JPS627247B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing a hot-rolled fine-grained steel material having an extremely fine ferrite crystal structure and excellent ductility. The fine-grained ferrite structure mentioned here is mostly
The temperature above 70-80℃ consists of fine ferrite crystal grains,
Depending on the desired mechanical properties, in addition to the ferrite phase, one or more of other fine structures such as pearlite, martensite, retained austenite, etc. may be present, or carbide, nitride, etc. may be present. It may contain precipitates. The structure called fine-grained ferrite in the present invention is almost isotropic without significant elongation of the grain shape, and refers to a structure consisting of crystal grains surrounded by so-called high-angle grain boundaries as the original side. Grain boundaries (low angle grain boundaries)
are not considered as grain boundaries. However, there may be some increase in dislocation density and formation of subgrain boundaries inside such grains. Among the various strengthening methods for steel, grain refinement is known to be the only method to increase both strength and toughness, and is most often used to improve the quality of steel materials used as hot-rolled. This is an important technology that must be considered first in most cases. What has been achieved industrially with conventional grain refining technology is as small as 4
It is about ~6μ. This is usually carried out using a method called controlled rolling, which is a technique for strongly rolling steel containing alloying elements such as Nb at relatively low temperatures. In this case, Nb needs to be in solid solution during rolling, so heating is performed at a high temperature of 1200°C or higher before rolling to dissolve Nb in solid solution, and then finish rolling is carried out at a low temperature range of 800°C or lower. Because it is done with
This method has industrial disadvantages, such as waiting for the temperature of the steel plate to drop, resulting in a significant drop in production efficiency, and a significant increase in deformation resistance during rolling, resulting in a heavy load on the rolling mill. Various other methods have been devised, such as heating and rolling in a low-temperature range or forced cooling after rolling, but all of these methods stay within the above grain size range, and the ultrafine grains ( 3~4μ
below) have not yet been obtained industrially. On the other hand, methods for experimentally obtaining ultrafine grain structures have recently been studied. For example, there is a method of repeatedly annealing Ni steel or the like several times before and after the transformation point.
However, it is clear that such heat treatment is difficult to implement industrially from an economic standpoint. The present invention relates to such an epoch-making ultra-fine-grained steel that has actually been obtained industrially, and in particular is a hypo-eutectoid steel whose main component is C without using any special alloying elements. It concerns steel obtained by processing. The gist of the present invention is that C0.3 in weight%
% or less, C or less alloying element content is 3% or less, hot working is carried out in the process of cooling from the temperature range above the Ac ₃ transformation point, and at the final stage (Ar ₁ + 50 ° C) ~ ( Hot working is carried out once or twice or more in a temperature range of (Ar ₃ +100℃) within 1 second, resulting in a total area reduction of 50% or more and 95% or less,
After the hot working is completed, cooling is performed at a cooling rate of 20° C./s or more and 2000° C./s or less to a temperature range of 600° C. or less. The reasons for the limitations of the present invention will be explained below. The reason for specifying the chemical composition of steel in the present invention is as follows. C0.3%: Generally, as the amount of C increases, the amount of ferrite decreases, and the steel becomes pearlite-based.
However, in the steel of the present invention, the amount of ferrite can be increased much more than in the normal case even with the same amount of C.
C: Up to 0.3%, a structure consisting mainly of ferrite can be obtained, but if it exceeds this amount, the amount of pearlite etc. increases and it becomes difficult to obtain a steel consisting mainly of ferrite. 3% or less in total of other alloying elements: In principle, the steel of the present invention can be obtained regardless of the presence or absence of alloying elements other than C, but the optimum temperature for hot working is between 700 and 900°C, and Ar ₃ transformation is possible. For the point, Ar1 + 50 ~ Ar3
Since there is a relationship that a temperature between +100℃ is desirable,
It is often desirable to adjust the Ar3 transformation point with alloying elements. However, if the total amount of alloying elements exceeds 3%, Ar3 becomes too low, making it difficult to obtain fine grains. Substantially free of Nb, Ta, Mo, and W:
Nb, Ta, Mo, and W are all known as elements that delay recrystallization. In the steel of the present invention, transformation and recrystallization occur during hot working and the grains become finer, so Nb,
Ta, Mo, W, etc. are elements that inhibit this and must not be included in the steel of the present invention. There are no particular restrictions on the steps leading to the final processing of the steel of the present invention. That is, normally produced molten steel may be made into a slab by continuous casting, or may be made into a slab by an ingot-blending process. The slab may be brought to the rolling process while still at high temperature, or
It may be reheated once cooled. It can be said that it is generally desirable that the heating and processing conditions for the slab be such that the austenite grain size of the slab becomes smaller just before the processing step of the present invention, but normal conditions may be used before the processing step of the present invention. Therefore, there are no restrictions. The feature of the present invention is that the steel is heated to the normal _Ar3 transformation point (refers to the temperature at which ferrite transformation begins during slow cooling from the temperature range where the steel is austenite; hereinafter simply referred to as Ar3
) and Ar1 transformation point (also refers to the temperature at which pearlite transformation begins during slow cooling, hereinafter simply referred to as Ar ₁ ) as a reference, (Ar1 + 50℃) ~ (Ar3 + 100℃)
The process involves applying a large pressure within a short period of time in a temperature range of (°C). The present inventors conducted a detailed study on the effect of large reduction working on the hot-worked structure, which had not been studied in the past, and obtained new knowledge that was completely unknown in the past. The results are schematically shown in FIG. In this figure, the region where the area reduction rate is less than 50% is relatively well known, and it has recently been known that austenite undergoes dynamic recrystallization at relatively high temperatures even under high pressure. However, it has now been revealed for the first time that transformation occurs during processing when a large pressure is applied around Ar3. It was also discovered that ferrite recrystallizes during rolling, partially overlapping with this. In response to this newly discovered phenomenon, it was found that ferrite grains were significantly refined as shown in FIG. 2. The range of hot working conditions of the present invention is clearly shown in FIGS. 1 and 2. In other words, if the area reduction rate exceeds 50% in an appropriate temperature range, dynamic transformation will occur and an average ferrite grain size of 3 to 4 μm or less will be obtained, but if the area reduction rate is further increased, the grains will become finer. becomes even more remarkable, and at about 75%, dynamic recrystallization of ferrite is probably added, resulting in ultrafine grains of 2μ or less, and above this, the grain refining effect is somewhat saturated. From this, it can be seen that the optimum area reduction rate is at least 50% or more, preferably 75% or more. Here, the upper limit of the area reduction rate is 95%. Applying an area reduction of 95% or more to a material within 1 second is extremely difficult and impractical with current rolling technology. Although it is best to apply this reduction in one pass, it has been found that cumulative strain applied in multiple passes over a short period of time as shown in FIG. 2 has an effect similar to this. This short time is 1 in normal rolling.
It was also found that it is sufficient if the time is within about seconds. Therefore, the above rolling reduction rate can be replaced by the accumulated total area reduction rate. Incidentally, such a short-time cumulative reduction can be realized in the finishing stage of wire rod rolling or in the latter half of hot strip rolling, as shown in the upper part of FIG. It is desirable that the above hot working is carried out at the final stage of the overall working, but in some cases even a small amount of hot or cold deformation to adjust the shape of the rolled material may significantly impair its properties. isn't it. In order to suppress grain growth after processing, it is desirable to cool at a high cooling rate. When the area reduction rate is sufficiently large or when the finishing temperature is within the appropriate temperature range and on the low side, fine grains can be obtained even if the steel cross section is small, so there is no need to limit the area reduction rate. When the temperature is close to the lower limit, when the product steel cross section is large, or when the finishing temperature is high, accelerated cooling is required, and the lower limit is 20℃/sec as shown in Figure 3.
becomes. There is no reason to limit the upper limit of the cooling rate, but if the cooling rate is high and even a small amount of untransformed austenite remains, that part becomes a hard second phase, which has the effect of further improving the strength. As described above, depending on the purpose, accelerated cooling can improve the material quality, especially the strength. It goes without saying that the temperature range in which accelerated cooling is performed should include a temperature range of 600° C. or higher in which ferrite grain growth or portions that are not transformed during rolling transform into ferrite or pearlite during cooling. In the present invention, the material after hot working is cooled at a cooling rate of 20° C./sec or more in order to suppress the grain growth of the fine ferrite structure induced by working. For the purposes stated above, it is necessary and sufficient for the cooling rate of the material after hot working to be 20°C/sec or higher, but at the current technological level, the limit is 2000°C/sec, so the cooling rate is The upper limit of 2000℃/sec
And so. The steel of the invention can be provided by various hot working methods. Examples include thick plate rolling, hot strip rolling, wire rod rolling, etc., and processing methods other than rolling such as hot extrusion or hot forging are also possible. Next, the effects of the present invention will be described. As mentioned above, it is well known that strength and toughness are improved by making the grains finer, but so far there has been no study of this effect using ultrafine grains of 4 μm or less. FIG. 4 shows the data for the fine-grained steel (black circles) obtained by the present invention together with the data for the conventional steel. Conventional data (white circles) can be well organized by the so-called Petch relation, but the steel according to the present invention shows a tendency to further improve from this extension. Furthermore, FIG. 5 shows the ductility of the steel of the present invention relative to its strength, and it can be seen that the steel of the present invention has higher ductility than the conventional steel at the same strength level. In addition, there are 2~
Ultrafine-grained steel with a diameter of 3μ or less exhibits distinctive properties such as a superplastic phenomenon in which ductility increases significantly at temperatures above 600°C. In this way, the steel of the present invention exhibits properties that far exceed those of conventional steel, so the effects of the present invention are extremely large, and it is possible to easily produce high-quality, high-strength steel, etc., at a very low cost and without adding alloying elements. It can be done. Example 1 The converter melted steel shown in Table 1 was continuously cast into a 200 mm slab, heated to 1100°C, and then rolled in a hot strip mill to form a 5 mm thick steel plate. In rough rolling, a 200 mm slab was rolled to 50 mm in 7 passes, and the finishing temperature was 900 to 1000°C. Table 2 shows the pass schedule for finish rolling.
A is based on the present invention and is a case where a total reduction of 58% is performed in the 5th and 6th passes performed within 1 second. B is an example of normal rolling for comparison, and the total reduction in the final two reductions is 27%. Table 3 shows the combinations of the above rolling conditions and the mechanical properties of the rolled steel sheets. Note that the finish rolling temperature range of the present invention is calculated from the transformation temperatures in Table 1, and is 680 to 870°C for steel and 660 to 890°C for steel. With the exception of trial numbers 3, 6, and 10, cooling after rolling was performed by spray cooling on a run-out table within the cooling rate range of the present invention of 20° C./sec. The effect of the present invention is clear from the mechanical properties.
It has a strength of more than mm ² and excellent ductility of more than 20%. Fig. 6 shows an example of the microstructure photograph (sample number 4), which is mostly occupied by fine equiaxed ferrite grains of 2 to 3μ, showing the typical characteristics of the steel of the present invention mentioned above. ing. On the other hand, comparative material Sample No. 5 was finished at a normal high temperature, and its strength increased as a result of rapid cooling, but its ductility was poor. As shown in FIG. 7, this structure is about 50% burnt, and the ferrite is also needle-shaped, indicating that it was transformed during the rapid cooling. Trial No. 6 is a case where the rolling temperature is lower than the range of the present invention, and the strength does not increase much because the ferrite grain size is not sufficiently small. Trial Nos. 7 and 11 are cases where the rolling reduction is small; in this case, the ferrite grains become considerably finer due to rapid cooling, and the second phase of the rapid cooling increases, so the strength increases somewhat, but it is not as good as the steel of the present invention. In this case, the proportion of the second phase (pearlite, bainite) is quite large, and therefore the ductility is not sufficient. From the above, it is clear that the effects of the present invention are remarkable.

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【table】 [Brief explanation of the drawing]

Figure 1 shows the amount of strain during hot working of 0.15C-1Mn steel.
A diagram schematically showing the relationship between temperature and the structure immediately after working (investigated using a structure that was rapidly cooled immediately after hot working). Figure 2 shows the amount of strain and ferrite grains during hot working of 0.15C-1Mn steel. Figure 3 is a diagram showing the relationship between the diameter and the cooling rate after processing and the ferrite grain size.
Figure 4 is a diagram showing the relationship between ferrite grain size, yield stress, and toughness of 0.1~0.15%C-0.5~1.5Mn steel.
Figure 5 is a diagram showing a comparison of strength-ductility balance between the steel of the present invention and conventional steel, Figure 6 is a photograph showing the metallographic structure of the steel of the present invention taken with an optical microscope, and Figure 7 is a comparison with a higher finishing temperature than that of the present invention. It is a photograph showing the metal structure of steel taken with an optical microscope.

Claims (1)

[Claims] 1 Steel with a C content of 0.3% or less and an alloying element content other than C of 3% or less in weight percent is hot worked during the process of cooling from a temperature range above the Ac ₃ transformation point, and in the final stage ( Ar ₁ +50℃) ~ (Ar ₃ +
100℃) in a temperature range of 1 second or more in which the total area reduction rate is 50% or more and 95% or less, and after the hot working is completed, the temperature is 20℃. /
1. A method for producing ultrafine-grained high-strength hot-worked steel material, characterized by cooling to a temperature range of 600°C or less at a cooling rate of S or more and 2000°C or less.