JP2003027137A - Method for reducing segregation in wire rod rolled from continuously cast billet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for stabilizing quality of a wire rod by reducing segregation in the wire rod, with respect to a cast billet produced with a billet continuous caster. SOLUTION: In the method for reducing the segregation in the wire rod, the wire rod is cooled at <=10 deg.C/sec cooling speed in temperature range from 1,050 deg.C to 850 deg.C by setting a rolling finished temperature in the wire rod rolling to >=1,050 deg.C. Further, the cooling speed from 1,050 deg.C to 850 deg.C according to the maximum segregation grain diameter in the billet measured in the cast billet and a target carbon segregation degree, is not higher than values obtained with the following equation (1). Equation (1) CR=D2×d<2> +D1×d+D0, D2=-430.9×C<3> +1027.8×C<2> +55.799×C-680.93, D1=343.32×C<3> -1855.5×C<2> +3157.4×C-1662, D0=97.648×C<3> -442.97×C<2> +654.05×C-310.89. Wherein, CR is the cooling speed ( deg.C/sec), d is the segregation grain diameter (mm) and C is the target carbon segregation degree.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for improving the segregation of a billet manufactured in a steelmaking process in the wire rod process and stabilizing the wire rod quality.

[0002]

2. Description of the Related Art In the steel industry, slabs have been manufactured by continuous casting for more than 20 years in order to save energy.

One of the problems in continuous casting is segregation of concentrated molten steel that accumulates at the center of the slab. When the concentration of concentrated molten steel in the segregated portion is high, for example, in the case of wire rod manufactured from billet or bloom, when the wire is drawn into a steel wire, fracture may occur due to partial difference in hardness of the steel material. . This tendency becomes remarkable when the carbon concentration is high among the steel components. The reason is that when the carbon concentration is high, pro-eutectoid cementite or micro martensite occurs during the production of a wire from a billet, and when pro-eutectoid cementite or micro martensite exists, cracks occur during wire drawing starting from it. ,
This is because the wire breaks.

Conventionally, casting was carried out in a bloom, and subsequently the size was reduced by slabbing and then the wire was rolled. Conventionally, with respect to the above-mentioned problem of segregation, a light reduction is carried out during bloom casting. However, in this case, the heating in the slabbing process and the billet-size rolling process after the bloom casting once again came between the casting and the wire rod rolling processes, and the energy saving was not achieved.

When a bloom is cast and rolled into a billet, if the bloom is heated for a long time before the billet is rolled, segregation in the slab can be diffused. Decarburization of the slab surface occurs due to long-term heating, but since the surface can be scarfed during billet rolling, the decarburized layer can be removed by scarfing, that is, segregation concentration can be reduced by long-term heating. .

[0006]

Recently, for the purpose of energy saving and process saving, a process for casting a billet by continuous casting, omitting the subsequent slabbing process, and producing a wire rod by direct wire rod rolling has been introduced. ing. In billet casting, segregation is also reduced, and the need for high-grade steel is increasing. As a method therefor, there is a technique of applying a light reduction during solidification.

Generally, as the carbon concentration of the base material increases, the segregation concentration of carbon in the slab also increases, and the influence of segregation on the wire quality also increases.

When a bloom is produced by continuous casting and a billet is produced from the bloom by billet rolling, the bloom is heated for a long time as described above to diffuse segregation and the decarburized layer on the surface is removed by scarfing. It was possible. However, when producing a billet by continuous casting,
Since the billet cannot be scarfed, the heating time in the billet should be long enough to diffuse the carbon segregation, because the decarburization of the billet surface layer will occur during heating and the heating atmosphere will not be a reducing atmosphere. As difficult as possible

An object of the present invention is to provide a method for stabilizing the quality of a wire produced by a billet continuous casting machine by reducing segregation of the wire.

[0010]

Means for Solving the Problems That is, the gist of the present invention is as follows: (1) The final rolling temperature of wire rod rolling is set to 1050 ° C. or higher, and 1050 to 850 ° C. is fed at a speed of 10 ° C./sec or less. A method for reducing segregation of a wire, which comprises cooling with. (2) The cooling rate from 1050 ° C. to 850 ° C. is set to be equal to or less than the value obtained by the formula (1) according to the maximum segregated grain size of the billet measured on the cast piece and the target degree of carbon segregation. The method for reducing segregation of a wire according to (1) above. Formula (1) CR = D2 × d ² + D1 × d + D0 where CR is a cooling rate (° C./sec) d is a segregated particle size (mm) D2, D1 and D0 are the coefficients shown below, and C is a target. Is the degree of carbon segregation. ^{D2 = -430.9 × C 3 + 1027.8} × C 2 + 55.799 × C-680.9
^{3 D1 = 343.32 × C 3 -1855.5} × C 2 + 3157.4 × C-1662 D0 = 97.648 × C 3 -442.97 × C 2 + 654.05 × C-310.89

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors examined how much segregation of carbon diffuses during heating of a wire while comparing the segregated grain sizes of a slab and a wire. As a result, it was found that the segregation concentration of carbon was reduced by 25% in the temperature history during normal wire rolling.

Numerical analysis was conducted assuming a billet cast slab and assuming a spherical segregation during heating of the slab and a cylindrical segregation during rolling of the wire rod. As a result, as shown in FIG. 1, it was found that segregation was reduced by about 22% in the wire heating furnace and about 3% even in the rolling time of about 100 seconds. The reason why segregation is reduced even during rolling time is that segregation is extended linearly in wire rod rolling and its deformation ratio is proportional to the area reduction rate, so the thickness of segregation may be thin and may diffuse even in a short time. Is.

Based on this analysis, an efficient method of reducing segregation in the wire rolling process from billet heating to wire winding was examined. As a result, the following conclusions were obtained. 1) It is effective to reduce segregation due to diffusion in the austenite phase region. Cementite tends to precipitate in the ferrite phase region and segregation does not decrease.
2) The segregated grains are deformed more easily after rolling than in the heating furnace because they are deformed. 3) The slower the cooling rate in the austenite region after rolling, the more segregated carbon decreases due to diffusion. 4) The larger the segregated grain size, the higher the initial carbon concentration and the longer the distance required for diffusion, and the higher the carbon concentration after diffusion.

Based on these results, heating and rolling experiments were carried out using actual billet slabs, and experiments were conducted to control the cooling rate during cooling after rolling. As a result, when the rolling end temperature is set to 1050 ° C. or higher and the cooling is continued to 850 ° C. and the cooling rate at that time is set to 10 ° C./second or less, carbon segregation in the wire is reduced due to diffusion of segregation, and the quality of the wire is reduced. Was found to improve.

A further analysis of the relationship between the segregated grain size observed in the slab, the cooling rate when the rolling end temperature is 1050 ° C. or higher and the subsequent cooling to 850 ° C. and the degree of segregation in the wire is shown in FIG. become. Based on FIG. 2, when a formula for deriving the cooling rate from the segregated grain size of the slab and the segregation degree of the wire was examined, the relationship shown in formula (1) was obtained. Formula (1) CR = D2 × d ² + D1 × d + D0 where CR is a cooling rate (° C./sec) d is a segregated particle size (mm) D2, D1 and D0 are the coefficients shown below, and C is a target. Is the degree of carbon segregation. ^{D2 = -430.9 × C 3 + 1027.8} × C 2 + 55.799 × C-680.9
^{3 D1 = 343.32 × C 3 -1855.5} × C 2 + 3157.4 × C-1662 D0 = 97.648 × C 3 -442.97 × C 2 + 654.05 × C-310.89 Note, since the equation that regression of the results It is considered that the error contains about 15%.

From this fact, it was found that more stable wire rod performance can be obtained by adjusting the cooling rate in the austenite region according to the segregation grain size of the cast piece and the target degree of carbon segregation. Here, the target degree of carbon segregation was set to 1.3 or less for 0.8% C material. This is the degree of segregation at which a harmful degree of pro-eutectoid cementite does not occur in the segregation part. In addition, 0.
The 6% C material also reduces the concentration of segregation, so the target degree of segregation is relaxed to about 1.7 (= 1.3 * 0.8 / 0.6).

The present invention relates to the production of high carbon steel wire rods,
This is particularly effective when a continuously cast billet slab is directly rolled into a wire rod. It is effective when the carbon concentration in the steel is 0.6% or more, and more effective when it is 0.8% or more.

[0018]

EXAMPLES Example 1 A 120 mm square billet slab was cast by a continuous casting machine, and a test was conducted using this slab.
The steel type is a high carbon steel of 0.6% -0.8% C. In the billet heating furnace, it was heated from room temperature to 1100 ° C. in about 50 minutes. Immediately after reaching the temperature of 1100 ° C., the billet was taken out of the heating furnace and wire-rolled. If the billet was held in the heating furnace in a high temperature region for a long time, decarburization of the surface would occur, so the billet was immediately extracted from the heating furnace without holding it for a long time.

After the heating furnace is extracted, the wire is rolled to a diameter of 5.5.
Rolled to mm. The rolling end temperature was 1050 ° C. Subsequently, the temperature range up to 850 ° C was cooled at a rate of 10 ° C / sec.

With the 0.6% C material and the 0.7% C material, cementite was not detected in the wire rod obtained by rolling a slab having a segregated grain size of 5 mm, which was good. With the 0.8% C material, cementite was not detected in the wire rod obtained by rolling the slab having a segregated grain size of 4 mm, which was good. Further, a slight amount of cementite was detected in the wire rod obtained by rolling the slab of 0.8% C material and having a segregated grain size of 5 mm, but the quality was good.

(Comparative Example) A slab having a segregation grain size of 5 mm was used in a conventional process in which a 0.6% C material and a 0.7% C material were rapidly cooled to 850 ° C. without rolling control after rolling. Cementite was detected in the rolled wire. Cementite was detected in the wire rod obtained by rolling the slab having a segregation grain size of 4 mm in the 0.8% C material.

Example 2 A test was conducted using a billet slab of 120 mm square. The steel grade is high carbon steel with 0.8% C. In a heating furnace, it takes about 50 minutes from room temperature to 1100 ° C
Heated up. Immediately after reaching the temperature of 1100 ° C., the wire was rolled out of the heating furnace. After extraction from the heating furnace, the product was rolled to a diameter of 5.5 mm. Rolling finish temperature is 10
It was 50 ° C. Since the segregation grain size of the slab was 5 mm, the target segregation degree was set to 1.3 or less, and 850
The temperature range up to ° C was cooled at a rate of 5 ° C / sec according to the formula (1). No cementite was observed in the obtained wire, and the quality was good.

[0023]

EFFECTS OF THE INVENTION This method stabilizes the quality of the wire rod manufactured directly from the billet, and enables the production of high carbon steel from the billet that does not pass through the slabbing process, which is superior to the conventional method of producing from bloom. And a manufacturing method with less energy can be realized.

[Brief description of drawings]

FIG. 1 is a diagram showing how the segregation concentration changes during heating-rolling of a wire.

FIG. 2 Influence on carbon segregation degree in billet segregated grains 10
It is a figure which shows the influence of the cooling rate from 50 degreeC to 850 degreeC.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Oga Tadanori             1 Kimitsu, Kimitsu-shi Mr. Nippon Steel Corporation             Tsu Steel Works F term (reference) 4K032 AA06 BA02 CA02 CC04 CD03

Claims (2)

[Claims] 1. A method for reducing segregation of a wire rod, wherein the final rolling temperature of the wire rod is set to 1050 ° C. or higher and cooling is performed from 1050 ° C. to 850 ° C. at a rate of 10 ° C./sec or less. 2. 1050 ° C. to 850 ° C. depending on the maximum segregated grain size of the billet measured on the cast piece and the target degree of carbon segregation.
The method for reducing segregation of a wire according to claim 1, wherein the cooling rate up to is less than or equal to the value obtained by the formula (1). Formula (1) CR = D2 × d ² + D1 × d + D0 where CR is a cooling rate (° C./sec) d is a segregated particle size (mm) D2, D1 and D0 are the coefficients shown below, and C is a target. Is the degree of carbon segregation. ^{D2 = -430.9 × C 3 + 1027.8} × C 2 + 55.799 × C-680.9
^{3 D1 = 343.32 × C 3 -1855.5} × C 2 + 3157.4 × C-1662 D0 = 97.648 × C 3 -442.97 × C 2 + 654.05 × C-310.89