JPH04313454A - Continuous casting method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention prevents the segregation of impurity elements found in the center of the thickness of continuously cast slabs, such as sulfur, phosphorus, and manganese in the case of steel slabs, and obtains homogeneous metal. Concerning continuous casting method.

0002

[Prior Art] In recent years, requirements for material properties for offshore structures, storage tanks, steel pipes for oil and gas transportation, high-tensile wire rods, etc. have become more severe, and providing homogeneous steel materials has become an important issue. .

Originally, steel should be homogeneous in its cross section, but steel generally contains impurity elements such as sulfur, phosphorus, and manganese, which segregate during the casting process and become partially concentrated. steel becomes brittle.

[0004] Continuous casting methods have become popular in recent years for the purpose of improving productivity, yield, and energy conservation, but there is usually significant component segregation near the center of the thickness of slabs obtained by continuous casting. is observed.

[0005] This component segregation significantly impairs the homogeneity of the final product and causes serious defects such as cracks that occur due to stress acting on the steel during the product usage process and wire drawing process, so it is important to reduce it. coveted.

[0006] Such component segregation is thought to occur because residual molten steel flows at the final stage of solidification due to solidification shrinkage force, washes out the concentrated molten steel near the solid-liquid interface, and progressively concentrates the remaining molten steel. There is. Therefore, in order to prevent component segregation, it is important to eliminate the cause of the flow of residual molten steel.

[0007] In addition to the flow caused by solidification shrinkage, the causes of such flow include flow caused by slab bulging between rolls and roll misalignment, but the most important cause of these is flow caused by solidification shrinkage. In order to prevent segregation, it is necessary to reduce the slab by an amount that guarantees this.

[0008] Attempts to improve segregation by reducing the slab have been made in the past, and during the continuous casting process, when the temperature at the center of the slab reaches from the liquidus temperature to the solidus temperature, the slab A method is known in which the pressure is reduced at a constant rate greater than the amount that guarantees solidification shrinkage.

However, conventional continuous casting methods have problems such as hardly any segregation improvement effect being observed depending on the conditions, or even worsening segregation in some cases.
It has been difficult to sufficiently improve component segregation.

As a result of various investigations into the causes of such problems in the conventional method, the present inventors have found that the reason why the conventional method does not have an effect on improving segregation or causes segregation to worsen is basically due to pressure. It was discovered that the timing and range of coagulation to be determined were inappropriate.

Based on these findings, the present inventor previously disclosed in Japanese Patent Application Laid-Open No. 62-275556 that the flow rate starts from the point at which the center of the slab reaches a temperature corresponding to a solid fraction of 0.1 to 0.3. The area until the temperature reaches the limit solid fraction is continuously reduced at a rate of 0.5 mm/min or more and less than 2.5 mm/min per unit time until the center of the slab reaches the flow limit solid fraction. We proposed a continuous casting method in which substantially no reduction is applied in the region from the point where the temperature corresponds to the temperature corresponding to , until the solidus temperature is reached.

Furthermore, by carrying out numerous experiments, the inventor of the present invention found that, as previously proposed in Japanese Patent Application No. 1-120295, there is a solidification period in which concentrated molten steel violently accumulates in the center of the slab. It was found that preventing flow during the accumulation period of concentrated molten steel is most important for improving segregation, and the solidification period when the amount of concentrated molten steel is particularly large differs depending on the solidification structure.

Based on this result, we researched a light reduction method to further improve segregation, and found that at least 1
In the continuous casting method of molten metal, in which the slab is pulled out while being compressed by a pair of rolls, if the upper surface equiaxed crystal ratio is less than 5%,
The temperature at the center of the slab has a solid phase ratio of 0.25, preferably 0.35.
At least one pair of rolls is installed at any position in the solidification time range from a position corresponding to the flow limit solid phase ratio to a position corresponding to the flow limit solid fraction, preferably on the upstream side within the solidification time range, and If the reduction is made by an amount that compensates for the total solidification shrinkage, and if the top surface equiaxed crystal fraction is 5% or more, the temperature at the center of the slab will reach the flow limit from the position corresponding to the solid phase fraction of 0.1, preferably 0.15. At least one pair of rolls is installed at any position in the solidification time range up to a position corresponding to the solid phase ratio, preferably on the upstream side within the solidification time range, and compensates for the total solidification shrinkage within the solidification time range. We have proposed a simple and efficient light reduction method that is characterized by reducing the amount of reduction and is capable of reducing the reduction range.

[0014]

[Problem to be Solved by the Invention] However, in continuous casting operations, the casting speed often slows down or stops due to changing pots, top processing, or unexpected troubles, so slabs are not allowed to reach the rolling zone. arrive late or
It is very difficult to eliminate the occurrence of unsteady slabs whose casting speed has slowed or stopped in the rolling zone, and the segregation of these unsteady slabs is worse than in the steady part.

In order to prevent troubles with wire rods containing slabs for which the segregation improvement effect of light reduction is insufficient due to such fluctuations in casting speed, it is necessary to sufficiently reduce the worst segregation of slabs by heating during blooming. It is necessary to heat the bloom at a high temperature and for a long time in order to diffuse and make it uniform, which results in overaction for the steady section slab with good segregation.

[0016] In addition, high-temperature heating causes problems such as reduced yields such as iron loss and generation of decarburized layers in the heating furnace, and deterioration of workability. Establishing technology to reduce the amount of pieces as much as possible is an important issue. In order to improve these problems, it is necessary to further improve the segregation of the slab in the steady part and at the same time select a slab with worse segregation in the unsteady part.

[0017] As a method for separating and selecting these slabs with deteriorated segregation, the present inventor has determined the average casting speed in a specific solidification period range during solidification of the slab, and the solidification time at which rolling of the slab starts or when the slab is solidified. We have previously proposed a method for separating and selecting slabs in unsteady areas where the segregation improvement effect of light reduction is insufficient, depending on the average casting speed in a specific solidification period range during solidification.

In view of the above-mentioned problems, the present invention has been made as a result of research into a method for separating such slabs with deteriorated segregation.The present invention has made it possible to determine a specific solidification time range in which the average casting speed should be controlled, which conventionally required a great deal of effort to determine. We found a simple method for quantifying, accurately selected the parts of slabs with sufficient segregation improvement effect and good segregation.
We have now provided a continuous casting method that allows the blooming heating conditions to be kept at a low temperature and for a short time.

[0019]

[Means for Solving the Problems] The present invention provides a method for continuous casting of molten metal in which a slab is pulled out while being compressed by one or more pairs of rolls, and a slab located in a continuous casting machine when the casting speed is decelerated and stopped. Among these, the solidification time range of the slab with worsened segregation is quantified in advance from the solidification time when the casting speed of the slab with worsened segregation starts to be slowed down, and the average casting time during which the slab in question solidifies within the solidification time range is calculated. The method is characterized in that the segregation level is evaluated only based on the speed and the solidification timing at which the slab starts rolling, or the average casting speed during which the slab solidifies within the solidification timing range, and the blooming heating conditions and process are selected. This is a continuous casting method.

[0020]

[Function] The present invention will be explained in detail below along with its function.

As a result of research into the segregation determining factors when the casting speed slows down and stops as shown in the example of FIG. 1, the inventors found that the parts of the slab where segregation worsens are as shown in FIG. , ■It was found that this is the case.

[0022]■ In a slab that is in a specific solidification period range A to B (B>A) at the start of deceleration of the casting speed, there is a solidification period A' where segregation is the worst, and a portion where solidification is more advanced than B. The segregation of is the same as in the stationary part.

[0023] ■ Segregation in the part of the slab where the solidification time at the start of rolling is delayed is large, and there are parts with good segregation and parts with bad segregation. It has been revealed that slabs with worsened segregation due to a delay in the time of solidification at the start of rolling, as shown in item (■), can be improved by optimizing the rolling conditions.

Based on the phenomena described in (1) and (2) above, we researched a method for separating slabs with insufficient segregation improvement effect under light reduction, and found that solidification requires controlling the average casting speed when the slab passes through. The present invention was developed based on the knowledge that the timing range A to B corresponds to the solidification timing range A(A') to B at the start of deceleration of the segregation-prone slab shown in (2) above, as shown in Fig. 3. accomplished.

Therefore, the solidification time range A in which the casting speed should be controlled can be quantified in advance with high accuracy through experiments. The values of A(A') and B can be determined based on Equations 1 and 2 by quantifying the solidification time at the start of deceleration of a slab with worsened segregation as shown in Equation 4. Further, the rolling start solidification time of the slab can be determined based on equation 3 by quantifying the rolling start solidification time as shown in equation 5.

[0026]

[Equation 1] Solidification time on the upstream side of a slab with deteriorated segregation:
A=f(tsA)

[0027]

[Equation 2] Solidification time on the downstream side of a slab with deteriorated segregation:
B=f(tsB)

[0028]

[Equation 3] Solidification time when the slab reached the rolling zone:
C=f(tc) where f(t) is the relationship between coagulation time and coagulation time

00
29]

[Equation 4] Solidification time at the start of deceleration of a slab with deteriorated segregation: tsA
=LsA/VsA, tsB=LsB/VsB (
min)

[0030]

[Equation 5] tc: Time required for the slab to move to the rolling zone inlet roll (min) LsA: Distance from the mold meniscus at the start of deceleration of the slab part with worsened segregation (m) LsB: slab with worsened segregation Distance from the mold meniscus at the start of deceleration of the part (m)

The range between LsA and LsB is a range where the segregation of slabs is particularly severe due to deceleration or stopping of the casting speed. Also, VsA and VsB are the slab positions LsA and Ls
This is the average casting speed (m/min) up to the time of deceleration of B.

Note that A' is determined based on Equations 1 and 4 in the same way as A and B, by employing the values of LsA' and VsA' at the portion where segregation is the worst.

According to the present invention, the length of a slab determined to be a slab with deteriorated segregation varies depending on the width of the solidification time range A to B of the slab, which controls the average casting speed. In order to accurately separate slabs with deteriorated segregation and shorten the length of slabs determined to be deteriorated segregation, it is sufficient to narrow the solidification period range to be controlled, and in the example of FIG. The best yield is achieved by adopting B, but which solidification time to adopt as A or B depends on the degree of segregation of the obtained slab, the variation in segregation, and process capabilities such as the blooming conditions. They are considered to be different, and it is necessary to decide based on the merits when considering the entire process.

According to the method of the present invention described above, it is possible to easily quantify the solidification time range A to B of the slab whose average casting speed should be controlled, and it is possible to easily quantify the solidification time range A to B of the slab whose average casting speed is to be controlled. It is possible to separate one part with high accuracy.

[0035] By keeping the heating conditions for blooming of the slab with good segregation selected in this way at a low temperature and for a short time, it becomes possible to significantly save the energy used and the iron yield.

[0036] The solidification period of a slab can be quantified by the central solid fraction, shell thickness, unsolidified thickness, or unsolidified ratio, but here it is considered that the solidification period has the most influence on the generation of segregation. It was quantified by the central solid fraction, which is related to the increase in fluid flow resistance at the center of the slab.

The central solid phase ratio is calculated as a function of the temperature at the center of the slab as shown in Equation 4 above, and is the proportion of the solid phase existing at the center. The temperature at the center of the slab is calculated in advance by heat transfer calculations based on operating conditions, or calculated during casting based on conditions such as cooling of the slab and casting speed.

[0038] This central solid fraction depends on the casting speed, cooling conditions,
Once the slab size and steel type are determined, it is a function of solidification time,
It can be easily converted into shell thickness, unsolidified thickness, and unsolidified ratio, which are also functions of solidification time. Further, the average casting speed in the solidification period range in which the slab solidifies from A to B is determined from equation 6 below.

[0039]

[Equation 6] Center solidus ratio of slab = (Tl-T)/(Tl-Ts) where Tl: Liquidus temperature of molten steel (℃) Ts: Solidus temperature of molten steel (℃) T: Slab Center temperature (℃)

[0040]

[Equation 7] Average casting speed during solidification time from A to B = L/t (m/min) where L: Movement length of the slab during solidification from A to B ( m) t: Solidification time for the slab to solidify from A to B
(min)

[0041] According to the present invention, the blooming conditions and process can be adjusted according to the segregation level by separating the slabs that have sufficient segregation improvement effect due to the fluctuation of casting speed from the slabs that have insufficient segregation improvement effect. A continuous casting method is provided that can be selected and can obtain homogeneous steel materials with a high yield and with less energy than conventional methods.

[0042]

[Examples] The present invention will be explained below with reference to Examples.

Example 1 The schematic structure of the continuous casting machine in which the test was conducted is shown in FIG. 4, and a typical example of the composition of the cast molten steel is shown in Table 1 below. In FIG. 5, 1 and 2 are electromagnetic stirring devices, 3 is a rolling zone, 4 is a segment of a continuous casting machine, and 5 is a cast slab.

[0044]

[Table 1]

The relationship between the solid fraction at the center of the slab when the casting speed starts to slow down and the slab segregation observed after cooling is shown in Figure 3 above.
Shown below. Based on this result, the solidification period range in which the average casting speed should be controlled was determined to be 0.15 to 0.3 in terms of central solid fraction.

FIG. 3 shows the average casting speed in the solidification period range during which the slab solidifies from 0.15 to 0.3 at the center solid fraction, which corresponds to the slab part where segregation is worsening, and The central solid fraction when the piece reaches the reduction zone is also shown.

Further, FIG. 5 shows the conditions under which wire rod segregation is improved when rolling is performed under the same conditions of heating the entire amount of bloom at a lower temperature and shorter time than in the past. Deterioration of wire rod segregation is observed in slabs where the average casting speed is slower than in the steady zone and where the slab is rolled while the slab solidifies from 0.15 to 0.3 at the central solid fraction. The slab has an initial solid fraction of 0.18 or more.

[0048] Wire rod obtained when the blooming conditions for the unsteady part slab with bad segregation separated by the method of the present invention are the same as before, and the blooming conditions for the steady part slab with good segregation are set at a lower temperature and for a shorter time than before. The segregation is shown in Figure 6. All wire rod segregation is good,
It has been proven that a homogeneous steel material with no segregation can be obtained with lower energy than the conventional method.

Example 2 Typical compositions of the molten steel cast in this example are shown in Table 2 below.

[0050]

[Table 2]

FIG. 7 shows the relationship between the central solid fraction of the slab and the slab segregation observed after cooling when the casting speed started to be reduced. Based on this result, the solidification period range in which the average casting speed should be controlled was determined to be 0.25 to 0.5 in terms of central solid fraction.

FIG. 7 shows the average casting speed during which the slab solidified from 0.25 to 0.5 at the center solid fraction, which corresponds to the slab part where segregation is worsening, and The central solid fraction when reaching the reduction zone is also shown.

FIG. 8 shows the conditions under which the wire rod segregation is good when the blooming heating conditions are set to a low temperature and a short time. The average casting speed during the time when the solidified state of the slab is solidified to a center solid fraction of 0.24 to 0.5 is the center solid phase when the slab reaches the rolling zone inlet roll. Deterioration of segregation of slabs with a ratio of 0.24 or more is observed.

[0054] As in the case of Example 1, the blooming heating conditions for the unsteady part of the slab with poor segregation separated by the method of the present invention were the same as before, and the blooming conditions of the regular part of the slab with good segregation were the same as the conventional ones. As a result of using a lower temperature and shorter time, the wire segregation was improved throughout, proving that a homogeneous steel material without segregation can be obtained efficiently with less energy than conventional methods.

Example 3 This example deals with preventing the deterioration of segregation due to the delay in the solidification timing at the start of rolling, which is caused by deceleration of the casting speed. The molten steel composition, cooling conditions, etc. are the same as in Example 1.

[0056] The solid fraction at the center of the slab at the time of starting deceleration;
Figure 9 shows the relationship with slab segregation observed after cooling. Figure 9 shows the slab corresponding to the slab part where segregation is worsening.
The average casting speed during solidification from 0.15 to 0.3 in the center solid fraction and the center solid fraction when the slab reaches the rolling zone are also shown.

The slab with worsened segregation can be separated only by the average casting speed during which the slab solidifies from 0.15 to 0.3 in central solid fraction.

Example 4 Table 3 below shows the average casting speed determined by the method of the present invention for various steel types and solidification structures, and the solidification timing range A to B to be controlled and the slab speed at which segregation worsens. This shows the time at which pressure starts to solidify.

[0059]

[Table 3]

[0060] Based on the results in Table 3, the parts of the slab where the effect of improving segregation that occurs with deceleration of the casting speed is insufficient are selected,
It becomes possible to improve the blooming conditions of areas with good segregation at lower temperatures and in a shorter time.

Furthermore, as shown in Table 3, the solidification period range in which segregation worsens as the casting speed decreases varies depending on the steel type, solidification structure, and mild deterioration conditions. It is most efficient to quantify and use the method shown in the invention to quantify the solidification period range of the slab in which segregation worsens and the solidification period in which the slab reaches the rolling zone.

[0062]

[Effects of the Invention] As explained above, according to the continuous casting method of the present invention, sulfur and
It prevents the segregation of impurity elements such as phosphorus and manganese, and also reduces the average casting speed, which conventionally required a great deal of effort to determine.
We found a specific solidification time range to be controlled using a simple method of quantifying, accurately selected the areas of slabs with sufficient segregation improvement effect and good segregation, and determined the blooming heating conditions for the separated slabs in unsteady areas with worsened segregation. By changing the blooming heating conditions of the steady-state slab with good segregation to a lower temperature and shorter time than before, it is possible to obtain a homogeneous continuously cast slab without segregation with a high yield and with less energy than before.

[Brief explanation of the drawing]

FIG. 1 is a drawing showing an example of variation in casting speed in continuous casting.

FIG. 2 is a diagram illustrating the solidification timing range A(A') to B at the start of deceleration of a slab with deteriorated segregation that occurs due to deceleration.

FIG. 3 is a drawing showing a method for determining the solidification timing range A to B of the slab in which the average casting speed should be controlled in Example 1.

FIG. 4 is a drawing showing a schematic structure of a continuous casting machine used in the test.

FIG. 5 is a diagram showing conditions for good wire rod segregation when the whole amount is subjected to low-temperature, short-time blooming heating in Example 1.

FIG. 6 is a drawing showing the level of wire rod segregation in Example 1 when the blooming conditions of the good segregation region were set at low temperature and for a short time, and the slab with bad segregation remained as before.

FIG. 7 is a drawing showing a method for determining the solidification timing range A to B of the slab in which the average casting speed should be controlled in Example 2.

FIG. 8 is a diagram showing conditions for good wire rod segregation when the whole amount is subjected to low-temperature, short-time blooming heating in Example 2.

FIG. 9 is a drawing showing a method for determining the solidification timing range A to B of the slab in which the average casting speed should be controlled in Example 3.

[Explanation of symbols]

1, 2 Electromagnetic stirring device 3　　　　　Reduction zone 4 Segment 5　　　　Slab

Claims (1)

[Claims] Claim 1: In a continuous casting method for molten metal, in which slabs are pulled out while being compressed by one or more pairs of rolls, cast slabs with worsened segregation among the slabs located in the continuous casting machine when the casting speed is slowed down and stopped. The solidification time range of the slab with deteriorated segregation is quantified in advance from the solidification time when the slab casting speed starts to be slowed down, and the average casting speed and the reduction of the slab are determined in advance. A continuous casting method characterized in that the segregation level is evaluated only based on the start solidification time or the average casting speed during which the slab solidifies in the solidification time range, and the blooming heating conditions and process are selected.