JP2001098322A - Method of producing steel having fine-grained ferritic structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of producing steel with a ferritic structure excellent in strength, toughness and ductility, and in which the average crystal grain size is extremely fine by using low carbon steel or low carbon alloy steel. SOLUTION: Steel containing, by weight, 0.05 to 0.3% C and 0.5 to 3% Mn or steel containing 0.05 to 0.3% C, 0.5 to 3% Mn, 0.01 to 0.3% Si, 0 to 0.05% Nb, 0 to 0.05 Ti, 0 to 0.08% V, 0 to 1% Cr and 0 to 1% Mo is cooled from >=Ac3 point at a cooling rate of 5 to <100 deg.C/s to <=650 deg.C, in the temperature range from the same to a temperature in which a low temperature phase starts to precipitate, working in which the reduction rate of the cross-sectional area after the finish of the working to the start of the working is >=60% is executed by one pass or multipass of at least >=30% per pass, and after that, the steel is cooled to >=400% deg.C at a cooling rate of air cooling or above the same.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength steel which does not contain a large amount of alloying elements, has excellent ductility, and has high toughness.

[0002]

2. Description of the Related Art Conventionally, methods of strengthening steel include a method of forming a solid solution of a specific element, a method of working in a cold state to give a working strain, a method of transforming into a high-strength structure by heat treatment, and
A method of precipitating fine particles such as 1N (aluminum nitride) and TiC (titanium carbide), a method of making crystal grains fine, and the like are known. These strengthening methods have both advantages and disadvantages, and in practical steels, the required steel performance is obtained by combining these strengthening methods.

In the case of steel, reinforcement by solid solution is usually obtained by including a large amount of alloying elements, for example, Si. For this reason, the influence of the added element strongly appears on surfaces other than the strength, such as changes in surface properties and deterioration of corrosion resistance. In addition, the alloying elements to be added are often more expensive than steel, and if it is attempted to increase the strength by this effect, the steel is inevitably expensive, and the inherent properties of steel that are cheap and strong are lost. .

[0004] The method of imparting work strain utilizes the effect of hardening by applying strain by cold working or the like. However, ductility decreases sharply with increase in strength, and toughness also deteriorates significantly, and the material becomes harder. There is a difficulty in becoming brittle, and the shape is further limited.

[0005] As a method of utilizing the transformation, generally C
Is used, and a quenching-tempering treatment is performed. The quenching is performed by rapidly cooling from a high temperature of about 900 ° C. by water cooling or oil cooling to form a metastable phase such as a martensite phase or a bainite phase. In order to obtain these extremely hard phases, it is necessary to select the chemical composition of the steel based on the size of the steel to be treated, and to obtain a steel having excellent properties by such quenching and tempering. Can be. However, an extra step for this heat treatment is required, and a heating furnace and a rapid cooling device are required. So, in recent years,
Various measures have been taken to shorten the process, for example, quenching is performed immediately after hot working in the high temperature state.

[0006] The hardening due to the precipitation of fine particles is performed by Ti, N
A small amount of elements that form carbides or nitrides such as b and V are added, hot-worked in a solid solution state of these elements, and then finely precipitated in a cooling process. Although there is an advantage that a large amount of hardening can be obtained with a small amount of added element, the toughness tends to be deteriorated, and the amount of addition needs to be strictly adjusted. Further, since the addition of alloying elements as described above is required, the price of steel materials also increases.

[0007] When the crystal grains are made finer, the strength, especially the yield point, is generally improved without lowering the ductility, and the toughness is also improved. In the case of ordinary steel, toughness tends to decrease when the strength is increased. However, by making the crystal grains fine, the toughness can be improved, that is, the toughness-brittle transition temperature can be lowered. Refining the crystal grains is usually a favorable result for improving the performance of steel, unless the workability is strongly required as in the case of a thin steel sheet used for press forming or when the creep strength at high temperatures is important. Bring. For this reason, any of the above various methods of strengthening steel is applied in combination with the refinement of crystal grains.

[0008] In a steel mainly containing an ordinary low-carbon ferrite phase, the grain refinement is basically performed by a method of applying a working deformation to break a coarse crystal of a material to make it finer, or a method of austenite-ferrite. It is performed by a method of making use of metamorphosis. In the case of non-ferrous metals such as Al, there is a method of adding a fine precipitation nucleation element to the molten metal to make the solidified structure finer, but in the case of steel, the solidified structure is usually coarse. However, various processes are usually performed until the final product shape is reached, and in the process, some degree of grain refinement proceeds.

Taking the case of a steel sheet as an example, a slab having a thickness of about 200 mm by a continuous casting method is hot rolled, and a coarse solidified structure is destroyed as the steel is deformed to form a rolled deformed structure. Become. Because of the high temperature, new recrystallized grains without deformation during processing are generated in the rolling deformation structure immediately after leaving the rolling roll, and these grow and the whole steel quickly becomes a recrystallized grain structure. In that case, as the degree of rolling increases, a larger number of recrystallized grains are generated, which tends to result in a fine grain structure. Further, when this rolling process is repeated to further reduce the thickness, the structure is destroyed and recrystallized each time, and the grain size is further reduced. Hot working is usually performed in the region of the austenite phase, and is transformed into a ferrite phase by cooling after working. During this transformation, crystal grains of the ferrite phase are generated from the crystal structure of the austenite phase, and eventually the entire steel has a ferrite grain structure. However, when the transformation from the high-temperature austenite phase to the low-temperature ferrite phase is performed, the ferrite phase structure generally has the same crystal grain size as that of the structure in the austenite phase.

As described above, the crystal grains can be made finer by repetition of processing and recrystallization. However, as the crystals become finer, the crystal grains become united and grow more easily. This is because the energy of the grain boundary is larger than that in the crystal grain, and the energy is released and stabilized. Therefore, the smaller the crystal grain, the stronger the tendency. For this reason, there is a limit to grain refinement only by simple processing and recrystallization. On the other hand, Al, Ti, Nb, V
Addition of a small amount of nitride or carbonitride forming element such as to form fine precipitates, thereby suppressing the movement of grain boundaries, inhibiting the growth of crystal grains, There is a way to Practical low-cost refined steels have been obtained by the addition of such carbonitride forming elements.

[0011] However, the demands on the performance of steel have become more and more severe, and there has been a demand for a steel having higher strength and higher toughness, and a method called thermomechanical heat treatment or controlled rolling or TMCP (Thermo Mechanical Control Process) has been developed. , Has come to practical use. This is intended to regulate the steel composition and to control the working temperature and the working degree in the process of hot working such as rolling to make a high-strength steel with higher toughness. The steel composition is usually lower in carbon than when conventional quenching-tempering is applied, and Ti, N
b, V, etc. are added. In particular, the addition of Nb has an effect of delaying the recrystallization in the austenite region, and can increase the accumulation of processing strain due to rolling at lower temperatures and repeated rolling, so that it is preferably used. Then, the hot working is expanded not only to the austenite region but also to the two-phase region of austenite + ferrite, and the working deformation is combined with the progress of recrystallization, precipitation, transformation and the like which occur with temperature change. Thereby, grain refinement is added to the transformation strengthening and the precipitation strengthening, the strength is improved, and the toughness is further improved.

As described above, in the thermomechanical treatment, the effect of increasing the strength and improving the toughness due to the refinement of crystal grains is particularly large.
The refinement of the crystal grains increases the strain energy before recrystallization after processing and the frequency of generation of recrystallization nuclei based on the release of energy due to the addition of the element that delays the recrystallization and forms a fine precipitate. This is because the grains are refined and grain growth is suppressed by inhibiting the movement of fine precipitates at the grain boundaries. This is further promoted by setting the processing temperature lower than usual. Further, it is considered that, by performing the processing in the two-phase region of austenite + ferrite, the energy of the transformation also increases the nucleation frequency, and the effect of suppressing the grain growth by inhibiting the movement of the grain boundary at the phase interface is considered to be added.

[0013] The thermomechanical heat treatment is a combination of metallographic change due to a temperature drop in the course of hot working after heating of a material and working, and rapid cooling or reheating is performed during the working. It may be. In addition, a method of processing a transformed structure obtained by cooling in a cold or warm state, raising the temperature to transform (reverse transformation), and refining crystal grains has also been practiced with high alloy steel. This is an example in which the crystal grains are most refined at present.However, in a metastable austenitic steel of high alloy steel, it is processed at room temperature and is subjected to a work-induced transformation to form a martensite phase, which is heated to austenite. It transforms into a phase, and an ultrafine grain structure is obtained.

As described above, various refinements of crystal grains have been studied to improve the strength and performance of steel, and the effect of improvement has been recognized in practical use. However, although ultra-fine-grained steel has been realized to some extent in high-alloy steels, sufficient material has not yet been obtained in low-carbon steels or low-alloy steels.

[0015]

As described above, even in a low-carbon steel or a low-carbon low-alloy steel, if the crystal grains are further refined, it is expected that a steel with better performance and a lower cost will be obtained. Is done. An object of the present invention is to provide a method for producing a low-carbon steel or a low-carbon low-alloy steel having an extremely small average crystal grain size and excellent strength, toughness, and ductility.

[0016]

Means for Solving the Problems By making the crystal grains fine,
Not only can the strength of the steel be increased, but also the toughness and ductility can be improved at the same time. That is, like other steel strengthening methods, with the increase in strength, toughness deteriorates,
There is no problem of poor workability, and it is considered to be an ideal method for strengthening steel.

As a method for refining crystal grains of a low-carbon steel or a low-carbon low-alloy steel, various thermomechanical treatment methods have been studied, and a steel having a fine crystal structure has been obtained. As described above, these methods crush and subdivide the base structure or crystal grains by processing, suppress the growth of recrystallized grains generated from the processed structure as much as possible, and obtain fine-grained steel. Fine graining has been realized to a close place, and further fine graining seems to be difficult. In the as-processed structure, the strain is too high, the toughness and the ductility are extremely poor, and the strain must be released in order to recover them.In the process of strain release, recrystallization and grain growth Is to advance. In addition, reverse transformation as in high alloy steel cannot be used for grain refinement in low carbon low alloy steel. This is because no matter how large the degree of cold working, the low-carbon low-alloy steel does not become anything other than the ferrite phase. This is because crystal nucleation and grain growth proceed, and when reverse transformation occurs, the grains have already grown considerably.

Therefore, the present inventors have studied a method of combining transformation with a method of crushing by processing and suppressing grain growth as a means for further promoting the fine graining of low carbon steel or low carbon low alloy steel.

[0019] heated above Ac ₃ point to quench the steel becomes an austenite phase usually becomes austenite phase in a state of being subcooled below ₃ points Ar, or held at that temperature, or even Continuing with cooling It transforms into a ferrite phase, a martensite phase or a bainite phase depending on the steel composition and cooling conditions at that time. When processing is performed in a supercooled state immediately before the transformation, the structure rapidly changes to a structure mainly composed of ferrite. This is presumably because the transformation induces and promotes the transformation. At that time, they found that by changing the processing temperature and the processing rate, it was possible to obtain a ferrite phase in which strain was released and a structure having an extremely small crystal grain size.

As a result of further investigation on conditions for obtaining a structure mainly composed of the fine-grained ferrite phase, it was found that if the temperature for processing was too high, the crystal grains would not be fine, and in that case, the deformation or the rolling reduction It was found that, if not sufficiently large, the ratio of the ferrite phase decreases and the martensite phase and the bainite phase increase. Cooling after this processing is initially for the purpose of suppressing grain growth of the microstructure,
It was thought that it was desirable to make it as quickly as possible, but it became clear that the growth of ferrite grains did not progress so much even with air cooling.

This is because austenite is processed and transformed in a state of being supercooled as much as possible, so that the ferrite formation temperature is as low as 650 ° C. or less, so that the temperature is in a temperature range where grain growth does not proceed, By applying strong processing, transformation proceeds while ferrite transformation recrystallization nuclei are generated rapidly and at high density, and at the same time, the processing strain is released, and the strain energy for promoting grain growth disappears. It is presumed that there are times. In this case, it is considered that the larger the deformation due to the processing, the more the transformation induced by the deformation is promoted, and the more the associated processing strain is released. If the degree of processing is insufficient, not only the refinement of the crystal grains will be insufficient, but also
The release of distortion is also insufficient. In this way,
The ferrite crystal structure that has become extremely fine due to the transformation induced by processing, unlike conventional thermomechanical treatment,
After the transformation, the fine structure is maintained even without rapid cooling.

In this way, the present invention was completed by clarifying the chemical composition of steel, cooling conditions, working temperature range, working degree, and other critical conditions. The gist of the present invention is as follows. (1) A steel containing, by weight%, C: 0.05 to 0.3% and Mn: 0.5 to 3%, with the balance being substantially Fe, was heated from a temperature of ₃ or more Ac to a temperature of 5 ° C./s or more. Cool at a cooling rate of less than ℃ / s to 650 ℃ or less, at the temperature range up to the temperature at which low-temperature phase such as ferrite phase, bainite phase, martensite phase starts to precipitate Processing with a cross-sectional area reduction rate of 60% or more in one pass or one
A method for producing steel having a fine-grained ferrite structure, wherein the steel is applied in multiple passes of 30% or more per pass, and then cooled to a temperature of 400 ° C or less at a cooling rate of air or higher. (2) In weight%, C: 0.05 to 0.3%, Mn: 0.5 to 3%,
Si: 0.01 to 0.3%, Nb: 0 to 0.05%, Ti: 0 to 0.05
%, V: 0 to 0.08%, Cr: 0 to 1%, and Mo: 0 to 1%
, The balance of which is substantially composed of Fe, is cooled from a temperature of ₃ or more Ac at a cooling rate of 5 ° C / s or more and less than 100 ° C / s to 650 ° C or less, a ferrite phase, a bainite phase, Alternatively, in a temperature range up to a temperature at which a low-temperature phase such as a martensite phase starts to be precipitated, processing in which the cross-sectional area reduction rate at the end of processing relative to the start of processing is 60% or more is performed in one pass or 30% or more per pass. A method for producing a steel having a fine-grained ferrite structure, wherein the steel is applied in a pass and thereafter cooled to a temperature of 400 ° C. or less at a cooling rate of air cooling or higher.

The ferrite formed by the low-temperature transformation of austenite is difficult to observe by ordinary optical microscopy because of its fine crystal structure. It is a ferrite structure composed of crystal grains with little distortion that can be found by direct observation with a microscope. (1) above and
In the steel of the present invention (2), this structure has an area ratio of 80
% Or more.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the chemical composition of steel in the method of the present invention are as follows. In addition, all the contents of the component elements are% by weight.

The content range of C is 0.05-0.3%. If the content is less than 0.05%, after transformation to an austenite phase of Ac ₃ points or more, transformation will start at high temperature even if quenched, so strong working in low temperature supercooled austenite phase It becomes impossible and fine-grained steel cannot be obtained. On the other hand, if C exceeds 0.3%, the deformation resistance increases, and it becomes difficult to perform strong working at low temperatures, and the pearlite structure becomes the main phase and does not become the structure of the ferrite main phase. Therefore, the content of C is in the range of 0.05 to 0.3%.

Mn is necessary for sufficiently lowering the temperature at which a low-temperature phase such as a ferrite phase, a bainite phase, or a martensite phase starts to precipitate when quenched from an austenite phase having _three or more Ac points. That is, Mn is important for stably realizing a low-temperature supercooled austenite phase. If the amount is small, it becomes difficult to stabilize the supercooled austenite phase, so 0.5%
The above content is necessary. However, the content of Mn is 3%
When it exceeds, the deformation resistance increases, and it becomes difficult to perform strong working.
In addition, the stabilizing effect of austenite, that is, the effect of suppressing transformation, is excessively large, does not cause transformation even by heavy working, becomes a low-temperature transformed phase such as bainite or martensite during cooling after working, and ferrite becomes the main phase. Organization. Therefore, the content of Mn is limited to 0.5 to 3%.

One of the methods of the present invention is the above-mentioned C and Mn.
It is intended for so-called low carbon steel that does not contain any special alloy components. That is, the balance other than C and Mn is substantially Fe, and “substantially Fe
"Means that the presence of impurities that are inevitably mixed in the production of steel is permissible. As inevitable impurities, there are P, S, O, N and the like, and it is desirable that these are as small as possible.

Although Al (aluminum) is not particularly necessary for the purpose of obtaining a fine grain structure, it is an essential element for deoxidizing molten steel in order to obtain a sound slab free of defects during casting. . Among the above unavoidable impurities, there is also included a residual amount of Al (preferably 0.01% or more) added for performing sufficient deoxidation of molten steel. However, the addition of a large amount of Al is meaningless because the effect is saturated and increases the price of steel. Therefore, it is better to keep the content at most 0.1% or less.

Another of the steels according to the present invention is that, in addition to C and Mn, S contributes to stably obtain an ultrafine grain structure.
This is a so-called low-carbon low-alloy steel containing at least one element of i, Nb, Ti, V, Cr and Mo in the following ranges. In addition, although the content of these elements was described as 0 to X%, it is not necessary to add the elements positively, and when they are added, the upper limit of the content is set to X%. It means to do.

When Si is contained, fine particles can be stably obtained even when the amount of C is relatively small. The effect is
If it is less than 0.01%, it is hardly recognized. Therefore, when it is added, its content is preferably made 0.01% or more. on the other hand,
If the content of Si exceeds 0.3%, the deformation resistance increases and it becomes difficult to perform strong working. Therefore, even when Si is added, the upper limit of the content is set to 0.3%.

When Nb or Ti is contained, a fine structure can be obtained sufficiently stably even when processing is performed at a temperature slightly higher than the temperature at which the low-temperature phase starts to precipitate.
This is presumably because the growth of crystal grains after transformation is suppressed by the precipitation of fine carbonitrides. In order to sufficiently obtain this effect, it is desirable that the content of Nb be 0.005% or more and that of Ti be 0.005% or more. However, if these elements become excessive, the toughness decreases, so that Nb should be 0.05% or less and Ti should be 0.05% or less. That is, when it is contained, Nb is 0.005 to 0.05% and Ti is 0.005 to 0.05%.
It is good to be in the range of.

By containing V, Cr and Mo, a fine grain structure can be obtained stably. These elements form carbides and the precipitates are N
As in the case of b or Ti, there is an action of suppressing the growth of crystal grains, but the effect is not so great. Rather, these elements have a strong effect of delaying the transformation, lowering the precipitation of the low-temperature phase at a lower temperature, delaying the precipitation time, and expanding the range of austenite at a low temperature in a supercooled state. This has the effect of facilitating the generation of a grain structure. In order to obtain such an effect, it is preferable that V contains 0.008% or more, Cr contains 0.05% or more, and Mo contains 0.05% or more. However, these elements tend to delay the transformation by large working like Mn, and if the content is excessively large, it becomes difficult to obtain a structure mainly composed of ferrite. Therefore, it is preferable that V is 0.08% or less, and that of Cr and Mo is 1% or less. In other words, the content of V is 0.008 to 0.08% for V,
Is preferably 0.05 to 1%, and Mo is preferably 0.05 to 1%.

The ferrite grains include coarse grains formed by high-temperature generation, grains surrounded by dislocation networks by processing, and recrystallized grains generated from a cold worked structure. In a state in which the ferrite is gathered, only ferrite produced at a low temperature can be observed with a transmission electron microscope. If the low-temperature formed ferrite structure is less than 80% of the whole, the steel does not have excellent toughness. This is because the part other than the low-temperature generated ferrite structure becomes a martensite phase or a bainite phase, resulting in steel with high strength but poor toughness. This is because the steel phase is inferior in strength and toughness. If the average crystal grain size exceeds 3 μm, this also results in a steel having poor strength and toughness. Therefore, the manufacturing method must be such that a steel having such a structure can be obtained.

The production method of the present invention uses a steel material having the above composition range, and cools from a temperature of ₃ or more points of Ac at a cooling rate of 5 to 100 ° C./s to 650 ° C. or less. In a temperature range up to the temperature at which a low-temperature phase such as a martensite phase starts to precipitate, processing in which the cross-sectional area reduction rate at the end of processing to the processing start is 60% or more per pass or 30% or more per pass And then cooling at a temperature with air cooling or a higher cooling rate.

Here, the cooling rate from the temperature of _three or more Ac to 650 ° C. or less is set to 5 to 100 ° C./s.
If the cooling rate is lower than
This is because it is difficult to bring the temperature down to 650 ° C. or lower, and it is transformed into ferrite by the time of processing, and the crystal grains become coarse. When the cooling rate is abruptly higher than 100 ° C./s, the temperature distribution of the material to be cooled is deteriorated, causing nonuniformity depending on the location, and may be lowered to a temperature lower than a temperature at which a low-temperature phase is precipitated. Because there is.

The material before the start of cooling may be a material heated from room temperature to a temperature of _three or more Ac in a heating furnace.
The material may be heated and processed into a required shape such as rough forging or rough rolling at a temperature of _three or more Ac, regardless of its prior history.

The reason why the temperature is cooled to 650 ° C. or less is that if the working is performed at a temperature exceeding 650 ° C., a sufficient fine structure cannot be obtained due to recrystallization immediately after working deformation. In addition, if processing is performed after the transformation has begun, a uniform fine structure cannot be obtained, and not only processing strain remains, but also deformation resistance increases, so that it becomes difficult to apply strong processing. . Therefore, processing is 650
It must be carried out in a temperature range of not more than 0 ° C and a temperature range until a low-temperature phase is precipitated.

Processing in this case is performed at a reduction rate of the cross-sectional area of 60%.
%. When the deformation amount is less than 60%, the deformation is insufficient and the structure does not have a sufficient fine grain structure, and further, there is a tendency that the release of the processing strain due to the transformation is insufficient. In the case of sheet rolling, there is almost no deformation in the width direction,
The cross-sectional area reduction rate is substantially the same as the sheet thickness reduction rate. This processing can achieve the same effect no matter how large the cross-sectional area reduction rate is 60% or more, but is generally limited to about 90% due to an increase in energy required for deformation and a temperature drop.

When performing the processing of 60% or more, the processing may be performed in one pass, but may be performed in a large number of times.
However, when dividing into many times, one processing must be 30% or more. This is because if the processing is less than 30%, the growth of crystal grains is rather promoted, and a fine grain structure may not be obtained. The interval between passes does not need to be particularly short as long as the interval is maintained in the above-mentioned processing temperature range, and the temperature may be kept as needed.

After the processing, the air is cooled to a temperature of 400 ° C. or less at a cooling rate of air or higher. Air cooling in this temperature range varies depending on the shape of steel and the like, but is about 0.2 to 5 ° C./s at an average cooling rate. On the other hand, the microstructure formed by the transformation has a low temperature of 650 ° C. or lower, so that the grain growth is slow, and the microstructure can be sufficiently maintained at such a cooling rate.

[0041]

EXAMPLE A steel having the composition shown in Table 1 was melted in a 50 kg high-frequency vacuum melting furnace, and an ingot was forged into a slab having a width of 150 mm and a thickness of 50 mm. The plate was 20 mm in length. After heating this base plate to 1000 ° C to austenitize, it is cooled by changing the cooling rate by spray cooling, and the temperature at which the low-temperature phase starts to precipitate after reaching the target temperature, that is, the temperature at which transformation starts Was rolled to a temperature just above the temperature, and cooled immediately after the rolling.

[0042]

[Table 1]

The rolling conditions for each of the steel numbers subjected to these rolling processes, ie, the working start temperature, in the case of multi-pass rolling, the lower limit of the working ratio per roll, and the total working ratio of the end thickness to the rolling start thickness , Etc. are shown in Table 2. Using a transmission electron microscope, a 7,000-fold photograph was taken using a transmission electron microscope to measure the ferrite particle size, and the 2,000-fold magnification was obtained from the thin-film specimens at the center of the sheet thickness, which were collected at arbitrary positions from the obtained rolling specimens at arbitrary positions. The ratio of the ferrite structure was determined from the photograph. In addition, JIS5
Cut out the tensile test piece of No.
An impact test was performed using a 2.5 mm JIS No. 4 subsize test piece to determine the fracture surface transition temperature.

[0044]

[Table 2]

Table 2 summarizes the test results of the average grain size of ferrite, the occupancy of the ferrite structure, the strength and the toughness.
Shown in As is clear from the results, the ferrite produced at a low temperature by the production method of the present invention accounts for 80% or more of the whole,
Further, it is understood that steel having an average crystal grain size of 3 μm or less has excellent toughness with respect to its strength. In addition, such ultrafine-grained steel must be manufactured by regulating the steel composition, the cooling rate from austenite to processing, the processing temperature, the degree of processing, and the cooling rate after processing as defined in the present invention. Is evident.

[0046]

According to the production method of the present invention, a steel having high strength and extremely excellent toughness can be obtained despite being a material steel having a small alloy composition content. This is due to the fact that ferrite generated by low-temperature transformation accounts for 80% or more of the steel structure and its average crystal grains are fine.

 ────────────────────────────────────────────────── ─── Continued on the front page (71) Applicant 000001199 Kobe Steel, Ltd. 1-3-18, Wakihama-cho, Chuo-ku, Kobe, Hyogo (72) Inventor Yoshitaka Adachi 4-5-Kitahama, Chuo-ku, Osaka, Osaka No. 33 Sumitomo Metal Industries, Ltd. (72) Inventor Toshiro Tomita 4-5-5 Kitahama, Chuo-ku, Osaka-shi, Osaka Prefecture No. 33 Sumitomo Metal Industries, Ltd. (72) Shuhei Shimokawa, Kitahama, Chuo-ku, Osaka, Osaka 4-5-233 Sumitomo Metal Industries, Ltd. (72) Inventor Masaaki Fujioka 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Research Institute (72) Inventor Tomoyuki Yokota 1 Marunouchi, Chiyoda-ku, Tokyo Kobe Steel Co., Ltd. Kobe Steel Ltd. Research house F-term (reference) 4K032 AA01 AA04 AA05 AA11 AA16 AA17 AA19 AA22 AA27 AA29 AA31 AA35 AA36 BA01 CA02 CB01 CB02 CD02 CD03 CD05

Claims (2)

[Claims] C. 0.05 to 0.3% and Mn: 0.5% by weight.
The steel containing about 3% and the balance substantially consisting of Fe is cooled from a temperature of ₃ points or more to a cooling rate of 5 ° C./s or more and less than 100 ° C./s to 650 ° C. or less. In a temperature range up to the temperature at which a low-temperature phase such as a phase, a bainite phase, or a martensite phase starts to precipitate, a process in which the cross-sectional area reduction rate at the end of processing relative to the start of processing is 60% or more per pass or per pass A method for producing a steel having a fine-grained ferrite structure, wherein the steel is applied in multiple passes of 30% or more, and then cooled to a temperature of 400 ° C or less at a cooling rate of air or higher. 2. C: 0.05 to 0.3%, Mn: 0.5% by weight.
-3%, Si: 0.01-0.3%, Nb: 0-0.05%, Ti: 0
-0.05%, V: 0-0.08%, Cr: 0-1% and Mo: 0
Comprises 1%, the steel balance being substantially Fe, Ac ₃
Cooling at a cooling rate of 5 ° C / s or more and less than 100 ° C / s from a temperature above the point to 650 ° C or less, until a temperature at which a low-temperature phase such as a ferrite phase, a bainite phase, or a martensite phase starts to precipitate. In the above temperature range, processing with a cross-sectional area reduction rate of 60% or more at the end of processing relative to the start of processing is performed in one pass or in multiple passes of 30% or more per pass, and then air-cooled or cooled at a cooling rate of 400 or more. A method for producing a steel having a fine-grained ferrite structure, characterized in that the steel is cooled to a temperature of not more than ℃.