JP2705414B2 - Slab light reduction method in continuous casting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for lightly reducing a slab in which the center porosity of a continuously cast slab is eliminated by pressing.

[0002]

2. Description of the Related Art In the steel industry in recent years, large sections of slabs, blooms,
Continuous casting of billets and the like has been performed. This slab,
With the enlargement of the cross section of blooms, billets and the like, continuous defects such as center porosity and Zaku are generated in the continuous cast material, which deteriorates the product quality in the subsequent process. In particular, the demand for steel is becoming more sophisticated, and in recent years, the demand for quality in the steel products used for automobile security parts, railway bogie parts, etc. is severe.
The importance of quality control for internal quality is increasing. The center porosity of the continuous cast slab is a shrinkage hole at the center of the slab caused by the occurrence of bridging in the final solidification region in continuous casting and solidification shrinkage.

[0003] As a countermeasure, there is a method of applying a gentle reduction with a drawing straightening roll (pinch roll) after secondary cooling in a continuous casting facility to prevent defects based on center porosity and zigzag defects over the entire length of the slab. However, it is extremely difficult to control the rolling force, and the defects are often increased. In particular, high-carbon steel and sulfur-added free-cutting steel tend to have internal defects, which are fatal in expensive stainless steel.

As a known technique for preventing the center porosity of a slab in the continuous casting line, there is a method of pressing a slab in a thickness direction with a reduction roll during continuous casting.
A method in which at least one crown roll is provided as the reduction roll, and the central portion of the cast slab and the vicinity thereof are reduced (JP-A-60-162560). A slab reduction roll having a flat central portion of the crown so that an unsolidified portion existing inside the formed and reduced slab is pressed in a flat state (Japanese Utility Model Laid-Open No. 1-106661). ) Downstream of the continuous casting machine, the range from the crater end of the liquidus line to the crater end of the solidus line of the slab derived from the continuous casting machine is reduced in the thickness direction to just match the solidification shrinkage. Apparatus in which a light reduction device for reducing the thickness of a slab is arranged, and a press device for forging in the thickness direction downstream of the crater end of the solid phase line of the slab is disposed downstream of the light reduction device Sho 61-23844 Publication), a forging press is arranged in a moving line of a slab, and a moving mechanism capable of moving in the moving line direction is provided in the forging press, and the forging press is reciprocated in the moving line direction via the moving mechanism. Device having a reciprocating drive device for causing
No. 38450) or a method of plastically deforming a slab which is continuously cast by a continuous casting apparatus by applying a rolling reduction to the slab by a press device (Japanese Patent Application Laid-Open No. 63-163).
-268548) and the like. Further, as another method, a continuous casting bloom (hereinafter referred to as CC bloom) to be subjected to slab rolling is pressed down over the entire length between a soaking furnace and a slab rolling mill to press the center porosity. Has been implemented.

[0005]

The technique disclosed in Japanese Patent Application Laid-Open No. 60-162560 or Japanese Utility Model Application Laid-Open No. 1-100661 using a stepped roll having a convex portion at the center as a pinch roll in the above continuous casting line. Fig. 6 shows a comparison with the case of using a flat roll as a pinch roll.
(A) As shown in FIG.
Of the center porosity 25 can be eliminated by pressure bonding. However, even in this case, depending on the casting conditions such as the casting speed and the roll load, the effect of pressure-removing the center porosity of the slab may not be effectively obtained.
As shown in FIG. 6B, this is the case where the pressure of the convex portion 22 of the step roll 21 does not penetrate into the center of the slab 23 and is absorbed by the deformation 24 of the surface of the slab 23. Further, Japanese Patent Application Laid-Open Nos. 61-238449 and 61-23845.
No. 0 and JP-A-63-268548 each require a new forging press device, which is not necessarily an advantageous method.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method of lightly reducing a slab in continuous casting that can be eliminated by pressing a center porosity of the slab in a continuous casting line.

[0007]

Means for Solving the Problems The present inventors have conducted various tests and studies to achieve the above object. As a result, the rolling force due to the rolling down of the convex portion of the pinch roll having the convex portion at the central portion is absorbed by the deformation of the slab surface because the strength of the slab surface is insufficient. Because it is. Therefore, by increasing the strength of the slab surface by cooling, the rolling force of the convex portion can be permeated to the central portion. In order to increase the strength of the slab surface, a mist spray or a spray band is provided in front of the step roll under light pressure, and the slab surface is reduced by 60 %.
After forcibly cooling to 0 ° C to 400 ° C, the pinch roll
By reduction by, without slab surface is 塑 deformation in the width direction, compressive force is generated toward the center portion, and investigate the possible rolling force by the rolls protrusions penetrate to the heart, to the present invention Reached.

[0008] In other words, in this invention, coagulation in a continuous casting line
In slab soft reduction method of crimping by pinch rolls and center porosity in continuously cast slab to form a convex portion in a central portion occurring after solid, placing the <br/> pinch rolls downstream of the withdrawal straightening rolls, The slab surface was cooled to 600 ° C. to 400 ° C. by a slab surface cooling facility provided immediately before the pinch roll.
After the cooling , the pressure is reduced by the pinch roll .

[0009]

According to the present invention, prior to the slab light pressure reduction, the slab surface cooling equipment such as a mist spray provided immediately before the pinch roll at the subsequent stage of the drawing straightening roll group having a convex portion at the center portion covers the entire slab surface. uniformly sprayed cooling water mist, by changing the spraying flow rate in accordance with the casting conditions such as casting speed and roll load, 600 ° C. suitable cast slab surface temperature on the crimping disappearance of center porosity to 4
It can be adjusted to an optimum temperature of 00 ° C. Therefore, when slab soft reduction, without slab surface is 塑 deformation in the width direction, compressive force is generated toward the center, rolling force by the pinch rolls protrusions penetrate to the center, of the slab center The porosity can be effectively eliminated by pressure bonding. The slab surface temperature after cooling by mist spray
Is set to 600 ° C. to 400 ° C. because the slab is over 600 ° C.
The surface is plastically deformed in the width direction, and the rolling force penetrates into the center of the slab.
In addition, when the temperature is lower than 400 ° C., the strength of the slab surface is high.
The rolling resistance becomes too large and the center porosity
This is because it becomes difficult to crimp the wire.

[0010]

【Example】

Embodiment 1 The details of the present invention will be described below with reference to FIGS. 1 to 3 showing an embodiment. FIG. 1 is a side view of a main portion of a pinch roll band of a continuous casting line provided with a light reduction device used in the method of the present invention, and FIG. 2 shows a drawing straightening roll.
The figure shows a flat roll, the figure (b) shows a front view of a stepped roll,
FIG. 3 is an explanatory view of a situation at the time of rolling down by a stepped roll. In FIG. 1 to FIG. 3, a drawing straightening roll (pinch roll) group 1 of the continuous casting line includes a front flat roll 2 and a rear step roll 3. A spray header 4 of mist spray is provided in front of each of the stepped rolls 3 in the subsequent stage, and the surface of the slab 5 is changed in accordance with the casting speed to change the spray flow rate so that the slab surface can be adjusted to the optimum temperature. .

[0011] Because of the above configuration, in the continuous casting line, the spray flow rate from the spray header 4 provided immediately before the stepped roll 3 after the drawing straightening roll group 1 in accordance with the casting speed of the slab 5 and the type of steel. Adjust the slab 5
Is forcibly cooled to 600 ° C to 400 ° C. To do this the slab 5 of the surface, forced cooling section indicated by hatching is generated in FIG. 3, reinforced slab 5 surface, without 塑 deformation in the width direction, compressive force is generated toward the center portion Then, the reduction caused by the convex portion at the center of the step roll 3 penetrates to the center and the center porosity 6 is pressed and disappears. In this case, the surface temperature of the slab 5 after forced cooling is in the range of 600 to 400 ° C., although there is a slight difference depending on the type of steel.
You .

[0012] To confirm the effect of the soft reduction method of Example 2 the present invention, the speed casting the steel compositions shown in Table 1 0.5 m / min, cast in a continuous casting under the conditions of the slab size 410 mm × 530mm In the process of forming the pieces, the last roll of the forced pulling roll group and the preceding roll are stepped rolls, and cooling water is sprayed at a specific water volume of 2.0 l / kg from a mist spray header provided immediately before the cast rolls to form a slab. 10mm after forcibly cooled to ℃
In the case of the method of the present invention in which light reduction is applied, in the case of the conventional method 1 in which light pressure is applied at a slab temperature of 800 ° C. without spraying cooling water from the mist spray header, and all of the forced pulling roll groups are not flat rolls. 800 ° C
For each of the conventional methods 2 in which light reduction was applied in
A flaw detection test was carried out using an ultrasonic flaw detector having a frequency of 1 MHz and a transducer diameter of 24 mm using a standard test piece for vertical flaw detection STB-G V15-2.8 specified in JIS Z 2345. The defect index was determined with the case of 1.0 as 1.0. FIG. 4 shows the results. The other conditions were the same, and the casting speed was changed, and light reduction was applied in each of the three methods, and the relationship between the casting speed and the light reduction effect index was determined for each method. The result is shown in FIG. In addition, the effect index under light reduction is obtained by performing an ultrasonic flaw detection test after rolling a slab into a billet of the same diameter and determining the number of defective billets.

[0013]

[Table 1]

As shown in FIG. 4, in the case of the method of the present invention in which step rolls and spray cooling are combined, the product defect index is approximately 30 times smaller than that of the conventional method 1 in which light reduction is simply applied with step rolls. % Defect index is decreasing. In addition, as shown in FIG. 5, in the method of the present invention, the effect index curve under light pressure is shifted toward a higher casting speed. This indicates that even when the unsolidified layer is thick, the amount of reduction is large, so that even during high-speed casting, the effect of light reduction is large, which can contribute to an improvement in productivity.

[0015]

As described above, according to the method of the present invention, the center porosity of the continuous cast slab can be significantly reduced by light pressure reduction in the continuous cast line, and the porosity of the continuous cast slab can be greatly reduced. Various defects in the post-process due to the center porosity can be suppressed.

[Brief description of the drawings]

FIG. 1 is a side view of a main part of a pinch roll band of a continuous casting line provided with a light reduction device used in the method of the present invention.

FIG. 2 shows a front view of a drawing straightening roll,
(A) is a flat roll and (b) is a step roll.

FIG. 3 is an explanatory view of a situation at the time of rolling down by a stepped roll.

FIG. 4 is a graph showing the relationship between each light reduction method and a product failure index in Example 2.

FIG. 5 is a graph showing the relationship between the casting speed and the effect index under light pressure in Example 2.

6A and 6B are explanatory diagrams of a situation of light reduction by a conventional stepped roll. FIG. 6A shows a case where the light reduction penetrates into a central portion, and FIG.
The figure shows the case where the light reduction is absorbed by the deformation of the surface.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Pull-out straightening roll group 2 Flat roll 3, 21 Step roll 4 Spray header 5, 23 Cast piece 6, 25 Center porosity 22 Convex part 24 Deformation

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-60-162560 (JP, A) JP-A-63-286260 (JP, A) JP-A-63-278655 (JP, A) JP-A-63-278655 242452 (JP, A) JP 2-45939 (JP, B2)

Claims (1)

(57) [Claims] In a slab light pressure reduction method in which a center porosity of a continuous cast slab generated after solidification in a continuous casting line is pressure-bonded by a pinch roll having a convex portion formed at a central portion , the center porosity is set at a subsequent stage of a drawing straightening roll group. Distribute pinch roll
After cooling the surface to 600 ° C ~ 400 ° C by the slab surface cooling equipment provided immediately before the pinch roll ,
A slab light reduction method in continuous casting, characterized in that reduction is performed by a pinch roll .