 and 8-8, the work-induced reverse transformation began to occur in earnest, and the austenite grains became quite fine, but all were not only fine austenite grains due to the work-induced reverse transformation, but also ordinary austenite. It has a mixed structure in which large austenite grains generated in the process remain. For this reason, the ferrite grains after cooling also have a mixed grain structure, and most of them are fine grains of 5 μm or less, and some very small grains of 15 μm or more are mixed.

After the test number 8-9, all the austenite grains are fine austenite grains with a grain size of 5 μm or less due to the work-induced reverse transformation. Therefore, in order to obtain an ultrafine austenite grain structure by reverse transformation heat treatment, it is necessary to set the rolling reduction amount to 20% or more, but preferably 30% or less.
It turns out that it is good to set it to% or more.

The results of Test Nos. 8-6 and later show that reverse transformation progresses and the mixed grain structure decreases when the material is kept isothermally at 900 ° C. after rolling. Therefore, in order to obtain a uniform fine austenite structure by the reverse transformation heat treatment, it is preferable to hold the material at an isothermal temperature for an appropriate time and then rapidly cool it, depending on the steel type and rolling conditions.

From the results of test Nos. 8-11 to 8-24, fine austenite grains of 10 μm or less can be realized even by conventional multi-pass rolling if devised so that rolling can be performed while raising the temperature by using auxiliary means. It can be confirmed that a fine ferrite grain structure of 5 μm or less can be obtained using this as a starting structure.

Test Example ix A rolling test similar to that of Test Example vii was carried out using the rolled materials having the plate thicknesses of the steels A, G and H shown in Table 1 of 20 mm. In this case, since the rolling is performed under high pressure, the temperature of the material to be rolled on the delivery side of the rolling mill rises due to the heat generated during processing, but the temperature changes depending on the rolling speed in the planetary mill. Therefore, the temperature of the material to be rolled at the end of rolling was variously adjusted by adjusting the rolling speed. Then, the rolled material after rolling was subjected to induction heating to keep the temperature at the end of rolling for 5 seconds or 1 minute by inductive heating, and then cooled with water.

As for the pre-structure of the material to be rolled, Steel A had a ferrite single phase, Steel G had a ferrite-pearlite-bainite structure and a quenched structure of martensite, and Steel H had a pearlite structure.

The results are shown in Table 10 together with the treatment conditions. In Test Nos. 9-1 to 9-10, the structure before rolling was a ferrite single phase and the rolling end temperature was thought to have changed to Ac ₃ point or more. For example, test Nos. 9-11 to 9-22 relate to medium carbon low alloy steels, in which the preceding structure is a mixed structure of ferrite / pearlite / bainite and the rolling end temperature is Ac ₁
This is an example of the case where it seems that the number of points has changed from ₃ points or less to ₃ points or more. And, in Table 10, test numbers 9-7 to 9-
Those marked with "*" in the "Austenite area ratio" and "Austenite grain size" in the column "Microstructure analysis before cooling" in 10 were cooled because the quenched structure could not be obtained and the austenite grain boundaries could not be clearly identified. It is a result estimated and judged from the subsequent ferrite structure. Also, The ferrite grain size in the same column is also marked with *, but this indicates that the appearance of ferrite before cooling was estimated from the ferrite structure after cooling because the austenite structure was not clear as described above. is there. The ferrite structure before rolling and before cooling has a grain size of 5 μm or less generated by reverse transformation, whereas the ferrite grains remaining untransformed are stretched and have a grain size of 5 μm.
From the above, it was possible to sufficiently discriminate and estimate both.

The following can be seen from the results shown in Table 10. That is,
In the case where the preceding structure is a ferrite single phase, it can be confirmed that all structures become reverse transformed austenite when the rolling end temperature exceeds 869 ° C. as seen from the results of trial number 9-7 and later.

In the case of medium carbon steel, the reverse transformation austenite structure appears only when the rolling end temperature reaches 740 ° C (see Test No. 9-13). Then, it is found that at 830 ° C., a 100% reverse transformation austenite structure is formed (see test number 9-18). Note that the austenite grain size tends to increase as the isothermal holding time becomes longer, and from both viewpoints of increasing the generation ratio of fine reverse transformation austenite grains and preventing growth coarsening of the austenite grains, the rolling end temperature and rolling Later retention time must be considered.

Further, when the pre-structure is martensite, the reverse transformed austenite grains are further effectively refined (test number 9).
-23 to 9-25), and even in the case of eutectoid steel in which the antecedent structure is all pearlite, it changes to a very fine structure when the rolling end temperature exceeds the Ac ₁ point (test numbers 9-26 and 9-27). See also).

Needless to say, if the austenite grains before cooling are sufficiently fine, the microstructure of the steel material after cooling is also extremely fine.

Test Example x Steel D shown in Table 1 having a plate thickness of 20 mm was used as a starting material, which was heated to 740 ° C, 780 ° C, and 850 ° C to change the ratio of austenite and ferrite. Test example vi
The same rolling test as i was conducted. The rolling end temperature was adjusted to about 810 ° C by adjusting the rolling speed. Also,
The structure before rolling was investigated with the material that was quenched after heating and without rolling, and after rolling, it was kept isothermal for 5 seconds and then quenched by water cooling or allowed to cool, and test numbers 10-7 and 10- in Table 11 were used. As shown in No. 8, the sample was kept isothermally for 1 minute after rolling.

The results are shown in Table 11.

The results shown in Table 11 show the following. That is,
As in Test Nos. 10-9 to 10-12, when there is no ferrite in the structure before rolling, the austenite grain size after rolling is about 30 μm, which is not much different from the conventional controlled rolling method. In the case where the ferrite structure exists before rolling such as 10-1 to 10-8, the austenite grains after rolling are remarkably refined.

As described above, if the austenite grains are remarkably refined, a sufficiently refined structure can be obtained after cooling.

Test Example xi Rolled material with steel G having a plate thickness of 20 mm shown in Table 1 After heating it to 875 ℃ in an infrared heating furnace, it is allowed to cool before rolling, and the temperature of the material is 675 ℃, 650 ℃, 625 ℃, 60 ℃.
When each temperature of 0 ° C was reached, rolling was carried out in the planetary mill in the same manner as in Test Example vii. At this time, the rolling speed was adjusted so that the rolling finish temperature of the material to be rolled was approximately 850 ° C. In addition, in order to confirm the steel structure before rolling, the same material was heated to 875 ° C and then left to cool to each temperature of 675 to 600 ° C.
Without rolling as it was, quenching and tempering were performed and the microstructure was observed to presume the microstructure before rolling.

Further, the steel G having a plate thickness of 20 mm shown in Table 1 was patented in a salt bath to form a "bainite structure".
The same material was oil-quenched and then tempered at 200 ° C. to obtain a “tempered martensite structure”, which was used as a material for rolling. These materials were also subjected to rolling and post-treatment under the same conditions as above, and the structure was observed.

Under the above-mentioned conditions, the results of the case where the reverse transformation rolling is performed on the pre-structure in which the temperature is adjusted and then the temperature is adjusted to change the ratio of the ferrite structure to
Shown in Table 12.

From the examples of test numbers 11-7 and after in Table 12, it is understood that the structure before cooling is austenite grains of 15 μm or less. Therefore, it can be confirmed that, by preparing 20% or more of the ferrite structure during hot rolling and then performing reverse transformation rolling, it is possible to manufacture an ultrafine structure steel material by reverse transformation heat treatment on a hot rolling line.

From Test Nos. 11-13 and 11-14, it is clear that the pre-structure prepared prior to reverse transformation rolling may be bainite or martensite.

Test Example xii Using a square bar of steel I having a cross section of 50 mm × 30 mm shown in Table 1, this was heated to 200 ° C. and then forged into a square bar of 20 mm × 30 mm cross section in a temperature range of 1050 to 700 ° C. with an air hammer. . Then, this
The structure was kept in a furnace at 700 ° C for 5 minutes to 2 hours to form a microstructure of austenite, spherical carbides and nitrides, ferrite and pearlite. Then 700 Test sample vii immediately after the above forged material taken out from the furnace
It was rolled in the same manner as above and allowed to cool. Then, the rolled material was allowed to cool to room temperature and then immediately tempered to measure the austenite grain size.

The results are shown in Table 13 together with the treatment conditions.

As is clear from the results shown in Table 13, the test number
In 12-1 and 12-2, since the ferrite phase structure (here, pearlite) is only 10%, the austenite grain size after rolling and before cooling is 20 μm or more, but 700 ° C after hot forging.
In Test Nos. 12-3 to 12-5 held for 20 minutes or more, since the pearlite is increased, the austenite grain size after reverse transformation rolling is 8 μm or less, and especially the structure of the ferrite phase is
When the test number 12-4, which is 13% recognized, is obtained, it is found that the austenite grain size is refined to 5 μm or less.

Therefore, it can be confirmed that a sufficiently fine and fine structure can be secured even after cooling.

Example 3 Five kinds of high carbon containing the main components shown in Table 14 Was melted, cast into an ingot, and then rolled by a wire rod mill to obtain a short steel wire rod having a diameter of 6 mm and a length of 1 m. Then, after heating these steel wire rods to a processing start temperature in a muffle furnace,
A block mill equipped with an auxiliary heating device between rolling stands provided a wire diameter of 3.2 to 5.2 mm under the hot working conditions (processing start temperature, processing end temperature, temperature rising rate, and degree of plastic working) shown in Table 14.
After that, it was water-cooled to 800 ° C., and then blast cooling was performed to complete the pearlite transformation.

The properties of the wire rod thus obtained are also shown in Table 14 above.

Then, thereafter, the obtained wire was cold-drawn to manufacture a filament, and its limit workability, ultimate strength, twist value, and 180-degree bending fracture probability were investigated. It is also shown in the table.

In Table 14, "Comparative Example" means that any of the C content, the processing start temperature, the processing end temperature, the temperature rising rate, and the plastic working degree is out of the range specified by the present invention. The “conventional example” is an example in which the conventional patenting process is performed.

In Table 14, Test Nos. 13-1 to 13-4 were examined for the influence of the C content. The test Nos. 13-2 and 13-3 in which the C content satisfies the conditions specified in the present invention were not tested. Also achieves the target ultimate strength (380 kgf / mm ² ), twist value (20 times or more), and bending failure probability (5% or less), while C
In the test number 13-1 in which the content is less than 0.70%, the ultimate strength and the twist value are low. Also, the C content is 0.
In the case of the test number 13-4 which is higher than 90%, it can be seen that the ultimate strength and the flexural fracture probability have not reached the target values.

Test Nos. 13-5 to 13-8 are for investigating the influence of the processing start temperature. However, Test Nos. 13-6 and 13-7 in which the processing start temperature satisfies the conditions specified in the present invention are both While the characteristics exceeding the target value are shown, the processing start temperature is 600
Test number 13-5, which is lower than ℃, and test number 13-8, in which the processing start temperature exceeded Ae ₁ point (735 ℃), the ultimate strength was 350.
It has reached only around kgf / mm ² .

Test No. 13-9 is an example in which the processing end temperature was lower than the Ac ₃ point. In this case, the ultimate strength was 317 kgf / mm ^2, which was extremely low, and the bending fracture probability was 10%, which was abnormally high. .

Test Nos. 13-10 and 13-11 investigated the effect of the heating rate, but in Test No. 13-10, the ultimate strength reached the target value because the heating rate was as low as 20 ° C / sec. On the other hand, it can be confirmed that in Test No. 13-11, in which the temperature rising rate is set according to the conditions specified in the present invention, all characteristics are superior to the target values.

Test Nos. 13-12 to 13-15 are for investigating the influence of the plastic workability, but in Test No. 13-12, the workability is out of the range of 15% and 20% or more specified in the present invention. Although the ultimate strength is extremely low at 301 kgf / mm ² and the bending fracture probability is as high as 6%, the test numbers 13-13 to 13-in which the workability according to the conditions specified in the present invention is added. In No. 15, it can be seen that all the characteristics are superior to the target values.

Test No. 13-16 is an example of patenting treatment, which is the conventional method. In this conventional example, the target ultimate strength (380 kgf / mm ² ), twist value (20 times or more), flexural fracture probability It can be seen that none of (5% or less) has been achieved.

<Summary of Effects> As described above, according to the present invention, it is possible to stably provide a hot-worked steel material having an ultrafine uniform structure that could not be practically realized by the conventional technology. ,
It enables stable supply of inexpensive steel materials with excellent toughness, and also has excellent tensile strength and ductility without the need for complicated punting as in the past (high carbon steel wire rod for wire drawing). ) Can be provided easily and at low cost, resulting in an extremely useful effect in industry.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the workability (ε) of a filament obtained from a conventional patented wire and the drawing and tensile strengths. FIG. 2 is a graph showing the relationship between the degree of processing (ε) of a filament obtained from a conventional patented wire and the twist value. FIG. 3 is a graph showing the relationship between the workability (ε) of the filament obtained from the conventional patenting-treated wire and the bending fracture probability. FIG. 4 is a graph showing the relationship between the pearlite block size and the limit workability. FIG. 5 is a graph showing the relationship between the temperature rising rate and the pearlite block size. FIG. 6 is a graph showing the relationship between the plastic workability and the pearlite block size. FIG. 7 is a schematic configuration diagram of a temperature-rising rolling facility using a multi-stand continuous rolling mill used in the examples. In the drawing, 1 ... induction heating furnace, 2 ... rolling rolls, 3 ... rolled material, 4 ... infrared heating furnace, 5 ... induction heating coil, 6 ... winding coiler, 7 ... heat-retaining furnace , 8 …… Water cooling nozzle.

Claims (3)

[Claims] 1. A steel having a structure in which the C content is 2.5% by weight or less and at least a part of which is ferrite, is heated to a temperature range of Ac ₁ point or more while being plastically processed with a strain amount of 20% or more. A method for producing an ultrafine structure steel material, characterized in that a part or all of a structure made of ferrite is once transformed into austenite and then cooled. 2. A steel having a structure in which the C content is 2.5% by weight or less and at least a part of which is ferrite, is heated to a temperature range of Ac ₁ point or more while being subjected to plastic working with a strain amount of 20% or more, Ae A method for producing an ultrafine structure steel material, which is characterized in that a part or all of a structure made of ferrite is once transformed into austenite while being kept in the temperature range of ₁ point or more, and then cooled. 3. A C content of 0.70 to 0.90% by weight at 600 ° C. to A
The structure of the steel at least partially composed of ferrite in a temperature range of e ₁ point, strain amount: while applying more than 20% of the plastic working rate of temperature increase: raising the temperature at 30 ° C. / sec or more while Ac ₃ point The structure is characterized in that the structure made of ferrite is once transformed into austenite by raising the temperature to the above temperature range or holding this temperature range after the temperature rise, and then the adjustment region cooling is performed to cause pearlite transformation. A method for producing an ultrafine structure steel material.