JP3404253B2 - Method and apparatus for producing billet for bar steel - 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 and an apparatus for producing a steel strip for bar steel having a good internal quality, which is used for rolling a steel bar or a wire rod from a continuously cast slab.

[0002]

2. Description of the Prior Art Bloom is generally used as a raw material for steel strips for rolling steel bars and wire rods, which is manufactured by a continuous casting method. The continuous casting bloom is cast with a large cross-sectional size having a cross-sectional area of about 4 to 17 times that of the steel strip for bar steel, and after cutting and heating, the slab and steel are cut by a double reversing type or VH continuous slab mill. It is rolled in multiple passes to an intermediate size of the piece, and then formed into a predetermined size steel billet for bar steel by a double continuous type or VH continuous type billet rolling machine or the like. This method is suitable for a large-scale steelmaking furnace by combining the large cross-section of the slab size and the slab-casting method, and greatly contributes to the expansion of the continuous casting ratio and the improvement of productivity. However, since the slab heating time is as long as about 2 to 3 hours and the number of passes of the slabbing is large as about 10 to 20 times, the energy consumption required for heating and rolling increases and the rolling yield decreases, and the steel There is a problem that the manufacturing cost increases significantly because the amount of flaw maintenance in single-sided adjustment increases.

In recent years, billet continuous casting has been adopted or the cross-sectional size of bloom continuous casting has been reduced in the field of high-grade rods for the purpose of omitting and simplifying the slabbing process. However, in the reduction of the cross-sectional size in billet continuous casting or bloom continuous casting, internal cracks and center porosities that are inevitably formed in the solidification process of continuous casting are not pressure-bonded and tend to remain in the steel slab, and Since the crystal grain size of the steel slab is coarse, it is difficult for ultrasonic waves to penetrate, and ultrasonic flaw detection of internal defects such as inclusions is not possible.In addition, the rolling ratio up to the steel bar is small, resulting in poor mechanical properties. However, there is a problem that the applicable bar steel product diameter is limited.

As a method for improving the center porosity and the like of continuously cast slabs, many reports have hitherto been made. For example, Japanese Patent Publication No. 59-16862 and Japanese Patent Publication No. 2
Japanese Patent No. 56982 describes a method of preventing center segregation and center porosity by lightly reducing a slab in the final stage of solidification. These methods reduce the segregation and the center porosity by applying a light reduction to the slab at the final stage of solidification in the continuous casting machine, and cannot completely prevent it.

Several methods have been reported for applying a large reduction to a cast piece in a continuous casting machine. For example, iron and steel, 60 (197)
4), p. In 875, an in-line reduction method was reported in which a 120 mm square slab was rolled in a continuous casting machine to a non-solidified rolling or rolling after solidification to 30 mm square to 50 mm square to prevent internal cracks and surface cracks. The conditions, pressure bonding of center porosity, and refinement of solidification structure are described. Further, in Japanese Examined Patent Publication No. 8-4891, the slab is heat-insulated or heated to perform a reduction at a reduction ratio of 1.4 or more and less than 3.0 immediately before completion of solidification, and then detect inclusions by ultrasonic waves. How to do is Japanese Patent Publication No. 5-73508
Japanese Patent Laid-Open Publication No. Hei 6 (1994) -96 is a method in which the slab after completion of solidification is kept warm or heated and a reduction of 9% or more is applied before cutting.
JP-A-28784 describes a method for producing a slab for thick steel plates having excellent internal soundness by pressing center porosity by a pair of sheet pressure devices immediately after completion of solidification.

Next, the center porosity of the steel piece in the rolling process is generally pressure-bonded by the shape ratio (roll contact arc length / roll contact arc length /
It is said that as the average thickness of the steel strip) increases, the compressive stress also acts on the central portion and is more effective for crimping. Regarding the relationship between the center porosity pressure bonding and the shape ratio in the rolling process,
For example, Japanese Patent Laid-Open No. 61-23840 discloses a method in which a temperature difference of 400 ° C. or more is provided between the surface and the central portion, a shape ratio is 0.5 or more, and a rolling is performed by a press or a roll. According to the publication, rolling is performed for one pass or several passes at a shape ratio of 0.4 or less, and then the shape ratio is set to 0.
As 7 or more, a method of rolling at least one pass or more is described. This method is to manufacture thick steel plate by accumulating work strain in the surface layer part in the former rolling to increase the deformation resistance of the surface part and increasing the compressive stress in the central part in the latter rolling to press the center porosity. Is.

As another method, Japanese Patent Laid-Open No. 5-6900
Japanese Unexamined Patent Publication (Kokai) No. 1 describes a method of manufacturing a thick steel sheet having no internal defects by completely crimping center porosity in a low pressure reduction rolling with a reduction ratio of 1.5 to 2.0 from a slab. For iron and steel, 67 (1981) and S339, the strong reduction rolling method by improving the pass schedule is used. For iron and steel, 67 (1981) and S369, temperature reduction rolling and strong reduction by partial cooling of the width center part are performed. A method of combining rolling is described. Each of them relates to a method for rolling a thick steel plate which increases the compressive stress in the central portion and improves the pressure-bonding property of the center porosity.

[0008]

The center porosity of a continuously cast slab is formed due to solidification shrinkage at the end of solidification, and if it is attempted to prevent this, the slab at the end of solidification in a continuous casting machine is attempted. On the other hand, it is necessary to apply a reduction so as to compensate for the solidification shrinkage, but there is a problem that under the overpressure, molten steel locally flows and segregation deteriorates. Therefore, the unsolidified cast large rolling method that prevents the formation of center porosity by squeezing the molten steel in the axial center of the slab in the final stage of solidification to the upstream side, and pressing the center porosity immediately after completion of solidification Although a large reduction method after solidification can be considered, these methods require a considerable reduction force and reduction amount, and at the same time, internal cracking is likely to occur due to the reduction, and 0.5 to 3 m / Since a large reduction is performed in synchronization with a casting speed as low as about min, there are problems such as the generation of slab surface cracks and the shortening of the life due to the generation of thermal cracks in the reduction roll.

In order to achieve the omission and simplification of the slab rolling process, it is necessary to establish a method for producing a steel strip for a steel strip by rolling a medium cross section bloom at a low draw ratio. That is, the steel piece is generally subjected to ultrasonic flaw detection of inclusions before being rolled into a rod wire, but the extension of inclusions is small and relatively minute,
It is necessary to solve problems such as internal cracks and noise remaining when center porosity remains without being crimped, and when the crystal grain size is coarse, the ultrasonic permeability decreases and noise signals similarly rise. . S as detection performance of inclusions
It is necessary to secure the / N ratio of 3 or more. For that purpose, it is necessary to surely crimp internal cracks and center porosity and to set the grain size number of the steel slab to 5 or more.

The present invention is a means for efficiently obtaining a high-quality steel slab that has no surface flaws and is crimped with internal cracks and center porosity when rolling a continuously cast slab to obtain a steel strip for a steel strip. The challenge is to provide.

[0011]

The present invention solves the above-mentioned problems. That is, according to claim 1 of the present invention, in a method for producing a billet rolling billet from a continuously cast billet, the size and shape of the cross section of the billet is 140 mm on a side.
~ 300 mm square or a rectangle having a cross-sectional area equivalent to this and having a long side to short side ratio of 1.6 or less is continuously cast, and after solidification is completed, the cast piece is cut into a predetermined length and a predetermined temperature. Shape ratio (roll contact arc length / average billet thickness) ≧
Rolling in the thickness direction and width direction satisfying 1.0 is performed by at least one pass each, and is performed by 2 to 4 pass rolling so that the stretch ratio from the slab to the steel slab is 1.5 to 2.8. It manufactures steel billets.

According to claim 2 of the present invention, the diameter is 950 mm.
The method according to claim 1, wherein rolling is performed with a roll having a diameter of φ or more, and the rolling completion temperature of the billet according to claim 3 is 940 ° C. or more.
Alternatively, the method according to claim 2 is the method according to claim 1 or 2, wherein rolling is performed with a flat roll having no hole shape.

Further, claim 5 of the present invention is such that one side is 140 mm
A continuous casting machine that continuously casts a rectangular slab of 300 mm square or a cross-sectional area equivalent to this with a long side to short side ratio of 1.6 or less, and a casting that cuts to a predetermined length after solidification is completed. A piece cutting machine, a slab heating furnace for charging a slab cut by the slab cutting machine and heating the slab to a predetermined temperature, and a diameter for rolling the heated slab into a steel slab having a predetermined cross-sectional size of 950 mmφ An apparatus for manufacturing a steel strip for a steel strip, characterized in that the rolling mill comprising one stand or a plurality of stands having the above flat rolls is sequentially arranged.

First, in the present invention, the stretch ratio (ratio of the cross-sectional area of the slab / the cross-sectional area of the steel slab) is 2 to 4 so as to be 1.5 to 2.8.
The reason for pass rolling to form a steel slab, and a slab size and shape of a square of 140 mm □ to 300 mm □ or a rectangle having a cross-sectional area equivalent to this and a long side to short side ratio of 1.6 or less. The reason for doing so will be described below. As mentioned above, the steel piece is subjected to ultrasonic flaw detection for the purpose of inspecting inclusions before rolling it into a rod or wire rod, but for this purpose it is necessary to refine the crystal grains of the steel piece to enhance ultrasonic permeability. is there. However, since the stretch ratio of medium cross-section bloom is smaller than that of large cross-section bloom, in slabbing, the amount of reduction per pass is increased to apply a reduction force from the surface of the slab to the center to destroy the cast structure. There is a need to.

According to the investigation by the inventors, there is a close relationship between the drawing ratio in the slab rolling, the number of passes and the austenite grain size of the obtained steel slab. As shown in FIG. 1, the drawing ratio is 1.5 or more. It was found that a fine structure of 5 or more can be stably obtained when 2 to 4 pass rolling is performed. As is clear from the figure, when the drawing ratio is less than 1.5 and 2 to 4 passes are rolled, or even when the drawing ratio is 1.5 or more, the reduction amount per pass is small and multi-pass rolling of 5 or more passes is performed. In that case, 5 or more cannot be obtained. When the austenite grain size is 5 or more, ultrasonic waves sufficiently penetrate into the steel slab, so S
The / N ratio is improved and inclusions can be detected. For the above reason, in the present invention, the stretching ratio from the cast slab to the steel slab is specified to be 1.5 or more, and the rolling is performed for 2 to 4 passes.

Next, the size of the billet for bar steel is 115 mm □
Although 180 mm □ is generally used, Table 1 can be obtained by obtaining a representative example of a slab size capable of rolling these steel slabs in 2 to 4 passes. Here, as a precondition, the maximum surface reduction rate per pass is 30% or less, the axial ratio (dimension ratio of the long side / short side of each pass) is 1.8 or less, and the width expansion due to the reduction is 1 / th of the reduction amount.
It was calculated by approximating to 3. If the area reduction ratio or axial ratio is larger than these values, the biting angle becomes too large and slippage occurs, or the steel pieces are easily twisted or fallen and the workability of rolling decreases. Value.

From Table 1, it can be seen that the 115 mm □ steel slab has two passes from a 140 mm □ slab with a draw ratio of 1.5 and a draw ratio of 1.5.
Stretching ratio of 2.8 in 3 passes from a 124 mm × 158 mm slab
It can be rolled in 4 passes from a 194 mm square slab. In addition, 180 mm square steel slab is two passes from 240 mm square slab with a stretch ratio of 1.8, three passes from 210 mm x 330 mm slab with a stretch ratio of 2.1, and 300 mm square slab with a stretch ratio of 2.8. Each can be rolled in 4 passes. If the stretch ratio exceeds 2.8, the area reduction ratio exceeds 30% and the axial ratio exceeds 1.8, so 4
It becomes difficult to form a billet by pass rolling. If the stretching ratio exceeds 2.8, multi-pass rolling of 5 passes or more is performed, and fine crystal grains cannot be obtained. For the reasons described above, in the present invention, the stretch ratio from the cast piece to the steel piece is 1.5 to 2.
The billet rolling is performed in 2 to 4 passes, which is defined as 8. Regarding the shape of the slab, the minimum cross-sectional size and the maximum cross-sectional size that can be formed under the rolling conditions according to the present invention are set to the general 115 mm □ to 180 mm □ as a steel strip for bar steel,
It is a square of 140 mm □ to 300 mm □ or a rectangle having a long side to short side ratio (flatness ratio) of 1.6 or less. still,
In the present invention, the slab shape is a square or a rectangle in consideration of the degree of difficulty in forming the steel slab, but it may be a circle that satisfies the above conditions.

[0018]

[Table 1]

Next, in the present invention, the reason why the rolling in the thickness direction and the width direction satisfying the following formula (1) is performed by at least one pass each will be explained. L _d / H _m = √ {R (h ₀ −h ₁ )} / {1/2 (h ₀ + h ₁ )} ≧ 1.0 (1) Here, L _d is the roll contact arc length, H _m is the average billet thickness,
R is the roll radius, h ₀ is the thickness of the entering steel piece, and h ₁ is the thickness of the exiting steel piece.

The inventors have changed the amount of reduction and the roll diameter from a slab of 140 mm □ to 300 mm □ to 115 mm □ to 180 mm.
An experiment in which the steel piece of □ was formed by 2 to 4 pass rolling was conducted, and the center porosity inside the steel piece and the residual state of internal cracks were investigated. The results of the center porosity investigation are shown in Fig. 2, and the results of internal cracking are shown in Fig. 3. In both figures, L _d / H _{m on} the horizontal axis is the first pass and the second pass in the case of 2-pass rolling, and the second pass and the third pass in the case of 3-pass rolling.
In the case of 4-pass rolling, the values of the 3rd and 4th passes are shown.

[0021] First, center porosity thickness 2 reduces with increasing _{L d / H m, L d} / H m ≧
It is crimped at 1.0 or more. Further, as shown in FIG. 3, the internal cracking decreases with an increase in L _d / H _m as in the case of the center porosity, and L _d / H _m ≧ 0.8.
The above is crimped. In this investigation, for the center porosity and internal cracking, the steel piece was ultrasonically flaw-detected and the thickness and length were measured by combining microscopic inspection of the defective portion.
Although not shown in the figure, the thickness of the center porosity in the cast slab was 0.5 to 3.0 mm, and internal cracks were 2 to 10 mm in which corner cracks, bulging straightening cracks, and center cracks were observed. . As is clear from both figures, by satisfying L _d / H _m ≧ 1.0 or more in at least one pass in each of the thickness direction and the width direction, it is possible to press-bond the internal crack and the center porosity at the same time. . From the above, when forming a steel slab by 2-pass rolling, the shape ratio of the first pass and the second pass must satisfy the formula (1), and when forming by 3-pass rolling and 4-pass rolling. Is such that the shape ratio of each one pass satisfies Expression (1) even if at least rolling in the thickness direction and the width direction is performed.

Next, the roll diameter according to claim 2 of the present invention is set to 95.
The reason why it is set to 0 mmφ or more will be described. Generally, the larger the roll diameter, the more the compressive force and shearing force acting on the inside of the steel slab increase, which is advantageous for pressure bonding such as center porosity.
If it is too large, equipment costs and running costs increase, which is a problem. Therefore, it is necessary to set the minimum roll diameter that satisfies the expression (1). As known from Table 1, 11
A steel piece of 5 mm square can be formed from a slab of 140 mm square to 194 mm square under the conditions of a draw ratio of 1.5 to 2.8 and rolling of 2 to 4 passes. In this case, the minimum roll diameter that satisfies the formula (1) is calculated from the relationship between the roll diameter and the shape ratio, and it is 950 mmφ.
Is obtained. Also, as a typical example, a 115 mm □ steel piece is used.
In the case of molding from a slab of 40 mm □ to 155 mm □ by two-pass rolling, the minimum roll diameter whose shape ratio satisfies the expression (1) in the second pass having a smaller shape ratio than the first pass is shown in FIG. 950 mmφ can be obtained. From the above, in the present invention, the roll diameter is defined as 950 mmφ or more. In addition, the roll diameter for rolling the billet for bar steel is 1300 mm from the viewpoint of equipment cost, running cost, maintainability, etc.
φ or less is desirable.

Next, the reason why the slab is cut into a predetermined length after completion of solidification and then heated and rolled will be described below. The inventors of the present invention have found that a roll diameter of 7 is provided upstream of the cutting machine in the continuous casting machine.
Install a 20mmφ VH type rolling mill, and immediately after completion of solidification, 2
An experiment was conducted in which a 222 mm square slab was pass-rolled into a 165 mm square slab. In this case, since the center temperature is as high as about 1300 to 1350 ° C. due to the sensible heat of the slab, internal cracking and center porosity were pressure-bonded with a relatively small shape ratio (L _d / H _m = 0.98, 0.84). However, deep cracks and flaws were observed mainly in the corners of the steel slab. That is, since the rolling is performed in synchronization with the casting speed (Vc = 1.0 to 2.0 m / min), the rolling speed is slow and the strain rate is small, and the billet temperature at the roll contact portion is lowered, so It was inferred that deep cracks had occurred at one corner. As a result of raising the rolling temperature by installing a heat insulating cover and a heating device in front of the rolling mill,
The flaw depth was reduced, but it was not completely eliminated and required maintenance of the billet, and the rolling roll had an extremely low rolling time, so the contact time with the billet was long and thermal cracking occurred, resulting in a significant decrease in life. did.

In the present invention, based on the above knowledge, the slab is cut after completion of solidification, heated, and then rolled. As a result, high speed rolling of about 20 to 100 m / min is possible, cracks can be prevented from occurring even at a low rolling temperature, and the problem of roll life shortening can be solved.

Next, in claim 3 of the present invention, after the cut slab is heated to a predetermined length, the rolling is completed at a steel slab surface temperature of 940 ° C. or higher. The reason for this will be described below. Two
In ~ 4 pass / low draw ratio rolling, it is necessary to improve hot ductility because the amount of reduction increases and the stress on the surface of the billet increases. Therefore, the present inventors
Casting into a 2 mm square slab, cutting and heating the slab, and performing 2-pass rolling to obtain a 165 mm square slab, the effect of the rolling temperature on the surface flaw was investigated. Rolling condition is roll diameter 1
Using a flat roll or dia square roll with no hole shape of 010 mmφ, the reduction amount per pass is 85 mm,
The shape ratio was 1.16 for the first pass and 1.00 for the second pass.
The results obtained are shown in FIG. 5, which is 940 under the above rolling conditions.
By completing the rolling at a temperature of ℃ or higher, no surface defects are generated. This indicates that the hot ductility is improved and cracks can be prevented by increasing the rolling completion temperature. From the above, in the present invention, rolling is completed at 940 ° C or higher.

On the other hand, comparing the effect of the roll shape in FIG. 5, the flat roll having no hole type has a shallower flaw depth than the dia square roll having a hole type, and the rolling completion temperature at which no flaw occurs is 910 ° C. It is low and it is advantageous. Therefore, FIG. 6 shows results obtained by performing FEM analysis on the relationship between the roll shape and the surface stress of the billet corner portion. The analysis conditions are: effective roll diameter 1010 mmφ, rolling temperature 1000 ° C., rolling speed 36 m / min, steel grade S4.
It was compared as 5C. As a result, it was confirmed that the flat roll was advantageous in that the tensile stress was small near the roll exit side. From the above experiment and analysis results, in claim 4 of the present invention, the roll is defined as a flat roll having no hole type.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION An example of an embodiment of the present invention is shown in FIG. 1 is a ladle, 2 is a tundish, 3 is a mold, 4 is a secondary cooling zone, 5 is a guide roll, 6 is a slab cutting machine, 7 is a slab after cutting, 8 is a slab heating furnace, and 9 is a slab heating furnace. A billet rolling machine consisting of one stand or a plurality of stands is a billet for steel strip 10.

Using the mold 3, the cross-sectional shape is 140 mm □
~ 300 mm square medium-section bloom of machine structural steel is cast, and after the solidification is completed, the slab 7 cut into a predetermined length by a slab cutting machine 6 is heated to a predetermined temperature in a heating furnace 8 and then billet-rolled. 115mm □ ～ 180 in 2 to 4 passes using machine 9
It was formed into a steel piece 10 of mm □. Billet rolling is 950mm
A flat roll with a diameter of φ or more and a shape ratio of L _d /
The rolling in the thickness direction and the width direction satisfying H _m ≧ 1.0 is rolled at least for each one pass so that the stretch ratio from the slab to the steel slab is 1.5 to 2.8, and the rolling is performed. The completion temperature was 940 ° C. or higher. The obtained steel piece was free of surface flaws and was subjected to ultrasonic flaw detection. As a result, internal cracks and center porosity were completely crimped, and the inclusion detection performance was satisfactory.

Incidentally, in the present invention, the cast slab 7 after cutting is inserted into the heating furnace as it is as a hot piece, but for the purpose of preventing the occurrence of surface defects due to heating or / and billet rolling,
If necessary, a slab cooling device may be installed between the continuous casting machine and the slab heating furnace, and the slab may be once cooled to a predetermined temperature and then charged into the heating furnace. Further, in the present invention, 2 to
Although 4-pass rolling is performed to form a steel slab, skin pass rolling for the purpose of adjusting the shape of the slab and improving dimensional accuracy may be performed, if necessary.

[0030]

EXAMPLES Examples will be described in detail below. Melting steel for machine structure in a converter with a heat size of 260 tons,
In the curved 6-strand bloom continuous casting machine with a radius of curvature of 12 m shown in FIG. 7, the molten steel in the ladle 1 is replaced with the tundish 2
Through the mold into the mold 3 and the cross-sectional size is 140 mm □ ～ 3
A square slab of 00 mm □ or a rectangular slab having a cross-sectional area equivalent to this and an aspect ratio of 1.6 or less is cast at a speed of 1.0 to 3.0.
Cast at m / min.

After the solidification is completed, the slab cutting machine 6 cuts the slab 7 into a predetermined length, and the slab 7 is heated in a heating furnace 8 to have an average cross-section temperature of 990 to 1
After heating to 140 ℃, roll diameter 950mmφ
Using a VH type continuous rolling mill 9 composed of flat rolls of ˜1300 mmφ, a steel piece 10 of 115 mm □ to 180 mm □ was formed so as to be completed in 2 to 4 pass rolling at 940 ° C. or higher. Table 2 compares the examples of the present invention with the comparative examples.

[0032]

[Table 2]

In the method according to the present invention, since the proper roll diameter, reduction amount, and number of passes are selected in accordance with each slab size, the predetermined stretch ratio and shape ratio are satisfied, and The resulting steel slab has no austenite defects and has an austenite grain size number of 5 or more, and internal cracks and center porosity are completely crimped, so the S / N required for ultrasonic flaw detection of the steel slab The ratio of 3 or more was satisfied, and the detection performance of inclusions was sufficient.

On the other hand, in the comparative example in which the slab is rolled for 2 to 4 passes and the shape ratio is less than 1.0, the center porosity is not pressure-bonded and noise is generated in ultrasonic flaw detection of the steel slab. No S / N ratio was obtained. Further, in the comparative example in which the billet rolling was not performed, the crystal grains were coarse and internal cracks and center porosity remained, so noise was generated during ultrasonic flaw detection, and inclusions could not be detected. On the other hand, in a comparative example in which a slab is formed by performing multipass slabbing and billet rolling from a large cross-section bloom, ultrasonic flaw detection of the slab was possible, but the manufacturing cost related to slab heating and rolling was a big problem. Met.

FIG. 8 shows a typical example of the temperature transitions of the slab and the steel slab from casting to completion of rolling (average cross-section temperature from casting to extraction, and center / surface / corner temperature from extraction to completion of rolling). Indicates. In the example of the present invention, a 222 mm square slab was cast at a casting speed of 1.8 m / min, charged into a heating furnace after cutting, heated for 35 minutes and extracted at an average cross-section temperature of 1040 ° C.
This is an example of forming into a 165 mm square steel piece by an H type continuous rolling mill. On the other hand, in the comparative example, a 400 × 500 mm slab was cast at a casting speed of 0.65 m / min, and after cutting, it was heated to 1170 ° C. at the average cross-section temperature in a heating furnace and extracted, and then 16 passes with a double reversing type rolling mill. This is an example in which a slab of 250 mm square was rolled to form a 165 mm square billet which was then subjected to 4-pass rolling by a double continuous rolling mill. Comparing the two, the rolling completion temperature is adjusted to 940 ° C. However, in the case of a large cross-section bloom, it takes about 7 to 8 minutes from the heating furnace extraction to the rolling completion, so it takes from extraction to rolling. Until the completion, the corner temperature has dropped by about 230 ° C. On the other hand, in the embodiment of the present invention, since the time from extraction to completion of rolling is short at about 40 to 60 seconds due to 2 to 4 pass rolling, the corner temperature from extraction to completion of rolling is about 100 ° C. It only needs to be lowered. As described above, in the example of the present invention, the heating furnace extraction temperature can be lowered, and the energy saving effect can be enjoyed.

Furthermore, FIG. 9 shows the relationship between the thickness of the slab, the thickness of the slab, and the stretch ratio in comparison. Comparative Example 1 shows a billet continuous casting method in which slabbing rolling is omitted, and Comparative Example 2 shows a large-section bloom continuous casting / slumping method in which slabbing rolling and billet rolling are performed. The present invention uses a slab of 140 mm □ to 300 mm □ to 115 mm □
This is a method of forming steel pieces of 180 mm square with a draw ratio of 1.5 to 2.8. As shown in Table 2, ultrasonic flaw detection of the steel pieces is possible, and the production cost of the steel pieces is significantly low. Can be said to be a method.

[0037]

Industrial Applicability According to the present invention, even when a medium cross-section bloom is heated for a short period of time and rolled with a small number of passes, the cast structure is efficiently refined and internal cracks and Since center porosity is crimped, it enables ultrasonic flaw detection of steel slabs and greatly simplifies the slab rolling process, reducing heating fuel and electric power for rolling slabs and improving rolling yield. It is possible to reduce the manufacturing cost,
Its industrial value is extremely high.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a draw ratio, the number of passes, and an austenite grain size of a steel slab.

FIG. 2 is a diagram showing the relationship between L _d / H _m and the billet center porosity thickness.

FIG. 3 is a diagram showing a relationship between L _d / H _m and a steel piece internal crack length.

FIG. 4 is a typical example of a relationship between roll diameter and L _d / H _m .

FIG. 5 is a diagram showing a relationship between rolling temperature and billet surface flaw depth.

FIG. 6 is a view showing a result of FEM analysis on a roll shape and a surface stress of a billet corner portion.

FIG. 7 is an overall schematic diagram showing an example of an embodiment of the present invention.

FIG. 8 is a comparative diagram of temperature changes of a cast slab and a steel slab from casting to completion of rolling.

FIG. 9 is a comparison diagram of the thickness of the slab, the thickness of the slab, and the draw ratio.

[Explanation of symbols]

1 Ladle 2 Tundish 3 Mold 4 Secondary Cooling Zone 5 Guide Roll 6 Slab Cutting Machine 7 Slab after Cutting 8 Slab Heating Furnace 9 Billet Rolling Machine 10 with One Stand or Multiple Stands 10 Steel Strip for Steel

Front page continuation (72) Inventor Yasushi Ishibashi 12 Nakamachi, Muroran City, Hokkaido Inside Nippon Steel Co., Ltd. Muroran Ironworks (72) Inventor Takashi Yatsuka 12 Nakamachi, Muroran Hokkaido Hokkaido Nippon Steel Co., Ltd. Muroran In-house (72) Inventor Koichi Isobe 12 Naka-machi, Muroran-shi, Hokkaido Nippon Steel Co., Ltd. Muroran In-house (56) Reference JP-A-10-180307 (JP, A) JP-A-53-1156 (JP) , A) JP 62-16802 (JP, A) JP 55-33840 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B21B 1/00-1/46 B22D 11/00-11/22

Claims (5)

(57) [Claims] 1. A method for producing a steel slab for bar steel from a continuously cast slab by billet rolling, wherein the slab has a transverse cross-sectional size and shape of 140 mm to 300 mm square or a long side having a cross-sectional area equivalent to this. Continuously cast into a rectangle having a ratio of the shorter side to 1.6 or less, and after solidification is completed, the slab is cut into a predetermined length and heated to a predetermined temperature, and then the shape ratio (roll contact arc length / average steel slab). (Thickness) ≧ 1.0, at least one pass each in the thickness direction and width direction is rolled so that the stretch ratio from the slab to the steel slab becomes 1.5 to 2.8.
A method for producing a billet for a strip steel, which comprises producing a billet by 4-pass rolling. 2. The method according to claim 1, which comprises rolling with a roll having a diameter of 950 mmφ or more. 3. The method according to claim 1, wherein the rolling completion temperature of the steel slab is 940 ° C. or higher. 4. The method according to claim 1, wherein rolling is performed with a flat roll having no hole shape. 5. A continuous casting machine for continuously casting a rectangular slab having a side length of 140 mm to 300 mm or a cross-sectional area equivalent thereto and having a long side to short side ratio of 1.6 or less, and after solidification is completed. A slab cutting machine for cutting to a predetermined length, a slab heating furnace for charging a slab cut by the slab cutting machine and heating the slab to a predetermined temperature, and a heated slab having a predetermined cross-sectional size An apparatus for producing a steel strip for a steel strip, characterized in that a rolling mill comprising one stand or a plurality of stands having a flat roll having a diameter of 950 mmφ or more for rolling on a steel strip is sequentially arranged.