JP3356092B2 - Continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method of an Fe-Ni alloy steel used as a material for a shadow mask and the like, and more particularly, to a method for processing a shadow mask into uneven stripes and internal cracks of a slab. The present invention relates to a continuous casting method for steel capable of obtaining a slab that does not cause defects caused by cracking.

[0002]

2. Description of the Related Art Fe-Ni alloy steel has a low coefficient of linear expansion and is therefore used as a shadow mask or a lead frame of a semiconductor integrated circuit. In particular, with the recent increase in the definition of computer displays and the increase in the size of consumer televisions, the demand for shadow masks made of this Fe-Ni alloy steel has increased significantly.

[0003] Conventionally, a shadow mask is produced by using a steel slab obtained by hot rolling a steel ingot by an ingot-making method as a raw material, and further subjecting a hot rolled sheet obtained by hot rolling to cold rolling to obtain several tens to several hundreds. μm
And then etching.

[0004] In a steel ingot formed by the ingot-making method, remarkable component segregation is liable to be generated therein, and component segregation tends to remain in a steel slab obtained by hot rolling such a steel ingot. If component segregation is present inside the steel slab, a streak-like pattern, that is, a defect referred to as a streak unevenness, may occur after etching when manufacturing a shadow mask using the steel slab as a raw material. Computer displays and televisions in which a shadow mask with uneven stripes is incorporated in a cathode ray tube have a problem in that the image becomes unclear.

[0005] Such component segregation can reduce or eliminate the degree of segregation in a state of a steel ingot, a billet or a thin coil by a heat treatment for a long time or the like, but the productivity decreases. And the processing cost is increased.

[0006] In Japanese Patent Application Laid-Open No. 6-128662, an electroslag ligmelting method (ESR method) is used.
A method has been proposed in which a solidified structure is refined to suppress segregation of Ni and Mn. If the steel slab is manufactured by using the ESR method, the segregation of components inside the steel slab can be greatly reduced, and the generation of defects such as uneven stripes in the shadow mask can be prevented. However, there is a problem that the manufacturing cost is significantly increased.

In Japanese Patent Application Laid-Open No. 2-54743, a continuous cast slab is used. At this time, an equiaxed crystal is present in the center of the slab, and the slab is heated at 1100 ° C. or more for 1 hour or more. Methods for suppressing component segregation have been proposed. It is stated that by making the center of the slab equiaxed, the size of crystal grains having a specific orientation can be reduced, and uneven streaks of the shadow mask can be reduced. In order to control the ratio of the equiaxed crystal region generated in the center of the slab to the thickness of the slab, an electromagnetic stirrer is installed in the mold or secondary cooling zone of the continuous casting machine, or the degree of superheat of molten steel in the tundish is reduced. Methods such as controlling or applying ultrasonic vibration to the slab surface have been proposed.

[0008] However, when the proposed electromagnetic stirring device is used, the center of the slab becomes equiaxed, but a negative segregation zone called a white band is generated so as to surround the equiaxed zone. . This white band remains on the hot-rolled sheet and the shadow mask, and causes uneven stripes during the etching process. In controlling the degree of superheat of the molten steel in the tundish, it is difficult to maintain a constant degree of heating of the molten steel from the beginning to the end of continuous casting. In addition, in order to apply ultrasonic vibration to the surface of the slab, there are problems that not only the equipment becomes excessively large but also that the ultrasonic vibration adversely affects the continuous casting equipment.

Japanese Patent Application Laid-Open No. 2-54744 proposes a method in which B is added to molten steel to form an equiaxed crystal at the center of a slab. However, when B is contained in the Fe—Ni-based alloy steel, internal cracks occur in the continuously cast slab due to the precipitated boride. The internal cracks in the slab due to the addition of B may make subsequent hot rolling difficult,
Even if hot rolling can be performed, surface cracks and internal cracks occur in the hot rolled sheet. Therefore, after the cold rolling, the shadow mask becomes a defect such as a surface flaw or an internal crack when it is processed into a shadow mask.

[0010]

DISCLOSURE OF THE INVENTION The present invention is directed to a method for producing a shadow mask, which is free from component segregation and does not cause defects due to uneven lines or internal cracks in a cast slab. An object of the present invention is to provide a continuous casting method of steel capable of obtaining a good quality slab without cracks.

[0011]

The gist of the present invention is to cast a molten steel having a superheat degree of 20 to 80 ° C. in a tundish to obtain, by weight%, C: 0.07% or less, and Si: 0. .
005-0.5%, Mn: 0.05-1.0%, P:
0.005% or less, S: 0.002% or less, Ni: 33
~ 43%, B: 0.001% or less, the balance being Fe
And a thickness of 100 to 250 mm composed of unavoidable impurities
When the position including the unsolidified portion of the slab is reduced by at least one pair of reduction rolls, the reduction ratio per pair of reduction rolls is 1.2% or less, and the total reduction is A continuous casting method for steel in which the reduction ratio is 1.2 to 10% of the thickness of the slab at the start of the reduction and the center solid phase ratio of the slab at the completion of the reduction is 0.5 or more.

In order to obtain a cast of good quality without center segregation and without internal cracks, it is effective to reduce the cast including an unsolidified portion. When the slab is unsolidified, internal cracks are likely to occur in the slab.

[0013] The metal structure of the Fe-Ni alloy steel at normal temperature is a single austenite phase. Since there is no phase transformation, the solidification structure generated with the progress of solidification tends to be columnar crystals, and columnar crystals easily develop to the center of the slab. For this reason, molten steel in which elements that tend to segregate are concentrated is accumulated in gaps at the center of the thickness where the columnar crystals abut, and significant center segregation occurs. When a slab including an unsolidified portion of such Fe-Ni-based alloy steel is rolled down, internal cracks are generated at austenite grain boundaries existing with the center segregation therebetween.

In the method of the present invention, the upper limit is set on the content of P, S and B elements and the upper limit is set on the reduction ratio per pair of reduction rolls. The problem was solved by providing.

The upper limits are set for the contents of the elements P, S, and B because any of the elements is liable to segregate at the grain boundaries upon solidification, and to cause grain boundary cracks and the like.

The reason why the upper limit is set for the reduction ratio per pair of reduction rolls is that the amount of strain applied to the final solidified portion at the center of the slab due to the reduction, ie, the center segregation portion and the austenite grain boundary, is determined by the internal characteristic of steel. This is to prevent the occurrence of internal cracks by setting the value to a small value equal to or less than the crack generation limit strain.

Although the method of the present invention as described above prevents the occurrence of internal cracks during unsolidification reduction of a slab, the improvement of center segregation by reducing the slab is further improved by the method of the present invention. Then, by setting the total reduction ratio to 1.2 to 10% with respect to the thickness of the slab at the start of the reduction, and by setting the center solid phase ratio of the slab at the completion of the reduction to 0.5 or more. Achieved.

The total reduction ratio is set to be 1.2 to 10% of the thickness of the slab at the start of the reduction because of the reduction in this range.
This is because an equiaxed crystal can be generated without generating a white band at the center of the slab. When an equiaxed crystal is formed at the center of the slab, the center segregation tends to be improved.

The center solid phase ratio of the slab at the time of completion of the reduction by the final reduction roll is set to 0.5% or more because the unsolidified molten steel is located near the center of the slab at the time of completion of the reduction. This is to suppress the existence of. If unsolidified molten steel is present, in the course of the subsequent solidification, the molten steel in which elements that tend to segregate are concentrated at the center of the slab, and center segregation occurs again.

In the method of the present invention, the increase of equiaxed crystals effective for improving the center segregation is further reduced by reducing the thickness of the cast slab to 10%.
This was achieved by setting the degree of superheating of the molten steel in the tundish to 0 to 250 ° C to 20 to 80 ° C.

In order to increase the number of equiaxed crystals, it is preferable that the thickness of the slab is thick. However, the upper limit is set on the thickness so that the rolling stress can easily act on the central portion of the slab.

In order to increase the number of equiaxed crystals, it is better that the superheat of the molten steel in the tundish is low, but the lower limit is set to ensure the cleanness of the slab and to prevent the clogging of the immersion nozzle. This is for the purpose of stabilizing the casting operation by suppressing such factors.

According to the method described above, a cast slab of good quality without center segregation can be obtained without causing internal cracks when the cast slab is reduced.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION The method of the present invention will be described more specifically. In addition,% display of each component means weight%.

(A) Chemical composition C: 0.07% or less If the C content exceeds 0.07%, carbides are easily formed, and if carbides are formed, toughness is deteriorated. Therefore, the C content is set to 0.07% or less. Desirably, it is 0.02% or less.

Si: 0.005 to 0.5% Si is an element for deoxidizing molten steel, and the lower limit of the Si content is 0.005%. When the content exceeds 0.5%, hot workability deteriorates. Therefore, the Si content is set to 0.005 to 0.5%.

Mn: 0.05-1.0% Mn is an element for improving the strength of steel,
And the lower limit of the Mn content is 0.1%.
05%. If the content exceeds 1.0%, not only the effect of improving the strength is saturated, but also Mn tends to segregate in the center of the slab as the slab solidifies. In addition, manufacturing costs increase. Therefore, the Mn content is set to 0.05 to 1.0%.

Ni: 33 to 43% Ni is one of the main elements constituting the Fe-Ni alloy steel. If the Ni content is less than 33%, the characteristic of low linear expansion required for the shadow mask cannot be satisfied. When the content exceeds 43%, the linear expansion coefficient increases and the manufacturing cost increases. Therefore,
The Ni content is 33 to 43%. Desirably 36 ~
43%.

P: 0.005% or less, S: 0.002%
And B: 0.001% or less The upper limits are set for the contents of P, S, and B because these are elements that easily segregate at the grain boundaries during solidification and embrittle the grain boundaries. Further, P and S are elements contained as impurities in the steel. In particular, in the case of a single-phase solidified austenitic steel, P and S are elements that easily segregate at grain boundaries and have a bad influence on hot workability. B has an effect of suppressing S from segregating at the grain boundary, but B is an element that causes segregation at the grain boundary because B segregates at the grain boundary. Therefore, P: 0.005% or less, S: 0.002% or less, and B: 0.001% or less.

(B) Casting conditions and slab rolling conditions The degree of superheat of the molten steel in the tundish is set to 20 to 80 ° C. When the degree of superheat of the molten steel in the tundish is less than 20 ° C., the degree of superheat of the molten steel in the mold becomes 0 ° C. or less, and the generation of crystal grains starts rapidly and the number of equiaxed crystals increases. However, at this time, even if the casting operation cannot be performed due to the clogging of the immersion nozzle and the casting operation can be continued, the oxides in the molten steel are less likely to float and be removed in the tundish or the mold. This causes a problem that the cleanliness of the cast slab is deteriorated. When the degree of superheat exceeds 80 ° C., it is difficult to form equiaxed crystals, and breakout may occur during operation. Therefore, the degree of superheat of the molten steel in the tundish is set to 20 to 80 ° C. Desirably, it is 30 ° C to 60 ° C.

The thickness of the slab before rolling is 100 to 250 mm
And When the thickness of the slab is less than 100 mm, the influence of the secondary cooling from the slab surface is large, and no equiaxed crystal is generated even when the slab is subjected to rolling. Further, when the thickness of the slab exceeds 250 mm, equiaxed crystals are easily formed, but the stress at the time of rolling hardly acts on the center of the slab, and the elements which are easily segregated are equiaxed at the center of the slab. It concentrates between the crystal grains, and eventually the center segregation remains. Therefore, the thickness of the slab is 100 to 250 mm.

At the time of reduction, the material is reduced by at least one pair of reduction rolls. The roll reduction is, for example,
In the case of rolling down by a mold that reciprocates in the thickness direction of the slab, since the amount of rolling down once becomes large, internal cracks may occur in the slab. Therefore, a rolling roll system is used. If the total amount of reduction is large, 1
Rolling is performed using a plurality of pairs of reduction rolls so that the reduction in strain at the center of the slab due to reduction by the pair of reduction rolls is about 2% or less. This is because the critical strain at which internal cracking occurs in Fe-Ni alloy steel is about 2%.

The reduction ratio per pair of reduction rolls is 1.2.
% Or less. FIG. 1 shows the effect of 1 on the occurrence of internal cracks in slabs.
It is a figure which shows the influence of the amount of reduction per pair of reduction rolls.
Using a bending type continuous casting machine having a bending radius of 8 m and a length of 15 m, an Fe-Ni-based alloy steel was formed to a thickness of 200 mm and a width of 120 mm.
It was cast at a speed of 0.6 m / min on a slab having a rectangular cross section of 0 mm. For the reduction, two pairs of reduction rolls were used, and the reduction ratio of each of the two pairs of reduction rolls was made equal, and the reduction ratio per pair of reduction rolls was changed. The final position of the pair of rolls is 10 m from the molten steel meniscus.

As can be seen from FIG. 1, the thickness of the slab is 20 mm.
In the case of 0 mm, when the pair of rolling reduction is about 2.5 mm or less, that is, when the rolling reduction is 1.2% or less, the occurrence of internal cracks can be prevented. The rolling reduction of 1.2% corresponds to a rolling strain of about 2% at the center of the slab during rolling. This is because the critical strain at which internal cracking occurs in Fe-Ni alloy steel is about 2%. Therefore, the reduction ratio per pair of reduction rolls is set to 1.2% or less.

The total reduction is 1.2 to 10% of the thickness of the slab at the start of the reduction. If it is less than 1.2%, it is difficult to improve center segregation. At 1.2% or more, near 1.2%,
Although the generation of equiaxed crystals does not occur, the stress due to the reduction acts on the center of the slab, and the molten steel in which the segregable elements are concentrated is discharged to the upstream side in the casting direction, so that the center segregation is improved.
When the content is 1.5% or more, an equiaxed crystal is generated by the reduction, so that the center segregation is further improved. However, if it exceeds 10%, the effect of forming equiaxed crystals is saturated, and a negatively segregated portion is formed near the center of the slab. Further, the equipment becomes large. Therefore, the total reduction ratio is set to 1.2 to 10% of the thickness of the slab at the start of the reduction. Desirably,
1.2 to 5%.

The center solid phase ratio of the slab at the completion of the reduction is 0.5 or more. When the center solid fraction is less than 0.5, center segregation is generated again. That is, when it is less than 0.5, there is still a lot of unsolidified molten steel at the time of completion of the reduction. Therefore, unsolidified molten steel is in a flowable state,
The molten steel enriched in the segregation-prone elements generated during the progress of the subsequent solidification moves in the center of the slab, and the center segregation occurs again. Therefore, the center solid phase ratio of the cast slab after rolling is set to 0.1.
5 or more. The upper limit of the center solid fraction is not particularly limited, but is desirably 0.99 or less. When it exceeds 0.99, the equipment for reduction incorporated in the continuous casting machine becomes large.

[0037]

EXAMPLE A Fe-Ni-based alloy steel having a thickness of 80 to 25 was prepared by using a curved continuous casting machine having a bending radius of 8 m and a length of 15 m.
0mm, 600-1200mm wide slab with rectangular cross section,
Casting was performed at a speed of 0.4 to 0.8 m / min. The rolling was performed by using two pairs of rolling rolls so as to obtain a uniform rolling reduction. The final roll was located at a position 6 to 10 m from the molten steel meniscus. The test was performed while changing the casting conditions and the slab rolling conditions. Also, in some tests,
The electromagnetic stirring device provided in the secondary cooling zone was operated.

The chemical composition of the steel used was C: 0.005 to
0.007%, Si: 0.03 to 0.04%, Mn:
0.21 to 0.28%, Ni: 36.2 to 36.8%
The contents of P, S and B are as shown in Table 1 described later. The balance is Fe and inevitable impurities. Prior to continuous casting test, capacity is 35t or 15
The mixture was melted in an electric furnace of t and refined by a ladle vacuum decarburization method (VOD method).

From the obtained slab, a longitudinal section sample having a length of 4 m in the casting direction at the center of the width and a transverse section sample having a total width of 0.5 m in the casting direction were collected. The macrostructure of the sample in the longitudinal section and the transverse section was observed, and the presence or absence of internal cracks and the crack length were measured. Furthermore, the presence or absence of the formation of the equiaxed crystal and the thickness thereof were measured, and the ratio of the thickness of the equiaxed crystal to the total thickness of the slab (equiaxed crystal ratio) was calculated. The degree of center segregation was determined by measuring the ratio C / C ₀ of the analysis value C ₀ of the ladle to the analysis value C of the ladle C by cutting a cut from the center of the cross-sectional sample with a drill blade having a diameter of 5 mm. Was evaluated.

The obtained slab was subjected to hot rolling, the obtained hot rolled sheet was annealed at 1050 ° C., then pickled, and cold rolled into a thin sheet having a thickness of 0.7 mm. This thin plate was etched to produce a shadow mask. The occurrence of defects such as streaks of the shadow mask and defects caused by internal cracks in the slab was investigated.

Table 1 shows the test conditions and test results.

[0042]

[Table 1]

Test No. of the present invention example. 1 to No. In 4,
The test was conducted under the chemical composition, slab thickness, molten steel superheat degree and slab pressure reduction conditions within the range specified in the present invention. for that reason,
No internal cracks occurred in any of the cast pieces, and the center segregation degree was 1.2 or less, which was a good result. Further, the shadow mask was good without any stripe unevenness and rolling flaws. In addition, the test No. In 1, the total reduction is 1.2
%, And the thickness of the slab was as thin as 150 mm.
No equiaxed crystals were formed. However, since the conditions defined in the present invention were satisfied, the quality of the slab and the shadow mask was good.

Test No. of Comparative Example 5-No. In 7,
The test was performed at a value where the content of P, S or B exceeds the upper limit of the range specified in the present invention. Furthermore, the test No. of the comparative example.
In No. 8, the test was performed at a rolling reduction per pair of rolling rolls of 1.6%, which exceeds the upper limit of the range specified in the present invention. In each of the tests, internal cracks occurred in the slab. Further, a defect (spot-like surface defect) caused by the internal crack of the slab also occurred in the shadow mask.

Test No. of Comparative Example In No. 9, the total reduction was tested at 0.9%, which was below the lower limit of the range specified in the present invention. Since the thickness of the slab was as thin as 150 mm, no equiaxed crystal was formed in the slab, and the degree of center segregation was poor at 1.4. Furthermore, stripe unevenness occurred in the shadow mask.

Test No. of Comparative Example In No. 10, the central solid fraction at the completion of the reduction was tested at 0.3 which was less than the lower limit of the range specified in the present invention. Center segregation occurred near the center of the slab, and the degree of center segregation was as poor as 1.6. Furthermore, stripe unevenness occurred in the shadow mask.

Test No. of Comparative Example In No. 11, the test was performed without electromagnetic pressure acting on the slab at the position of the secondary cooling zone, and without reducing the slab. Equiaxed crystals were formed in the slab, and the degree of center segregation was as good as 1.1. However, a white band was generated near the center of the slab, so that the shadow mask had uneven stripes.

Test No. of Comparative Example At 12, the superheat of the molten steel in the tundish was tested at 18 ° C. below the lower limit of the range specified in the present invention. The degree of center segregation of the slab is 1.2
Although there was no particular problem, an etching defect occurred when etching the shadow mask. This is because the cleanliness of the slab was poor.

Test No. of Comparative Example In No. 13, the superheat of the molten steel in the tundish was tested at 88 ° C., which exceeded the upper limit of the range specified in the present invention, but a breakout occurred during casting and the operation was not possible.

[0050]

According to the method of the present invention, a slab which is free from unevenness or defects due to internal cracks in the slab during etching of the shadow mask, that is, has no component segregation and no internal cracks. High quality Fe-Ni alloy steel slabs can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of the amount of reduction per pair of reduction rolls on the occurrence of internal cracks in a slab.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-2-151354 (JP, A) JP-A-4-305350 (JP, A) JP-A-54-107831 (JP, A) JP-A-5-107 228598 (JP, A) JP-A-7-204813 (JP, A) JP-A-63-108955 (JP, A) JP-A-6-128662 (JP, A) JP-A-2-54743 (JP, A) JP-A-2-54744 (JP, A) JP-A-2000-96190 (JP, A) JP-A-2000-80447 (JP, A) JP-A-10-64420 (JP, A) JP-A-8-273554 (JP , A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/128 350 B22D 11/00 B22D 11/10 310

Claims (1)

(57) [Claims] 1. A superheat degree in a tundish of 20 to 80.
C. 0.07% by weight by casting molten steel
% Or less, Si: 0.005 to 0.5%, Mn: 0.05
1.0%, P: 0.005% or less, S: 0.002%
In the following, Ni: 33 to 43%, B: 0.001% or less, the balance being Fe 1 and a thickness 1 comprising unavoidable impurities.
When a continuous cast slab of 00 to 250 mm is cast and the position including the unsolidified portion of the slab is reduced by at least one pair of reduction rolls, the reduction ratio per pair of reduction rolls is 1.2% or less, The total reduction rate is 1.2 to the thickness of the slab at the start of the reduction.
A continuous casting method for steel, wherein the center solid phase ratio of the slab at the completion of rolling is 0.5% or more at 10%.