JPS59153550A - Mold for continuous casting - Google Patents

Abstract

PURPOSE:To obtain a billet having superior surface properties by delaying the initial freezing velocity by providing a heat insulating layer comprising primarily of ceramics backed by a water-cooled copper plate to the face confronting a freezing shell at the top of a mold. CONSTITUTION:A hollow part is formed by machining previously a part of a copper plate by a necessary thickness and a thin film of an Ni base alloy powder is formed to the hollow part by spraying. The spraying is further continued with powder comprising primarily of ceramics to form a heat insulating layer having 1-10mm. thickness. Also, a hollow part is formed by previously machining a part of a water-cooling copper plate by a necessary thickness, and a heat- insulating layer is formed thereon with a flat plate comprising primarily of tile- shaped ceramics having 3-10mm. thickness and 5-50mm. edge length by adhering firmly with an adhesive consisting essentially of alumina.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to mold G for continuous casting, and in particular to surface modification casting of continuously cast slabs, in which solute components are negatively segregated,
This is intended to make it possible to produce continuously cast slabs that have fewer inclusions such as alumina clusters, are free from minute surface cracks, and have excellent surface properties.

Continuous casting technology has made remarkable progress, and recently continuous casting slabs with less inclusion accumulation and center segregation have been mass-produced.

However, cold-rolled steel sheets used for automobile exterior panels are required to have very few defects on the surface of the steel sheet, but a large number of inferior products due to alumina clusters are often produced from cold-rolled steel sheets manufactured from continuously cast slabs. In rolled steel sheets, scarfing is often required at the slab stage after continuous casting due to minute surface cracks.

In other words, even with the accumulation of continuous casting technology, there is still considerable room for improvement in the surface quality of slabs.

This invention discloses the main part of a series of development results aimed at fundamentally solving the surface quality problem of continuously cast slabs.

It is already known that when molten steel is stirred during solidification of a slab, negative segregation of solute components occurs as the molten steel flow rate increases.For example, when electromagnetic stirring is performed in the spray cooling zone of a continuous casting machine, solidification during stirring is known. Negative segregation of solute components occurs in areas where solubility is progressing. It is known that the degree of negative segregation is proportional to the stirring flow rate and inversely proportional to the solidification rate.

However, even with this fact, there are many difficulties and it is generally impossible, especially when trying to expand the negative segregation in the surface layer of the slab.

That is, the solidification rate is determined by the commonly used expression 6 for the solidification thickness d, which includes the solidification fixation number 25 jntn/min +
Given by the microcube of the formula %), &ci/at= 12.57. /r tfjt &d/at=312.5/d.

By the way, the solidification rates at the 2, 5 and 10 depth positions on the surface of the slab are 156.68 and 81, respectively.
”/min+, which is quite fast, and therefore does not meet the intended purpose of this invention.

The above calculations are based on a copper water-cooled mold commonly used in continuous casting.

The inventors conducted a number of experiments in order to solve the problem that the copper water-cooled mold cannot meet the objectives of the present invention. As a result, the length of the mold in the casting direction was increased from the normal length to 120%, and the thickness was increased on the solidified shell side in the upper casting direction, which corresponded to 10 to 40% of the total length of the mold. By using a composite structure continuous casting mold in which a refractory layer of 0.5 to 30 mm is backed up with a copper plate, and the rest is made of conventional copper layers, the initial solidification rate can be increased to 20 to 50 mm. Between/min”4G
We have determined how to adjust this suppression.

In this way, a slow cooling zone whose length can be extended is formed at the top of the water-cooled mold, and electromagnetic stirring is applied to the molten steel injected into this zone, so that the solidification coefficient k in the slow cooling zone is 2 to 1''/
min+, stirring flow rate U: 12,000 to 120,000
11Im/min dekatsuni/U: (0.3~8.0
) X], O-'min+, and by forming a negative segregation layer with a thickness of 2 to 1 Orryn on the surface layer, this is a means to solve the problem of surface modification of continuously cast slabs.

When making the length of the continuous casting mold longer than usual,
The initial solidification of the slab inside the mold is delayed and the thickness of the solidified shell directly under the mold becomes thinner, creating the risk of breakout.In addition, attempts to avoid this will reduce the casting speed, i.e. the productivity of the continuous casting machine. It is particularly effective as a coping strategy, but it is not essential.

As already mentioned, this invention solves the above problems by providing a heat insulating layer mainly made of ceramics backed up by a water-cooled copper plate on the surface facing the solidified shell on the mold head in order to significantly slow down the initial solidification speed in the continuous casting machine. Achieve your purpose.

That is, a part of the Cu plate is cut to the required thickness in advance to form a recess, a thin film of Ni-based alloy powder is formed by thermal spraying, and then thermal spraying is continued with powder mainly composed of ceramics to form a heat insulating layer. The amount of heat removed is compared to normal water cooling aU.
It is adjusted to 14 to 1/2 degrees compared to the case of a plate.

Of course, the level difference between the head slow cooling zone and the solidified shell side surface of a normal water-cooled copper plate will directly lead to trouble during continuous casting, so it is important to adjust the level to 100μ or less by cutting and polishing the sprayed layer. be.

In addition, a part of the water-cooled copper plate is cut to the required thickness in advance to form a recess, and a tile-like ceramic-based flat plate is firmly bonded and fixed using an alumina-based adhesive. form a layer. In this case, the so-called joints and the level difference at the part of the OU plate where the flat plate is not bonded will cause trouble during continuous casting, so the shape of the gate and the dimensional accuracy of the ceramic flat plate should be set to 50μ or less.
Avoid this kind of problem.

Example 1 An experiment was conducted using a 1tQn experimental continuous casting machine. The cross-sectional size of the mold is 150 mln square, and the length is usually 700 mm, but it is extended to 900 mm, and the mold head is 150 mm square.
All the sections from 00 to 300 were subjected to water plasma spraying.

The plasma sprayed brand is Y2O3 stabilized zirconia.
y2oB was set to 4 wt%, and the thickness of the sprayed layer was set to 5 wt%.

On the other hand, for a fine powder mixture of 30 wt % MO and 70 wt % stabilized zirconia, the thickness of the sprayed layer was 10 am.

Casting tests were conducted twice using each of the above two types of molds with a force of 1 and a normal Qu top mold. Note that the casting conditions were not much different from those in the normal case, and powder was used.

The heat removed by the mold during casting was calculated from the temperature difference between thermocouples embedded at different depths within the mold. In addition,
The total mold thickness was kept constant at (30±1) mm. As a result, if the maximum amount of heat removed during casting in the slow cooling zone, the solidification coefficient determined by artificial breakout is as shown in Table 1, and by setting the slow cooling zone, the heat removed can easily be approximately (' /2 to 1//8).

Table 1 Example 2 An experiment was conducted using a 1 ton experimental continuous casting machine. The cross-sectional size of the mold is l5Qss square, and the length is extended from the usual 700mm to 900111111mm, and a ceramic plate is attached to the entire section 100 to 300mm from the mold head to form a slow cooling zone. . The brand of Xeramix flat plate is Y2O3 stabilized zirconia and Y2O3 is 4
wt%, shape was 50x50x5 thickness, and on the other hand, two types of flat plates molded based on a fine powder mixture of MO80wt% stabilized zirconia 70wt% with the same shape were used.

In addition to these molds, casting tests were conducted twice using ordinary Qu molds. Note that the casting conditions were not much different from those in the normal case, and powder was used.

The heat removed by the mold during casting was calculated from the temperature difference between thermocouples embedded at different depths in the mold. The total mold thickness was kept constant (30±1). As a result, the maximum value of heat removal during casting in the slow cooling zone and the solidification coefficient determined by artificial breakout are as shown in Table 2. By setting the slow cooling zone, the heat removal can easily be approximately (1// 2 to 1//8).

Table 2

Claims (1)

[Scope of Claims] 1. A mold for continuous casting, characterized in that the head of the mold is provided with a heat insulating layer made mainly of ceramic, which is punched up with a water-cooled copper plate, on the surface facing the solidified shell. i The mold according to l, wherein the heat insulating layer portion is a plasma or water plasma sprayed layer with a thickness of 1 to 10 mm. & The mold according to ], wherein the heat insulating layer portion is made of a rectangular ceramic-based material with a thickness of 3 to 1 Qfflfi and a side length of 5 to 5 Qmm.