JP3289132B2 - Method of manufacturing 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 for producing a billet for a bar and a bar, and more particularly to a method for producing a billet for a bar and a bar having a good internal quality to be used for rolling a bar or a wire from a continuous cast slab. The present invention relates to a method for manufacturing a billet for a bar.

[0002]

2. Description of the Related Art Conventionally, blooms manufactured by a continuous casting method have been generally used as a raw material of a steel slab for rolling a bar or a wire rod. Here, this continuous casting bloom usually has about 4 to
It is cast with a large cross-sectional size having a cross-sectional area of about 15 times, cut into a required size, and after heating, is subjected to slab rolling to form a steel slab of a predetermined size.

[0003] In recent years, there has been a demand for reducing the cross-sectional size of a continuous casting bloom for the purpose of simplifying the slab rolling process. However, as described above, as the cross-sectional size of the continuous casting bloom is reduced, and the reduction in area in the slab rolling is reduced, the center porosity formed in the solidification process of the continuous casting is reduced during the slab rolling. However, this remains in the steel slab without being crimped, and this causes a disadvantage that it is easily determined as a defect during ultrasonic flaw detection.

[0004] There have been many reports on means for improving internal defects such as center porosity in such continuous cast slabs. As a typical example,
JP-B-59-16682 and JP-B-2-56982
The publication describes a means for preventing center segregation and center porosity by slightly reducing the slab at the end of solidification. Also, in Japanese Patent Publication No. 64-4868,
Means for making the cast structure equiaxed by electromagnetic stirring and preventing semi-macro segregation by using light reduction at the end of solidification are further disclosed in CAMP-ISIJ, Vol.
91), 297 describe means for preventing internal cracking at the end of solidification under light pressure by making the solidification interface structure equiaxed. Each of these improvement means improves segregation and center porosity by applying a light reduction to a slab having unsolidified molten steel at the center in a continuous casting machine. The reason why the solidified structure is equiaxed is to prevent internal cracking and promote dispersion of segregation under light pressure.

On the other hand, with respect to a means for applying pressure to the slab immediately after the solidification is completed to compress the center porosity, for example, Japanese Patent Publication No. Hei 6-28784 discloses that a central portion of a solidified slab is subjected to equiaxial crystal solidification. Means for lightly reducing the pressure by a pair of planar pressing devices immediately after the completion of the solidification is described. And, in this case, the central portion is made to be equiaxed because of the dispersion of segregation, and the center porosity which is inevitably formed by equiaxed crystallization is subjected to planar pressing immediately after solidification. The slab for thick steel plates with excellent internal soundness is manufactured by crimping using a device.

[0006] In the rolling process, the center porosity of the steel slab is press-bonded following the deformation and shrinkage of the center porosity.
Generally, the shape ratio (Ld
/ Hm) increases, the compressive stress also acts on the central portion and is effective for pressure bonding.

[0007]

(Equation 3)

Here, R is the roll radius, h0 is the thickness of the incoming slab, h1 is the thickness of the outgoing slab, Hm is the average slab thickness, and Ld is the contact length of the roll.

The relationship between the center porosity compression and the shape ratio in the rolling process is described in, for example, Japanese Patent Application Laid-Open No. 3-47916.
Rolling of several passes or several passes, and then reducing the shape ratio by 0.7
The means for performing at least one pass of rolling is described above. This rolling means increases the deformation resistance of the surface layer by accumulating processing strain in the surface layer in the preceding rolling,
In the subsequent rolling, the compressive stress at the center is increased, and the center porosity is pressed to produce a thick steel plate.

[0010] In addition, "Iron and Steel", 67 (1981), S339, a strong rolling method by improving the pass schedule,
"Iron and Steel", 67 (1981), S369, each describes a means for combining temperature difference rolling with partial cooling at the center of the width and high-pressure rolling. All of them relate to a method of rolling a thick steel plate in which the compressive stress at the center portion is increased to improve the compressibility of the center porosity.

[0011]

The center porosity of the continuous cast slab is formed by the solidification shrinkage of the unsolidified molten steel in the central part at the end of solidification. Although it is necessary to apply a reduction so as to completely compensate for the solidification shrinkage of the piece, there is a disadvantage that the molten steel flows locally due to an overpressure and the segregation worsens. To cope with this, the unsolidified slab large rolling method that prevents the formation of center porosity by squeezing the molten steel at the shaft center to the slab at the end of solidification, A large reduction method in which the center porosity is pressed by pressing the slab immediately after the large slab is also considered, but in each of these large reduction methods, in addition to requiring a considerable reduction force and reduction amount, casting is also required. 0.5 ~ 3m / min depending on the size
Since the rolling must be performed in synchronization with the slow speed, the strain rate is extremely small, causing slab surface cracks due to large reduction, and shortening of the life due to thermal cracking of the rolling roll. There was a point.

Accordingly, it is an object of the present invention to provide a method for producing a steel strip for a bar by slab rolling from a continuously cast slab, which avoids the above-mentioned problems and reduces the cross-sectional size of the slab. It is an object of the present invention to provide a method of manufacturing a steel slab for a bar and bar, in which center porosity can be surely pressed by slab rolling even when the total area reduction in the slab rolling is small.

[0013]

According to a first aspect of the present invention, there is provided a method for producing a steel strip for a steel bar by continuous slab rolling from a continuous cast slab. 10% including slab center by low temperature casting
% Of the solidified structure in the region of more than
After cutting the slab and heating it to a predetermined temperature, rolling in the thickness direction and the width direction satisfying the following equation (1) is performed at least one pass at a time for each pass to produce a slab. Features.

[0014]

(Equation 4)

Here, R is the roll radius, h0 is the thickness of the incoming slab, h1 is the thickness of the outgoing slab, Hm is the average slab thickness, and Ld is the roll contact arc length.

According to a second aspect of the present invention, there is provided a method for producing a steel slab for a bar by slab rolling from a continuous cast slab, wherein the continuous casting of the slab is performed by electromagnetic stirring or / and low-temperature casting. The solidification structure in the area of 10% or more including the center of the slab is equiaxed, and the solid fraction in the center is 0.1%.
After applying light pressure to the slabs at the end of solidification in the range of 5 to 0.75 with a plurality of pairs of upper and lower rolls, solidification was completed, and the slabs were further cut and heated to a predetermined temperature. The above is characterized in that the billet is manufactured by performing at least one pass in each of the rolling in the thickness direction and the width direction satisfying the following equation (2) and [2].

[0017]

(Equation 5)

Here, R is the roll radius, h0 is the thickness of the incoming slab, h1 is the thickness of the outgoing slab, Hm is the average slab thickness, and Ld is the contact length of the roll.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a method for manufacturing a steel slab according to the present invention will be described below in detail with reference to FIGS.

FIG. 1 is a schematic diagram for explaining a production process of a steel slab for a steel bar including a schematic configuration of a continuous casting apparatus and a rolling apparatus to which a production method according to an example of the present embodiment is applied. FIG. 2 is an explanatory view showing the definition of the equiaxed crystal ratio of the slab.
FIG. 2A shows a case of a vertical continuous caster, and FIG. 2B shows a case of a curved continuous caster. Further, FIG. 3 is a graph showing the relationship between the equiaxed crystal ratio and the shape ratio of the slab and the thickness of the center porosity of the steel slab. FIG. 3 (a) shows the case where the equiaxed crystal ratio is less than 10%. FIG. 3B shows a case where the equiaxed crystal ratio is ≧ 10%. FIG. 4 is a graph showing the relationship between the equiaxed crystal ratio of the slab and the center porosity of the slab.

In the apparatus configuration shown in FIG. 1, 1 is a ladle, 2 is a tundish, 3 is a mold, 4 is an electromagnetic stirring device in the mold, 5 is a guide roll group, 6 is a secondary cooling zone,
Reference numeral 7 denotes a group of pinch rolls, and reference numeral 8 denotes a slab continuously cast. Reference numeral 9 denotes an end-of-solidification light reduction device for the slab 8,
Reference numeral 10 denotes a machine for cutting the slab 8, and reference numeral 11 denotes a slab after cutting. Further, 12 is a slab heating furnace for heating the slab 11 after cutting, 13 is a slab rolling machine for rolling the slab 11, 14 is a slab on the entry side in the slab rolling machine 13, Numeral 15 denotes a billet on the exit side of the ingot rolling mill 13, and numeral 16 denotes a billet after rolling.

In the case of this embodiment, whether or not the casting slab 8 is subjected to stirring flow by the electromagnetic stirring device 4 in the mold,
And / or by adjusting the temperature of the molten steel in the tundish 2 to a low temperature, the solidified structure in the region of 10% or more including the central part of the slab is equiaxed, and after the solidification is completed, the slab 8 Is cut using a cutting machine 10 and heated to a predetermined temperature by a heating furnace 12, and for example, each of rolling in the thickness direction and the width direction is performed by a pair of upper and lower rolls by a slab rolling machine 13 by at least one pass. Go to form the desired billet. Here, by appropriately selecting the roll diameter and the rolling pass schedule, the respective shape ratios in the rolling in the thickness direction and the rolling in the width direction are set to the above [1] when light reduction by the final solidification light reduction device 9 is not applied. When the equation is applied, the steel slab 16 is rolled so as to satisfy the above equation [2] to produce the intended steel slab 16 to obtain a target.

That is, in the present embodiment, as described above, the main purpose is to make the 10% region including the center of the slab be equiaxed by electromagnetic stirring or / and low-temperature casting in continuous casting. Here, the equiaxed crystal ratio of the slab 8 is, as shown in FIG. 2, a case of a vertical continuous caster with a slab cross section (FIG.
In (a)), it is defined as d / D × 100 (%), and in the case of a curved continuous caster (FIG. 2 (b)), it is defined as l / L × 100 (%). As the above-mentioned electromagnetic stirring method, the stirring flow is given to the unsolidified molten steel by the electromagnetic stirring device 4 provided in the mold 3 or the secondary cooling zone 6, and the stirring flow rate is appropriately set, so that the casting slab 8 or the like is used. Ensure an axial crystal ratio of 10% or more. In addition, the degree of superheat of the molten steel in the tundish 2 is adjusted to a low value in accordance with the type of steel and the casting conditions so that the equiaxed crystal ratio of the slab 8 becomes 10% or more.

FIGS. 3 (a) and 3 (b) show the billet 1 after rolling.
6 shows the results of an investigation by the inventors regarding the relationship between the equiaxed crystal ratio of the slab and the rolling conditions on the thickness of the center porosity remaining in the center of No. 6. This is a 220mm square slab 11
140mm square to 195mm by changing the rolling amount and roll diameter
This is a result of an investigation in the case where the square steel slab 16 is formed by two-pass or four-pass rolling, and the shape ratio of the horizontal axis is the value of the first pass and the second pass in the case of two-pass rolling. In this case, the values of the third pass and the fourth pass are respectively shown.

As is apparent from FIG. 3 (a), when the slab 8 having the equiaxed crystal ratio of less than 10% is rolled, it is defined by the equation [3] regardless of the presence or absence of the last stage of solidification. Even if the shape ratio increases, the center porosity is not crimped. On the other hand, as is clear from FIG. 3 (b), the equiaxed crystal ratio is 10%.
When the above slab 8 is rolled, the center porosity is Ld / Hm ≧ 1.0 when there is no light reduction, and Ld / Hm ≧ 0.8 or more when there is light reduction. The light reduction is performed by applying a reduction amount of 7 to 9 mm with 10 pairs of upper and lower rolls within the range of 0.15 to 0.75 of the solid fraction in the center.

As described above, when the slab is formed by two-pass rolling, the shape ratio of the first pass and the second pass must satisfy the lower limit. It is preferable that the shape ratio of at least one pass (normally, the third pass and the fourth pass in which a large shape ratio is obtained) of rolling in the thickness direction and the width direction satisfies the above lower limit.

The reason why the equiaxed crystal ratio of the slab 8 is set to 10% or more will be described below. Fig. 4 shows the results of an investigation on the relationship between the equiaxed crystal ratio and the center porosity of the slab. When the equiaxed crystal ratio is less than 10%, the center porosity is reduced regardless of the presence or absence of light reduction. Has increased in thickness and its variation has increased. For this reason, as shown in FIG. 3 (a), even if the shape ratio is increased and the compressive stress on the central portion of the slab 11 is increased, the columnar crystal is formed from the upper and lower surfaces and both side surfaces of the slab 8 at the central portion. Further, since it is grown, the deformation resistance in the central region is large, and it is difficult to press the center porosity by pressure.

On the other hand, when the equiaxed crystal ratio becomes 10% or more, in addition to the effect of stabilizing the center porosity thickness of the slab 8, the central region of the slab 8 is made of equiaxed grains. Since it is filled and has low deformation resistance, as shown in FIG.
It is considered that the equiaxed grains are sufficiently pressed by the reduction, and the center porosity disappears by the pressing. As shown in FIG. 4, when the final solidification light reduction is applied to the slab 8, the cast slab 8 is compared with the case where the light reduction is not applied in the region where the equiaxed crystal ratio is 10% or more. The center porosity thickness of No. 8 is reduced, and as a result, as shown in FIG. 3 (b), it is considered that the crimping is performed with a smaller shape ratio than when no light reduction is applied.

As described above, in the present embodiment, a steel slab 11 is manufactured by subjecting a 10% or more region including the center of the slab 8 to equiaxed crystallization, cutting, heating, and then rolling. Regarding the shape ratio in this case, the slab thickness, the slab thickness, the reduction amount, and the roll ratio are set so as to satisfy the formula [1] when light reduction is not applied as described above and to satisfy [2] when applied. The diameter should be selected appropriately. As a result of the investigation, as the shape of the rolling roll, there are a flat roll, a diagonal square, and a box caliber shape, but any shape may be used since there is no difference in the press-bonding property of the center porosity.

In addition, the rolling speed can be as high as that of ordinary slab rolling, since the slab 8 is independent after cutting. There is no fear of causing problems.

[0031]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a specific embodiment will be described in detail. A steel for machine structural use is melted in a converter having a heat size of 270 tons / heat, and a curved mold 6 having a radius of curvature of 12 m shown in FIG.
Superheat degree of molten steel in tundish in ladle 1 through tundish 2 in bloom continuous caster of strand
The molten steel is poured into the mold 3 at 15 to 45 ° C., and the molten steel is stirred by the electromagnetic stirring device 4 in the mold at a flow rate of 40 cm / sec.
A slab 8 of 205 mm square to 292 square was cast at a casting speed of 1.0 to 2.0 m / min. When electromagnetic stirring is not applied, low-temperature casting with a superheat degree of 20 ° C or less in the tundish is performed, and the solidification rate at the center of the slab 8 is within the range of 0.15 to 0.75, A reduction device (light reduction zone 5.0 m) 9 was installed, and a reduction of a total of 7 to 9 mm was applied to some of the slabs 8 with 10 pairs of upper and lower rolls.

The slab 8 after solidification is cut into a predetermined length by a cutting machine 10 and the cut slab 11 is heated at 1100 ° C. in a heating furnace 12.
162 mm square steel slab 16 was produced by two-pass rolling to four-pass rolling in a double reversing slab rolling mill 13.

Tables 1 and 2 show this embodiment and a comparative example. In this embodiment, after rolling, the 162 mm square
In the case of pass rolling, the slab 11 is 205 mm square to 227 mm square,
In the case of four-pass rolling, the slab 11 was 247 mm square to 292 mm square.

[0034]

[Table 1]

[0035]

[Table 2]

In the manufacturing method according to the present embodiment, the equiaxed crystal ratio of the slab 8 is reduced by applying electromagnetic stirring in a mold or low-temperature casting.
It is 10% to 30%, and a predetermined shape ratio is secured by selecting an appropriate roll diameter and reduction amount in accordance with each of the above slab sizes. When the steel slab 16 thus obtained was inspected by ultrasonic flaw detection, it was confirmed that the center porosity was completely pressed.

On the other hand, a comparative example in which the shape ratio does not satisfy the predetermined value even when the equiaxed crystal ratio of the slab 8 is 10% to 30%, and even when the shape ratio does not satisfy the predetermined value, In Comparative Examples having a crystallinity of less than 10%, the center porosity was not pressed, and the center porosity was detected by ultrasonic flaw detection.

Further, using the steel slab 16 manufactured by the manufacturing method according to the present embodiment, the steel slab 16 was secondarily rolled into a steel bar or a wire, and the results were examined. As a result, mechanical properties, cold workability, drawability, etc. A significant quality improvement effect was observed. This is considered to be because the center segregation formed in the solidification process was deformed and reduced in size by the effect of the infiltration under pressure.

[0039]

As described above, according to the embodiment, according to the method of the present invention, the center porosity can be efficiently crimped even when the small-section slab is reduced with a small number of passes. In addition to this, it is possible to simplify the slab rolling process, and to achieve a reduction in the fuel consumption per unit of slab, a reduction in the rolling power, and an improvement in the rolling yield. There are excellent features such as being possible.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a manufacturing process of a steel slab for a steel bar, including a schematic configuration of a continuous casting device and a rolling device to which a manufacturing method according to an example of an embodiment of the present invention is applied.

FIG. 2 is an explanatory view showing the definition of the equiaxed crystal ratio of the cast slab, and FIG. 2 (a) shows the case of a vertical continuous caster;
(b) shows the case of a curved continuous caster.

FIG. 3 is a graph showing the relationship between the equiaxed crystal ratio and the shape ratio of the slab and the thickness of the center porosity of the slab, and FIG. 3 (a) shows the case where the equiaxed crystal ratio is less than 10%. , FIG. 3 (b)
Indicates the case where the equiaxed crystal ratio is ≧ 10%.

FIG. 4 is a graph showing the relationship between the equiaxed crystal ratio of the cast slab and the center porosity of the slab.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ladle 2 Tundish 3 Mold 4 In-mold electromagnetic stirrer 5 Guide roll group 6 Secondary cooling zone 7 Pinch roll group 8 Cast piece 9 End solidification light pressure reduction device 10 Cutting machine 11 Cast piece after cutting 12 Cast piece heating furnace 13 Bloom rolling mill 14 Rolling mill entry side slab 15 Rolling mill exit side slab 16 Rolled slab

──────────────────────────────────────────────────の Continued on the front page (72) Inventor Makoto Baba, Nakamachi, Muroran City Nippon Steel Corporation Muroran Works (72) Inventor, Takeshi Sakamoto Nakamachi, Muroran City 12, Nippon Steel Corporation Muroran Steel (56) References JP-A-59-159258 (JP, A) JP-B-6-28784 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) B21B 1/46 B21B 1 / 02 B22D 11/128

Claims (2)

(57) [Claims] 1. A method for producing a steel strip for bar steel by continuous slab rolling from a continuous cast slab, wherein at the time of continuous casting of the slab, at least 10% or more including the center of the slab is subjected to electromagnetic stirring or / and low-temperature casting. After the solidification structure in the region is equiaxed and solidification is completed, the slab is cut and heated to a predetermined temperature,
A method for producing a steel slab for a strip steel, characterized in that a slab is produced by performing rolling in the thickness direction and the width direction satisfying the following expressions (1) and (1) at least for each pass. (Equation 1) Here, R is the roll radius, h0 is the thickness of the incoming slab, h1 is the thickness of the outgoing slab, Hm is the average slab thickness, and Ld is the roll contact arc length. 2. A method of manufacturing a steel slab for a steel bar by slab rolling from a continuously cast slab, wherein at the time of continuous casting of the slab, at least 10% or more including the center of the slab is subjected to electromagnetic stirring or / and low-temperature casting. The solidification structure in the region is equiaxed, and the slabs at the end of solidification with the solid fraction in the center in the range of 0.15 to 0.75 are subjected to light reduction with multiple pairs of upper and lower rolls before solidification. After completion of the slab, the slab is cut and heated to a predetermined temperature, and rolling in the thickness direction and the width direction satisfying the following equations (2) and (2) is performed by at least one pass for each slab. A method for producing a billet for a bar steel, comprising: (Equation 2) Here, R is the roll radius, h0 is the thickness of the incoming slab, h1 is the thickness of the outgoing slab, Hm is the average slab thickness, and Ld is the roll contact arc length.