JPS6241294B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to cooling equipment for thick steel plates, and in equipment that accelerates cooling treatment of thick steel plates after normal rolling or controlled rolling, in particular, a plurality of cooling zones are arranged at the front and rear for cooling, and one of the cooling zones is placed on the inlet side of the cooling zone. is defined as a strong cooling zone, while the cooling zone outlet side is defined as a weak cooling zone, and the ratio Ws/Ww of the water volume density of these strong cooling zones and the water volume density of the weak cooling zone is 2.
-12, we aim to refine the structure and improve hardenability with a specific cooling pattern, which not only improves the mechanical strength that thick steel plates should have, but also creates thick steel sheets with particularly excellent toughness. It is designed to provide steel plates. As is well known, thick steel plate (10mm or more) after hot rolling
In the past, it was naturally air-cooled and then quenched again to ensure the desired mechanical properties, but the fuel cost for this was high and mass production was lacking, so a so-called accelerated cooling system was used. The current situation is that it has been newly adopted and is being brought up close. This accelerated cooling system applies water injection cooling to thick steel plates after normal rolling or controlled rolling using laminar flow or spray flow, and is considered to be a very effective means to solve the various problems of the conventional method. Attention has been paid. However, with current equipment, the thick steel plate after rolling is only cooled with an even distribution of water flow and density in the threading direction. Therefore, for example, as shown in the cooling pattern X shown in Fig. 1, Ar _{1 is}
While the cooling rate up to the point near the point is slower than the so-called average cooling rate between the Ar ₃ point and the Ms point, the cooling rate between the Ar ₁ point and the Ms point tends to be faster than the average cooling rate. Therefore, as shown in the same figure, if the temperature at a specific point on the steel plate is captured over time, the cooling pattern will take on an upwardly convex shape as shown by As a result, the mechanical properties of thick steel plates, especially the mechanical strength (tensile strength) and toughness, could not be greatly improved as a result of the adoption of accelerated cooling. This invention was made by paying attention to these facts, and the feature here is that a thick steel plate of 10 mm or more is threaded in one direction after finish rolling or hot leveling, and a cooling zone is used. In the cooling equipment for thick steel plates that cools the steel plate in the temperature range from the vicinity of the Ar ₃ point to the vicinity of the Ms point by water-cooling the steel plate, the cooling zone is arranged as a plurality of control zones in the front and rear of the sheet threading direction. , the inlet side of the cooling zone is made a strong cooling zone, while the outlet side is made a weak cooling zone, and the ratio Ws/Ww of the water volume density Ws of the strong cooling zone and the water volume density Ww of the weak cooling zone is set to 2 to 12. In addition, an air cooling zone is provided between the strong cooling zone and the weak cooling zone.Hereinafter, an example of cooling equipment according to a comparative example of the present invention will be described. Figure 3 shows an example of the layout when the cooling equipment is incorporated. In the figure, this equipment is arranged between the finishing mill 1 and the leveler 2.
It may be configured after leveler 2. This equipment has three rows of curtain lamina control zones 3, 3, 3 facing each other vertically in the front stage, while a pipe lamina control zone 4 in the rear stage from the top and a spray cooling control zone 5 from the bottom.
are arranged facing each other in two banks, forming a cooling zone in the front stage and a cooling zone in the rear stage, with the entrance side of the front stage forming a strong cooling zone and the exit side of the latter stage forming a weak cooling zone. be. Note that the specific configuration of each cooling zone is not limited to the above, and there are plans to implement it in various ways by combining lamina cooling, spray cooling, mist cooling, etc. In this way, both strong and weak cooling zones were arranged at the front and rear, and the cooling pattern represented by Y in FIG. 1 was completed. This pattern Y exhibits a downward convex shape, which is the opposite of that of X, compared to the average cooling rate mentioned above.In other words, from the cooling start point (Ar ₃ points or more) to around the Ar ₁ level, the cooling rate is faster than the average cooling rate. Mochi, conversely Ar ₁
This means that a cooling rate slower than the average cooling rate can be obtained from the vicinity to the Ms point level. Therefore, there is a cooling rate in the strong cooling zone.
The condition is that the ratio between Cs and the cooling rate Cw in the weak cooling zone, that is, the cooling rate ratio, satisfies the relationship Cs/Cw≧1. This Cs/Cw ratio is determined by the ratio between the water volume density Ws in the strong cooling zone and the water volume density Ww in the weak cooling zone, that is, the relationship between the water volume density ratio Ws/Ww shown in Figure 4. Therefore, it was found that if the Ws/Ww ratio is set to 2 or more, the Cs/Cw ratio becomes 1 or more. However, Ws/
This does not mean that there will be no problems no matter how much the Ww ratio is 2 or more, but there is a limit of its own.In other words, when considering the actual layout of the cooling zone, the Ws/Ww ratio cannot be set too high. Taking a large value is because the length of the strong cooling region Ls is too short compared to the length of the weak cooling region Lw, and therefore it is unrealistic.
The upper limit of the Ws/Ww ratio was set at 12. Figure 5 shows a case in which these facilities were applied to two types of A and B thick steel plates having the main components shown in Table 1 under the condition of Ws/Ww≈6.

【table】

[Table] In this comparative example, the strong cooling zone and weak cooling zone are determined from the intermediate temperature using the ferrite transformation end temperature or bainite transformation start temperature as a guide, and from the implementation results shown in Table 2. As is clear from the above, by using this equipment, strong cooling was achieved in the front stage and weak cooling was achieved in the latter stage, which not only resulted in an increase in strength of 2 to 3 kg/ ^mm2 , but also in terms of toughness in particular. We were able to obtain a thick steel plate with an excellent vTrs value of around 40°C.On the other hand, in the example shown in Table 8, which shows the results when the cooling start temperature was started from Ar ₃ points or lower, the conventional equipment In some cases, a decrease in mechanical strength (including toughness) was observed when the cooling start temperature was delayed to some extent.
By using this equipment, it was possible to prevent this, and as a result, we were able to obtain stable properties with respect to changes in temperature in the longitudinal direction on the entrance side of the cooling zone of the steel plate.

[Table] As mentioned above, the implementation results based on the comparative example were obtained by creating a cooling pattern that is opposite to the conventional one based on the pattern expressed by the average cooling rate. After fully considering other general transformation characteristics of metal structures, etc., and considering the cooling pattern by dividing it into the first stage and the second stage, we came to the following theoretical basis, which we would like to introduce. That is, first of all, the reason why the cooling rate in the first stage (from Ar ₃ or more to around Ar ₁ ) was made faster than the average cooling rate is that, as is already well known, ferrite grains improve strength and toughness by making them finer. Accelerated cooling after rolling is aimed at promoting the refinement of these ferrite grains, and rolling control can increase the austenite grain boundary area, deformation bands, etc. If the number of ferrite nucleation sites is increased by introducing a processed substructure, the refinement of ferrite grains will be further promoted. Furthermore, it is important that accelerated cooling not only refines the ferrite grains but also produces fine bainite (upper bainite). In the past, it was thought that the upper bainite structure could contribute to an increase in strength, but at the same time significantly impair toughness, but fine-grained bainite, such as that produced by accelerated cooling, has almost no effect on toughness deterioration. Therefore, if it is possible to obtain finer ferrite than conventional accelerated cooling techniques and to generate upper bainite that can further reduce the degree of loss of toughness, further toughening effects can be expected.
A major feature of the present invention is that this effect can be fully exhibited when compared at the same cooling rate. That is, after accelerated cooling, ferrite α + bainite B, α + B + pearlite P, or α +
In the steel plate having P, α nucleates and grows in the pre-accelerated cooling stage. Considering this kind of cooling, even if the average cooling rate in the forced cooling temperature range is the same as shown in Figure 1, the cooling of the first stage is faster in the comparative example than in the conventional method; In the comparative example, the degree of supercooling per unit time is large in the temperature range from when α begins to generate until it stops, so α
In addition, when looking at the growth of α after nucleation, the time from the start to the end of α generation is shorter in the comparative example, so the growth of α is suppressed compared to the conventional method. This allows the miniaturization of α to be promoted more than in the past. Next, the reason why the cooling rate in the latter stage was made smaller than the average cooling rate is that in the latter stage of forced cooling (from around Ar ₁ to Ms), B is often generated, and the mechanism that governs the toughness of this B is as follows. Although there are still many unknown points, it is thought that the size and the island-like martensite that constitutes bainite have a large effect on toughness. Generally, the bainite grain size is determined by the austenite γ grain boundary area finally obtained through hot rolling control, and basically, the size does not change significantly depending on the cooling rate in the forced cooling range. You can think about it. On the other hand, island-like martensite that forms within bainite grains is known to significantly deteriorate the toughness of steel sheets, and if the formation of this martensite can be suppressed, the deterioration of toughness that accompanies bainite formation can be prevented. It becomes possible. In this case, there are many unknowns about the formation mechanism of island martensite, but
It is generally believed that concentration of alloying elements such as C and Mn accompanying the formation of acicular α occurs on the γ side of the α/γ interface, and island-like martensite as highly concentrated martensite is formed at this position. Therefore α/
If the concentration of these alloying elements can be alleviated by sufficiently causing diffusion of C, Mn, etc. at the γ interface, the formation of island-like martensite can be suppressed. As a method of alleviating this concentration, it is effective to delay the cooling below the B formation start temperature, and this is the reason why the cooling rate in the latter stage was made slower in the comparative example than before. Next, according to the comparative equipment, rolling in the non-recrystallized γ range (low temperature range), which has a great effect on toughness, can be strengthened more than before, and accelerated cooling can be performed, which is effective for refining the α grains. In other words, in steels that do not contain Nb, such as carbon steel and V-containing steel, the temperature range in which they are in the unrecrystallized γ state is narrower (100 to 150°C than in Nb steel), regardless of the rolling pass.
Therefore, rolling with strong reduction in the non-recrystallized γ range of such steels may extend over the entire temperature range, and the formation of α may occur at the same time as or before the start of accelerated cooling after rolling. begins. In this case, when conventional accelerated cooling is performed, as shown in Figure 1, after the start of cooling there is little temperature drop and the cooling rate during this period becomes extremely slow, so the frequency of α nucleation is low and growth is rapid at high temperatures. Therefore,
The α grains become coarse, and the effect of obtaining uniform and fine α grains by accelerated cooling is impaired. On the other hand α
If we try to accelerate cooling in a temperature range that is effective for grain refinement, cooling in a temperature range where there is little temperature drop after the start of accelerated cooling will inevitably result in an austenite temperature range, and the total reduction rate in the non-recrystallized γ range will decrease. I have no choice but to do it. According to the comparative example equipment, the temperature drop immediately after the start of cooling is greater than that of the conventional method. Therefore, even when strong pressure is applied over the entire unrecrystallized γ region, cooling becomes effective for refining the α grains immediately after the start of cooling, which is different from the conventional method. Furthermore, we can expect a toughening effect. The reason why we targeted steel plates with a thickness of 10 mm or more is because if we target steel plates with a thickness less than that, the cooling pattern will not change significantly from the previous one, and the toughening effect will also decrease accordingly. It is something. Furthermore, although various average cooling rates are set above, the range of 1 to 30°C/sec is selected for equipment implementation.
The reason for this is that if the cooling rate is faster than that, the amount of α produced is small and the proportion of B or M (martensite) increases, and the strength increases, but if the steel is kept as rolled, there is a significant deterioration in toughness. In this way, the above-mentioned thick steel plate cooling device (for example, the third
However, the cooling rate in the strong cooling zone is large, and the temperature distribution in the thickness direction is particularly poor when dealing with thick steel plates of 25 mm or more. As this occurs significantly, the temperature near the surface of the steel plate becomes lower than the Ms point. In this case, depending on the type of steel, it becomes hard near the surface due to the hardenability effect, resulting in a hardness distribution in the thickness direction of the steel plate. The present invention solves these problems in the comparative example, and in the present invention, as shown in FIG. 6, an air cooling zone is provided between the front stage strong cooling zone and the rear stage weak strong cooling zone. Furthermore, the 6th
Figure Lac shows the air cooling zone length. Furthermore, in terms of operation, in the equipment shown in FIG. 1, it is also possible to use this area as an air cooling zone instead of partially injecting water from the weak cooling zone closer to the strong cooling zone. Here, Fig. 7 shows the state of steel plate cooling when using the equipment shown in Fig. 3 (hereinafter referred to as equipment A) and when using the equipment shown in Fig. 6 (hereinafter referred to as equipment B). show. In this case, in Figure 7, by providing an air cooling zone after the strong cooling zone, Ms.
The supercooled zone below the point exhibits a rapid reheating effect, which causes the surface to cool down in the next weakly cooled zone.
It became possible to perform cooling above the Ms point, and as a result, it was possible to alleviate the hardening properties near the surface. Figure 8 shows the relationship between the Vickers hardness Hv as seen from the distance from the center of plate thickness when using equipment A, and Figure 9 shows the relationship when using equipment B, that is, adding an air cooling zone. It is clear from these results that the hardening phenomenon in the surface layer is clearly observed in the equipment A of FIG. 8, while the softening of the surface layer is observed in the equipment B of FIG. As described above, according to the cooling equipment according to the present invention, it is possible to produce strong non-thermal steel having extremely superior strength and toughness compared to conventional steels, and it is also possible to reduce the hardenability near the surface of the steel plate.

[Brief explanation of the drawing]

Fig. 1 is a comparison diagram of steel plate cooling patterns, Fig. 2 is a diagram of cooling heat transfer characteristics using water, Fig. 3 is an explanatory diagram showing a process layout and an example of steel plate cooling equipment in a comparative example of the present invention, and Fig. 4 is an explanatory diagram showing an example of a steel plate cooling equipment. A graph showing the relationship between the water volume density ratio and the steel sheet cooling rate ratio in the strong cooling region and the weak cooling region, and FIG. 5 is a steel sheet cooling curve diagram according to the comparative example.
Fig. 6 is an example of a facility in which an air cooling zone is provided between strong and weak cooling zones, showing an embodiment of the present invention, and Fig. 7 is a cooling diagram showing a comparison of surface and center temperatures of a steel plate using equipment A and equipment B. FIG. 8 is a diagram showing the results of an implementation showing the Vickers hardness over the plate thickness using the equipment A, and FIG. 9 is a diagram showing the results of implementing the Vickers hardness over the thickness using the equipment B. 1...Finishing mill, 2...Leveler,...Strong cooling zone,...Weak cooling zone,...Air cooling zone, X...Example of cooling pattern using conventional equipment, Y...
...An example of a cooling pattern using the equipment of the present invention.

Claims (1)

[Claims] 1 A thick steel plate in which a thick steel plate of 10 mm or more is threaded in one direction after finish rolling or hot leveling and is water-cooled in a cooling zone to cool the steel plate in a temperature range from near the Ar ₃ point to near the Ms point. In the cooling equipment for use, the cooling zone is arranged as a plurality of control zones in the front and rear with respect to the sheet passing direction, and the zone on the entrance side of the cooling zone is a strong cooling zone, while the exit side is a weak cooling zone, The ratio Ws/Ww of the water volume density Ws in the strong cooling zone and the water volume density Ww in the weak cooling zone is 2.
12, and an air cooling zone is provided between a strong cooling zone and a weak cooling zone.