JPH04313453A - Continuous casting method - Google Patents

Abstract

PURPOSE:To shorten length of a cast slab part, at where segregation deteriorates, by selecting the cast slab position, where segregation improving effect under light rolling reduction due to variation of the casting speed is insufficient, in order to select blooming heating condition and process in light rolling reduction continuous casting. CONSTITUTION:This continuous casting method is the one, in which solidified time in the segregation deteriorated case slab developed according to reduction of casting speed and stop of the casting is selected with the average casting speed in the range of the specific solidifying time during solidifying by quantifying with center solid phase ratio, shell thickness, unsolidified thickness or unsolidified ratio, and the blooming rolling condition and proess according to the segregation level is selected. The sufficient cast slab part and the insufficient cast slab part to the segregation improving effect under the light rolling reduction according to variation of the casting speed, can be accurately separated, and the blooming heating condition can be selected according to the segregation level, and the continuous cast slab having uniform quality can be obtd. in good yield.

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] It is thought that such component segregation occurs because residual molten steel flows at the final stage of solidification due to solidification contraction force, washes out 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.

Attempts have been made to improve segregation by reducing slabs, and in the continuous casting process,
A method is known in which the core temperature of the slab is reduced at a constant rate greater than the amount that guarantees solidification shrinkage of the slab during the period from the liquidus temperature to the solidus temperature.

[0009] However, conventional continuous casting methods have problems such as hardly any segregation improvement effect being observed depending on the conditions, and in some cases, segregation may even worsen, making it difficult to sufficiently improve component segregation. there were.

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 when the temperature corresponds 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 top 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 0.3.
At least one pair of rolls is installed at any position in the solidification time range from the position corresponding to No. 5 to the position corresponding to the flow limit solid fraction, preferably on the upstream side within the solidification time range, and within the solidification time range. Reduce the amount to compensate for the total solidification shrinkage of
In addition, when the top surface equiaxed crystallinity is 5% or more, the solidification time range is from the position where the temperature at the center of the slab corresponds to the solid phase ratio of 0.1, preferably 0.15 to the position corresponding to the flow limit solid phase rate. A reduction range, characterized in that at least one pair of rolls is installed at any position, preferably upstream within the solidification time range, and the rolls are rolled down by an amount that compensates for the total solidification shrinkage within the solidification time range. We have proposed a simple and efficient light reduction method that can reduce the

[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 problems 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 make sure that the segregation components of the unsteady part of the slab, where segregation is the worst, are also sufficient. It is necessary to heat the bloom at high temperature and for a long period of time to enable diffusion, which results in overaction for steady-state slabs with good segregation.

[0016] In addition, high-temperature heating causes problems such as a decrease in yield due to iron loss in the heating furnace and generation of a decarburized layer, and deterioration of workability. In order to improve these problems, it is necessary to further improve the segregation of the slab in the steady area, and at the same time to separate the slab with worsened segregation in the unsteady area and select the blooming heating conditions according to the segregation level of the slab. is an important issue that needs to be solved.

[0017] As a method for separating and selecting these slabs with deteriorated segregation, the present inventors determined the average slab speed in a specific solidification period range during the solidification of the slab, and the light reduction based on the solidification timing of the start of reduction of the slab. We previously proposed a continuous casting method in which slabs in unsteady areas with insufficient segregation improvement effects are separated and the blooming heating conditions are selected.

[0018] As a result of further research on the light reduction method, the present inventors found that the worsening of segregation caused by a delay in the solidification time at the start of reduction can be improved by further optimizing the light reduction conditions. In the case of such a new light reduction method, even slabs for which the segregation improvement effect by light reduction is sufficient in the separation selection method of slabs with deteriorated segregation will be judged as slabs with deteriorated segregation, making it difficult to judge that the yield is good. It is necessary to improve the yield by identifying slabs with deteriorated segregation with even greater precision, rather than using a method.

In view of the above-mentioned problems, the present invention has further advanced the research on the factors determining the segregation of slabs whose casting speed has fluctuated, and as a result, it is possible to make slabs as long as possible by keeping the blooming heating conditions at a low temperature and for a short time. We have now provided a continuous casting method that allows accurate determination.

[0020]

[Means for Solving the Problems] The gist of the present invention is as follows.

(1) In a continuous casting method for molten metal in which a slab is pulled out while being compressed by one or more pairs of rolls, the solidification time of the slab is determined by the center solid fraction, shell thickness, unsolidified thickness, or unsolidified ratio. Quantify and select slabs with worsened segregation that occurs due to deceleration and stoppage of casting speed, based on the average casting speed in a specific solidification period range during solidification of the slab, and apply blooming conditions and conditions according to the segregation level. This is a continuous casting method characterized by the selection of processes.

(2) In the continuous casting method described in (1) above, segregation occurs at the average casting speed while the slab solidifies to a central solid fraction of 0.1 to 0.6, or as the casting speed slows down. Slabs with worsened segregation are separated based on the average casting speed in the solidification period range of the slab that is most degraded, and the blooming conditions and process are selected according to the segregation level.

[0023]

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

[0024] As a result of our research on the segregation determining factors when the casting speed slows down and stops as shown in the example of Fig. 1, we found that the slab with worsened segregation as shown in Fig. 2 Specific solidification time range A to B during solidification of slab
It was found that the average casting speed of the slab was reduced, and the rolling start time C of the slab was delayed.

[0025] As a result of further research into the segregation determining factors, the present inventor found that among the above-mentioned slabs with worsened segregation, the slabs whose segregation worsened due to a delay in the rolling start time C of the slabs were found to be affected by optimization of light rolling conditions. The present invention was accomplished by discovering that improvements can be made by.

In other words, if the light reduction conditions are set appropriately,
The present invention was achieved based on the finding that the segregation-degraded slab that occurs as the casting speed slows down can be separated with high accuracy only by the average casting speed in a specific solidification period range A to B during the solidification of the slab. Here, the length of a slab determined to be a slab with deteriorated segregation becomes longer as the solidification time range A to B of the slab, which controls the average casting speed, is widened.

In order to accurately separate slabs with deteriorated segregation and to shorten the length of slabs that are determined to be slabs with deteriorated segregation, the solidification period range should be controlled so that only slabs with the worst segregation can be separated. All you have to do is narrow A to B. Which solidification period to adopt as A or B is considered to differ depending on the degree of segregation of the obtained slab, the variation in segregation, and process capabilities such as the blooming heating conditions, and is determined based on the merits when considering the entire process. There is a need to.

Note that the specific solidification time range A to B during which the slab in question, whose average casting speed should be controlled, is solidified can be determined in advance based on data on changes in casting speed over time and data on the position of slabs where segregation has worsened. .

According to the method of the present invention shown above, FIG.
As shown in Figure 2, it is possible to accurately separate the parts of the slab where segregation is worsening only by the average casting speed during which the slab solidifies from A to B. As a result, it becomes possible to accurately select slabs with good segregation as shown in FIG. By setting the heating conditions for blooming of the selected slab with good segregation at low temperature and short time, it is possible to significantly save the energy used and the iron yield.

[0030] The solidification period of a slab can be quantified by the central solid fraction, shell thickness, unsolidified thickness, or unsolidified ratio, but here, solidification time is considered to have the most influence on the generation of segregation. , was quantified by the central solid fraction, which is estimated to be related to the increase in fluid flow resistance at the center of the slab.

The central solid phase ratio can be calculated as a function of the temperature at the center of the slab, as shown in the example of Equation 1 below, 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.

[0032] 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, and 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 by Equation 2.

[0033]

[Equation 1] 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 (℃)

[0034]

[Equation 2] 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)

[0035] According to the present invention, the effect of improving segregation due to light reduction that occurs with fluctuations in casting speed makes it possible to accurately separate sufficient slabs from insufficient slabs, and to separate the slabs according to the segregation level. A continuous casting method is provided in which heating conditions can be selected and a homogeneous steel material can be obtained with a high yield and with less energy than conventional methods.

[0036]

[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.

[0038]

[Table 1]

The light reduction conditions are equiaxed crystal solidification and a central solid phase ratio of 0.
This is a light reduction method with a reduction gradient of 0.02 to 0.1 of 0.7 mm/m, a center solid fraction of 0.1 to 0.7, and a total reduction of 12 mm.

As shown in Figure 2, slabs with worsened segregation are slabs whose average casting speed during solidification from 0.15 to 0.3 at the center solid fraction is slower than in the steady zone. Therefore, the segregation of slabs whose solidification time at the start of rolling is delayed is the same as in the steady state.

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 when the central solid fraction of the slab corresponding to the wire rod is between 0.15 and 0.15.
This is a slab whose average casting speed during solidification was slower than that in the steady zone, and no deterioration of slab segregation due to the delayed start of rolling solidification was observed.

[0042] Wire rod obtained when the blooming conditions of the unsteady part slab with bad segregation separated by the method of the present invention are the same as before, and the blooming conditions of 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 A typical example of the composition of the molten steel cast in this example is shown in Table 2 below.

[0044]

[Table 2]

The light reduction conditions were columnar crystal solidification and a central solid fraction of 0.
05~0.25 rolling gradient 1mm/m, center solid phase rate 0
．． This is a light reduction method with a total reduction of 16 mm between 25 and 0.7 mm.

As shown in FIG. 7, slabs with worsened segregation are slabs whose average casting speed during solidification from 0.25 to 0.5 at the center solid fraction is slower than in the steady zone. Therefore, the segregation of slabs whose solidification time started at a later stage is the same as in the steady state.

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. If the average casting speed is reduced while the slab solidifies to a central solid fraction of 0.25 to 0.5, deterioration of segregation is observed.

As in the case of Example 1, the blooming heating conditions of the unsteady part slab with bad segregation separated by the method of the present invention were kept as before, and the blooming conditions of the steady part slab with good segregation were changed as before. 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 Table 3 below shows conditions for worsening segregation in various steel types and solidification structures. B
) is shown.

[0050]

[Table 3]

[0051] Based on the results in Table 3, parts of the slab where the effect of improving segregation that occurs due to 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.

[0052]

[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 has the effect of improving segregation due to light reduction that occurs due to fluctuations in casting speed, making it possible to accurately separate slabs with sufficient and insufficient slabs. The blooming heating conditions can be selected according to the level, and homogeneous continuously cast slabs can be obtained with a high yield and with less energy than conventional methods.

[Brief explanation of drawings]

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

[Fig. 2] A drawing showing the average casting speed during the time when slabs with worsened segregation solidify from A to B in a light reduction method in which measures were taken to prevent deterioration of segregation due to a delay in the start of reduction. be.

FIG. 3 is a drawing showing a comparison of the length of a slab determined to be a slab with deteriorated segregation between the method of the present invention and the conventional method.

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

FIG. 5 is a drawing 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, where the blooming condition of the good segregation area selected by the method of the present invention was set to a low temperature and short time, and the slab with bad segregation was kept as before.

FIG. 7 is a drawing showing the average casting speed during solidification from A to B of a slab corresponding to a slab with worsened segregation in Example 2.

FIG. 8 is a drawing showing light rolling conditions under which wire rod segregation is good when the whole amount is subjected to short-time blooming heating at a low temperature in Example 2.

[Explanation of symbols]

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

Claims (2)

[Claims] [Claim 1] In a continuous casting method of molten metal in which a slab is pulled down while being rolled down by one or more pairs of rolls, the solidification time of the slab is determined by the central solid fraction, shell thickness, unsolidified thickness, or unsolidified ratio. Select slabs with worsened segregation that occurs due to deceleration and stoppage of casting speed according to the average casting speed in a specific solidification period range during solidification of the slabs, and apply blooming conditions and processes according to the segregation level. A continuous casting method characterized by selecting. 2. The average casting speed while the slab solidifies to a central solid fraction of 0.1 to 0.6, or the average casting speed of the slab corresponding to the slab where segregation becomes the worst as the casting speed decreases. 2. The continuous casting method according to claim 1, wherein slabs with worsened segregation are separated by an average casting speed in a solidification period range, and blooming conditions and steps are selected according to the segregation level.