JPH11323481A - Steel with fine grained structure, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel composed of low carbon steel or low carbon alloy steel, excellent in strength, toughness and ductility, and having extremely fine average crystalline grain size, and a production method of the steel. SOLUTION: The steel with fine grains has a composition consisting of, by weight, 0.05-0.3% C, 0.5-3% Mn, and the balance essentially Fe and further containing, if necessary, small amounts of one or more elements among Si, Nb, Ti, V, Cr and Mo and also has a metallic structure in which ferrite formed from austenite by low temperature transformation is contained by >=80% and average crystalline grain size is regulated to <=3 μm. In the course of cooling from a temperature not lower than the Ac3 point at (5 to <100) deg.C/s cooling rate, working of >=60% is carried out in the temperature range higher than the temperature at which the precipitation of a low temperature phase is initiated and not higher than 650 deg.C and then cooling is performed at >=40 deg.C/s cooling rate down to <=350 deg.C, by which the steel can be produced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a high-strength steel which does not contain a large amount of alloying elements, has excellent ductility, and has high toughness.
And its manufacturing method.

[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.

As a method utilizing the transformation of steel, a quenching-tempering treatment is generally performed. 900 ° C for quenching
It is rapidly cooled by water cooling or oil cooling from a high or low temperature 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 sufficiently select the chemical composition based on the size of the steel to be treated. However, it is necessary to obtain a steel having excellent properties by this quenching and tempering. it can. However, an extra step for this heat treatment is required, and a heating furnace and a rapid cooling device are required. Therefore, in recent years, various measures have been taken to shorten the steps, for example, quenching is performed in the high temperature state immediately after hot working.

[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 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 an austenite phase region, 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 the processing and recrystallization. However, when the crystals become finer, the crystal grains are united to each other 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, mere processing and recrystallization alone
There is a limit to grain refinement. On the other hand, Al, Ti, Nb,
The addition of a small amount of a nitride or carbonitride forming element, such as V, forms fine precipitates, thereby suppressing the movement of grain boundaries, inhibiting the growth of crystal grains, and reducing the structure of the steel. There is a method of granulating. 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 working in the two-phase region of austenite + ferrite, the energy of transformation also increases the nucleation frequency, and the effect of suppressing grain growth by inhibiting the movement of grain boundaries 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 and reheating are 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 steel, it has not yet been obtained in low-carbon steel or low-alloy steel.

[0015]

As described above, even in a low carbon steel or a low alloy steel, if the crystal grains are further refined, it is expected that a steel with better performance and lower cost can be obtained. . An object of the present invention is to provide 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, and a method for producing the steel.

[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, in the case of low-carbon low-alloy steel, reverse transformation such as in high-alloy steel does not become anything other than the ferrite phase even if the degree of cold working is increased. In the above temperature range, the processing strain is released, the recrystallization nucleus generation and the grain growth progress, and when the reverse transformation takes place, the grains have already grown considerably and cannot be used for grain refinement.

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.

When the steel which has been heated to the Ac ₃ point or more and is in the austenite phase is rapidly cooled, the steel usually becomes an austenite phase in a state of being supercooled to the Ar ₃ point or less.
Or, if the cooling is further continued, it transforms to a ferrite phase, a martensite phase or a bainite phase depending on the steel composition and the 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. It is considered that the transformation induces 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 the conditions for obtaining a structure mainly composed of the fine-grained ferrite phase, it was found that if the temperature at which the working was applied was too high, the crystal grains would not be fine, and the deformation or the rolling reduction would be sufficiently large. If not, it was found that the ratio of the ferrite phase decreased and the martensite phase and the bainite phase increased. Furthermore, it became clear that the crystal grains would become large unless cooled as quickly as possible after processing.
In other words, it was presumed that by performing strong working immediately before transformation to the low-temperature phase, transformation recrystallization nuclei of ferrite were rapidly and densely formed and transformation proceeded rapidly, and at the same time, the processing strain was released. . In this case, it is considered that the greater the deformation due to the working, the more sufficiently the working strain accompanying the transformation induced by the working is released. If the degree of work is insufficient, not only the grain refinement becomes insufficient, but also the strain release becomes insufficient. Then, the transformation induced by the working resulted in an extremely fine ferrite crystal structure, but it was also found that the grain growth proceeds even after the transformation unless cooling as rapidly as possible after the transformation.

Thus, 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) In terms of% by weight, C: 0.05 to 0.3% and Mn:
0.5 to 3%, with the balance being substantially Fe, 80% of ferrite generated by low-temperature transformation of austenite
A steel characterized by having a metal structure occupying the above and having an average crystal grain size of 3 μm or less.

(2) In weight%, C: 0.05-0.3%, Mn:
0.5-3%, Si: 0.01-0.3%, Nb: 0-0.05%, T
i: 0 to 0.05%, V: 0 to 0.08%, Cr: 0 to 1%, and Mo: 0 to 1%, with the balance being substantially Fe and 80% ferrite formed by low-temperature transformation of austenite. A steel characterized by having a metal structure occupying the above and having an average crystal grain size of 3 μm or less.

(3) The steel having the composition of the above (1) or (2) is
_In the process of cooling at a cooling rate of 5 ° C / s or more and less than 100 ° C / s from a temperature of ₃ points or more, a temperature higher than a temperature at which a low-temperature phase such as a ferrite phase, a bainite phase, or a martensite phase starts to precipitate. And at a cross-sectional area reduction rate of 60% or more in a temperature range of 650 ° C or less, and then 40 ° C /
The method for producing steel according to the above (1) or (2), wherein the steel is cooled to a temperature of 350 ° C. or less at a cooling rate of s or more.

The ferrite formed by 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 low distortion that can be found by directly observing with a transmission electron microscope using a thin film sample collected from steel. In the steels of the present invention (1) and (2), this structure accounts for 80% or more of the area ratio of the cross-section observation.

[0026]

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

C is a basic component of the steel of the present invention. If the content is less than 0.05%, after transformation to an austenite phase of ₃ points or more of Ac, transformation will start at high temperature even if quenched, so that strong working in low temperature supercooled austenite phase is required. 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. 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%
If it exceeds, deformation resistance increases and strong machining becomes difficult,
Moreover, the steel obtained is very prone to rust. Therefore,
The content of Mn is limited to 0.5-3%.

One of the present invention is a so-called low carbon steel which does not contain any special alloy components other than the above-mentioned C and Mn. That is, the balance other than C and Mn is substantially Fe. The phrase “substantially Fe” means that the presence of impurities that are inevitably mixed in the production of steel is allowed. Inevitable impurities include P, S, O,
There are N and the like, but it is desirable that these are as small as possible.

Al (aluminum) is not particularly necessary for the purpose of obtaining a fine grain structure, but is an essential element for deoxidizing molten steel in order to obtain a sound slab without 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 one 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. Although the contents of these elements are described as 0 to X%, it is not necessary to add the elements positively, and when they are added, the upper limit of the contents is set to X%. It means to do.

When Si is contained, fine grains 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, the microstructure can be sufficiently stabilized to obtain a fine structure even when working 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 including 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, even if the content is excessively increased, the effect is saturated and the cost is unnecessarily increased. Therefore, it is preferable that the content is 0.08% or less for V and 1% or less for Cr and Mo, respectively. That is, the content in the case of containing V is 0.008 to 0.08% for V, 0.05 to 1% for Cr,
Mo is preferably set to 0.05 to 1%.

The steel of the present invention has the above composition, and the metal structure of the steel is ferrite generated by low-temperature transformation of austenite (hereinafter referred to as low-temperature generated ferrite) as a whole.
The steel occupies 80% or more and has an average crystal grain size of 3 μm or less. As described above, the low-temperature-generated ferrite, which can be directly observed by a transmission electron microscope technique using a thin film sample, has a low dislocation density and is clearly a recrystallized grain formed by transformation.

Ferrite crystal 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.

The method for producing the steel of the present invention uses a steel material of the above composition range, 5 to 100 ° C. from Ac ₃ point or more temperature / s
At a cooling rate of 650 ° C or lower, the temperature is reduced to a temperature range not lower than the temperature at which a low-temperature phase such as a ferrite phase, a bainite phase, or a martensite phase starts to precipitate.
% Strong processing.

Ac The cooling rate from a temperature of ₃ points or more is 5 to 10
The reason why the temperature is set to 0 ° C./s is that when the cooling rate is lower than 5 ° C./s, it is difficult to bring the supercooled austenite state to 650 ° C. or less, and the ferrite grains become coarse. . Further, 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 a place, and may be lowered to a temperature at which a low-temperature phase is precipitated. is there.

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 for cooling to 650 ° C. or less is that if 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 until a low-temperature phase is precipitated. In such a case, it is necessary that the rate of reduction in the cross-sectional area is 60% or more. 60
When the amount of deformation is less than%, deformation tends to be insufficient and a fine grain structure cannot be obtained, and further, there is a tendency that release of processing strain due to transformation is insufficient. In the case of sheet rolling, since there is almost no deformation in the width direction, the reduction rate of the cross-sectional area is substantially the same as the reduction rate of the sheet thickness. The same effect can be obtained no matter how large the working ratio is 60% or more, but is generally limited to about 90% due to an increase in energy required for deformation and a temperature drop.

After the strong working or immediately after the transformation, the steel is cooled to a temperature of 350 ° C. or less at a cooling rate of 40 ° C./s or more. This is because the grains are extremely fine immediately after the transformation, so that if the temperature is maintained immediately after the transformation or is cooled at a slow rate of less than 40 ° C./s, the grain growth proceeds, and the average grain size becomes 3 μm.
This is because there is a possibility that the crystal grains may exceed m. Since the processing strain is sufficiently released at the time of transformation, the cooling rate may be high, but the cooling rate is naturally limited by the cooling means that can be adopted such as water cooling.

[0042]

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 the base plate to 1000 ° C to austenitize, the cooling rate is changed by spray cooling, and if cooled at that cooling rate, the temperature at which the low-temperature phase starts to precipitate, that is, the temperature immediately above the temperature at which transformation starts Rolling at temperature
It cooled immediately after rolling.

[0043]

[Table 1]

Table 2 shows the rolling conditions of each of the steel numbers subjected to the rolling. 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, a JIS No. 5 tensile test piece was cut out from the rolled test piece, the tensile strength was measured, and an impact test was performed using a JIS No. 4 subsize test piece having a width of 2.5 mm to obtain a fracture surface transition temperature.

[0045]

[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.
It was also described in. As is clear from the results, it is understood that the low-temperature formed ferrite of the present invention accounts for 80% or more of the whole and the steel having an average crystal grain size of 3 μm or less is a steel having excellent toughness with respect to its strength. Also, in order to produce such ultrafine-grained steel, it is necessary to regulate 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. it is obvious.

[0047]

The steel of the present invention has high strength and extremely excellent toughness, despite being a steel having a small alloy composition content. This is because ferrite formed by low-temperature transformation accounts for 80% or more, and the average crystal grain is 3 μm.
The following is due to the fineness. According to the method of the present invention, an extremely fine ferrite structure is obtained by large strain processing of supercooled austenite, and even in a low-carbon low-alloy steel,
Steel with excellent strength and toughness can be obtained.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yoshitaka Adachi 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. (72) Inventor Masaaki Fujioka 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Steel Research Laboratory (72) Inventor Shigenobu Namba 1-5-5 Takatsukadai, Nishi-ku, Kobe City, Hyogo Prefecture Inside Kobe Research Institute, Kobe Steel Ltd. (72) Inventor Tomoyuki Yokota Chiyoda, Tokyo 1-1-2, Marunouchi-ku, Japan Kokan Co., Ltd. Technology Development Division

Claims (3)

[Claims] C. 0.05 to 0.3% and Mn: 0.5% by weight.
Steel having a metal structure containing at least 80% of ferrite produced by low-temperature transformation of austenite and having an average crystal grain size of 3 μm or less. . 2. C: 0.05 to 0.3%, Mn: 0.5% by weight.
-3%, Si: 0.01-0.3%, Nb: 0-0.05%, Ti:
0 to 0.05%, V: 0 to 0.08%, Cr: 0 to 1% and M
o: containing 0 to 1%, the balance being substantially Fe, and ferrite generated by low-temperature transformation of austenite
A steel having a metal structure occupying 80% or more and having an average crystal grain size of 3 μm or less. 3. The process of cooling a steel having the composition according to claim 1 or 2 at a cooling rate of 5 ° C./s or more and less than 100 ° C./s from a temperature of ₃ or more Ac. ,
Processing is performed at a temperature higher than the temperature at which a low-temperature phase such as a bainite phase or a martensite phase starts to precipitate, and at a cross-sectional area reduction rate of 60% or more in a temperature range of 650 ° C. or less, and then at a rate of 40 ° C./s or more. The method for producing steel according to claim 1, wherein the steel is cooled to a temperature of 350 ° C. or less at a cooling rate.