JPH0245536B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for reducing solidification segregation that causes surface flaws and cracks in slabs obtained by continuous casting and material defects in finished steel products. Prior Art Conventionally, in continuous casting, segregation of solutes during solidification causes surface flaws and cracks in slabs and deteriorates the quality of finished products, so improvements have been desired. These improvement methods include adding Ca to molten steel, reducing solutes that cause harmful segregation in advance through refining, shortening the roll interval of continuous casting machines to suppress bulging, and Methods such as electromagnetic stirring to reduce center segregation are being used. In addition, from the point of view of energy saving and labor saving, surface cracking of the slab during rolling is prevented during hot rolling or direct rolling or hot charge rolling in which the slab is charged into a heating furnace and rolled without being cooled to room temperature. Therefore, a method for suppressing surface cracks in slabs has been proposed in which ultra-slow cooling is performed during the cooling process following melt solidification until the start of hot rolling (Japanese Patent Application Laid-Open No. 84203-1983). In the above method, P, S, O, which is harmful to hot workability,
In the specific temperature range where elements such as N segregate and precipitate as non-metallic inclusions, simulation experiments show that the minimum value of cross-sectional shrinkage is in the temperature range of 1300 to 900℃.
Focusing on the fact that surface cracking occurs frequently when the content is less than 60%, hot cracking in slabs is suppressed by controlling the precipitation form of these elements. Further, Japanese Patent Application Laid-open Nos. 55-109503 and 55-110724 also disclose a method in which continuous cast pieces are slowly cooled before hot rolling and then directly rolled. Furthermore, Japanese Patent Publication No. 49-6974 discloses a method of preventing cracks by cooling and heating the cast strand in order to prevent the temperature difference between the surface and the center liquid from becoming too large. Purpose of the Invention The present inventor has discovered that deterioration in slab quality is not simply due to the amount of solidification segregation, and that α stabilizing elements (P,
When γ-stabilizing elements (Si, S, Cr, Nb, V, Mo, etc.) and γ-stabilizing elements (C, Mn, Ni, etc.) are concentrated in the same area, the synergistic negative effects due to overlapping segregation become even more significant. We also focused on the fact that these α-stabilizing elements and γ-stabilizing elements have different solubility in the δ phase and γ phase, and found that the effective temperature range for separating these solutes is different from the above-mentioned known technology. The present inventors have discovered that the present invention exists in a specific temperature range that is not disclosed, and has completed the method of the present invention for reducing solidification segregation. Structure of the Invention That is, the present invention utilizes the phase change of peritectic reaction, Ar ₄ transformation, or both that occurs during continuous casting secondary cooling of steel with a carbon concentration of 0.005 to 0.53% by weight. The solute elements in the segregated areas are separated from each other by being slowly cooled at a cooling rate of 30°C/min or higher, thereby eliminating defects in the material due to solidification segregation. This is a continuous steel casting method that is characterized by reducing Effect Steel in a molten state is cooled and as the temperature decreases, a solid phase crystallizes out. Figure 1 shows the relationship between the change in state and the carbon concentration. Carbon concentration is 0.17
Steel with a concentration between ~0.53% (weight%, the same applies hereinafter) changes from the liquid phase (area above line 1) to (liquid phase + δ phase) by cooling to (liquid phase) below 1495℃ (line 3 in the figure). phase + γ phase), and further cooling progresses to form straight line 6.
All become γ phase at the following temperatures. At the interface between the liquid phase and the δ phase (liquid phase + δ
Using the so-called peritectic reaction, which changes from phase) to (liquid phase + γ phase), P, which is an α-stabilizing element,
Si, S, Cr, etc., especially problematic P and S, are incorporated into the δ phase, and the γ stabilizing elements C, Mn,
Incorporate Ni, especially Mn into the γ phase. When the cooling progresses further and the total amount reaches the γ phase, the γ
The α-stabilizing elements described above are unevenly distributed in the portion transformed into a phase. As a result, for example, a portion where a peak concentration of P exists is separated from a portion where a peak concentration of Mn exists, and overlapping segregation of P and Mn is avoided. In steel with a carbon content of 0.005 to 0.08%, cooling changes the phase from liquid phase to (liquid phase + delta phase) to delta phase to gamma phase. In this case, the transformation from the δ phase to the γ phase is called Ar ₄ transformation, which begins at the temperature of line 4 in FIG. 1 and continues up to the temperature of line 5. During this time, in the Ar ₄ metamorphosis area,
Taking advantage of the coexistence of the δ phase and the γ phase, the α-stabilizing element and the γ-stabilizing element are temporarily separated using the difference in solubility. For example, P in the δ phase and Mn in the γ phase.
transfer and separate once. Even when the cooling progresses further and the entire amount changes to the γ phase, the α stabilizing element is unevenly distributed in the portion that transformed to the γ phase most late. As a result, for example, a portion where a P concentration peak exists is separated from a portion where a Mn concentration peak exists, and overlapping segregation of P and Mn is avoided. For steels with a carbon concentration of 0.08% to 0.17%, both the peritectic reaction and the separation in Ar ₄ transformation described above can be used. Figure 2 shows the relationship between the time required for phase change, that is, the cooling rate in actual operation, and the degree of separation of P and Mn. In the figure, 7 indicates the cooling rate of 2.7℃/min, 8 indicates the cooling rate of 27℃/min, and 9 indicates the cooling rate of the center of the slab in the current continuous casting machine. By slow cooling in minutes or less, we were able to obtain a degree of separation more than twice that of the current continuous casting method. Here, the following three indicators were used as the degree of separation. Concentration resolution C1 = (Mn ^* /Mn゜ /P ^* /P゜) / (k〓 ^/L / _Mo /k〓 ^/L / _Mo
/k〓 ^/L / _Mo /k〓 ^/L / _Mo ) = (Mn ^* /Mn゜ /P ^* /P゜) /1.80 Concentration resolution C2 = (P ^* /P゜ /Mn ^* /Mn゜) / (k〓 ^/L / _Mo /k〓 ^/L / _Mo
/k〓 ^/L / _Mo /k〓 ^/L / _Mo ) = (P ^* /P゜ /Mn ^* /Mn゜) / 1.80 Area separation A = 1-
Overlap area ratio of Mn high concentration area and P high concentration area/area ratio of Mn high concentration area Mn ^* , P ^* is the area where the δ phase first transforms into the γ phase at concentration separation degree C1, concentration separation degree C2 , represents the concentrations of Mn and P in the part where the δ phase is finally transformed into the γ phase. Also, Mn゜,
P゜ is the average concentration of Mn and P, respectively, and K ^a/b _i
represents the equilibrium distribution coefficient of component i between a phase and b phase. For the equilibrium distribution coefficient, the values shown in Table 1 were used. In the area separation degree A, the area ratio of the high concentration portion of Mn and P was set to 5%.

[Table] Figure 2 is for 50Kg/ ^mm2 steel (C0.13%).
This value is obtained when cooling is performed at different rates between 1500 and 1450°C, and then rapidly cooled at 4500°C/min. To explain this point in more detail, if the cooling rate of the solute element is too fast as in the prior art, there will be no margin for separation and results cannot be expected. The lower limit may be determined based on economic efficiency. Furthermore, after the phase change or transformation separation is completed, a single solid phase is formed, so separation due to differences in solubility does not occur. Therefore, unless the temperature within the single phase is rapidly lowered, the elements that have been separated will tend to diffuse again. As a result of various studies, it has been found that the cooling rate after treatment is preferably 30°C/min or more. Furthermore, in order to increase the efficiency of the peritectic reaction and transformation separation described above, steel that has once dropped from these temperature ranges is rapidly heated, then raised to the separation range again, and the slow cooling and rapid heating are repeated. , can be separated, and furthermore, after those operations, 30 as before
It was found that separation can be ensured by cooling at a rate of ℃/min or more. Specific examples of these will be described later as examples. Example 1 In 50Kg/ ^mm2 steel (carbon concentration 0.13%), 1450
After cooling at a cooling rate of 2.7℃/min to 4500℃/
Cooled down to room temperature in minutes. The degree of separation of P and Mn in the segregated part of this steel is 0.67, 1.00, and 1.00 when expressed as concentration separation C1 and C2 and area separation A, respectively.
It was hot. Mn, Si, P by 2D EPMA analysis
A characteristic X-ray image of the coagulated tissue (14 mm on the photograph corresponds to 200 μ) is shown in Fig. 4 (displayed using 5 levels of density difference through image processing). (a) Mn (1.4~
(1.6%Mn displayed in 5 levels), (b) Si (0.03~0.04%Si displayed in 5 levels), (c) P (0.006~0.021%P displayed in 5 levels), and the white part indicates each element. It can be seen that in the high concentration area, Si and P overlap, but are clearly separated from Mn. Furthermore, Fig. 5 shows the area ratio of 5% in the high concentration area of Mn and P (14 mm in the photograph corresponds to 200 μ). (a)
Mn, (b) P are shown, and the white part is the area ratio of 5% of the high concentration area. This figure also shows that Mn and P are clearly separated. Note that the thermal history graph of this example is shown in 1 of FIG. Example 2 Steel similar to Example 1 was cooled between 1500 and 1450°C at a cooling rate of 27°C/min. The degree of separation of P and Mn in the segregated part of this steel is concentration separation degree C1,
When expressed as C2 and area separation A, each is 0.41,
It was 0.40 and 0.38. Note that the thermal history graph of this example is shown in 2 of FIG. Example 3 A slab with a carbon concentration of 0.30% was heated at 30°C/1500°C to 1470°C.
Cool at a rate of 60°C/min and then 1500°C at a rate of 60°C/min.
The degree of separation when heating to ℃, cooling at the same rate, and repeating heating and cooling again is the concentration separation.
C1 and C2, expressed as area separation A, respectively
They were 0.32, 0.30, and 0.28. Note that a graph of the thermal history of this example is shown in 3 of FIG. Example 4 After performing the same operation as in Example 3, the temperature was increased to 4500℃/
The degrees of separation when cooled to room temperature at a cooling rate of 10 minutes were 0.40, 0.42, and 0.38 when expressed as concentration separation degrees C1 and C2, and area separation degree A, respectively. The thermal history graph of this example is shown in FIG. 3. Effects of the Invention As detailed above, the present invention makes it possible to avoid the solutes in the segregated areas, especially the combined segregation of Mn and P, which is a problem in continuous casting production methods, and to eliminate the causes of quality defects in slabs and finished products. This has greatly contributed to the development of the steel industry, such as by improving the quality of lamellar tear-resistant steel and sour-resistant steel.

[Brief explanation of drawings]

Figure 1 is a state diagram of carbon steel, Figure 2 is a diagram showing the relationship between the cooling rate and degree of separation of slabs, Figure 3 is an explanatory diagram showing the thermal history of examples, and Figure 4 is a diagram showing Mn, Si, FIG. 5 is a photograph showing the distribution of Mn and P in the structure of the steel. FIG.

Claims (1)

[Claims] 1. A peritectic reaction that occurs during secondary cooling of continuous casting of steel with a carbon concentration of 0.005 to 0.53% by weight,
Utilizing the phase change of _Ar4 transformation or both,
The cooling rate of the central part of the slab in the temperature range from the start to the end of the phase change is 40°C/min or less, and the cooling rate after the completion of the phase change is 30°C/min or more to cool the solute in the segregated part. A continuous steel casting method characterized by separating elements from each other and reducing defects in the material due to solidification segregation. 2. The continuous steel casting method according to claim 1, characterized in that the slab is repeatedly heated and cooled in the phase change region so that the heating rate is equal to or higher than the cooling rate.