JPS6234460B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing continuously cast slabs free from semi-macro segregation. In recent years, in the continuous casting method, the unsolidified molten steel in the slab is electromagnetically stirred, thereby stably generating equiaxed crystals in the center of the slab, thus dispersing and eliminating macroscopic central segregation. It became possible to do so. FIG. 1 shows the state of component segregation in the center of the slab depending on the presence or absence of electromagnetic stirring. As is clear from FIG. 1, when electromagnetic stirring is performed, the center segregation of the slab is significantly reduced compared to when electromagnetic stirring is not performed. The results shown in Figure 1 are obtained by chemically analyzing the chips of a 250 mm thick x 230 mm wide slab cast at a casting speed of 0.9 m/min and a molten steel superheat of 25°C, step-cut every 1 mm in the thickness direction. This was obtained by However, as shown in the solidification structure diagram of the cross section in the pouring direction in Fig. 2b, the slab obtained by electromagnetically stirring the unsolidified molten steel in the slab does not always have V-shaped formations. Segregation bands are present. FIG. 2a shows a solidification structure diagram without electromagnetic stirring, in which center segregation exists but V segregation does not occur. When observing the solidified structure at the center of the slab where the V segregation occurs, magnified approximately 10 times, as shown in Figure 3, island-like segregation between the equiaxed grains, that is, semi-macro segregation. It can be seen that this segregation is intermittently connected to form a V-shaped segregation pattern. This V segregation is thought to be formed when the liquid phase in the solid-liquid remaining phase is sucked toward the high solid phase ratio side, flows, and accumulates in the center of the slab at the final stage of solidification. The V-segregated area not only contains extremely high concentrations of solutes left behind during solidification, but also becomes a place where MnS is generated as Mn and S are concentrated, and is also prone to voids due to delayed solidification. This can cause various problems. As a way to solve the above problems, efforts are being made to disperse semi-macro segregation by making the solidified structure as fine as possible, as typified by electromagnetic stirring, but a complete solution has not yet been found. . This is because the conventional method could not sufficiently suppress the flow of the liquid phase within the solid-liquid coexistence phase. In other words, the driving force for sucking and flowing a liquid phase with a high solute concentration toward a high solid phase ratio is solidification shrinkage and the bulging phenomenon of the slab between adjacent rolls, so-called bulging, and if these are not controlled, semi-macro Segregation cannot be completely eliminated. On the other hand, if the above-mentioned semi-macro segregation exists in the slab, it becomes a particular problem when manufacturing hydrogen-induced cracking-resistant steel for line pipes, so-called HIC-resistant steel, for which demand has been increasing recently. That is, large semi-macro segregation is
Since it is hardly reduced in subsequent steps, low-temperature transformed structures such as bainite and martensite are formed in the rolling to cooling process, increasing cracking susceptibility and making HIC (hydrogen-induced cracking) more likely to occur. Additionally, in HIC-resistant steel, it is common to add Ca to control the morphology of inclusions, but even under conditions where MnS does not occur between dendrites,
MnS precipitates in semi-macro segregation areas where Mn and S concentrations are higher than the surrounding area, which becomes the starting point for HIC. At this time, if excessive Ca is added, large cluster-like inclusions will occur, which will also become the starting point for HIC. The inventors of the present application have conducted various studies in order to solve the above-mentioned problem, that is, when electromagnetic stirring of unsolidified molten steel is performed in the continuous casting method, semi-macro segregation is generated in the slab. . As a result, the following findings were obtained. Semi-macro segregation is where extremely high concentrations of solutes exist. In other words, as shown in Figure 1, the concentration distribution at the center of the slab subjected to electromagnetic stirring is as follows:
Although it is extremely flat in macroscopic check analysis, as shown in Figure 4,
It can be seen that there are semi-macro segregated grains with Pmax/Po≒12 and Mn max/Mno≒2.0. Figure 4 shows the casting speed of 0.8 to 1.0 using an X-ray microanalyzer (XMA) using a 50μΦ electron beam.
m/min, molten steel superheating degree 10-32℃, slab size 250
Line analysis of P and Mn in the semi-macro segregation of slabs cast under conditions of m thickness x 2300 mm width, roll pitch 500 mm, and electromagnetic stirring, and shows the relationship between the peak value and the ratio of the raw steel value. . FIG. 5 shows the relationship between the thickness and number of semi-macro segregation existing in the same slab as in FIG. 4, and the maximum thickness reaches about 3 mm. In such a high concentration portion, the solute may disappear due to the diffusion of the solute during the high temperature holding in the furnace during the rolling process. In other words, in terms of ease of solute diffusion, the smaller the thickness of semi-macro segregation, the more advantageous it is, and generally speaking, the critical thickness that disappears during the rolling process is:
Since it has been found that the particle diameter is approximately 300μ, particles larger than this remain as a high concentration band in the finished product. Furthermore, it has been recognized that the peak value of solute concentration within semi-macro segregated grains is correlated with the size of the segregated grains. That is, in Fig. 6, the casting speed is 0.8.
~1.0m/min, molten steel superheating degree 10~32℃, slab size 250mm thick x 2300mm width, roll pitch 500mm, equiaxed crystal ratio 25% sample cast with electromagnetic stirring, beam diameter 50μφ The relationship between the semi-macro segregation thickness measured by XMA and Pmax/Po is shown, and it can be seen that as the segregation thickness increases, solute concentration also increases. On the other hand, the thickness of semi-macro segregation and the area ratio of semi-macro segregation in the longitudinal section of the slab (the total area of semi-macro segregation observed after conducting the HCl corrosion test were measured using an image processing device, and the value was divided by the cross-sectional area of the slab). There is a correlation between the results (obtained), and as the segregation area ratio increases, the thickness of the segregated grains also tends to increase. This relationship is shown in FIG. 7, and this tendency exists regardless of the magnitude of the equiaxed crystallinity. As long as steel undergoes volumetric contraction during solidification and bulging between rolls of slab occurs due to the static pressure of molten steel, such semi-macro segregation will inevitably be formed. It becomes necessary to completely stop the flow of high solute concentration molten steel in the liquid coexistence phase. At this time, bulging cannot be suppressed simply by strongly cooling the slab. Further, simply stopping the flow does not compensate for the volumetric contraction, so voids are generated. Based on the above, the inventors of the present application have determined that in order to eliminate semi-macro segregation, the occurrence of bulging can be suppressed by narrowing the roll pitch, and in addition, as solidification of the solid-liquid coexistence phase progresses, the volume shrinkage of the slab can be reduced. We obtained knowledge that it would be better to reduce the thickness of the material. This invention has been made based on the above knowledge, and when manufacturing slabs by continuous casting method, while stirring molten steel by electromagnetic force, it Roll pitch is at least 2 m from the crater end of the solidus line toward the upstream side with a rolling reduction rate of 0.5 mm/m or more.
A light reduction is performed using reduction rolls set to 450 mm or less, thereby preventing bulging that occurs in the slab and preventing the flow of molten steel within the solid-liquid coexistence phase at the center of the slab. It is characterized by producing slabs without segregation. One embodiment of this invention will be described with reference to the drawings. FIG. 8 is a schematic explanatory view showing a state in which a slab is being rolled down by the method of the present invention. In Fig. 8, 1 is the electromagnetically stirred molten steel, 2 is the liquidus line of the solid-liquid coexisting phase, 3 is the solidus line of the solid-liquid coexisting phase, 4 is the slab, 5 is the crater end of the liquidus line, 6 is a solidus crater end, 7 is a reduction roll having a hydraulic cylinder 8, 9 is a pinch roll,
And 100 is a guide roll. The reduction roll 7 is installed within a range of at least 2 m toward the upstream side from the crater end 6 of the solidus line. As the solidification of the solid-liquid coexistence phase existing in the above range progresses, a light reduction is applied to the slab 4 by the amount of volumetric shrinkage. This is to prevent the flow of The roll pitch of the reduction roll 7 is 450 mm or less. This is to prevent the occurrence of bulging of the slab 4 in the above range. The reduction rate by the reduction roll 7 is 0.5 mm/m. That is, every time the slab 6 is pulled out by 1 m, it is reduced by 0.5 mm or more in the thickness direction by the reduction roll 7. In this way, the slab 4 is stirred electromagnetically while the molten steel 1 is stirred.
By lightly reducing a predetermined range of , it is possible to produce slabs with extremely low semi-macro segregation. Next, embodiments of the invention will be described. Example 1 Slabs were produced according to the casting conditions shown in Table 1, and the relationship between the semi-macro segregation thickness and the number of semi-macro segregations in the longitudinal section of the slab was investigated. The results are shown in FIG. In Table 1, the method of the present invention applies a light reduction to the slab using the roll arrangement shown in Figure 8, and the comparative method uses the same roll arrangement as the method of the invention, but applies a light reduction to the slab. This is the case where there is no. Also in the following examples, the roll arrangement is the same between the method of the present invention and the comparative method.

[Table] As is clear from FIG. 9, according to the comparative method, there are semi-macro segregated grains with a maximum thickness of about 3 mm, whereas according to the method of the present invention, the thickness is less than 0.3 mm. Example 2 A slab was manufactured according to the casting conditions shown in Table 2, and the solute concentration in the semi-macro segregation of the manufactured slab was measured by XMA. The results are shown in FIG.

[Table] As is clear from Figure 10, the semi-macro segregation concentration of the slab produced by the method of the present invention, in which light reduction was applied to the slab while electromagnetically stirring the molten steel, was higher than that of the comparative method using only electromagnetic stirring. It is within the minimum level range of the case. Example 3 Slabs were manufactured according to the casting conditions shown in Table 3, and the relationship between the semi-macro segregation area ratio and the light reduction rate in the longitudinal section of the manufactured slabs was investigated. The results are shown in FIG.

[Table] As is clear from FIG. 11, if the roll pitch is set to 450 mm or less as in the method of the present invention, the semi-macro segregation area ratio can be reduced to 0.12% or less, which is the critical value for defect generation. On the other hand, if the roll pitch exceeds 450 mm as in the comparative method, the semi-macro segregation area ratio cannot be reduced to 0.12% or less even if the light reduction rate is the same as in the method of the present invention. As is clear from FIG. 11, when the light reduction rate is less than 0.5 mm/m in the method of the present invention, the semi-macro segregation area ratio increases even if the other conditions satisfy the method of the present invention. Example 4 A slab was manufactured according to the casting conditions shown in Table 4, and the relationship between the semi-macro segregation area ratio and the light reduction length of the slab was investigated. The results are shown in FIG.

[Table] As is clear from Figure 12, the light rolling length is 2
m or more, the semi-macro segregation area rate is
It can be seen that the value is less than 0.12%. Example 5 A slab was produced according to the casting conditions shown in Table 5, and the relationship between the equiaxed crystallinity and the semi-macro segregation area ratio of the slab was investigated. Comparative method () is for low-temperature casting. The results are shown in FIG.

[Table] As is clear from Fig. 13, even if the slabs produced by the method of the present invention have the same equiaxed crystallinity, they are different from those produced by electromagnetic stirring alone or by low-temperature casting. It can be seen that the semi-macro segregation area ratio is stable at a low level compared to the slab in which axial crystals are generated. In addition, the semi-macro segregation area ratio of the slab produced by the comparative method is 0.15% or more. Example 6 A slab was manufactured under the casting conditions shown in Table 6, and a rolled product for pipelines equivalent to API×52×80 was manufactured from this slab, and a HIC test was conducted. Figure 14 shows the relationship between CLR (crack length ratio), which is a measure of HIC resistance, and the semi-macro segregation area ratio of the raw slab.

[Table] As is clear from FIG. 14, the semi-macro segregation area ratio of slabs produced by the method of the present invention was reduced to 0.12% or less, and as a result, no HIC occurred. On the other hand, all of the slabs produced by the comparative method had a semi-macro segregation area ratio of 0.15% or more, and as a result,
It can be seen that HIC is occurring. Example 7 Calculation of Ca in molten steel according to the casting conditions shown in Table 4
Slabs were produced with various ratios of S and S, and the relationship between Ca/S and the amount of MnS and Ca-based large cluster-like inclusions, which are the starting points of HIC, was investigated for the obtained slabs. The results are shown in FIG. As is clear from Figure 15, 1.0<Ca/S<
It can be seen that no problematic inclusions occur within the range of 4.5. As explained above, according to the present invention, extremely useful effects such as being able to manufacture slabs free of semi-macro segregation are brought about.

[Brief explanation of the drawing]

Figure 1 shows the segregation situation in the center of the slab with and without electromagnetic stirring, and Figure 2 shows the solidification structure of a cross section in the pouring direction of the slab with and without electromagnetic stirring, where a is for the case without electromagnetic stirring, and b is for the case where there is no electromagnetic stirring. Figure 3 is a metallurgical microscopic structure showing the formation of semi-macro segregation in the slab, and Figure 4 shows the peak values of P and Mn relative to the raw steel values when semi-macro segregation was analyzed by XMA. Figure 5 is a diagram showing the relationship between the thickness and number of semi-macro segregation in a longitudinal cross section of an electromagnetically stirred cast slab. Figure 6 is the result of measuring the thickness of semi-macro segregation and solute concentration by XMA. Diagram showing the relationship between
The figure shows the relationship between the area ratio of semi-macro segregation in a slab and the thickness of semi-macro segregation, Figure 8 is a conceptual diagram of the light reduction method of the present invention, and Figure 9 shows the thickness and number of semi-macro segregation in a longitudinal section of a slab. Figure 10 is a diagram showing an example of solute concentration during semi-macro segregation of slabs measured by XMA, Figure 11 is a diagram showing the relationship between Diagram showing the relationship with the semi-macro segregation area ratio when changed, 1st
Figure 2 is a diagram showing the relationship between the light reduction length and the semi-macro segregation area ratio, and Figure 13 is a diagram showing the relationship between the equiaxed crystallinity and the semi-macro segregation area ratio of slabs produced by the method of the present invention and slabs produced by the comparative method. , FIG. 14 is a diagram showing the relationship between the HIC test results and the semi-macro segregation area ratio of the corresponding slab, and FIG. 15 is a diagram showing the relationship between Ca/S and the number of inclusions. In the drawings, 1... Molten steel, 2... Liquidus line, 3
...solidus line, 4 ... slab, 5, 6 ... crater end, 7 ... reduction roll, 8 ... hydraulic cylinder,
9...Pinch roll, 10...Guide roll.

Claims (1)

[Claims] 1 When manufacturing slabs using the continuous casting method,
While molten steel is stirred by electromagnetic force, the roll pitch is set to 450 mm or less in an area of at least 2 m toward the upstream side from the crater end of the solidus line of the slab cast by a mold. A light reduction is performed using a reduction roll at a reduction rate of 0.5 mm/m or more, thereby preventing bulging that occurs in the slab and preventing the flow of molten steel within the solid-liquid phase in the center of the slab. Thus, a method for producing continuously cast slabs free from semi-macro segregation is characterized by producing slabs free from semi-macro segregation.