JPH0437456A - Production of continuously cast slab having excellent internal quality - Google Patents

Abstract

PURPOSE:To produce a cast slab having excellent internal quality by setting plural steps of rolling reduction rolls having the specific diameter at end stage of solidifying the cast slab, setting the specific % of draft at each step of rolling reduction roll and executing the rolling reduction. CONSTITUTION:At the end stage of solidification containing unsolidified molten steel part 4 in the cast slab 3 continuously cast through a mold, the rolling reduction rolls 5 having diameter of 2-5 times the thickness of the cast slab 3 providing rolling reduction cylinders 6 are set in two or three steps. Then, by setting the draft of rolling reduction roll 5 in the first step to 1.5-4.0% and the draft of rolling reduction roll in the second step to 2.0-4.0%, the rolling reduction are executed. Further, by setting the draft of rolling reduction roll in the third step to 2.0-4.5%, the rolling reduction is executed. By this method, the cast slab having little center segregation and porosity and no internal cracking can be produced.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for producing continuously cast slabs with excellent internal quality, and particularly relates to a method for producing continuously cast slabs with improved center segregation and porosity. .

[Conventional technology]

Conventionally, positive segregation of carbon, etc., or so-called center segregation, has occurred in the center of continuously cast slabs (hereinafter referred to as slabs), causing problems in the quality of the slabs. This central segregation is generally thought to be generated as follows. In other words, the component elements of the molten steel in the solid-liquid coexistence phase are concentrated due to the distribution of solutes during solidification, and this concentrated molten steel flows through solidification shrinkage and hulling between rolls and accumulates in the center of the slab, resulting in the formation of a cast at the final stage of solidification. At the center of the piece, the accumulated concentrated molten steel just before solidification is sucked into the negative pressure section caused by solidification shrinkage and inter-roll bulging, and center segregation is generated.

On the other hand, based on this generation mechanism of center segregation, as a measure to improve center segregation, light pressure is applied by rolls to the final stage of solidification of the slab to suppress the generation of negative pressure areas caused by solidification shrinkage and bulging between the rolls. Therefore, techniques have been proposed to suppress the flow of the liquid phase of the solid-liquid coexistence phase as much as possible to reduce center segregation and porosity. For example, in Japanese Patent Publication No. 62-34460, in addition to stirring the molten steel by electromagnetic stirring to refine the casting structure, it is also proposed that the cast iron be at least 2 m away from the crater end of the in-phase line at the final stage of solidification of the slab toward the upstream side. The range of
A light reduction is carried out at a reduction rate of 0.5cm/m or more using reduction rolls set at a roll pitch of 450 mm or less, thereby preventing bulging from occurring in the slab and preventing the coexistence of solid and liquid in the center of the slab. A method is provided for preventing phasewise molten steel flow and producing segregation-free continuously cast slabs.

[Problem to be solved by the invention]

By the way, when setting the reduction rolls so that the roll pitch is 450 m or less, as in the above-mentioned method for manufacturing slabs, it is necessary to use rolls with a diameter of 450 m or less, which makes them difficult to use with small diameter rolls. Become. On the other hand, as one of the indicators necessary for improving center segregation, the difference in slab thickness before and after rolling [
The rolling reduction rate (i.e., rolling gradient (-so/m)), which is the total rolling reduction amount (an) divided by the length of the rolling zone (m), is generally used, and by this, each roll has a rolling reduction I (Mongolia) are equally multiplied.

Therefore, when the final stage of solidification of a slab slab with a width larger than its thickness is rolled down using a small diameter roll using the above-mentioned slab manufacturing method, center segregation and porosity are reduced and a good slab is obtained; When the rolling reduction of the rolls is large, internal strain increases, and slabs with internal cracks are sometimes produced.

On the other hand, when the slab is a plume or billet, the cross-sectional shape of the slab is closer to a square than that of a slab, so internal deformation is less likely to occur even if the final stage of solidification is rolled down with a small diameter roll. In order to obtain the effect of improving center segregation, it is necessary to reduce the slab by a large amount. However, if the amount of reduction is large, large tensile strain (hereinafter referred to as internal strain) will occur in the solidified interface and the non-strength solidified shell in the vicinity, making internal cracks more likely to occur.In addition, the reduction will cause flattening of the slab. If the slab is too thick, it will interfere with the rolling process in the next step.For this reason, when the slab is a plume or billet, it has not yet been put to practical use.

The present invention has been made in view of the above problems, and includes:
The purpose is to provide a method for manufacturing slabs, plumes, etc., which not only reduce center segregation and porosity but also have no internal cracks and excellent internal quality.

[Means to solve the problem]

In order to achieve the above object, the method for manufacturing a continuously cast slab with excellent internal quality according to the present invention provides a method for manufacturing a continuously cast slab with excellent internal quality. 2 to 3 stages of rolling down rolls having a diameter of
Rolling is performed by setting the rolling reduction ratio of the second stage rolling roll to 2.0 to 4.5%.

When rolling down using the third stage rolling roll, it is desirable to set the rolling reduction rate to 2.0 to 4.5%.

The present invention will be explained in detail below.

Conventionally, a slab pulled out from a mold in continuous casting has unsolidified molten steel inside, and the static pressure of the molten steel causes the slab to bulge, which is called bulging. Therefore, in order to suppress this bulging, the guide rolls in the cooling zone under the mold should be made with the smallest possible roll diameter (usually less than 1.5 times the thickness of the slab). The spacing is made as narrow as possible. For this reason, it is considered preferable to provide the roll-down rolls that roll down the final stage of solidification of the slab as described above, with the distance between the rolls as narrow as possible. In this way, if the roll diameter of the reduction roll is made small, in order to reduce center segregation and porosity of the slab, it becomes necessary to increase the rolling reduction ratio of the slab, but on the other hand, , when the reduction ratio is large,
Since the internal strain increases, there is a concern about internal cracking of the slab.

Therefore, as a result of intensive research by the present inventors, it was found that at the final stage of solidification of slabs that are continuously cast using molds,
Two to three stages of reduction rolls having a diameter five times as large are arranged, and the reduction ratio of the first reduction roll is 1.5 to 4.0.
%, and the rolling reduction rate of the second rolling roll is 2.0 to 4.5%.
It has been discovered that by setting the steel sheet to 100% and rolling down, it is possible to not only reduce center segregation and porosity, but also to produce slabs without internal cracks. Furthermore, by adding a reduction of 2.0 to 4.5% by the third stage reduction roll,
It was discovered that slabs with improved center segregation can be produced.

In this way, when the reduction roll is a large-diameter roll, as shown in Fig. 1, for the same reduction rate (reduction amount Δh/slab thickness before reduction h x 100%), the small-diameter roll (1) In comparison, the large diameter roll (2) has a longer contact length (f
fi, -j! □), and as a result, the contact area increases, so the surface of the large diameter roll (2) and the slab (
The surface layer of the slab (3) that is in contact with the surface of the large-diameter roll (2) where the surface friction of 3) can be effectively obtained is
The deformation in the longitudinal and width directions of the small diameter roll (1)
This will result in greater suppression than in the case of . In this way, when the deformation of the slab (3) in the longitudinal direction and the width direction is greatly suppressed, a large amount of deformation in the rolling direction is obtained, and therefore, the large diameter roll (2) In this case, the rolling reduction ratio is made smaller than in the case of the small diameter roll (1), and the same amount of deformation in the rolling direction as in the case of the small diameter roll (1) can be obtained. In addition, by being able to reduce the rolling reduction ratio,
Since the flattening of the slab (3) due to rolling is small, rolling can be applied to thick slabs such as blooms without interfering with the rolling process in the next step.

Next, in the present invention, the diameter of the reduction roll is set to 2 to 2 of the thickness of the slab.
Explain the reasons for specifying the range in 5.

The graph shown in Figure 2 was obtained by numerical analysis using the finite element method, and shows that the final solidification stage of a slab with a thickness of 300+ sm is rolled at the same reduction rate (approximately 2.7%) and the roll diameter is 400 mm.
, 600 mm, and 1000 mm and are graphs showing changes in the internal strain of the slab, the amount of cross-sectional area compression of the unsolidified part, and the amount of deformation of the unsolidified part. As is clear from Figure 2, if the rolling reduction ratio is the same, when the roll diameter becomes more than twice the ratio of the slab thickness, the amount of cross-sectional area compression of the unsolidified area and the amount of deformation of the unsolidified area will increase dramatically. However, if it is less than double, the cross-sectional area pressure of the unsolidified area will increase! 1, the amount of deformation in the unsolidified portion is small, and no improvement effect on center segregation etc. can be expected. For this reason, the diameter of the reduction roll was specified to be at least twice the thickness of the slab. On the other hand, the reason why the upper limit of the diameter of the reduction roll is specified to be 5 times or less than the thickness of the slab is because if the diameter is more than 5 times, the distance between the reduction rolls becomes too large, and bulging of the slab occurs between the reduction rolls. This is because the lowering effect of the final stage of solidification cannot be obtained sufficiently.

Next, the reason for specifying the reduction ratio of the reduction rolls arranged in two or three stages in the present invention will be described.

The reason why the rolling reduction ratio of the first rolling roll is specified to be 1.5 to 4.0% is because if the rolling reduction ratio is less than 1.5%, the rolling reduction is small and segregation, particularly V segregation, occurs. On the other hand, if the reduction ratio exceeds 4.0%, the reduction becomes too thick (excessively resulting in internal cracks).
The reduction ratio of the reduction roll of the stage was specified to be 1.5 to 4.0%.

The reason why the rolling reduction ratio of the second stage reduction roll was specified to be 2.0 to 4.5% is because if the rolling reduction ratio is less than 2.0%, the rolling reduction is small and segregation and retention occur, as in the case of the first stage reduction roll. ■ Segregation occurs. On the other hand, if the reduction rate exceeds 4.5%, the reduction becomes too large, causing internal cracks, and unsolidified molten steel flows upstream, causing reverse segregation. For this reason, the rolling reduction ratio of the second rolling roll was specified to be 2.0 to 4.5%.

Furthermore, in the present invention, if a third stage reduction roll is provided to apply a reduction, it is preferable to apply a reduction of 2.0 to 4.5% in terms of reduction ratio in addition to the above-mentioned first and second stage reductions. The reason is that although the reduction by the second stage reduction roll is sufficient, center segregation and porosity are further improved.
If the rolling reduction rate is less than 2.0%, no improvement effect can be obtained, and 4.
This is because, if it exceeds 5%, the reduction becomes too large, causing internal cracks, and unsolidified molten steel flows upstream, causing inverted V segregation.

In addition, the position at which the reduction is applied is the final stage of solidification of the continuously cast slab, but preferably the solidification end point (crater The length (a) to the end (end) is good, and the first rolling roll is preferably provided in the range of a - a / 2.

(Solidification formula) χ=K (A/V)-""χ: Solidified shell thickness (■) K: Solidification coefficient (ms・5in-"") A: Distance from meniscus (m) ■: Casting speed (m/win) [Example] The present invention will be described in detail below.

FIG. 3 is a schematic diagram of a three-stage rolling down device applied to the method of the present invention, in which (3) is the slab in the final stage of solidification, (4) is the unsolidified molten steel part within the slab, (5) shows a roll-down roll with a roll diameter of 1000+u+, and (6) shows a cylinder for rolling down the upper roll-down roll (5).

Using the steel types shown in the table below, slabs with a cross section of 300 x 430 mm were continuously produced using the three-stage rolling device described above.

The manufacturing conditions at this time were: casting speed: 0.69 m/min, reduction roll diameter: 1000 mm, roll pitch 1.2.
m. Pressure roll position (distance from meniscus) 1
Tier: 22.6m, 2nd tier: 23.8m, 3rd tier: 25
．． It was Om.

Under the above conditions, the rolling reduction ratio of the first rolling roll was varied, and internal cracks at this time were investigated. The results are shown in Figure 4. According to the figure, internal cracks occurred when the reduction ratio of the reduction roll exceeded 4.0%, and no internal cracks occurred when the reduction ratio of the reduction roll was 4.0% or less.

Next, add a first stage reduction of 1.5 to 4.0%,
Furthermore, the rolling reduction ratio by the second rolling roll was varied, and the maximum C segregation ratio (maximum C segregation/average C segregation) of the slab obtained thereby was investigated. For comparison, a similar investigation was conducted with the first stage rolling reduction being less than 1.5%. These results are shown in FIG. According to the figure, the former is within the limit where troubles due to segregation (for example, wire breakage during wire drawing) can be prevented in the processing steps after casting when the rolling reduction in the second stage is in the range of 2.0 to 4.5%. Segregation was improved, but
In the latter case, segregation was not sufficiently improved even when the rolling reduction was in the range of 2.0 to 4.5%.

Next, the first stage reduction is 1.5 to 4.0%, the second stage is 2.0 to 4.5%, and the third stage reduction roll is applied to the reduction ratio. The maximum C segregation ratio of the obtained slab was investigated. The results are shown in Part 6.

As shown in the figure, when the rolling reduction was in the range of 2.0 to 4.5%, segregation was improved to a greater extent than the segregation improvement effect up to the second stage.

Furthermore, the slab obtained by the above-mentioned second rolling was rolled to a diameter of 81111, and the cross section of the obtained rolled product was evaluated by macro etching.

The evaluation results are shown in Figure 7. As is clear from the figure, in the examples of the present invention, the number of (good) marks increased, whereas in the comparative examples, it was only (slightly good).

〔Effect of the invention〕

As described above, according to the method for manufacturing continuously cast slabs with excellent internal quality according to the present invention, slabs, blooms, etc., with relatively low center segregation and porosity, and with excellent internal quality without internal cracks can be produced. Pieces can be manufactured.

[Brief explanation of drawings]

Figure 1 is an explanation for comparing the method of the present invention with the conventional method, and Figure 2 shows the roll diameter, internal strain of the slab, and A graph diagram showing the relationship between the amount of cross-sectional area compression of the solidified part and the amount of deformation of the unsolidified part,
Fig. 3 is a schematic diagram of a three-stage rolling down device applied to the method of the present invention, Fig. 4 is a diagram showing the relationship between the rolling reduction rate by the first stage rolling roll and internal cracks in the slab, and Fig. 5. is a diagram showing the relationship between the reduction rate by the second reduction roll and the maximum C segregation ratio of the slab when the second reduction is applied in addition to the first reduction,
Figure 6 shows the relationship between the rolling reduction ratio by the third rolling roll and the maximum C segregation ratio of the slab when the third rolling is added in addition to the first rolling and second rolling. FIG. 7 is an evaluation diagram obtained by macro-etching the cross sections of rolled products obtained by the method of the present invention and the comparative method. 1 Small diameter roll 2 Large diameter roll Cast slab reduction roll Unsolidified molten steel part reduction cylinder contact length Δh Reduction amount

Claims (2)

[Claims] (1) Two reduction rolls with a diameter 2 to 5 times the thickness of the slab are installed at the final stage of solidification of the slab that is continuously cast by the mold.
In addition to arranging three stages, the rolling reduction ratio of the first stage reduction roll is set to 1.5 to 4.0%, and the rolling reduction ratio of the second stage reduction roll is set to 2.0 to 4.5%. A method for producing continuous cast slabs with excellent internal quality, characterized by rolling. (2) The reduction ratio of the third stage reduction roll is 2.0 to 4.
The method for manufacturing a continuously cast slab with excellent internal quality according to claim 1, characterized in that the rolling reduction is set at 5%.