JPH10305301A - Manufacturing method and device for bar steel billet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain bar steel billet having no surface defects and internal check or center porosity therein is ensurely crimped, by specifying the size and form of the cross-section of the continuous casting piece and after the completion of solidification, by cutting the casting piece in a given length, and heating to a given temperature, and by rolling it so that the form ratio and the drawing ratio stand for given values. SOLUTION: The continuous casting is conducted so that the cross-section of the casting piece is formed in a perfect square of which a side length is to be 140 to 300 mm, or is to be formed in a rectangular which has the equivalent section area of the square and the ratio of the major side to the minor side is less than 1.6. The billets are cut off in a given length after the completion of the solidification, and are heated to a given temperature. The billets are rolled 1 pass each of the perpendicular and cross direction so that the form ratio meets (the arc of contact/the average casting thickness) >=1.0. Two to four pass rolling is carried out so that the drawing ratio of the casting thickness to the billet is 1.5 to 2.8. It is desirable that the rolling works are conducted by flat rolls without pores having a diameter above 950 mmϕ. It is also desirable that the temperature upon completion of the rolling is higher than 940 deg.C.

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 manufacturing a steel slab having good internal quality, which is used for rolling a bar or a wire from a continuously cast slab.

[0002]

2. Description of the Related Art Bloom manufactured by a continuous casting method is generally used as a raw material of a steel slab for rolling a bar or a wire rod. 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 a steel slab, cut and heated, and then slabs and steel are cast by a double reversing type or VH continuous type bulking mill. It is rolled to an intermediate size of a piece in multiple passes, and then formed into a strip of steel of a predetermined size by a double continuous or VH continuous billet rolling mill or the like. This method matches a large steelmaking furnace by combining a large section of the slab size and the sizing method, and greatly contributes to an increase in the continuous casting ratio and an improvement in productivity. However, since the slab heating time is as long as about 2 to 3 hours and the number of passes of the slab rolling is as large as about 10 to 20 times, the energy consumption required for hot rolling increases and the rolling yield decreases, and There is a problem that the production cost is greatly increased due to an increase in the amount of flaw repair in one-sided adjustment.

[0003] In recent years, in the field of high-grade rods and bars, the use of continuous billet casting or the reduction of the cross-sectional size in bloom continuous casting has been carried out for the purpose of omitting and simplifying the slab rolling process. However, in reducing the cross-sectional size in billet continuous casting or bloom continuous casting, internal cracks and center porosity unavoidably formed in the solidification process of continuous casting are likely to remain on the billet without being pressed, and Ultrasonic penetration becomes difficult due to the coarse grain size of the slab, making it impossible to ultrasonically detect internal defects such as inclusions.In addition, the mechanical properties deteriorate due to the small rolling ratio up to the bar. However, there is a problem that the diameter of the applicable bar steel product is limited.

[0004] Numerous reports have hitherto been made on methods for improving the center porosity and the like of continuous cast slabs. For example, Japanese Patent Publication No. 59-16682 and Japanese Patent Publication No.
JP-A-569982 describes a method for preventing center segregation and center porosity by reducing the slab at the end of solidification slightly. These methods are intended to reduce segregation and center porosity by applying a slight reduction to a slab at the final stage of solidification in a continuous casting machine, but do not completely prevent them.

[0005] Several methods have been reported for applying a large pressure reduction to a slab in a continuous casting machine. For example, iron and steel, 60 (197
4), p. 875 reports an in-line reduction method in which unsolidified rolling or solidification rolling is performed so that a 120 mm square slab becomes 30 mm square to 50 mm square by a rolling mill provided in a continuous caster, and prevention of internal cracks and surface cracks Conditions, center porosity crimping, and refining of the solidified structure are described. Further, Japanese Patent Publication No. 8-4891 discloses that a slab is kept or heated and reduced immediately after completion of solidification, at a reduction ratio of 1.4 or more and less than 3.0, and then inclusions are detected by ultrasonic waves. The method of doing is Japanese Patent Publication 5-73508.
Japanese Patent Application Laid-Open No. Hei 6 (1994) -118555 discloses a method in which a slab after solidification is completed is kept warm or heated and a reduction of 9% or more is applied before cutting.
Japanese Patent No. 28784 describes a method of manufacturing a slab for a thick steel plate having excellent internal soundness by pressing center porosity by a pair of planar presses immediately after completion of solidification.

[0006] Next, in the rolling process, the center porosity of the steel slab is generally compressed by the shape ratio (roll contact arc length / roll contact length / roll length).
It is said that as the average billet thickness increases, the compressive stress also acts on the central portion and is effective for pressure bonding. For the relationship between center porosity compression and shape ratio in the rolling process,
For example, Japanese Patent Application 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 center, the shape ratio is 0.5 or more, and pressing is performed by a press or a roll. In the publication, rolling is performed in one or several passes at a shape ratio of 0.4 or less, and then the shape ratio is reduced to 0.
A method of performing rolling of at least one pass or more as 7 or more is described. This method is to produce thick steel plates by accumulating work strain in the surface layer by rolling in the first stage to increase the deformation resistance of the surface layer, and by increasing the compressive stress in the center by rolling in the second stage and pressing the center porosity. It is.

Another method is disclosed in Japanese Patent Application Laid-Open No. 5-6900.
No. 1 describes a method of completely pressing a center porosity in a low reduction ratio rolling with a reduction ratio of 1.5 to 2.0 from a slab to produce a thick steel plate having no internal defects. For iron and steel, 67 (1981) and S339, the heavy rolling method by improving the pass schedule is used, and for iron and steel, 67 (1981) and S369, the temperature difference rolling and the strong rolling method by partial cooling at the center of the width are used. A method of rolling combination is described. All of these methods relate to a method of rolling a thick steel plate that increases the compressive stress at the center and improves the pressure resistance of the center porosity.

[0008]

The center porosity of a continuous cast slab is formed at the end of solidification due to solidification shrinkage. 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 the molten steel flows locally due to overpressure and the segregation deteriorates. In the late stage of solidification, the molten steel at the shaft center is squeezed to the upstream side to prevent the formation of center porosity. Although a large rolling method after solidification is also conceivable, these methods require considerable rolling force and reduction, and at the same time, internal cracks are easily generated by rolling, and depending on the slab size, 0.5 to 3 m / m Since the large reduction is performed in synchronization with the low casting speed of about min, there are problems such as generation of cracks on the surface of the slab and shortening of service life due to generation of thermal cracks of the reduction roll.

In order to achieve omission and simplification of the slab rolling process, it is necessary to establish a method of manufacturing a billet for a bar by low-drawing-ratio rolling from a medium-section bloom. In other words, the billet is generally subjected to ultrasonic flaw detection of inclusions before rolling into a rod or wire, but the elongation of the inclusions is relatively small,
It is necessary to solve the problem that an internal crack or a center porosity remains without being pressed, resulting in a noise signal, and that if the crystal grain size is large, the permeability of ultrasonic waves is reduced and the noise signal is similarly increased. . S as detection performance of inclusions
It is necessary to ensure that the / N ratio is 3 or more. For this purpose, it is necessary to securely press the internal cracks and center porosity and to make the grain size number of the steel slab 5 or more.

[0010] The present invention is a means for efficiently obtaining a high-quality steel slab having no surface flaws and having internal cracks and center porosity crimped thereon in rolling a continuous cast slab to obtain a steel slab for a bar. The task is to provide

[0011]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems. That is, claim 1 of the present invention relates to a method for producing a steel strip for billet by billet rolling from a continuous cast slab, wherein the size and shape of the cross section of the slab are 140 mm on a side.
It is continuously cast as a square having a cross section of ~ 300 mm or a rectangle having an equivalent cross-sectional area and a ratio of long side to short side of 1.6 or less. After solidification is completed, the slab is cut into a predetermined length and subjected to a predetermined temperature. Shape ratio (roll contact arc length / average billet thickness) ≧
Rolling in the thickness direction and width direction satisfying 1.0 at least one pass at a time, and rolling in two to four passes so that the draw ratio from the slab to the slab is 1.5 to 2.8. It is used to produce billets.

The second aspect of the present invention has a diameter of 950 mm.
The method according to claim 1, wherein the rolling is performed by a roll having a diameter of not less than φ, and the rolling completion temperature of the slab is set to 940 ° C or more.
The method according to claim 2 or 4 is the method according to claim 1 or 2, wherein rolling is performed by a flat roll having no hole shape.

[0013] Further, claim 5 of the present invention is that a side of 140 mm or more.
A continuous casting machine for continuously casting a 300 mm square or rectangular cast piece having a cross-sectional area equivalent to this and having a long side to short side ratio of 1.6 or less, and a casting for cutting to a predetermined length after solidification is completed. A slab 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 of 950 mmφ for rolling the heated slab to a slab of a predetermined cross-sectional size. An apparatus for producing a steel bar for billet, characterized in that a rolling mill comprising one or a plurality of stands having the above flat rolls is sequentially arranged.

First, in the present invention, the stretching ratio (ratio of slab cross-sectional area / steel slab cross-sectional area) is set to 2 to 4 so as to be 1.5 to 2.8.
The reason for forming into a slab by pass rolling, and a slab size and shape of a square of 140 mm □ to 300 mm □ or a rectangle having an equivalent cross-sectional area and a ratio of long side to short side of 1.6 or less. The reason for this will be described below. As described above, before the slab is rolled into a rod or wire, ultrasonic inspection is performed for the purpose of inspecting inclusions, but for this purpose it is necessary to refine the crystal grains of the slab to increase the permeability of ultrasonic waves. is there. However, since the middle section bloom has a smaller elongation ratio than the large section bloom, in slab rolling, the rolling reduction per pass is increased and the rolling force is applied from the surface of the slab to the center to destroy the cast structure. There is a need to.

According to the investigations by the inventors, there is a close relationship between the draw ratio, the number of passes, and the austenite grain size of the obtained steel slab in the slab rolling, and as shown in FIG. , And it was found that a fine structure of 5 or more was obtained stably when rolling was performed in 2 to 4 passes. As is clear from the figure, when the stretching ratio is less than 1.5 and rolling is performed in 2 to 4 passes, or when the stretching ratio is 1.5 or more, the rolling reduction per pass is small and multi-pass rolling is performed in 5 passes or more. In this case, 5 or more cannot be obtained. If the austenite grain size number is 5 or more, ultrasonic waves sufficiently penetrate into the
The / N ratio is improved, and inclusions can be detected. For the above reasons, in the present invention, the stretching ratio from the slab to the slab is set to 1.5 or more, and rolling is performed in 2 to 4 passes.

Next, the size of the billet for the bar steel is 115 mm square or more.
180 mm is generally used. Table 1 is obtained by obtaining a typical example of a slab size that can roll these steel slabs in two to four passes. Here, as a precondition, the maximum area reduction rate per pass is 30% or less, the axial ratio (the dimensional ratio of the long side / short side of each pass) is 1.8 or less, and the width expansion due to the reduction is 1/1 of the reduction amount.
Approximately 3. If the area reduction ratio or the axial ratio is larger than these values, the bite angle becomes too large, causing slipping or twisting or tipping of the steel slab, which reduces the workability of rolling, so the upper limit is set. Value.

From Table 1, it can be seen that a 115 mm square slab can be obtained from a 140 mm square slab having an elongation ratio of 1.5 in two passes.
Of 124mm x 158mm slab in 3 passes with a draw ratio of 2.8
194mm slab can be rolled in four passes. In addition, a 180 mm steel slab is drawn from a 240 mm slab with a draw ratio of 1.8 in two passes, a 210 mm x 330 mm slab with a draw ratio of 2.1 in three passes, and a 300 mm slab with a draw ratio of 2.8. Each can be rolled in four passes. When the stretching ratio exceeds 2.8, the area reduction rate exceeds 30% and the axial ratio exceeds 1.8, so that
It becomes difficult to form into a billet by pass rolling. On the other hand, when the stretching ratio exceeds 2.8, multi-pass rolling of 5 passes or more is performed, so that fine crystal grains cannot be obtained. For the reasons described above, in the present invention, the draw ratio from a slab to a steel slab is 1.5 to 2.
8, and billet rolling is performed in 2 to 4 passes. And for the slab shape, take the minimum cross section size and the maximum cross section size that can be formed under the rolling conditions according to the present invention from 115 mm square to 180 mm square as a steel bar for strip steel,
It is a square of 140 mm 300 to 300 mm or a rectangle having a ratio of long side to short side (flat ratio) of 1.6 or less. still,
In the present invention, the shape of the slab is square or rectangular in consideration of the difficulty of forming the slab, but it may be a circle satisfying the above conditions.

[0018]

[Table 1]

Next, in the present invention, the reason why the rolling in the thickness direction and the width direction in which the shape ratio satisfies the following equation (1) is performed at least for each one pass will be described. L _d / H _m = {R (h ₀ −h ₁ )} / {1/2 (h ₀ + h ₁ )} ≧ 1.0 (1) where L _d roll contact arc length, H _m is the average steel strip thickness,
R is roll radius, h ₀ enters side slab thickness, h ₁ is the outlet side steel strip thickness.

The inventors changed the rolling amount and the roll diameter from a 140 mm square to 300 mm square slab to 115 mm square to 180 mm square.
An experiment in which the slab of □ was formed by rolling in 2 to 4 passes was conducted, and the center porosity inside the slab and the remaining state of internal cracks were investigated. FIG. 2 shows the results of the center porosity investigation, and FIG. 3 shows the results of internal cracking. In both figures, L _d / H _{m on} the horizontal axis is the first and second passes in the case of two-pass rolling, the second and third passes in the case of three-pass rolling,
In the case of four-pass rolling, the values of the third and fourth 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. Also, as shown in FIG. 3, the internal cracks decrease with an increase in L _d / H _m , as in the case of the center porosity, and L _d / H _m ≧ 0.8.
This is the crimping. In the present investigation, the center porosity and internal cracks were measured for thickness and length by using a steel slab to detect flaws with ultrasonic waves and combining microscopic inspection of defects.
Although not shown in the figure, the center porosity thickness of the slab was 0.5 to 3.0 mm, and the internal cracks were 2 to 10 mm in length with corner cracks, bulging correction cracks, and center cracks 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, the internal crack and the center porosity can be bonded simultaneously. . As described above, when the slab is formed by the two-pass rolling, the shape ratio of the first pass and the second pass must satisfy the expression (1), and when the slab is formed by the three-pass rolling and the four-pass rolling, Is such that the shape ratio of each pass at least satisfies the expression (1) at least in rolling in the thickness direction and the width direction.

Next, the roll diameter according to claim 2 of the present invention is set to 95
The reason for setting the diameter to 0 mmφ or more will be described. Generally, the larger the roll diameter, the greater the compressive and shear forces acting inside the slab, which is advantageous for crimping such as center porosity.
If it is excessive, facility costs and running costs increase, which is a problem. Therefore, it is necessary to set a minimum roll diameter that satisfies the expression (1). As can be seen from Table 1, 11
A 5 mm square steel slab can be formed from a 140 mm square to 194 mm square slab under the conditions of a draw ratio of 1.5 to 2.8 and 2 to 4 pass rolling. From the relationship between the roll diameter and the shape ratio in this case, the minimum roll diameter that satisfies the expression (1) is 950 mmφ.
Is obtained. A typical example is a 115 mm square steel slab.
In the case of forming from a slab of 40 mm □ to 155 mm □ by two-pass rolling, the minimum roll diameter that satisfies the equation (1) in the second pass having a smaller shape ratio than the first pass is shown in FIG. 950 mmφ is obtained. As described above, in the present invention, the roll diameter is specified to be 950 mmφ or more. In addition, the roll diameter for the rolling of the billet for bar steel is 1300 mm from the viewpoint of facility costs, running costs, and maintainability.
φ or less is desirable.

Next, the reason why the slab is cut into a predetermined length after the solidification is completed and then heated and rolled will be described below. The inventors set the roll diameter 7 in the continuous caster upstream of the cutting machine.
Install a VH type rolling mill of 20 mmφ, and immediately after solidification is completed,
An experiment was conducted in which pass rolling was performed to form a 222 mm square cast piece into a 165 mm square steel piece. In this case, since the center temperature was as high as about 1300 to 1350 ° C. due to the slab sensible heat, the internal cracks and the center porosity were pressed with a relatively small shape ratio (L _d / H _m = 0.98, 0.84). However, deep cracks were observed mainly at the corners of the 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 low, the strain rate is low, and the billet temperature at the roll contact portion is lowered. It was presumed that a deep crack was generated at one corner. As a result of raising the rolling temperature by installing a heat insulating cover and a heating device before the rolling mill,
The flaw depth was reduced, but it was not eliminated, and the billet had to be maintained.In addition, the rolling roll was extremely slow rolling, so the contact time with the billet was long and thermal cracks occurred, resulting in a significant reduction in life. did.

In the present invention, based on the above findings, the slab is cut after completion of solidification, heated, and then rolled. Thereby, high speed rolling at about 20 to 100 m / min becomes possible, and the generation of cracks can be prevented even at a low rolling temperature, and the problem of reduction in roll life can be solved.

Next, in claim 3 of the present invention, after the cut slab is heated to a predetermined length, rolling is completed at a slab surface temperature of 940 ° C. or higher. The reason will be described below. 2
In 4-pass low-ratio elongation rolling, it is necessary to improve hot ductility because the amount of reduction increases and the stress on the surface of the slab increases. Therefore, the present inventors have developed 22 steel for machine structural use.
The cast slab was cast into a 2 mm square slab, the slab was cut, heated, and then rolled in two passes to form a 165 mm square slab, and the effect of rolling temperature on surface flaws was investigated. The rolling condition is roll diameter 1
Using a flat roll or dia square roll with 010 mmφ and no hole shape, the rolling reduction per pass is 85 mm,
The shape ratio was 1.16 in the first pass and 1.00 in the second pass.
FIG. 5 shows the obtained results.
The completion of the rolling at a temperature of not less than ℃ eliminates the occurrence of surface flaws. This indicates that the increase in the rolling completion temperature improves hot ductility and prevents cracks and flaws. As described above, in the present invention, the rolling is completed at 940 ° C. or higher.

On the other hand, a comparison of the influence of the roll shape in FIG. 5 shows that the flat roll without a hole has a smaller flaw depth than the diamond-shaped square roll with a hole, and the rolling completion temperature at which no flaw occurs is 910 ° C. This is advantageous. Then, the result obtained by performing FEM analysis on the relationship between the roll shape and the surface stress of the billet corner is shown in FIG. The analysis conditions were as follows: the effective roll diameter was 1010 mmφ, the rolling temperature was 1000 ° C., the rolling speed was 36 m / min, and the steel type was S4.
5C was compared. As a result, it was confirmed that the flat roll is advantageous because the tensile stress is small near the roll exit side. Based on the results of the above experiments and analysis, in claim 4 of the present invention, the roll is defined as a flat roll having no hole shape.

[0027]

FIG. 7 shows an embodiment of the present invention. 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 mill 10 having one stand or a plurality of stands is a billet for a bar.

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

In the present invention, the cast slab 7 after cutting is inserted into a heating furnace as a hot slab. However, in order to prevent surface flaws caused by heating and / or billet rolling,
If necessary, a slab cooling device may be provided 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, under the above conditions,
Four pass rolling is performed to form a slab, but if necessary, skin pass rolling may be performed for the purpose of adjusting the shape of the slab and improving the dimensional accuracy.

[0030]

Embodiments Hereinafter, embodiments will be described in detail. Melting steel for machine structural use in a converter with a heat size of 260 tons,
In the curved 6-strand bloom continuous caster having a radius of curvature of 12 m shown in FIG.
Into the mold 3 and the cross-sectional size is 140mm □ ～ 3
A 100 mm square or a rectangular slab having a cross-sectional area equivalent to this square and an aspect ratio of 1.6 or less is cast at a casting speed of 1.0 to 3.0.
Cast at m / min.

After the solidification is completed, the slab 7 is cut to a predetermined length by a slab cutter 6, and the slab 7 is heated in a heating furnace 8 so that the average cross-sectional temperature is 990 to 1
After heating to 140 ° C, roll diameter 950mmφ
A VH type continuous rolling mill 9 composed of flat rolls of up to 1300 mmφ was formed into a steel slab 10 of 115 mm □ to 180 mm □ so as to complete rolling at 940 ° C. or higher by 2 to 4 pass rolling. Table 2 shows a comparison between examples of the present invention and comparative examples.

[0032]

[Table 2]

In the method according to the present invention, since the appropriate roll diameter, reduction amount, and number of passes are selected in accordance with each slab size, they satisfy the predetermined stretching ratio and shape ratio, respectively. Since the obtained steel slab has no surface flaw and has an austenite crystal grain size number of 5 or more, and the internal cracks and the center porosity are completely pressed, the S / N required for ultrasonic inspection of the steel slab is required. 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 cast slab was rolled in 2 to 4 passes and the shape ratio was less than 1.0, the center porosity was not crimped, and noise was generated in ultrasonic flaw detection of the steel slab. No S / N ratio was obtained. In the comparative example in which billet rolling was not performed, since the crystal grains were coarse and internal cracks and center porosity remained, noise was generated during ultrasonic flaw detection, and it was impossible to detect inclusions. On the other hand, in the comparative example in which a large cross-section bloom was subjected to multi-pass slab rolling and billet rolling to form a steel slab, ultrasonic testing of the steel slab was possible, but the production cost involved in heating and rolling the slab was a major problem. Met.

FIG. 8 shows typical examples of temperature transitions of slabs and steel slabs from casting to rolling completion (average cross-sectional temperatures from casting to extraction, and center, surface and corner temperatures from extraction to rolling completion). Is shown. In the present invention, a 222 mm slab was cast at a casting speed of 1.8 m / min, cut, charged in a heating furnace, heated for 35 minutes, extracted at an average cross-sectional temperature of 1040 ° C.
This is an example in which a slab of 165 mm square is formed by an H-type continuous rolling mill. On the other hand, in the comparative example, a slab of 400 × 500 mm was cast at a casting speed of 0.65 m / min, cut, heated at a mean temperature of 1170 ° C. in a heating furnace and extracted, and then passed through a double reversing mill for 16 passes. This is an example in which a slab of 250 mm square was rolled to form a slab of 165 mm square and then subjected to four-pass rolling with a double continuous rolling mill to form a slab of 165 mm square. Comparing the two, the rolling completion temperature is set to 940 ° C., but in the case of large section bloom, multi-pass rolling is performed and it takes about 7 to 8 minutes from heating furnace extraction to rolling completion. Until completion, the corner temperature has dropped by about 230 ° C. In contrast, in the embodiment according to the present invention, since the time from extraction to the completion of rolling is as short as about 40 to 60 seconds for 2 to 4 pass rolling, the corner temperature from the extraction to the 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.

FIG. 9 shows a comparison between the thickness of the slab, the thickness of the slab and the draw ratio. Comparative Example 1 shows a billet continuous casting method omitting the bulk rolling, and Comparative Example 2 shows a large-section bloom continuous casting / bulking method that performs the bulk rolling and the billet rolling. The present invention uses a slab of 140 mm
This is a method of forming a steel slab of up to 180 mm square at a draw ratio of 1.5 to 2.8. As shown in Table 2, ultrasonic inspection of the steel slab is possible, and the manufacturing cost of the steel slab is significantly low. It can be said that the method.

[0037]

According to the present invention, even when the medium-section bloom is heated for a short time and rolled with a small number of passes, the cast structure can be efficiently refined without generating surface defects on the billet, and internal cracks and the like can be reduced. Because the center porosity is pressed, ultrasonic inspection of steel slabs is possible, and the slab rolling process is greatly simplified, reducing the heating fuel and rolling power for slabs and improving rolling yield. Production cost can be reduced,
Its industrial value is extremely large.

[Brief description of the drawings]

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

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

FIG. 3 is a diagram showing the relationship between L _d / H _m and the length of internal slab cracks.

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

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

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

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

FIG. 8 is a comparison diagram of temperature transition of a slab and a slab from casting to completion of rolling.

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

[Explanation of symbols]

DESCRIPTION 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 consisting of one stand or plural stands 10 Steel slab for steel bar

(72) Inventor Yasushi Ishibashi 12 Nakamachi, Muroran, Hokkaido Nippon Steel Corporation Muroran Works (72) Inventor Takashi Yatsuka 12 Nakamachi, Muroran, Hokkaido New Japan Works Muroran Works (72) Inventor Koichi Isobe 12 Nakamachi, Muroran-shi, Hokkaido Nippon Steel Corporation Muroran Works

Claims (5)

[Claims] 1. A method of manufacturing a steel strip for billet by billet rolling from a continuously cast slab, wherein the cross section of the slab has a size and shape of a square having a side of 140 mm to 300 mm or a cross-sectional area equivalent to a square having a long side. Continuous casting is performed as a rectangle having a ratio of short 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. Thickness) The rolling in the thickness direction and the width direction satisfying ≧ 1.0 is rolled at least one pass at a time, and the stretching ratio from the slab to the steel slab is 1.5 to 2.8.
A method for manufacturing a steel slab for a bar steel, comprising manufacturing a steel slab by four-pass rolling. 2. The method according to claim 1, wherein the material is rolled on a roll having a diameter of 950 mmφ or more. 3. The method according to claim 1, wherein the rolling completion temperature of the slab is 940 ° C. or higher. 4. The method according to claim 1, wherein rolling is performed by a flat roll having no groove. 5. A continuous casting machine for continuously casting a square slab having a side length of 140 mm to 300 mm or a rectangular cross section having an equivalent cross-sectional area and a ratio of long side to short side 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 to a predetermined temperature, and heating the heated slab to a predetermined cross-sectional size An apparatus for producing a steel strip for strip steel, wherein 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 into a steel slab is sequentially arranged.