JP7307339B2 - Rolling method for rectangular cross-section billet, continuous casting and rolling equipment, and rolling equipment - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for rolling rectangular cross-sectional steel billets, continuous casting and rolling equipment, and rolling equipment for rolling steel billets having a substantially rectangular cross-sectional shape, which are used as materials for plate products.

In a continuous casting machine that continuously solidifies molten steel into billets with a rectangular cross section, there have been problems with porosity and segregation in the central part of the thickness of the billet, internal defects due to the formation of columnar crystals, and subsequent processes caused by inhomogeneous structures. The problem is the occurrence of product defects (surface defects and mechanical property defects) such as billet cracking in rolling and plate breakage during rolling.

For example, Patent Literature 1 discloses a method of improving the structure of the central part by rolling after the center of the billet is completely solidified, by specifying the reduction ratio (amount) according to the temperature difference between the surface layer and the central part. disclosed. Compared to when the temperature difference between the surface layer and the center is small, this technology increases the hydrostatic stress in the center of the billet, increases the amount of plastic strain, and improves porosity defects and center segregation. can get.

Further, Patent Literature 2 discloses a technique for reducing center segregation by reducing a deformed section steel billet having a thick portion at the center of the width thereof after the center of the billet has completely solidified. In this technique, the deformed cross section of the steel slab increases the plastic strain due to rolling at the center of the steel slab and also increases the hydrostatic stress, compared to the technology disclosed in the above-mentioned Patent Document 1. The organization of the core is improved.

Furthermore, in Patent Document 3, as a technique for controlling the flatness of a metal plate, an apparent Techniques have been disclosed for improving the critical buckling stress of the upper corrugation generation limit and improving the flatness of the metal plate after cooling.

In addition, Patent Document 4 discloses a rolling facility equipped with an induction heating device that is provided between a plurality of mill stands and performs heating in the longitudinal direction and the width direction of the steel plate during specific or each reverse rolling. ing.

Japanese Patent Application Laid-Open No. 2004-237291 JP 2016-016450 A Japanese Patent No. 4392115 Japanese Patent No. 6362151

However, with respect to Patent Document 1, the temperature difference between the surface layer and the central portion is generally determined by the heat transfer coefficient of the surface and the billet thickness, and the reduction amount is generally set by the billet thickness and the product thickness. The difference in thickness of the intermediate product (usually called a rough bar) limits the upper limit of the reduction. For this reason, there are many cases where a sufficient amount of reduction cannot be achieved, and due to the lack of amount of reduction, when manufacturing a product with a particularly large thickness, a sufficient effect of improving the structure and quality may not be obtained. many.

In addition, in Patent Document 2, the plastic strain generated in the vicinity of the surface layer from the position of 1/4 of the thickness of the billet from the surface layer is drastically reduced, and the columnar crystal structure remains. For this reason, segregation in this portion is extremely aggravated. Moreover, continuous casting of deformed cross-section steel billets itself is difficult, and the equipment cost is enormous.

Furthermore, in the technique described in Patent Document 3, although it is possible to improve the flatness of the metal plate after cooling, it does not consider the improvement of internal quality such as porosity reduction or grain refinement. , improvement of internal quality by such technology is not expected.

Further, according to Patent Document 4, it is possible to intentionally adjust the temperature distribution in the width direction and the longitudinal direction before rolling. Since the adjustment by the induction heating device is for compensating for the temperature drop that occurs during rolling, the target temperature distribution is basically uniform. In other words, in Patent Document 4 as well, it is difficult to expect that internal quality such as porosity reduction or grain refinement will be improved by rolling.

Therefore, the present invention has been made in view of the above-mentioned problems, and the purpose of the present invention is to solve the problem of internal defects and heterogeneity due to porosity and segregation in the central part of the thickness of a steel slab, and the formation of columnar crystals. Rolling method and continuous casting of rectangular cross-section billet that can suppress the occurrence of billet cracking in subsequent processes due to the structure and plate breakage during rolling, and suppress the occurrence of product defects such as surface defects and mechanical property defects. It is to provide rolling equipment and rolling equipment.

In order to solve the above problems, according to one aspect of the present invention, there is provided a method for rolling a rectangular cross-section steel billet for rolling a steel billet having a substantially rectangular cross-sectional shape, the steel billet being in contact with a rolling roll prior to rolling. At least one of the upper and lower surfaces of the billet is intermittently heated or retained in the longitudinal direction of the billet to form localized heated portions scattered in the longitudinal direction of the billet, and the billet with the locally heated portions formed A method for rolling a rectangular section billet is provided.

In such a rolling method, the local heating portions may be formed periodically in the longitudinal direction of the billet.

Further, in this rolling method, the local heating part has a local heating depth from the surface of the billet in the billet thickness direction within the range of 0.01 times or more and 0.40 times or less of the thickness of the billet. Yes, the local heating length in the rolling direction may be in the range of 0.01 to 1.5 times the thickness of the billet.

Furthermore, in this rolling method, the surface area of the local heating part is in the range of 0.01 to 0.50 times the total surface area of the upper and lower surfaces of the billet in contact with the rolling rolls. may be formed.

Further, in this rolling method, the local heating part is formed so as to have an interval in the longitudinal direction of the billet that is 1/60 to 2 times the arc length of contact with the rolling rolls when the billet is rolled. good too.

Furthermore, in this rolling method, the local heating part has a temperature of +10 ° C. or more with respect to the average temperature of the portion other than the local heating part on the surface of the steel slab in contact with the rolling roll, and the steel It may be formed to be below the melting point temperature of the piece.

The local heating portions may be formed dispersedly in the width direction of the billet.

The width of the local heating portion may be 0.1 times or more the thickness of the billet.

When the billet is viewed from above, the shape of the local heating portion is oblique, polygonal, curved, periodic function, or zigzag that intersects a straight line extending along the width direction of the billet. good too.

Localized heating may be formed to include corner supercooling of the billet.

According to another aspect of the present invention, a continuous casting machine that casts a steel slab, and at least one of the upper and lower surfaces of the steel slab cast by the continuous casting machine and disposed downstream of the continuous casting machine. a heating device that intermittently heats or retains the heat in the longitudinal direction of the billet to form localized heated portions scattered in the longitudinal direction of the billet; and a rolling mill for rolling billets.

Furthermore, according to another aspect of the present invention, at least one of the upper and lower surfaces of the billet heated in the heating furnace is intermittently heated or maintained in the longitudinal direction of the billet to A rolling facility is provided that includes a heating device for forming scattered localized hot spots in a billet, and a rolling mill located downstream of the heating device for rolling the billet with the localized hot spots.

As described above, according to the present invention, billet cracking in subsequent processes and strips during rolling due to internal defects and heterogeneous structures due to porosity and segregation in the central part of the thickness of the billet and columnar crystal formation It is possible to suppress the occurrence of breakage and suppress the occurrence of product defects such as surface defects and poor mechanical properties.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic configuration diagram showing one configuration example of a continuous casting and rolling facility that implements a method for rolling a rectangular cross-section steel billet according to one embodiment of the present invention; It is a schematic block diagram which shows the example of a changed completely type of the continuous casting-rolling equipment of FIG. FIG. 2 is a schematic configuration diagram showing one configuration example of a rolling facility that implements the method for rolling a rectangular cross-section steel billet according to the same embodiment. FIG. 4 is a conceptual diagram showing the state of plastic deformation inside the steel slab under and near the contact region with the rolling rolls when the steel slab in which the localized heating portion is formed is rolled. FIG. 4 is an explanatory diagram showing the distribution of hydrostatic pressure stress generated at the center of the thickness of the billet at the roll bite and its vicinity, depending on the presence or absence of the local heating portion. FIG. 4 is an explanatory diagram showing the distribution of plastic strain in the billet longitudinal direction at the thickness center position and the 1/4 thickness position from the surface layer of the billet after rolling, depending on the presence or absence of the local heating portion. 4 is a graph showing an example of the relationship between the value obtained by dividing the local heating depth by the thickness of the billet and the plastic strain increment. 5 is a graph showing an example of the relationship between the value obtained by dividing the local heating depth by the thickness of the billet and the hydrostatic pressure increment. 4 is a graph showing an example of the relationship between the value obtained by dividing the local heating length by the thickness of the billet and the plastic strain increment. 5 is a graph showing an example of the relationship between the value obtained by dividing the local heating length by the thickness of the billet and the hydrostatic pressure increment. It is a graph which shows the relationship example of a heating area ratio and a plastic strain increment. It is a graph which shows the relationship example of a heating area ratio and a hydrostatic pressure increment. 5 is a graph showing an example of the relationship between the value obtained by dividing the contact arc length by the spacing of the local heating portions and the plastic strain increment. 5 is a graph showing an example of the relationship between the value obtained by dividing the contact arc length by the spacing of the local heating portions and the hydrostatic pressure increment. It is a graph which shows the relationship example of the temperature rise amount of a local heating part, and a plastic strain increment. 7 is a graph showing an example of the relationship between the temperature rise amount of the localized heating portion and the hydrostatic pressure increment. FIG. 5 is an explanatory view showing an example of locally heated portions of a steel billet formed dispersedly in the width direction of the billet; FIG. 10 is an explanatory view showing another example of locally heated portions of the billet formed dispersedly in the width direction of the billet; 4 is a graph showing the relationship between the width of a locally heated portion and the plastic strain increment. 4 is a graph showing an example of the relationship between the width of the localized heating portion and the hydrostatic pressure increment; FIG. 10 is an explanatory view showing another example of locally heated portions of the billet formed dispersedly in the width direction of the billet; FIG. 10 is an explanatory view showing another example of locally heated portions of the billet formed dispersedly in the width direction of the billet; FIG. 10 is an explanatory view showing another example of locally heated portions of the billet formed dispersedly in the width direction of the billet; FIG. 10 is an explanatory view showing another example of locally heated portions of the billet formed dispersedly in the width direction of the billet; FIG. 10 is an explanatory view showing another example of locally heated portions of the billet formed dispersedly in the width direction of the billet; FIG. 10 is an explanatory view showing another example of locally heated portions of the billet formed dispersedly in the width direction of the billet; FIG. 10 is an explanatory view showing another example of locally heated portions of the billet formed dispersedly in the width direction of the billet; FIG. 10 is an explanatory view showing another example of locally heated portions of the billet formed dispersedly in the width direction of the billet; FIG. 4 is an explanatory diagram showing the relationship between a corner supercooled portion and a locally heated portion of a billet; FIG. 11 is an explanatory diagram showing a local heating portion formed in Example 2;

Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In the present specification and drawings, constituent elements having substantially the same functional configuration are denoted by the same reference numerals, thereby omitting redundant description.

<1. Equipment configuration>
First, based on FIGS. 1 and 2, an example of equipment configuration for carrying out a method for rolling a rectangular cross-section steel billet according to an embodiment of the present invention will be described. FIG. 1 is a schematic configuration diagram showing one configuration example of a continuous casting and rolling facility for carrying out a method for rolling a rectangular cross-section steel billet according to the present embodiment. FIG. 2 is a schematic configuration diagram showing one configuration example of a rolling facility for carrying out the method for rolling a rectangular cross-section steel billet according to the present embodiment.

[1-1. Rolling of steel slabs produced by a continuous casting machine]
The method for rolling a rectangular cross-section steel billet according to the present embodiment can be carried out, for example, by a continuous casting and rolling facility 1 as shown in FIG. Such a continuous casting and rolling facility 1 includes, for example, a continuous casting machine 10 for casting billets 5, a heating device 20, and a rolling mill 30, as shown in FIG. The type and size of the billet 5 manufactured by the continuous casting machine 10 are not particularly limited. The billet 5 may be, for example, any of slabs, billets and blooms. In the following description, a slab is assumed as an example of the steel piece 5 .

(continuous casting machine)
The continuous casting machine 10, as shown in FIG. 1, is an apparatus for continuously casting molten metal using a mold 13 for continuous casting to produce a billet 5 such as a slab. A continuous casting machine 10 includes a tundish 11 , a submerged nozzle 12 , a mold 13 and a secondary cooling device 14 .

A tundish 11 is positioned above the mold 13 and stores molten metal conveyed by a ladle (not shown). While the molten metal is stored in the tundish 11, inclusions in the molten metal are removed. The bottom of the tundish 11 is provided with an immersion nozzle 12 for supplying molten metal to a mold 13 . The immersion nozzle 12 continuously supplies the molten metal from which inclusions have been removed in the tundish 11 to the mold 13 .

The casting mold 13 is a mold having a substantially rectangular hollow formed according to the width and thickness of the steel billet 5 to be manufactured. The mold 13 is configured by, for example, combining mold plates made of four water-cooled copper plates. The molten metal supplied into the mold 13 through the immersion nozzle 12 is cooled by being brought into contact with the mold plate to form a solidified shell 5a in which the molten metal is solidified. With the outer shell solidified, the billet 5 is withdrawn from the mold 13 .

The secondary cooling device 14 is provided on the downstream side of the mold 13 in the casting direction, supports the steel slab 5 pulled out from the lower end of the mold 13, and cools it while transporting it. The secondary cooling device 14 includes a plurality of pairs of support rolls 14a arranged on both sides in the thickness direction of the steel slab 5 and a plurality of spray nozzles (not shown) for injecting cooling water to the steel slab 5. have. An unsolidified portion 5b exists inside the solidified shell 5a of the billet 5 immediately after being pulled out of the mold 13, but solidification of the internal unsolidified portion 5b progresses while the billet 5 moves through the secondary cooling device 14, The thickness of the solidified shell 5a of the outer shell gradually increases. When the steel slab 5 is almost completely solidified, it is continuously transported from the continuous casting machine 10 to the heating device 20 .

The continuous casting machine 10 according to the present invention is not limited to the vertical bending type continuous casting machine 10 shown in FIG.

(heating device)
The heating device 20 is arranged downstream of the continuous casting machine 10 and intermittently heats at least one of the upper and lower surfaces of the steel slab 5 cast by the continuous casting machine 10 in the longitudinal direction of the steel slab. As a result, local heating portions 5c scattered in the longitudinal direction of the billet 5 are formed. The heating device 20 shown in FIG. 1 has heating units arranged above the upper surface of the steel slab 5 and below the lower surface of the steel slab 5 so that the upper and lower surfaces of the steel slab 5 can be heated. there is

The heating device 20 shown in FIG. 1 is an induction heating type heating device. The heating unit is connected to, for example, an AC power source 21 and includes a plurality of induction heating coil portions 23 arranged at predetermined intervals in the longitudinal direction of the billet. The intervals between the induction heating coil portions may be uniform or non-uniform. For example, as shown in FIG. 1, the induction heating coil unit 23 is configured by winding a coil connected to the AC power supply 21 around a magnetic material such as iron.

The induction heating coil portion 23 is arranged so as to cover at least a range substantially equal to the plate width of the billet 5 in the plate width direction. A switch 25 is connected to the induction heating coil section 23 for switching whether or not the current is supplied from the AC power supply 21 . When the switch 25 is turned on, current flows through the induction heating coil portion 23, and the high-frequency magnetic field heats the surface of the steel piece 5 immediately below and in the vicinity thereof. When the switch 25 is off, no current flows through the induction heating coil portion 23 and the surface of the steel piece 5 is no longer heated. By appropriately turning on and off the switch 25 of the induction heating coil portion 23, any position in the longitudinal direction of the billet can be locally heated.

The heating device 20 turns on and off the switch 25 of the induction heating coil portion 23 based on the moving speed of the billet 5 so that the same position of the billet 5 is heated while the billet 5 passes through the heating device 20 . As a result, locally heated portions 5c are formed on the surface of the billet 5 at predetermined intervals in the longitudinal direction of the billet.

(rolling mill)
The rolling mill 30 is arranged on the downstream side of the heating device 20 and rolls the billet 5 on which the locally heated portions 5 c are formed by the heating device 20 . The rolling mill 30 comprises a pair of rolling rolls 31a and 31b, and rolls the steel billet 5 at a predetermined rolling reduction. The rolling rolls 31a and 31b are flat rolls. The rolling mill 30 may be a two-high rolling mill as shown in FIG. 1, a four-high rolling mill, or any other type of plate rolling mill. The rolling rolls 31a and 31b may be rolls other than flat rolls and may have any cross-sectional shape.

In the continuous casting and rolling facility 1 shown in FIG. 1, the heating device 20 forms the locally heated portion 5c on the surface of the billet 5. However, even if the surface of the billet 5 is heated instead of being heated, the locally heated portion 5c can be similarly heated. can be formed. For example, as shown in FIG. 2, the continuous casting and rolling facility according to this embodiment may be a continuous casting and rolling facility 2 in which a heat retaining device 40 is installed instead of the heating device 20 shown in FIG.

(Heat retaining device)
As shown in FIG. 2, the heat retaining device 40 is arranged downstream of the continuous casting machine 10, and extends at least one of the upper and lower surfaces of the billet 5 cast by the continuous casting machine 10 in the longitudinal direction of the billet. Keep warm intermittently. As a result, local heating portions 5c scattered in the longitudinal direction of the billet 5 are formed. The heat retaining device 40 shown in FIG. 2 is provided above the upper surface of the steel slab 5 and below the lower surface of the steel slab 5 so as to intermittently retain heat on the upper and lower surfaces of the steel slab 5 in the longitudinal direction of the steel slab. A heat retention unit is arranged in each.

The heat retaining unit prevents the surface of the billet 5 from being locally air-cooled by, for example, arranging a plurality of heat retaining members 45 in the longitudinal direction of the billet at predetermined intervals. The heat-retaining member 45 is a plate-like member made of a heat-insulating and heat-resistant material. The heat-retaining member 45 has at least a length substantially equal to the width of the billet 5 in the width direction. In the heat retaining device 40 shown in FIG. 2, the heat retaining unit has a configuration in which a plurality of heat retaining members 45 are arranged on the outer peripheral surface of an endless belt 43 supported by a pair of rollers 41a and 41b. The endless belt 43 rotates at the same speed as the moving speed of the billet 5 along the longitudinal direction of the billet (that is, the casting direction). The intervals between the heat retaining members 45 provided on the endless belt 43 may be equal or may be uneven.

The surface portion of the steel piece 5 covered with the heat-insulating member 45 of the heat-insulating unit is difficult to be air-cooled. Since the endless belt 43 rotates at the same speed as the steel billet 5 , the same portion of the steel billet 5 continues to be heat-retained while the steel billet 5 passes through the heat retaining device 40 . As a result, on the surface of the billet 5, a locally heated portion 5c having a relatively higher temperature than the surrounding area due to local heat retention is formed in the longitudinal direction of the billet at intervals at which the heat retaining members 45 are installed. .

[1-2. Rolling of billet heated by heating furnace]
The method for rolling a rectangular cross-section steel billet according to this embodiment can also be carried out by a rolling facility 3 as shown in FIG. 3, for example. The rolling facility 3 includes, for example, a heating device 70 for further heating the steel slab 5 heated by the heating furnace 50, and a rolling mill 80, as shown in FIG.

For example, when the steel slab 5 produced by a continuous casting machine or the like is cooled once and then rolled, as shown in FIG. After being heated, the steel slab 5 is transported by the rolls 18 , scales on the surface of the steel slab 5 caused by heating are removed by the scale removing device 60 , and then transported to the heating device 70 . For steel billets that do not require descaling, the descaling device 60 does not need to be operated, and installation itself is unnecessary.

(heating device)
The heating device 70 intermittently heats at least one of the upper and lower surfaces of the billet 5 heated in the heating furnace 50 in the longitudinal direction of the billet. As a result, local heating portions scattered in the longitudinal direction of the billet are formed in the billet 5 . The heating device 70 shown in FIG. 3 is arranged above the upper surface of the steel piece 5 and is configured to locally heat only the upper surface of the steel piece 5 .

The heating device 70 may be, for example, an induction heating type heating device similar to the heating device 20 in FIG. The heating device 70 is connected to, for example, an AC power source 71 and includes a plurality of induction heating coil portions 73 arranged at predetermined intervals in the longitudinal direction of the billet. The intervals between the induction heating coil portions 73 may be uniform or non-uniform. The induction heating coil portion 73 is configured by winding a coil connected to the AC power supply 71 around a magnetic material such as iron. The induction heating coil part 73 is arranged so as to cover at least a range substantially equal to the plate width of the billet 5 in the plate width direction. When an electric current flows through the induction heating coil portion 73, the surface of the steel piece 5 immediately below and in the vicinity of the induction heating coil portion 73 is heated, and the surface of the steel piece 5 can be locally heated.

The heating device 70 shown in FIG. 3 is configured to be able to reciprocate in the conveying direction (ie, rolling direction) of the steel billet 5 and in the reverse direction. The movement of the heating device 70 in the conveying direction starts at the starting point position (broken line position in FIG. 3) and moves to the end point position (solid line position in FIG. 3) at the same speed as the billet 5 moving speed. When the heating device 70 reaches the end position, it reverses its movement direction and moves in the opposite direction to the start position at a higher speed than the movement speed in the conveying direction. The heating device 70 repeats the movement in the conveying direction and the movement in the opposite direction a number of times corresponding to the length of the steel piece 5 . The same position of the billet 5 continues to be heated while the heating device 70 moves in the conveying direction. By repeating the reciprocating movement of the heating device 70, locally heated local heating portions 5c are formed on the surface of the billet 5 at intervals of the induction heating coil portions 73 operating in the longitudinal direction of the billet.

In the heating device 70, it is not always necessary to operate all the induction heating coils 73 during movement in the conveying direction. good. The moving speed of the heating device 70 in the opposite direction is, for example, a value obtained by multiplying the moving speed of the billet 5 by {(conveyance direction distance between the start point position and the end point position)/(interval between the local heating portions 5c)}. Higher speed is sufficient.

In addition, in the rolling facility 3 shown in FIG. 3, a heat retaining device may be provided instead of the heating device 70 . In this case, the heat retaining device may be the heat retaining device as shown in FIG.

(rolling mill)
The rolling mill 80 is arranged on the downstream side of the heating device 70 and rolls the billet 5 on which the locally heated portions 5 c are formed by the heating device 70 . The rolling mill 80 comprises a pair of rolling rolls 81a and 81b, and rolls the steel billet 5 at a predetermined rolling reduction. The pressure rolls 81a and 81b are flat rolls. Note that the rolling mill 80 may be a two-high rolling mill as shown in FIG. 3, a four-high rolling mill, or any other type of plate rolling mill. The rolling rolls 81a and 81b may be rolls other than flat rolls and may have any cross-sectional shape.

An example of equipment for carrying out the method for rolling a rectangular cross-section steel billet according to the present embodiment has been described above. 3 may be used instead of the heating device 20 in the continuous casting and rolling equipment 1 of FIG. Similarly, the heating device 20 of FIG. 1 may be used instead of the heating device 70 in the rolling mill 3 of FIG. Further, the equipment for carrying out the rolling method for the rectangular cross-section steel billet according to the present embodiment is not limited to the examples shown in FIGS. The rolling method of the rectangular cross-section steel bill according to the present embodiment can be carried out in any facility equipped with a heating device or a heat retaining device that disperses the local heating portions 5c in the longitudinal direction of the billet.

<2. Application of strain by local heating before rolling>
In the method for rolling a rectangular cross-section steel billet according to the present embodiment, equipment such as that shown in FIGS. At least one of the upper and lower surfaces is intermittently heated or retained in the longitudinal direction of the billet to form locally heated portions 5c scattered in the longitudinal direction of the billet 5 . By rolling the billet 5 in which such a locally heated portion 5c is formed by a rolling mill, the deformation state such as stress and strain inside the material during rolling can be significantly and intentionally controlled under the same billet reduction amount condition. can be changed to improve the billet quality without changing the billet thickness. That is, an increase in hydrostatic stress at the center of the thickness is effective for porosity crimping, and an increase in plastic strain leads to improvement in segregation and grain refinement.

[2-1. Action by local heating before rolling]
First, with reference to FIGS. 4 to 6, the reason why the locally heated portion 5c is formed in the steel billet 5 prior to rolling in the method for rolling a rectangular cross-section steel billet according to the present embodiment will be described. FIG. 4 shows the plastic deformation inside the billet under and near the contact area with the rolling rolls 31a (hereinafter referred to as "roll bite") when the billet 5 in which the localized heating portion 5c is formed is rolled. It is a conceptual diagram showing states (stress, plastic strain and metal flow). FIG. 5 is an explanatory diagram showing the distribution of hydrostatic pressure stress generated at the center of the thickness of the billet 5 at the roll bite and its vicinity, depending on the presence or absence of the local heating portion 5c. In addition, in FIG. 5, the compressive side of the hydrostatic pressure stress is shown as positive. FIG. 6 is an explanatory diagram showing the distribution of plastic strain in the billet longitudinal direction at the thickness center position and the 1/4 thickness position from the surface layer of the billet after rolling, depending on the presence or absence of the local heating portion 5c.

When the locally heated portions 5c scattered in the longitudinal direction of the billet 5 are formed in the billet 5 and then rolled, the deformation state near the surface of the billet 5 remarkably fluctuates in the longitudinal direction of the billet. propagate to the center. That is, as shown in FIG. 4, when the steel slab 5 is rolled by the rolling rolls 31a, the local heating is effected by the restraint by the front and rear regions of the local heating portion 5c, which is relatively hard and difficult to deform, in the local heating portion 5c. Plastic deformation concentrates on the portion 5c, and the plastic strain of the billet 5 increases. That is, the locally heated portion 5c becomes a starting point of deformation, causing a large plastic strain inside the billet 5. As shown in FIG. In FIG. 4, a portion where a large plastic strain is generated from the locally heated portion 5c is shown as an increased deformation portion 5e. As shown in FIG. 4, one deformation increase portion 5e is generated from one local heating portion 5c.

The increased deformation portion 5e is generally similar to the portion where the hydrostatic pressure stress increases. The solid line in FIG. 5 shows the rolling direction distribution of the hydrostatic pressure stress at the center of the thickness of the billet when the billet 5 without the locally heated portion 5c is pressed by the rolling rolls. The dashed line in FIG. 5 indicates the rolling direction distribution of the hydrostatic pressure stress when the steel slab 5 having the locally heated portion 5c is pressed by the rolling rolls. FIG. 5 shows the distribution when the increased deformation portion 5e in FIG. 4 passes through the rolling direction position where the hydrostatic pressure stress is maximum.

At this time, the plastic strain propagates through the area of the deformation increase portion 5e starting from the local heating portion 5c and reaches the center of the thickness. The main plastic deformation in the region of the increased deformation portion 5e is extensional deformation in the rolling direction. Hydraulic stress changes to compression side. Therefore, the maximum compressive stress value (that is, peak value of hydrostatic pressure stress) σ ^peak _1/2 in the vicinity of the exit from the approximate center of the contact area between the rolling rolls and the billet 5 is The hydrostatic pressure increment value dσ ^peak _1/2 is increased compared to the case without. From this, it can be seen that by forming the locally heated portion 5c in the billet 5, the hydrostatic pressure stress can be increased more than when the locally heated portion 5c is not formed.

The increased deformation portion 5e is generated substantially periodically in the longitudinal direction of the billet. If the deformation-increased portion 5e and other portions coexist in the same roll bite, a hydrostatic stress gradient occurs, and secondary metal flow from the deformation-increased portion 5e to other portions occurs. The recurrence of large and small plastic deformation portions within the roll bite with a length approximately equal to the contact length makes the secondary metal flow more pronounced, resulting in a significant increase in plastic strain. Therefore, by forming the local heating portion 5c in the steel piece 5, the plastic strain can be increased more effectively.

FIG. 6 shows the distribution of plastic strain in the longitudinal direction of the billet after rolling when the billet 5 in which the locally heated portions 5c are formed is rolled. As shown in FIG. 6, by rolling the billet 5 in which the localized heating portion 5c is formed, the plastic strain is It has an asymmetrical waveform and changes almost periodically.

It is considered that the variation width of the plastic strain at the central position in the thickness direction corresponds to the size of the locally heated portion 5c on the surface of the billet 5 and the temperature difference (yield stress difference) with the surroundings. At this time, the position closer to the surface of the billet than the center of the thickness, for example, the position toward the surface of the billet by 1/4 of the thickness of the billet from the center of the thickness (hereinafter also referred to as "1/4 thickness position"). The amplitude is larger at the central position. Also, the phase at the center position is delayed from the phase at the quarter thickness position. As shown in FIG. 4, this phase delay is such that the surface vicinity portion of the increased deformation portion 5e is positioned upstream in the rolling direction from the thickness center portion, and the increased deformation portion 5e is inclined with respect to the thickness direction. corresponds to

For comparison, FIG. 6 shows the plastic strain of the steel slab 5 in the case where the steel slab 5 in which the local heating portion 5c is not formed is rolled. The amount of reduction when rolling the billet 5 without the locally heated portions 5c was the same as when rolling the billet 5 with the locally heated portions 5c. At this time, by forming the local heating portion 5c in the steel piece 5 both at the center position in the thickness direction of the steel piece 5 and at the quarter thickness position, the plastic strain increases in most of the length direction. , and the average strain value also increases. In FIG. 6, localized heated portions 5c are formed due to plastic strain when rolling the billet 5 in which the localized heated portions 5c are not formed at the central position and the quarter thickness position of the steel bill 5 in the thickness direction. The peak values of the plastic strain increment (hereinafter also referred to as “plastic strain increment”) when rolling the steel slab 5 are peak strain increments dε ^peak _1/2 and dε ^peak _1/4 , respectively. represent.

[2-2. Formation of Local Heated Part]
In order to increase the plastic strain of the billet 5 as described above, the billet 5 needs to be appropriately provided with the local heating portion 5c. As a result of examining this condition, the inventors of the present application have found that the same The plastic strain inside the billet can be significantly increased over the entire thickness direction under the conditions of the billet reduction amount, and the hydrostatic stress near the center of the thickness can be increased toward the compression side. It was found that not only the crimping effect of porosity and the improvement of center segregation, but also the micro-segregation due to the formation of columnar crystals near the surface layer of the billet was improved. The local heating depth refers to the thickness direction depth of the local heating part 5c, the local heating length refers to the rolling direction length of the local heating part 5c, and the local heating part interval refers to the longitudinal direction of the billet. It is the interval between adjacent local heating portions 5c, and refers to the distance between the centers of the billets in the longitudinal direction of the local heating portions 5c.

(1) Local Heating Depth, Local Heating Length The local heating portion 5c has a local heating depth from the surface of the billet 5 in the billet thickness direction of 0.01 times the thickness of the billet 5 or more. 40 times or less, and the local heating length in the rolling direction is preferably in the range of 0.01 to 1.5 times the thickness of the billet 5 . The local heating depth is the depth of the locally heated portion 5c in the thickness direction of the billet immediately before being rolled down by the rolling rolls. The locally heated length refers to the length in the rolling direction of the locally heated portion 5c just before being rolled down by the rolling rolls. The local heating depth and the local heating length can be estimated, for example, from the measured surface temperature at the entry side of the rolling mill using heat conduction calculation or the like. Note that the local heating portion size factor, which will be described later, can also be estimated in the same manner as the local heating depth.

FIG. 7 shows the relationship between the value obtained by dividing the local heating depth by the thickness of the billet 5 (that is, "local heating depth/slab thickness") and the plastic strain increment. In FIG. 7, the horizontal axis indicating the coordinates of the value obtained by dividing the local heating depth by the thickness of the steel billet 5 is represented on a logarithmic scale. The same applies to the horizontal axis of FIG. 8, which will be described later. As shown in FIG. 7, for the peak strain increments dε ^peak _1/2 and dε ^peak _1/4 at the center position and 1/4 thickness position in the thickness direction of the billet 5 shown in FIG. It has a convex curve. Here, considering that the rolling strain in normal rolling is on the order of 0.1, the strain increment from the plastic strain when rolling the billet 5 in which the local heating portion 5c is not formed is 0.01 (ie, 1%) or more, the effect is considered substantially significant. From FIG. 7, the range where the peak strain increments dε ^peak _1/2 and dε ^peak _1/4 for the central position and quarter thickness position in the thickness direction of the billet 5 exceeds 0.01 is the local heating depth The value obtained by dividing the thickness by the thickness of the billet 5 is in the range of 0.01 or more and 0.4 or less. That is, the local heating depth of the local heating portion 5c may be set in the range of 0.01 to 0.40 times the thickness of the billet.

If the local heating depth of the local heating portion 5c is 0.01 times or more the thickness of the steel slab, a significant strain increasing effect can be obtained. In addition, by setting the local heating depth of the local heating part 5c to 0.40 times or less of the thickness of the billet, the effect of the material due to the temperature rise due to the formation of the local heating part 5c can be used to evaluate the quality of the entire product. It does not reach, and is suitable.

Therefore, in order to significantly increase the plastic strain, it is preferable to set the local heating depth of the local heating portion 5c in the range of 0.01 to 0.40 times the thickness of the steel slab. As shown in FIG. 5, the hydrostatic stress in the roll bite is on the order of 100 MPa, and an increase in the hydrostatic stress of 10 MPa or more is considered to have a substantially significant effect. Therefore, as shown in FIG. 8, the value obtained by dividing the local heating depth by the thickness of the billet 5 (i.e., “local heating depth/slab thickness”) is 0.01 or more and 0.4 or less. If the range, that is, the local heating depth of the local heating portion 5c is set in the range of 0.01 to 0.40 times the thickness of the billet, a hydrostatic pressure stress increment that greatly exceeds 10 MPa is always obtained. , Practically sufficient effects can be obtained with respect to porosity crimping in the vicinity of the center in the thickness direction of the billet.

Further, FIG. 9 shows the relationship between the value obtained by dividing the locally heated length by the thickness of the billet 5 (that is, “locally heated length/slab thickness”) and the plastic strain increment. In FIG. 9, the horizontal axis indicating the coordinates of the value obtained by dividing the local heating length by the thickness of the steel billet 5 is expressed in a logarithmic scale. The same applies to the horizontal axis of FIG. 10, which will be described later. As in the case of the local heating depth, as shown in FIG. 9, for the local heating length, the peak strain increase at the center position and quarter thickness position in the thickness direction of the billet 5 shown in FIG. Both of the quantities dε ^peak _1/2 and dε ^peak _1/4 have convex curves. Here, considering that the rolling strain in normal rolling is on the order of 0.1, the strain increment from the plastic strain when rolling the billet 5 in which the local heating portion 5c is not formed is 0.01 (ie, 1%) or more, the effect is considered substantially significant. From FIG. 9, the range where the peak strain increments dε ^peak _1/2 and dε ^peak _1/4 for the center position and 1/4 thickness position in the thickness direction of the billet 5 exceeds 0.01 is the local heating length The value obtained by dividing the thickness by the thickness of the billet 5 is in the range of 0.01 or more and 1.5 or less. That is, the local heating length of the local heating portion 5c may be set in the range of 0.01 to 1.5 times the thickness of the billet.

If the local heating length of the local heating portion 5c is 0.01 times or more the thickness of the steel slab, a significant strain increasing effect can be obtained. In addition, by setting the local heating length of the local heating part 5c to 1.5 times or less than the thickness of the billet, the effect of the material due to the temperature rise due to the formation of the local heating part 5c is reflected in the quality evaluation of the entire product. It does not reach, and is suitable.

Therefore, in order to significantly increase the plastic strain, it is preferable to set the local heating length of the local heating portion 5c from the surface of the billet 5 to a range of 0.01 to 1.5 times the thickness of the billet. good. As shown in FIG. 5, the hydrostatic stress in the roll bite is on the order of 100 MPa, and an increase in the hydrostatic stress of 10 MPa or more is considered to have a substantially significant effect. Therefore, as shown in FIG. 10, the value obtained by dividing the locally heated length by the thickness of the billet 5 (that is, “locally heated length/slab thickness”) is 0.01 or more and 1.5 or less. If the range, that is, the local heating length of the local heating portion 5c is set in the range of 0.01 to 1.5 times the thickness of the billet, a hydrostatic pressure stress increment that greatly exceeds 10 MPa is always obtained. , Practically sufficient effects can be obtained with respect to porosity crimping in the vicinity of the center in the thickness direction of the billet.

(2) Local heating surface area The local heating surface area, which is the surface area of the local heating part 5c, is 0.01 to 0.5 times the total surface area of the upper and lower surfaces of the billet 5 in contact with the rolling rolls. is preferably formed so as to be within the range of Here, the value obtained by dividing the locally heated surface area by the total surface area (that is, "locally heated surface area/total surface area") is referred to as the heated area ratio.

FIG. 11 shows the relationship between the heating area ratio and the plastic strain increment. In FIG. 11, the horizontal axis indicating the coordinates of the heating area ratio is expressed in a logarithmic scale. The same applies to the horizontal axis of FIG. 12, which will be described later. As shown in FIG. 11, for the peak strain increments dε ^peak _1/2 and dε ^peak _1/4 at the center position and 1/4 thickness position in the thickness direction of the billet 5 shown in FIG. The larger the heating area ratio, the larger the plastic strain increment. Here, considering that the rolling strain in normal rolling is on the order of 0.1, the strain increment from the plastic strain when rolling the billet 5 in which the local heating portion 5c is not formed is 0.01 (ie, 1%) or more, the effect is considered substantially significant. From FIG. 11, the range where the peak strain increment dε ^peak _1/2 and dε ^peak _1/4 for the central position and 1/4 thickness position in the thickness direction of the billet 5 exceeds 0.01 is the heating area ratio is in the range of 0.01 to 0.5. That is, the locally heated portion 5c may be formed so that the locally heated surface area is in the range of 0.01 to 0.5 times the total surface area of the upper and lower surfaces of the billet 5 in contact with the rolling rolls.

If the heating area ratio is 0.01 or more, the strain increasing effect can be significantly obtained. Further, by setting the heating area ratio to 0.5 or less, the quality evaluation of the entire product is not affected by the material quality caused by the temperature rise due to the formation of the local heating portion 5c, which is preferable. Regarding the effect of increasing the hydrostatic pressure stress, as shown in FIG. It is possible to enjoy practically sufficient effects with respect to.

(3) Spacing between Local Heating Parts The local heating parts 5c are arranged in the longitudinal direction of the billet with a gap of 1/60 to 2 times the arc length of contact with the rolling rolls when the billet 5 is rolled. It is preferably formed so as to have The local heating interval is represented by the length corresponding to the contact arc length between the billet 5 and the rolling rolls in the billet 5 immediately before rolling.

FIG. 13 shows the relationship between the value obtained by dividing the contact arc length by the local heating portion interval (that is, “contact arc length/local heating portion interval”) and the plastic strain increment. As shown in FIG. 13, for the peak strain increments dε ^peak _1/2 and dε ^peak _1/4 at the center position and 1/4 thickness position in the thickness direction of the billet 5 shown in FIG. It has a convex curve. Here, considering that the rolling strain in normal rolling is on the order of 0.1, the strain increment from the plastic strain when rolling the billet 5 in which the local heating portion 5c is not formed is 0.01 (ie, 1%) or more, the effect is considered substantially significant. From FIG. 13, the peak strain increments dε ^peak _1/2 and dε ^peak _1/4 at the center position and quarter thickness position of the billet 5 in the thickness direction exceed 0.01, the contact arc length is in the range of 0.5 or more and 60 or less. That is, the interval between the local heating portions may be set in the range of 1/60 to 2 times the arc length of contact with the rolling rolls when the billet 5 is rolled. If the interval between the local heating portions is 1/60 to 2 times the arc length of contact with the rolling rolls when the billet 5 is rolled, a significant strain increasing effect can be obtained.

As shown in FIG. 5, the hydrostatic stress in the roll bite is on the order of 100 MPa, and an increase in the hydrostatic stress of 10 MPa or more is considered to have a substantially significant effect. Therefore, as shown in FIG. 14, if the interval between the local heating portions is set in the range of 1/60 times or more and 2 times or less of the contact arc length with the rolling rolls when the billet 5 is rolled, the pressure is always 10 MPa. is obtained, and a practically sufficient effect can be obtained with respect to porosity crimping near the center in the thickness direction of the billet.

(4) Temperature of Local Heating Part The average temperature of the local heating part 5c (that is, the temperature of the local heating part) is the average of the parts other than the local heating part 5c on the surface of the billet 5 in contact with the rolling rolls. It is preferably formed so as to be within the range of +10° C. or more with respect to the temperature and below the melting point temperature.

FIG. 15 shows the relationship between the temperature rise amount of the local heating portion 5c and the plastic strain increment. Here, the amount of temperature rise of the local heating portion 5c is the amount of increase of the temperature of the local heating portion with respect to the average temperature of the portions other than the local heating portion 5c. In FIG. 15, the horizontal axis indicating the coordinates of the temperature rise amount of the local heating portion 5c is expressed in a logarithmic scale. The same applies to the horizontal axis of FIG. 16, which will be described later.

As shown in FIG. 15, for the peak strain increments dε ^peak _1/2 and dε ^peak _1/4 at the center position and 1/4 thickness position in the thickness direction of the billet 5 shown in FIG. The plastic strain increment increases as the temperature rise amount of the local heating portion 5c increases. Here, considering that the rolling strain in normal rolling is on the order of 0.1, the strain increment from the plastic strain when rolling the billet 5 in which the local heating portion 5c is not formed is 0.01 (ie, 1%) or more, the effect is considered substantially significant. From FIG. 15, the range where the peak strain increment dε ^peak _1/2 and dε ^peak _1/4 for the center position and 1/4 thickness position in the thickness direction of the billet 5 exceeds 0.01 is the local heating part The amount of temperature rise in 5c is 10° C. or more, and the temperature of the local heating part is in the range of less than the melting point temperature. That is, the local heating portion 5c may be formed so that the amount of temperature rise of the local heating portion 5c is within the range of +10° C. or less and the temperature of the local heating portion is lower than the melting point temperature.

If the temperature change of the steel slab 5 due to local heating is +10° C. or more, a significant strain increasing effect can be obtained. Further, by setting the temperature of the locally heated portion to be lower than the melting point temperature, it is possible to avoid problems such as occurrence of surface defects and material incompatibility due to re-solidification after melting of the locally heated portion 5c.

As for the effect of increasing the hydrostatic pressure stress, as shown in FIG. 16, if the temperature rise amount of the local heating portion 5c is +10° C. or more and the temperature of the local heating portion is in the range of less than the melting point temperature, the hydrostatic pressure stress increases. Since the amount is always 10 MPa or more, a practically sufficient effect can be obtained for porosity crimping.

(5) Arrangement in Billet Width Direction The local heating portions 5c do not necessarily need to be formed continuously over the entire width of the billet 5 in the width direction, and may be formed dispersedly.

For example, when the localized heating portion 5c is formed over the entire width of the billet 5, non-uniform surface scale of the billet may be generated or peeled off during the formation of the localized heating portion 5c, resulting in deterioration of the surface quality of the product. They may come. In such a case, for example, as shown in FIG. 17A, the local heating portions 5c may be dispersed in the width direction to make the surface properties uniform.

Further, for example, near the width direction end of the billet 5 (for example, a position La that enters the width center side from the width direction end of the billet 5 by a distance of about 0.5 times the billet height) Porosity and center segregation may occur intensively near the center of the billet thickness. In such a case, for example, as shown in FIG. 17B, a local A heating portion 5c may be formed. The width W of the locally heated portion 5c is preferably 0.1 times or more the thickness of the billet 5. The width W of the local heating portion 5c is the length of the local heating portion 5c when the continuous local heating portions 5c are projected in the billet width direction.

FIG. 18 shows the relationship between the width W of the local heating portion 5c and the plastic strain increment. The horizontal axis of FIG. 18 indicates the value obtained by dividing the width W of the locally heated portion 5c by the thickness of the billet (that is, the width W of the locally heated portion 5c/the thickness of the billet). The same applies to the horizontal axis of FIG. 19, which will be described later. As shown in FIG. 18, for the peak strain increments dε ^peak _1/2 and dε ^peak _1/4 at the center position and 1/4 thickness position in the thickness direction of the billet 5 shown in FIG. The larger the width W of the local heating portion 5c, the larger the plastic strain increment. Here, considering that the rolling strain in normal rolling is on the order of 0.1, the strain increment from the plastic strain when rolling the billet 5 in which the local heating portion 5c is not formed is 0.01 (ie, 1%) or more, the effect is considered substantially significant. From FIG. 18, the range where the peak strain increment dε ^peak _1/2 and dε ^peak _1/4 for the central position and 1/4 thickness position in the thickness direction of the billet 5 exceeds 0.01 is the local heating part The width W of 5c is in the range of 0.1 times or more the thickness of the billet 5. If the width W of the locally heated portion 5c of the steel billet 5 formed by local heating is 0.1 times or more the thickness of the steel billet 5, a significant effect of increasing strain can be obtained.

As for the effect of increasing the hydrostatic stress, as shown in FIG. Since it is 10 MPa or more, it is possible to enjoy a practically sufficient effect regarding porosity crimping.

20A to 20H show examples of the local heating portions 5c dispersedly formed in the width direction of the billet. As shown in FIGS. 20A and 20B, the local heating portion 5c does not necessarily need to be formed parallel to the billet width direction (X direction). The shape may be oblique, polygonal, curvilinear, periodic function, or zigzag that intersects the straight line Lx extending along the width direction of the billet.

When the local heating portions 5c are formed substantially parallel to the width direction of the billet, the pulsation of the deformed state inside the billet caused by the local heating portions 5c occurs substantially simultaneously in the width direction of the billet. Therefore, depending on the rolling conditions and the state of the rolling mill (the degree of so-called backlash), significant vibration may occur during rolling, which may lead to a decrease in the stability of the rolling operation and the life of the rolling mill. In such a case, the local heating portion 5c is significantly inclined with respect to the width direction of the billet, and is oblique (for example, FIGS. 20A and 20B), polygonal (for example, FIG. 20C), or curved (for example, FIG. 20D). and FIG. 20E), staggered (eg, FIGS. 20F-20H), or a combination thereof. By dispersing the local heating portions 5c in the width direction of the billet in this way, the pulsations in the deformed state described above are suppressed from occurring substantially simultaneously in the width direction of the billet. As a result, significant vibration does not occur during rolling, and reduction in the stability of the rolling operation and the life of the rolling mill can be avoided.

In addition, when the local heating portion 5c is arranged so as to be inclined with respect to the billet width direction, the requirements for the above-mentioned local heating depth and local heating portion interval are the billet thickness direction and the billet length direction (rolling (substantially the same as the direction) is defined as a dimensional requirement evaluated on a plane. In addition, in order to satisfy the requirements for the local heating depth and the interval between the local heating portions, the inclination angle of the local heating portion 5c with respect to the billet width direction is preferably 45° or less.

Furthermore, as in the downstream side of the continuous casting process by a continuous casting machine, by cooling from both the side surface and the front and back surfaces of the billet 5, as shown in FIG. Also referred to as "corner supercooled portion 5f") may occur. At this time, depending on the material of the steel piece 5 and its temperature dependence, the toughness of the corner supercooled portion 5f may significantly decrease. In such a case, if the local heating portion 5c is formed so as to include the corner supercooled portion 5f, the average temperature of the corner supercooled portion 5f increases, improving product quality and preventing operational troubles due to billet breakage. can be done. In addition, the width We of the corner supercooled portion 5f in the billet width direction is included in the range from the end portion in the billet width direction to the central position about 1.0 times the billet thickness.

[Example 1]
In Example 1, a rolling mill was installed at the end of a continuous casting machine capable of casting a slab having a thickness of 100 mm and a width of 600 mm, and a continuous casting rolling test was conducted. The components of the molten steel were C: 0.09%, Si: 0.31%, Mn: 1.68%, P: 0.016%, S: 0.0008%, and Al: 0.007%. The molten steel temperature was 1570° C. and casting was performed at a speed of 1.2 m/min. The slab surface temperature at the entry side of the rolling mill was 1100°C.

The installed rolling mill was a two-high rolling mill, and the diameter of the work rolls was 600 mm. The reduction amount in the rolling mill was set to 30 mm. In an application example of the technology of the present invention, as in the continuous casting and rolling facility shown in FIG. Localized heating zones scattered along one longitudinal direction were formed.

In Example A to which the present invention was applied, the local heating portion was formed only on the upper surface side of the steel piece, and the lower surface side was air-cooled. Further, in Example B to which the present invention is applied, local heating portions are formed on both the upper surface and the lower surface of the steel slab. In either case, the local heating area extends over the entire width of the slab, the local heating depth is about 10 mm and the thickness is about 0.1 times the thickness of the billet, and the local heating length is about 15 mm and the thickness is about 0.1 times the thickness of the billet. It was formed so as to be 15 times larger. The interval between the local heating portions was approximately 60 mm (approximately 0.7 times the contact arc length). The heating area ratio was about 0.1 times in Example A and about 0.2 times in Example B. The temperature of the local heating portion was within the range of 1150-1200°C.

On the other hand, in Comparative Example a to which the prior art was applied, the steel slab was rolled without local heating. At this time, the surface temperature of the steel slab was approximately uniform and about 1100°C. Moreover, as a comparative example b, the case where the steel slab was not reduced at all by the rolling mill is shown.

Table 1 shows the rolling results of this example. λ0, d0, and A0 in Table 1 are the secondary arm spacing of the dendrite structure in Comparative Example b, and the average grain size (both are the average width of 100 mm in the center of the width of the billet after rolling × length 100 mm length × total thickness value) and the space factor of the porosity remaining in the center of the thickness (within a length of 1 m and a width of 600 mm). That is, λ/λ0, d/d0, and A/A0 represent the reduction rates of the secondary arm spacing, grain size, and porosity space factor. is high. In the remarks column, the shape of the material to be rolled after rolling or the surface properties during rolling and after cooling are described.

As shown in Table 1, in Examples A and B, compared with Comparative Example a, the primary arm spacing of the dendrite structure, the average crystal grain size, and the porosity space factor all decreased significantly. In Example A, a slight downward warp occurred after rolling, but it was so slight that there was no problem in subsequent sheet threading. From this, it is clear that the application of the present invention significantly improves the segregation, crystal structure, and internal defects of the slab delivered to subsequent processes without causing the problem of surface defects.

[Example 2]
In Example 2, as shown in FIG. 22, a slab having a thickness of 100 mm and a width of 600 mm was continuously cast in the same manner as in Example 1 using the same continuous casting machine as in Example 1 and a two-stage rolling mill at the end of the casting machine. A casting rolling test was performed. The components of the molten steel, the molten steel temperature, the casting speed, the amount of reduction, and the slab surface temperature at the entry side of the rolling mill were also substantially the same as in Example 1.

In Examples C to E to which the present invention is applied, as shown in FIG. 22, a local heating device is installed between the continuous casting machine and the rolling mill to locally heat both the upper and lower surfaces of the billet. As a result, localized heating zones were formed in the longitudinal direction of the billet. In either case, the local heating part has a local heating depth of about 10 mm (about 0.1 times the thickness of the billet) in a cross section configured in the billet thickness direction and the billet length direction. The distance between the parts was about 60 mm (about 0.7 times the contact arc length), and the length of the billet longitudinal direction of the local heating part was about 15 mm.

In Example C to which the present invention is applied, as in Example 1, a local heating portion is formed over the entire width of the rectangular steel piece. In Example D to which the present invention is applied, local heating portions having a width direction length of about 200 mm are formed on the left and right sides in a section from about 30 mm to about 230 mm based on the left and right width end face positions of the steel billet. In Example E to which the present invention is applied, a local heating portion is formed in the same section as in Example D so that the width center side of the billet is inclined 15° with respect to the width direction of the billet prior to the rolling direction. The heating area ratio was about 0.2 times for Example C and about 0.15 times for Examples D and E. The temperature of the local heating portion was within the range of 1150-1200°C.

On the other hand, in Comparative Example c to which the prior art was applied, the steel slab was rolled without performing local heating as in Example 1. At this time, the surface temperature of the steel slab was approximately uniform and about 1100°C.

Table 2 shows the rolling results of Example 2. A0 in Table 2 is the space factor (within a length of 1 m and a width of 600 mm) remaining at the center of the thickness of the billet after rolling in Comparative Example c. That is, A/A0 represents the reduction rate of each porosity space factor, and the smaller the value, the higher the degree of improvement in the internal quality of the billet. Also, in the remarks column, fluctuations in rolling torque during rolling and changes in the state of the rolling mill are described.

As shown in Table 2, in all of Examples C, D and E, the porosity space factor was significantly reduced compared to Comparative Example c. In Example C, rolling torque fluctuated significantly and some rolling mill vibration was detected. Including Example C, it was confirmed that the load fluctuations and vibrations during rolling did not pose a problem in terms of operation and facility maintenance. From this, it is clear that the application of the present invention can remarkably improve the internal defects of the billet delivered to the succeeding process without causing any problems in terms of rolling operation and facility maintenance.

Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention belongs can conceive of various modifications or modifications within the scope of the technical idea described in the claims. It is understood that these also naturally belong to the technical scope of the present invention.

Reference Signs List 1 mold 5 billet 5a solidified shell 5b unsolidified portion 5c local heating portion 5e increased deformation portion 5f corner supercooling portion 10 continuous casting machine 11 tundish 12 immersion nozzle 13 mold 14 secondary cooling device 14a support rolls 20, 70 heating device 21, 71 AC power supply 23, 73 Induction heating coil section 25 Switch 30, 80 Rolling mill 31a, 31b, 81a, 81b Rolling roll 40 Heat retaining device 41a, 41b Roller 43 Endless belt 45 Heat retaining member 50 Heating furnace 60 Scale removing device

Claims (12)

A method for rolling a rectangular cross-section steel billet for rolling a steel billet having a substantially rectangular cross-sectional shape, comprising:
Prior to rolling, at least one of the upper and lower surfaces of the billet in contact with the rolling rolls is intermittently heated or maintained in the longitudinal direction of the billet, so that the locally heated portions scattered in the longitudinal direction of the billet are heated. to form
A method for rolling a rectangular cross-section steel billet, wherein the steel billet on which the local heating portion is formed is rolled. 2. The method of rolling a rectangular cross-section billet according to claim 1, wherein the local heating portions are periodically formed in the longitudinal direction of the billet. The temperature of the local heating part is +10°C or higher than the average temperature of the portion other than the local heating part of the surface of the steel billet in contact with the rolling roll, and the temperature of the steel billet is 3. The method of rolling a rectangular cross-section steel billet according to claim 1 or 2, wherein the rectangular cross-section steel billet is formed so as to be below the melting point temperature. The local heating unit is
The local heating depth from the surface of the billet in the thickness direction of the billet is in the range of 0.01 times or more and 0.40 times or less as large as the thickness of the billet,
4. The method of rolling a rectangular cross-section billet according to claim 3 , wherein the local heating length in the rolling direction is in the range of 0.01 to 1.5 times the thickness of the billet. The local heating part is formed so that the surface area of the local heating part is in the range of 0.01 to 0.50 times the total surface area of the upper and lower surfaces of the billet in contact with the rolling rolls. The method for rolling a rectangular cross-section steel billet according to claim 3 or 4 . The said local heating part is formed so as to have an interval in the longitudinal direction of the billet that is 1/60 to 2 times the arc length of contact with the rolling rolls when the billet is rolled. 6. A method for rolling a rectangular cross-section billet according to any one of 3 to 5 . The method for rolling a rectangular cross-section billet according to any one of claims 1 to 6, wherein the local heating portions are formed dispersedly in the width direction of the billet. The temperature of the local heating part is +10°C or higher than the average temperature of the portion other than the local heating part of the surface of the steel billet in contact with the rolling roll, and the temperature of the steel billet is Formed to be below the melting point temperature,
8. The method of rolling a rectangular cross-section billet according to claim 7, wherein the width of said locally heated portion is 0.1 times or more the thickness of said billet. When the billet is viewed in plan, the shape of the local heating portion is oblique, polygonal, curved, periodic function, or zigzag that intersects a straight line extending along the width direction of the billet. The method for rolling a rectangular cross-section steel billet according to claim 7 or 8, wherein: The method for rolling a rectangular cross-section billet according to any one of claims 7 to 9, wherein the local heating section is formed so as to include a corner supercooled section of the billet. a continuous casting machine for casting billets;
is disposed downstream of the continuous casting machine and intermittently heats or retains at least one of the upper and lower surfaces of the billet cast by the continuous casting machine in the longitudinal direction of the billet to a heating device for forming local heating portions scattered in the direction of the billet;
a rolling mill disposed on the downstream side of the heating device for rolling the billet on which the local heating portion is formed;
Continuous casting and rolling equipment. At least one of the upper and lower surfaces of a billet heated in a heating furnace is intermittently heated or maintained in the longitudinal direction of the billet to form locally heated portions scattered in the longitudinal direction of the billet. a heating device that
a rolling mill disposed on the downstream side of the heating device for rolling the billet on which the local heating portion is formed;
A rolling facility.

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