JPS6239229B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to ultra-fine-grained ferritic steel, especially ultra-fine-grained ferritic steel that is as hot-rolled and that does not contain special alloying elements such as Nb, Ta, Mo, and W, and its production. It is about the method. The fine-grained ferrite structure mentioned here is mostly
More than 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 in principle refers to a structure consisting of crystal grains surrounded by so-called high-angle grain boundaries, and is subcrystalline. Grain boundary (low angle grain boundary)
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 6 μm. 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 as rolled, so heating is performed at a high temperature of 1200°C or higher before rolling to dissolve Nb, and then finish rolling is performed at a low temperature of 800°C or lower. 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. In addition, various methods have been proposed, such as heating and rolling in a low temperature range or forced cooling after rolling, but all of these methods remain within the above grain size range, and the ultrafine grains ( 3-4
μm or less) has 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 can actually be obtained industrially, and in particular is a hypo-eutectoid steel whose main component is C without using any special alloying elements. It relates to steel that can be obtained as is, and in particular to an epoch-making technology that allows for easy production by reducing impurities such as P, S, and N. That is, the gist of the present invention is as follows. (1) C: 0.3% or less, Si: 1.5% or less, in weight%
Consisting of Mn: 2% or less, N: less than 0.002%, balance Fe and unavoidable impurities, the metal structure contains deformation-induced equiaxed ferrite crystal grains surrounded by large-angle grain boundaries with an average size of 4 μm or less at a volume fraction of 70%. Ultra-fine grained ferrite steel with high strength and excellent ductility. (2) C: 0.3% or less, Si: 1.5% or less, in weight%
Steel consisting of Mn: 2% or less, N: less than 0.002%, the balance Fe and unavoidable impurities is heated to a temperature above the Ac ₃ transformation point, and then hot worked in the process of cooling. ℃〜
(Ar ₃ + 100°C) temperature range, one or more times within 1 second with a total area reduction of 35% or more. Method for manufacturing fine-grained ferrite steel. The details of the present invention will be explained below. The steel of the present invention is an ultra-fine-grained ferrite steel as described above, but the structure called fine-grained ferrite in the present invention does not involve significant elongation of the grain shape and is almost isodirectional.
Furthermore, as a general rule, it refers to a structure consisting of crystal grains surrounded by so-called high-angle grain boundaries, and subgrain boundaries (low-angle grain boundaries) are not considered grain boundaries. The reason for specifying the chemical composition of steel in the present invention is as follows. The reason for setting the C content to 0.3% or less is that C
When the amount of C increases, the amount of ferrite decreases and the steel becomes mainly pearlite. However, in the steel of the present invention, the amount of ferrite can be increased much more than usual even with the same amount of C. However, if this amount is exceeded, the amount of pearlite etc. increases and it becomes difficult to obtain a steel mainly composed of ferrite. Si is usually added to steel for the purpose of deoxidizing and is contained to some extent, and in the present invention, it is sometimes added intentionally because it has the effect of increasing the amount of ferrite. However, if it is added in an amount exceeding 1.5%, the ferrite crystal grains tend to become coarse, so the content was set at 1.5% or less. Mn adjusts the transformation point, makes deformation-induced transformation more likely to occur, and also contributes to grain refinement by preventing rapid grain growth of deformation-induced ferrite.
If it is added in excess of 2%, the transformation temperature may drop too much and the ferrite amount may not reach 70%.
The following was made. P and S are elements that are normally contained in steel to some extent, but if they are contained in large amounts, they impair the ductility and toughness of steel. In the present invention, the smaller the amount, the more preferable. This is because deformation-induced transformation and recrystallization, which are the formation mechanisms of the steel of the present invention, are more likely to occur as P and S are reduced, and it is desirable that P and S be at most 0.015% and 0.010%, respectively. Although some amount of N is contained in steel as an impurity element, for the same reason as P and S mentioned above, the lower the content, the better the properties and the easier it is to obtain the steel of the present invention. If this amount exceeds 0.002%, it becomes difficult to produce the inventive steel unless hot working under large pressure, which is industrially quite difficult, is applied, especially in steels containing nitride-forming elements such as Al and Ti. So 0.002%
less than Al is usually contained to some extent in steel for deoxidization, but if it is normally contained at 0.1%, it generally does not have a large effect on the properties of the steel of the present invention. Further, Nb, Ta, Mo, W, etc. are all known as elements that delay the recrystallization and transformation of austenite. Since the steel of the present invention promotes transformation and recrystallization of austenite during hot working to make the grains finer, Nb, Ta, Mo, W, etc. are elements that inhibit this, so they are not included in the steel of the present invention. It must not be allowed to happen. Steel having such a composition is manufactured by the following manufacturing method. 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 making-slaking 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 manufacturing method of the present invention is that the steel is heated to 600°C
(Ar ₃ +100°C), applying a large pressure for a short period of time. However, the Ar ₃ transformation point here refers to the temperature at which the steel starts to transform into ferrite during slow cooling from the temperature range where it is austenite.
(hereinafter simply referred to as Ar ₃ ) The present inventors conducted a detailed study on the effect of large reduction on the hot-worked structure, which had not been studied in the past, and discovered new knowledge that was completely unknown in the past. Obtained. The results are schematically shown in FIG. The region of small area reduction in this figure is relatively well known, and it has recently been known that austenite undergoes dynamic recrystallization at relatively high temperatures even under high pressure. By the way, when high-purity steel according to the present invention is used, transformation occurs during processing by applying a large reduction between Ar ₃ +100°C and 600°C, and the fine-grained ferrite produced at this time is further regenerated during rolling. It was found that the particles crystallized and became even finer. The range of hot working conditions of the present invention is shown in Figures 1 and 2.
clearly shown in the diagram. That is, normal purity steel (P:
0.015-0.030%, S: 0.008-0.015%, N:
0.0025 to 0.0050%), the area reduction rate exceeds 50% in an appropriate temperature range. Dynamic transformation and recrystallization of ferrite occur to obtain an average ferrite grain size of 3 to 4 μm or less (Fig. 1 ferrite dynamic recrystallization region), but in the case of high purity steel according to the present invention, the area reduction rate is Similar fine grains can be obtained even at 35%, and at lower temperatures, the grain refinement becomes even more remarkable due to complete dynamic recrystallization of ferrite, resulting in ultrafine grains of 2 μm or less (Fig. 1 Ferrite dynamic recrystallization ). From this, it can be seen that purifying steel according to the present invention is extremely effective. Figure 2 shows the relationship between the cumulative area reduction rate (%) and the average ferrite grain size.In the case of the steel according to the present invention, the ferrite grain size shows that it becomes smaller. In this case, the processing temperature of the present invention is 650~
700℃, and 750-800℃ for comparative steel. Although it is best to apply this reduction in one pass as in thick plate rolling, we have found that cumulative strain applied in multiple passes over a short period of time can have almost the same effect as shown in Figure 2. Obtained. It has also been found that this short time may be within about 1 second in normal rolling. Therefore, the above rolling reduction rate can be replaced by the accumulated total area reduction rate. Incidentally, such a short-time cumulative reduction can actually be realized at 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 often desirable to cool at a high cooling rate. When the area reduction rate is sufficiently large or when the finishing temperature is on the low side within the appropriate temperature range, fine grains can be obtained even if the steel material cross section is small, even if it is allowed to cool. However, when the area reduction rate is close to the lower limit, When the cross section of the product steel material is large or the finishing temperature is high, accelerated cooling at a rate of 20°C/sec or more is often effective, for example. 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. In this case, it goes without saying that the temperature range for accelerated cooling should include a temperature range of 500°C or higher, where ferrite grain growth or portions that are not transformed during rolling transform into ferrite or pearlite during cooling. be. The steel of the present invention is characterized by the formation of such fine-grained ferrite immediately after hot rolling, and for specific purposes (improving texture, etc.) It's possible. Next, the metallographic characteristics of the steel of the present invention produced in this manner will be explained. As mentioned above, the present invention is characterized in that when high-purity low-carbon steel is subjected to a reduction exceeding a certain limit in one pass or multiple passes at an appropriate temperature range, transformation is easily induced during processing, and the transformation is extremely fine. This is based on new knowledge that there is. At this time, the transformation nuclei generated during machining do not stop moving immediately after machining, and the machining strain (due to high-density dislocations induced by machining) does not recover immediately, so the boundary surface moves. Because the driving force is maintained to a certain extent, the fine-grained ferrite grows slightly after processing, and its volume fraction also increases. In such cases, it is difficult to experimentally strictly distinguish between the dynamically transformed ferrite part that has transformed during processing and the quasi-dynamic transformation point ferrite part that has grown following processing, and there is no practical need to do so. Therefore, the state up to a time period of about 1 second or less in the temperature range that becomes a problem after processing is defined as processing-induced transformation. In addition, in the case of high reduction machining even in one pass, and especially in the case of multi-pass machining of two or more passes, the ferrite generated in the first half of the machining or before the final pass of the multi-pass continues to undergo machining, and as a result, the machining distortion is recovered. Therefore, recrystallization may occur. In fact, after one pass is processed, if the next process is added within a short time of about 1 second or less, if the total reduction ratio of these two passes is as large as 35% or more, recrystallization nuclei will form during the process. This may result in finer grained ferrite compared to the state in the first half of processing or the state after the previous pass and before the next pass. Even in such a case, it is still difficult to distinguish between recrystallization nuclei that are dynamically generated immediately after processing (during processing) and growth that occurs continuously immediately after processing (quasi-dynamic recrystallization). Furthermore, the structure in which such dynamic and quasi-dynamic recrystallization phenomena are added is extremely similar to the structure in which only the dynamic and quasi-dynamic transformations are described above, and it is difficult to clearly distinguish between the two. . Therefore, in the present invention, these are collectively referred to as deformation-induced ferrite. Such processing-induced ferrite is extremely fine and has the following characteristics. The shape is approximately equiaxed and no significant stretching is observed during processing. This is clear from FIG. 3, which is a typical optical microscopic structure of the steel of the present invention (sample number 2). When processing is carried out in the normal transformation region, processed ferrite grains are formed that are stretched in the rolling direction. Such a structure is called a processed ferrite structure or a dynamic recovery structure, and when observed using a special corrosive solution, subgrain boundaries can be detected. A structure called (sub-boundary) is observed, but unlike normal ferrite grain boundaries (high-angle grain boundaries), there is little difference in the mutual orientation between grains, and the effect of grain boundaries on mechanical properties is completely different. Not only is there almost no improvement in properties due to fine grains, but the mechanical properties exhibit significant anisotropy due to stretching, and it is generally difficult to obtain excellent properties such as those of the steel of the present invention. Dislocation density is high on average and generally non-uniform. In the pro-eutectoid ferrite transformation that generally occurs during cooling, the transformation temperature is as high as 600 to 700°C, so it is characterized by extremely few dislocations in the produced ferrite. On the other hand, in the steel of the present invention, although there are some places where the dislocation density is high and some places where the dislocation density is low, overall it shows a considerably high dislocation density.
Such a high dislocation density is observed in bainite or martensite, which has a low transformation temperature (approximately 500°C or less), and is extremely abnormal in the case of Figure 3, which is thought to have been formed at around 800°C. The reason for this is that dynamic transformation or dynamic recrystallization of ferrite that occurs during processing continues to be processed, so that the dislocation density increases as processing progresses. For this reason, differences in dislocation density occur depending on the timing of ferrite formation, resulting in such a non-uniform appearance. Carbide particles are finely distributed and almost uniformly distributed. In the structure of the invention steel shown in Figure 1, the carbon content is 0.07%.
95% ferrite is produced, and no clear pearlite structure is observed. In trial number 10 described below
At 0.12% C, 98% ferrite was obtained. The maximum ferrite content predicted from the equilibrium phase diagram is 91% and 85% for these two steels, respectively.
In the steel of the present invention, ferrite is generated far more than this. On the other hand, even if only a very small amount of pearlite or bainite structure containing a large amount of carbide, or martensite structure containing a large amount of carbon as a solid solution is seen in such an optical microscopic structure, electron microscopic observation shows that the grains between fine grains are visible. Fine precipitates (carbides) are observed in the boundaries or in areas with high dislocation density. In this way, carbides are precipitated finer and more uniformly than in ordinary ferrite pearlite steel. The reason for the formation of such an organization is as follows. As mentioned above, ultrafine-grained ferrite is produced near or above the normal transformation point, and in large quantities that would not be produced at that temperature.This is because the energy that promotes transformation is not simply chemical, but processing. This is because it is supplied by. On the other hand, as can be seen from the fact that a large amount of ferrite is produced within a short period of time, such as during rolling, unlike normal ferrite transformation, carbon diffusion cannot keep up. (In such a case, it is disadvantageous in terms of energy compared to the case where transformation occurs due to carbon diffusion, but this shortage is also supplied through processing.) Therefore, carbon atoms are dissolved in supersaturated solid solution in ferrite. It is thought that during the process of cooling to room temperature after processing, precipitation occurs on ferrite grain boundaries or high-density dislocations that are fairly uniformly distributed from a macroscopic perspective. The characteristics described above and in the section above are significantly different from ordinary pro-eutectoid ferrite. The fact that there is no diffusion of C is similar to the so-called mushy ferrite transformation seen in pure iron, etc., and the fact that the transformation progresses in an extremely short period of time during hot working is also consistent with this. And the reason why the metastasis density is extremely high is
This is because the steel undergoes processing at the same time as it is formed, and a steel with a new structure completely different from that of conventional steel, which can be called "deformation-induced pine ferrite", has been created. 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 (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. In addition, it has good ductility, and as seen in the examples below, even at the same strength, it has superior ductility compared to conventional steel, showing an excellent strength-ductility balance. This is considered to be due to the combination of the above-mentioned structural uniformity and the effect of ultra-fine grain formation. In addition to the above properties, it has been observed that it also exhibits excellent properties in terms of durability (fatigue strength), formability (hole expandability, press formability), etc., but these are all due to the characteristics of the structure mentioned above. It is derived from In addition, especially in ultra-fine grain steel of 2 to 3 μm or less,
It exhibits distinctive properties such as a superplastic phenomenon in which ductility increases significantly at temperatures above 600℃. 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, making it 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. The steel was made using a special melting technique. 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 and B are according to the present invention, and are cases where a total reduction of 58% or 44% was performed in the 5th and 6th passes performed within 1 second. C 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. With the exception of trial numbers 2 and 6, cooling after rolling was performed by powerful spray cooling on a run-out table. 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. 3 shows an example of the microstructure photograph (trial number 2), which is mostly occupied by fine equiaxed ferrite grains of 1 to 3 μm, showing the typical characteristics of the steel of the present invention described above. ing. On the other hand, in the comparative material Trial No. 4, since the reduction ratio was small, fine grained ferrite was not generated and quenching occurred, resulting in poor ductility. Steel trial No. 5 has a rolling reduction that satisfies the requirements of the present invention, but due to the large amount of impurity elements, the ferrite does not dynamically recrystallize, so sufficient grain refinement does not occur and the ductility is poor. In sample No. 6, dynamic recrystallization of ferrite does not occur, resulting in elongated ferrite, which has high strength but extremely high ductility.

【table】

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[Table] From the above, it is clear that the effects of the present invention are remarkable.

[Brief explanation of the drawing]

Figure 1 shows the amount of strain during hot working of 0.07C-1Mn steel.
Figure 2 shows the relationship between temperature and structure immediately after processing.
A diagram showing the relationship between the amount of strain during hot working and the average ferrite grain size of 0.07C-1Mn steel, Figure 3 is a photograph showing the metallographic structure of the invention steel taken with an optical microscope, and Figure 4 is
FIG. 2 is a diagram showing the relationship between ferrite grain size, yield stress, and toughness of 0.07-1Mn steel.

Claims (1)

[Claims] 1% by weight: C: 0.3% or less, Si: 1.5% or less,
Consisting of Mn: 2% or less, N: less than 0.002%, balance Fe and unavoidable impurities, the metal structure contains deformation-induced equiaxed ferrite crystal grains surrounded by large-angle grain boundaries with an average size of 4 μm or less at a volume fraction of 70%. Ultra-fine grained ferrite steel with high strength and excellent ductility. 2 C: 0.3% or less, Si: 1.5% or less, in weight%
Steel consisting of Mn: 2% or less, N: less than 0.002%, the balance Fe and unavoidable impurities is heated to a temperature above the Ac ₃ transformation point, and then hot worked in the process of cooling. ℃〜(Ar ₃ +100
Ultra-fine grained ferrite steel with high strength and excellent ductility, characterized by being processed once or twice or more in a temperature range of 35% or more within 1 second in a temperature range of 35% or more. manufacturing method.