JP2008093711A - Continuous casting method of steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain sufficient improvement effect of internal quality of steel without requiring complicated device configuration. <P>SOLUTION: In the continuous casting method of the steel, the amount and the thrusting force of rolling reduction are controlled for each pair of reduction rolls or for each segment including a plurality of pairs of reduction rolls, when conducting rolling-reduction of a cast slab 2 along a thickness direction using a plurality of pairs of reduction rolls in the upstream side and the downstream side of a crater end where solidification in the inner part of a cast slab is completed. When applying rolling-reduction to the cast slab 2, the rolling-reduction is conducted such that the amount of the rolling-reduction of the cast slab 2 executed by each of four pairs of reduction rolls or each of segments 1a, 1b reaches the target amount of rolling-reduction. Thereafter, the pressure thrusting value acting on each of four pairs of reduction rolls or each of segments 1a, 1b is controlled so as to become the target pressure thrusting value, by increasing or decreasing the amount of the rolling-reduction of the cast slab 2 executed by each of four pairs of reduction rolls or each of segments 1a, 1b. Thus, generation of center segregation and porosity are effectively prevented, so that a cast slab having excellent internal quality is obtained. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a continuous casting method of steel intended to reduce segregation and porosity generated at the center of a cast slab for carbon steel, stainless steel, and high alloy steel.

  When a slab is manufactured by continuous casting, an internal defect called center segregation or center porosity (hereinafter, simply referred to as porosity) occurs in the thickness center portion.

Among these, center segregation is a phenomenon in which molten steel components such as C, S, P, and Mn are concentrated in the central portion in the thickness direction, which is the final solidified portion of the slab, and is segregated positively. It becomes.
On the other hand, the porosity is generated because the molten steel hardly flows in the final solidified portion, and solidification is completed without the molten steel being replenished in a narrow gap caused by volume shrinkage during solidification.

  This porosity, for example, the porosity generated in the bloom slab for hard steel wire rod, remains as a defect in the center of the wire rod after hot rolling. When a wire rod in which such a defect remains is cold drawn, a disconnection accident referred to as “cappy rupture” occurs.

  Also, when the slab with porosity is hot-rolled and processed into a steel bar, the porosity of the slab remains as a defect in the center of the steel bar. Defects called cracks occur.

  As described above, a slab in which an internal defect called center segregation or porosity has occurred cannot obtain a product with good internal quality even if it is rolled.

Therefore, a method for preventing the occurrence of such center segregation and porosity is employed, and among these, a light reduction method for a slab is common. As a specific example of the control of the amount of reduction under light pressure, the roll group in contact with the slab is pressed through a hydraulic cylinder, and the roll group is positioned by an insertion spacer that suppresses pressing of the hydraulic cylinder. A control method is disclosed in Patent Document 1.
Japanese Patent Laid-Open No. 3-90261

  However, in the method disclosed in Patent Document 1, the amount of roll reduction is determined by the thickness of the insertion spacer. Therefore, the thickness of the insertion spacer depends on changes in the operating conditions when trying to reduce the slab, such as slab dimensional variation, slab hardness variation due to changes in ambient temperature, and slab material. However, the thickness of the insertion spacer cannot be changed.

  Therefore, in recent years, by inserting a plurality of insertion spacers before rolling the slab and making the position of the roll adjustable in multiple stages, it is possible to fix the roll at a preferred position and lightly reduce it. ing.

  By the way, it is necessary to change the position of the roll in accordance with the state of reduction to the slab, so the insertion spacer is frequently inserted and removed, but once the insertion spacer is removed, it can be inserted again. difficult. Therefore, in order to insert the insertion spacer online, the device configuration becomes complicated and excessive equipment is required.

As another method, Patent Document 2 discloses an apparatus that changes the position of light pressure reduction in response to fluctuations in operating conditions. That is, a method is disclosed in which the position of a roll group in contact with a slab is monitored to control a hydraulic cylinder, and the roll group is lightly reduced while adjusting the position of the roll group to a predetermined position.
Japanese Patent Laid-Open No. 5-8004

  By the way, in continuous casting, even if the segments are at the same position due to fluctuations in casting speed, outside air temperature, and the like, the thickness of the solidified shell and the central solid phase ratio change, so that the appropriate reduction amount changes.

  However, since the method disclosed in Patent Document 2 is controlled so as to always give a constant amount of reduction, depending on the slab situation, internal cracking may occur due to overpressure, or conversely, sufficient due to insufficient reduction. There is a problem that the quality improvement effect cannot be obtained.

  As described above, in the method disclosed in Patent Document 1, although the reduction amount is constant and stable as compared with the method disclosed in Patent Document 2, in order to follow the fluctuation of casting conditions, the device configuration Becomes complicated and requires excessive equipment.

  On the other hand, in the method disclosed in Patent Document 2, although the reduction amount can be arbitrarily adjusted, control is performed so as to always provide a constant reduction amount. There is a problem that the quality improvement effect cannot be obtained and stability is applied.

  The problem to be solved by the present invention is that in the conventional continuous casting method, in order to reduce the segregation and porosity generated in the center of the slab, the apparatus configuration becomes complicated and excessive equipment is required. Or a sufficient internal quality improvement effect cannot be obtained and stability is applied.

The steel continuous casting method of the present invention is
In order to obtain a sufficient quality improvement effect without requiring a complicated device configuration,
When rolling down the slab in the thickness direction using a plurality of reduction roll pairs on the upstream side and downstream side of the crater end where solidification inside the slab is completed, for each reduction roll pair or a plurality of reduction roll pairs A method of continuously casting steel by controlling the amount of reduction and the reduction driving force for each arranged segment,
When rolling down the slab,
First, the slab is squeezed so that the reduction amount of the slab by each reduction roll pair or each segment becomes the target reduction amount,
Thereafter, by increasing or decreasing the reduction amount of the slab by each rolling roll pair or each segment, it is possible to control so that the pressure driving value acting on each rolling roll pair or each segment becomes the target pressure driving value. The most important feature.

  The present invention does not control the amount of reduction that requires a complicated apparatus configuration, but controls the pressure driving value acting on each reduction roll pair or each segment to be within a set range, thereby reducing the reduction of the slab. The optimum amount of reduction can be applied according to the change of the situation at the time, the occurrence of center segregation and porosity can be effectively prevented, and a slab having good internal quality can be obtained.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the accompanying drawings together with the processes until the invention is established.

  In order to solve the above-described problems, the inventors have studied a method for eliminating the occurrence of center segregation and porosity using a reduction technique based on the conventional problems. As a result, it was found that controlling the pressure value applied to the slab via the rolling roll pair (each segment) by the rolling cylinder is extremely effective in reducing the center segregation and the porosity, and the present invention has been realized. It came.

  As described above, the conventional reduction control method is used to reduce the size of the slab, the hardness of the slab due to changes in ambient temperature, and the material of the slab. In order to change the amount of reduction according to the change of conditions, the equipment configuration becomes complicated, and excessive equipment is required, or it is difficult to give the amount of reduction according to the slab condition even if it is changed. there were.

  Thus, as a result of earnest research on what is controlled as an index other than the reduction amount, the inventors have made extensive studies, and as a result, pressure values applied to each reduction roll pair or each segment, for example, applied to a hydraulic cylinder If the pressure value could be controlled within the set range, it was confirmed that the internal quality was stable even when the operating conditions were changed.

The present invention has been made based on the above findings,
When rolling down the slab in the thickness direction using a plurality of reduction roll pairs on the upstream side and downstream side of the crater end where solidification inside the slab is completed, for each reduction roll pair or a plurality of reduction roll pairs A method of continuously casting steel by controlling the amount of reduction and the reduction driving force for each arranged segment,
When rolling down the slab,
First, the slab is squeezed so that the reduction amount of the slab by each reduction roll pair or each segment becomes the target reduction amount,
After that, by increasing or decreasing the rolling reduction amount of each slab by each rolling roll pair or each segment, the steel is controlled so that the pressure driving value acting on each rolling roll pair or each segment becomes the target pressure driving value. It is a continuous casting method.

This steel continuous casting method according to the present invention is, for example, shown on the upstream side and downstream side (positions indicated by A to D in FIG. 4) of the crater end where solidification inside the slab of the continuous casting equipment shown in FIG. 1 to 3 is carried out by installing a reduction device.
1 is a view from the top of the segment, FIG. 2 is a view and a perspective view from the side of the segment, FIG. 3 is a view for explaining a configuration for controlling the amount of reduction, and FIG. 4 is a view showing a continuous casting facility It is.

  The segments 1a and 1b shown in FIGS. 1 to 3 are arranged on the upstream side and the downstream side of the crater end, which is indicated by A to D in FIG. Then, at least two pairs or more (in the example of FIG. 2, six pairs of six on the upper frame 3a and the lower frame 3b) are attached so as to be freely rotatable, and the four corners of the upper frame 3a are mounted. Is provided with a cylinder 5 with position control. The cylinder 5 with position control is called a so-called hydraulic stepping cylinder, and is widely used as a mold width variable device for continuous casting equipment.

  The four cylinders 5 installed at the four corners of the upper frame 3a can be controlled independently. For example, by providing a servo valve 8 in the middle of a pipe 7 connecting the hydraulic unit 6 and the cylinder 5, the amount of oil supplied to each cylinder 5 is controlled, and the amount of rod movement of each cylinder 5 is controlled. By changing the inclination of the upper frame 3a by such control, the slab 2 can be tapered or stepped down.

  In the present invention, when the slab 2 is reduced by the segments 1a and 1b, the reduction starts by position control so that a target reduction amount is obtained. Then, after each segment 1a, 1b reaches a predetermined reduction position, for example, the cylinder 5 is always set so that the pressure driving value acting on each segment 1a, 1b falls within the target pressure driving value range. Control is performed, and casting is performed while increasing or decreasing the amount of rolling of the slab 2 by the segments 1a and 1b.

  For example, when the casting speed is increased and the unsolidified thickness at a predetermined reduction position is increased, if the amount of reduction is constant, liquid phase squeezing due to reduction is insufficient and segregation remains. If the reduction propulsion value is made constant as in the method, the amount of reduction actually applied increases, and segregation is improved.

  As the target pressure propulsion value, set the pressure value calculated by statistical analysis from the data of the actual pressure value when the quality of internal quality (for example, slab center segregation degree, porosity occurrence etc.) is good Is desirable. Alternatively, it is desirable to set the target pressure propulsion value so that the standard deviation of the actual pressure value is within 1σ.

  The control range of the target pressure propulsion value is not particularly limited because it differs depending on the steel type, but according to the experimental results of the inventors using high carbon steel, the target pressure propulsion value should be ± 3 tons. The desired result was obtained. When exceeding the target pressure propulsion value +3 tons, internal cracking occurred due to overpressure. Moreover, when the target pressure propulsion value was less than -3 tons, there was a case where the reduction was insufficient. This value does not change even if the shape of the slab is different, and is the same whether the segment is controlled by the roll-down roll pair alone or the segment.

  In the present invention, in the same segment, the target pressure propulsion values of the inlet two axes and the outlet two axes (see FIG. 2B) with respect to the casting direction may be set to the same value, respectively. A separate value may be set for each axis.

Hereinafter, experimental results performed to verify the effects of the present invention will be described.
Using the reduction device and continuous casting equipment shown in FIGS. 1 to 4, high carbon steel (mass%, C: 1.0%, Si: 0.2 to 0.3%, Mn: 0.3 to 0.45%, P: 0.08 to 0.015%, S: 0.05 to 0.02%) A cast piece having a width of 400 mm and a thickness of 300 mm was cast from the molten steel. The casting speed at that time was in the range of 0.7 to 0.8 m / min, and the specific amount of secondary cooling water was constant at 0.3 l / kg · steel.

  In each zone with a zone length of 2 m, including 5 pairs of rolling rolls, on the upstream side and downstream side of the crater end where the solidification inside the slab is completed (the rolling zone indicated by A to D in FIG. 4) The rolling was performed such that the slab was tapered with a rolling amount of 1.0 mm / m (the total rolling amount was 8 mm).

The pressure propulsion value applied to the cylinder is statistically analyzed from the data of the actual pressure propulsion value when the internal quality (slab center segregation degree, presence or absence of porosity) is good, the reduction zones A and B are 30 tons, and C is 40 Ton and D were set to 50 tons and controlled.
Table 1 below shows test conditions and test results.

  An 800 mm long slab was taken from the cast slab for internal investigation, and five cross-sectional samples were taken at equal intervals in the longitudinal direction, and the presence or absence of internal cracks was investigated by the sulfur print. .

  The internal crack was defined as “with internal crack” when the length was 5 mm or more, and “no internal crack” when the length was less than 5 mm. Further, the porosity was investigated by sulfur printing, and when the diameter was 2 mm or more, “porosity remained” was determined, and when the diameter was less than 2 mm, “no porosity remained”. The carbon concentration center segregation ratio is determined by taking the analysis value C (mass%) of 26 chips from a position corresponding to the center of the slab in the thickness direction with a 7 mm diameter drill blade with a 2 mm diameter drill blade. C / Co was determined by dividing by the pan analysis value Co (mass%), and the average value was calculated as the center segregation ratio.

  Test numbers 1 and 2 are tests for the inventive examples, and test numbers 3 to 6 are tests for the comparative examples in which at least one of the conditions defined in the present invention is not satisfied.

  Test No. 3 is a case where the pressure propulsion value applied to the cylinder in the reduction zone A was not controlled, but because the pressure propulsion value was not controlled, the pressure propulsion value when the set reduction amount was reached. Was as low as 22 tons, and segregation deteriorated.

  Test No. 4 is a case where the pressure propulsion value applied to the cylinder in the reduction zone D was not controlled, but also in this case, the pressure propulsion value was not controlled. The pressure value was as low as 31 tons, and the porosity deteriorated.

  Test No. 5 is the case where the pressure propulsion value applied to the cylinder was not controlled in all zones, but the pressure propulsion value was deviated in all zones, internal cracks occurred and porosity remained. Segregation worsened.

  In Test No. 5, the cause of the reduction in the reduction zones A and C being higher than the set value and the decrease in the reduction zones B and D is as follows.

  The segments are periodically replaced, and the interval is initially set at the time of replacement. However, if the initial value is deviated at that time, the set reduction amount and the actual reduction amount may be different. In test number 5, in the reduction zones A and C, since the actual reduction is greater than the set reduction amount, the reduction force increases. On the contrary, in the reduction zones B and D, the actual reduction is less than the set reduction amount. Because the amount is small, the rolling force has decreased.

  In contrast to the above comparative examples of test numbers 3 to 5, the invention examples of test numbers 1 and 2 all controlled the pressure propulsion value applied to the cylinder within the set range in all four zones. There was no generation or porosity remaining, and segregation was also good.

  That is, even if the same amount of reduction is applied, the quality will change due to the outside air temperature, the secondary cooling water temperature, the error of the interval value due to the initial value setting at the time of segment replacement, etc. When the reduction reaction force, which is information from the slab, is different from the target pressure propulsion value when the interval is adjusted, the reduction amount is increased or decreased so as to reach the target pressure propulsion value. Since it is controlled, the occurrence of internal cracks and no residual porosity are eliminated.

  The present invention is not limited to the above-described example, and it goes without saying that the embodiments may be appropriately changed within the scope of the technical idea described in each claim.

  For example, FIG. 2 shows a case where the reduction is performed by a segment having six pairs of reduction rolls, but the reduction may be performed by a single reduction roll.

  The present invention can be applied not only to high carbon steel slabs as shown in the Examples but also to continuous casting of low carbon steel, medium carbon steel, stainless steel, high alloy steel, and the like.

It is a figure which shows the upper surface of a rolling-down segment. (A) is a figure which shows the side surface of a rolling-down segment, (b) is a perspective view of a rolling-down segment. It is a figure explaining rolling-down amount control. It is a figure which shows a continuous casting installation.

Explanation of symbols

1a, 1b Segment 2 Cast pieces 3a Upper frame 3b Lower frame 4 Rolling-down roll 5 Cylinder with position control 6 Hydraulic unit 8 Servo valve

Claims (1)

When rolling down the slab in the thickness direction using a plurality of reduction roll pairs on the upstream side and downstream side of the crater end where solidification inside the slab is completed, for each reduction roll pair or a plurality of reduction roll pairs A method of continuously casting steel by controlling the amount of reduction and the reduction driving force for each arranged segment,
When rolling down the slab,
First, the slab is squeezed so that the reduction amount of the slab by each reduction roll pair or each segment becomes the target reduction amount,
Thereafter, by increasing or decreasing the reduction amount of the slab by each rolling roll pair or each segment, it is possible to control so that the pressure driving value acting on each rolling roll pair or each segment becomes the target pressure driving value. A method for continuous casting of steel.

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