JP2000343191A - Method for producing nitriding steel with improved center segregation by continuous casting and nitriding steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the center segregation of a cast slab and suppress the generation of a white band in a cast slab of a steel for nitriding manufactured by a vertical continuous casting apparatus. To provide a homogeneous, nitriding steel free from cracks. SOLUTION: In vertical type continuous casting of steel for nitriding, a rolling speed at an optimum rolling position of an axial core solid phase ratio fs = 0.7 to 0.9 at the time of casting is set to 2.5 to 4.0 mm / min,
Applying a strong reduction with a reduction ratio of 0.03, the suction to the center of the concentrated molten steel due to the solidification shrinkage of the slab is performed at the end of solidification by the above-described strong reduction to suppress shrinkage and suppress segregation, and
Without performing electromagnetic stirring at the final solidification position, eliminating concentrated molten steel extraction at the solidification interface due to the influence of electromagnetic stirring,
As shown in FIG. 2, a steel slab for nitriding having a good center segregation degree C / C _{0 and a} good D / 4 is obtained by avoiding the formation of a white band.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for improving center segregation in continuous casting of steel for nitriding.

[0002]

2. Description of the Related Art In the final stage of solidification of a slab formed in continuous casting of steel, in the final stage of solidification, molten steel flows in the center of the slab, while non-uniform solidification occurs near the boundary with the molten steel and bridging occurs. As a result, the molten steel between the bridge and the surrounding area is attracted to the center by solidification shrinkage downstream of the bridge, so that center segregation is accumulated at the center of the slab. As a method of improving the occurrence of the center segregation, it is well known to use a method of electromagnetically stirring unsolidified molten steel inside a slab at a final solidification position. This is a method of preventing bridging by sedimenting dendrites on a solidified surface by electromagnetic stirring to generate fine equiaxed crystals. However, this method of improving center segregation by electromagnetic stirring has the following problems.

When the casting speed Vc, that is, the drawing speed, is high, the solidification end position of the molten steel inside the slab drawn from the mold is located lower, and the casting speed Vc
Is slower, the casting speed Vc
The position at which the solidification of the molten steel ends changes greatly depending on the variation in the temperature. However, the installation position of the electromagnetic stirrer is fixed, and the electromagnetic stirrer cannot be moved up and down with a change in the casting speed Vc. For this reason, the center segregation of the slab cannot be completely avoided only by electromagnetic stirring near the final solidification position by the electromagnetic stirring device installed at the fixed position.

When the casting speed Vc is low, the time required to pass through the portion of the electromagnetic stirrer becomes longer. Therefore, when electromagnetic stirring is applied, the washing time of the solidification interface becomes longer due to the stirring.
As shown in the figure, 1/4 of the diameter D called white band
At the position of D / 4 of the negatively segregated portion of the strength (that is, the segregation degree C /
C ₀ ≦ 0.90). The segregation degree C / C ₀ is the ratio C / C ₀ of the carbon concentration C in the axial center portion to the carbon concentration C _{0 in} the normal portion of the slab.

[0005] When the casting speed Vc is high, a white band is not generated because the passage time of the electromagnetic stirring device is short.
However, due to the high casting speed Vc, the center segregation is greatly deteriorated.

[0006]

SUMMARY OF THE INVENTION The problem to be solved by the present invention is to improve the center segregation of the slab of a steel slab for nitriding manufactured by a vertical continuous casting apparatus and to reduce the white band. An object of the present invention is to suppress generation and provide a homogeneous nitriding steel free from segregation.

[0007]

SUMMARY OF THE INVENTION The present invention is directed to a continuous casting of steel for nitriding, wherein the solid phase ratio fs = 0.7 to 0.7% at the time of casting.
A strong reduction with a reduction ratio of 0.03 at an optimum reduction position of 0.9 and a reduction speed of 2.5 to 4.0 mm / min is applied. By applying strong pressure at the end of solidification, the suction to the center of the concentrated molten steel due to the solidification shrinkage of the slab,
Segregation is suppressed by performing shrinkage compensation. Further, by not performing the electromagnetic stirring at the final solidification position, the extraction of the concentrated molten steel at the solidification interface due to the influence of the electromagnetic stirring is eliminated, so that the generation of a white band (negative segregation) can be avoided.

[0008] That is, the means for solving the above-mentioned problems of the present invention is, in the invention of claim 1, a solidification shrinkage portion in a region where the axial solid phase ratio fs is more than 0.0 and less than 1.0 by continuous casting. On the other hand, the unsolidified region at each position inside the isotherm having an axis solid phase ratio fs of 0.7 to 0.9 of the slab is reduced by a reduction ratio of 0.03 and a reduction speed of 2.5 to 4.0 mm / min. Nitriding is characterized by improving center segregation by rolling down one of the wide sides of the slab in the thickness direction, and by not using electromagnetic stirring at the end of solidification to prevent the generation of negative segregation (white band). It is a method of manufacturing steel for use.

According to the second aspect of the present invention, the segregation degree C / C ₀ at the D / 4 position by the manufacturing method according to the first aspect is 0.99 to 1.01, and the C / C ₀ is 0.9 or less. A nitriding steel comprising a homogeneous cast structure having substantially no negative segregation portion (white band).

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION As an embodiment of the present invention, in a vertical continuous casting apparatus, a multistage high-pressure roll is set in a draw roll section to cast a bloom of 380 mm × 490 mm nitriding steel, for example, SACM645. Continuous casting is performed at a speed of 0.50 m / min. In this case, a drawing roll at a position where the axial core solid phase ratio fs is 0.7 to 0.9 is used as a high pressure reduction roll, and the slab is cast at a reduction ratio of 0.03 and a reduction speed of 3.3 mm / min. One wide side is strongly pressed down in the thickness direction, and is cast at the end of solidification without using electromagnetic stirring. As a result, the optimum Vc range shown in FIG. 2A was obtained, and the value of the center segregation degree C / C ₀ indicating the center segregation rate of the slab was 0.98 to 1.02, which was an extremely good value. Further, the optimum Vc range shown in FIG. 2B is obtained, and the segregation degree C / C ₀ at the D / 4 position is obtained.
Was 0.99 to 1.01, making it possible to avoid the formation of white bands and perform casting. As a result, the obtained cast slab was a cast slab having substantially no negatively segregated portion (white band), and the steel made of this cast slab was a nitriding steel having a uniform cast structure. Note that light reduction was not possible with nitriding steel.

On the other hand, when continuous casting is performed without strong pressure with electromagnetic stirring at the final solidification position according to the conventional method, the center segregation rate indicating the center segregation rate of the slab in the optimum Vc range shown in FIG. The value of the degree C / C ₀ is 0.98 to 1.02
However, as shown in FIG. 3 (b), when the casting speed Vc is lower, the value of the segregation degree C / C ₀ at the D / 4 position is low, indicating a strong negative segregation portion. . Then, as the casting speed Vc increases, the segregation degree C /
The value of C ₀ will gradually approach 1. However, as shown in FIG. 3 (a), the degree of center segregation is greatly deteriorated at a high casting speed Vc.

[0012]

As described above, in the present invention, in the continuous casting apparatus, a multi-stage high-pressure roll is installed in the roll section of the drawing roll, and the suction to the center of the concentrated molten steel due to the solidification shrinkage of the slab is carried out. The segregation can be suppressed by compensating for shrinkage under high pressure at the optimum position where the core solid phase ratio fs is high, and the white band of the steel slab for nitriding can be suppressed by using no electromagnetic stirring at the final solidification position. And an excellent effect that has not been achieved in the past.

[Brief description of the drawings]

FIG. 1 is a view schematically showing a center segregation degree (C / C ₀ ) and a white band generation region in a slab when electromagnetic stirring is performed at a final solidification position and there is no strong pressure reduction.

FIG. 2 is a graph showing the relationship between the casting speed (Vc) and the degree of center segregation (C / C ₀ ) in a slab in the case of application under strong pressure without electromagnetic stirring at the final solidification position of the present invention.

FIG. 3 is a graph showing the relationship between the casting speed (Vc) and the degree of center segregation (C / C ₀ ) in a conventional slab when electromagnetic stirring is performed at a final solidification position and there is no strong pressure reduction.

Claims (2)

[Claims] 1. The solid-phase ratio fs of 0.
With respect to the solidification shrinkage portion of the region exceeding 0 and less than 1.0, the unsolidified region at each position inside the isotherm where the axial solid phase ratio fs of the slab is 0.7 to 0.9 has a reduction ratio of 0. 03 and a reduction speed of 2.5 to 4.0 mm / min to reduce center segregation by rolling down one of the wide sides of the slab in the thickness direction, and to eliminate negative segregation by using no electromagnetic stirring at the final stage of solidification. A method for producing a steel for nitriding, wherein a part (white band) is not generated. 2. A method of manufacturing a slab according to claim 1.
The segregation degree C / C ₀ at the / 4 position is 0.99 to 1.01
A steel for nitriding comprising a homogeneous cast structure substantially free of negatively segregated portions (white bands).