Molds for continuously casting steel

A mold of copper or copper alloy for continuously casting steel comprises a rough inner surface, a plating of nickel, cobalt or alloy thereof formed over the inner surface and a chromium plating formed over said plating. The mold has a prolonged mold life, gives steel slabs with improved qualities and reduces amounts of lubricating material used in the molding operation.

This invention relates to molds for continuously casting steels such as 
low-carbon steels, medium-carbon steels, high-carbon steels, stainless 
steels, alloy steels of special grades and the like. 
The mold for continuously casting steel slabs comprises a pair of opposed 
rectangular plates dimensionally identical with each other and each having 
a lengthwise long side for defining the thickness of the slab and a pair 
of opposed oblong plates dimensionally identical with each other and each 
having a vertically long side for defining the width of the slab. 
Conventionally plates forming the mold are made of copper or copper alloy 
with high thermal conductivity and have a plating or like protective 
coating formed over the inner surface of the plates (hereinafter referred 
to as "base surface"). In casting operation, a vitreous powder or like 
lubricating or friction-reducing material is placed between the coated 
inner mold surface (hereinafter referred to as "coated surface") and 
molten steel, to reduce the friction between molten steel or steel slabs 
and the coated surface. In this case, the powder melts upon receipt of 
heat from the molten steel, thereby serving as the lubricant. 
We conducted extensive research to mitigate difficulties frequently posed 
by conventional molds for continuous casting such as: the reduction of 
mold life which is attributable to the damage to a portion of the coated 
surface brought into contact with molten steel being poured; the breakout 
resulting from the adhesion of molten steel droplets to the coated 
surface; etc. Our research matured to numerous inventions. However, it is 
now desired to provide molds for continuous casting with more improved 
properties and performance because molds have been recently used under 
more severe casting conditions such as higher casting rates with the 
progress in the technique of continuous casting. 
The object of this invention is to provide molds for continuously casting 
steel which have enhanced properties and performance and thus a prolonged 
mold life. 
Other objects and features of this invention will become more apparent from 
the following description. 
This invention provides molds of copper or copper alloy for continuously 
casting steel comprising a rough base surface, a plating of nickel, cobalt 
or alloy thereof formed over the base surface and a chromium plating 
formed over the foregoing plating. 
Our research shows that the foregoing object is accomplished by forming 
specific plating layers over a rough base surface in contrast to the 
concept known in the art. The conventional concept has been that the 
coated surface should be as smooth as possible, although subject to 
economy, to achieve the lowest friction between molten steel or steel 
slabs and coated surface and to provide steel slabs with the smoothest 
surface. In other words, it has been thought that the smoother the coated 
surface, the longer the mold life and the better the surface of steel 
slabs. Heretofore there have been used even molds which have the base 
surface and coated surface mirror-finished. However, our research reveals 
that the following problems arise from the molds having an extremely 
smooth surface. (i) When the coated surface has a high smoothness, the 
lubricant is easily moved upon contact with moving steel slabs, and thus 
unevenly distributed over the coated surface. In the extreme cases, little 
or no lubricant material is present between molten steel and the coated 
surface over some parts of the surface. These parts are abraded by steel 
slab, thereby increasing the friction between abraded parts and slab and 
the resistance against the withdrawal of slab. As a result, a thin outer 
layer of solidified steel is broken so that there occurs breakout. (ii) 
Uneven distribution of lubricating material results in the presence of 
excess lubricant on some parts of the coated surface. In such parts, there 
arises insufficient cooling of molten steel and slab from the low thermal 
conductivity of the lubricant. Thereby an extremely thin film of 
solidified steel is formed, which is liable to lead to breakout. This 
phenomenon is likely to occur when a large amount of lubricant is used in 
an attempt to spread it over the entire coated surface. (iii) Large 
amounts of lubricating material tend to be present in the corner portions 
of casting mold. The excess lubricant in corners is likely to produce 
breakout therein and to delay the formation of solidified surface layer in 
the corners of slab which tends to create star cracks. (iv) Since the 
lubricant is easily discharged from the mold together with the slab being 
withdrawn, a new supply of lubricant must be frequently placed in the 
mold, consequently necessitating cumbersome operations and large amounts 
of lubricant, hence economically unfavorable. (v) When at least two 
protective platings are formed over the base surface, the difference in 
elongation between the metal materials is apt to produce large stress and 
strain in the outermost plating, thereby forming cracks therein. The 
formation of cracks reduces the mold life and impairs the quality of 
slabs. 
From the conventional viewpoint, it may be assumed that the surface of 
steel slabs would be deteriorated when using the molds of this invention 
with an uneven coated surface. Unexpectedly, however, the present molds 
are found to be able to cast steel slabs with the quality comparable with 
or even superior to that of the slabs given by conventional molds. With 
the present molds, the lubricant is retained uniformly in the fine valleys 
of the uneven surface. This retention substantially precludes any breakout 
or cracks at corners from occurring due to the shortage or excess of 
lubricant. Since the lubricating material initially supplied are mostly 
left and maintained in the fine valleys of the non-flat surface, the 
frequency of supplying the lubricating material and the total amount of 
the lubricant needed are sharply reduced. Further according to this 
invention, virtually no crack is produced over the alloy plating, its 
oxidized layer or chromium plating formed as the topcoat so that the mold 
life is extended and steel slabs are cast with enhanced quality. More 
specifically stated, the formation of cracks in the plating layer is 
prevented because the difference between the base surface and the plating 
layer formed over the base surface or between the respective layers in 
thermal stress and strain is moderated by the enlarged surface area 
resulting from the uneveness of the surface. 
The mold of this invention is similar in the basic structure to 
conventional molds of copper or copper alloy for continuously casting 
steel. The present mold has a surface roughness of about 20 to about 200S, 
preferably about 50 to about 150S, according to Japanese Industrial 
Standards B0601. With a surface roughness of less than 20S, it is 
difficult to provide improved lubricity as desired and to prevent 
formation of crack in the outermost plating to a satisfactory extent. The 
uneven surface having a surface roughness of over 200S is unfavorable 
because casting operations markedly wear out crests of the ridges of the 
irregular surface. The desired uneven surface may be such that 
infinitesimal ridges and valleys are regularly distributed when viewed 
microscopically and macroscopically or that they are almost uniformly 
distributed from a microscopic view although irregularly distributed from 
a macroscopic view. Also desirable are wavelike arrangements comprising 
series of ridges and valleys running in parallel. With wavelike 
arrangements, it is more preferred to align the series of ridges and 
valleys in the direction of flow of molten steel being poured, although 
the direction of the alignment is not particularly limited. The non-flat 
surface can be produced by any suitable method such as shot blasting, 
mechanical machining by a shaper or the like, a method comprising forming 
partly masked minute portions and selectively etching unmasked portions 
over the base surface, a method comprising moving over the base surface a 
roll having small protrusions or minute wavelike pattern to press the base 
surface, etc. Usually plating layers to be described below are formed over 
the rough surface thus formed. Further, with new molds of copper or copper 
alloy, it is possible to perform the treatment for giving an irregular 
surface after forming a single plating layer, two plating layers or three 
plating layers directly over the base surface. 
According to this invention, one of the platings (a) to (d) to be described 
below is formed over the base surface. (a) A first layer of nickel, cobalt 
or alloy thereof is formed by electroplating over the base surface and a 
second layer of chromium is applied over the first layer. Although 
variable depending on the kind of steel material, dimensions of the mold, 
etc., the preferred thickness of the nickel and/or cobalt plating is about 
195 to about 2950 .mu.m and that of the chromium plating about 5 to about 
50 .mu.m, hence the desired total thickness being about 200 to about 3000 
.mu.m. More preferred are a first layer about 300 to about 1000 .mu.m 
thick and a second layer about 10 to about 20 .mu.m thick, these layers 
having a total thickness of about 310 to about 1020 .mu.m. When the nickel 
and/or cobalt plating is over 2950 .mu.m thick, cracks are prone to occur 
at a level in the interior of the mold at which the coated surface is in 
contact with the meniscus of molten steel placed in the mold. In this 
case, large cracks run deep sometimes into 2.5 times the thickness of the 
platings, namely into the copper material portion of the mold. With a 
thickness of less than 195 .mu.m, the first layer is low in abrasion 
resistance so that part of the copper material is likely to be exposed 
particularly at the lower portion of the coated surface in an early stage 
of continuous casting operation. The chromium plating layer over 50 .mu.m 
thick tends to produce cracks locally which contribute to the separation 
of the layer and likely reach the nickel and/or cobalt layer, even when 
the surface is so uneven as to be able to distribute and moderate the 
thermal stress of the topcoat. The second layer less than 5 .mu.m thick is 
apt to become poor locally in adhesion to the first layer or to produce 
pinholes or the like, consequently failing to achieve the desired effect, 
hence undesirable. The term nickel used herein includes nickel materials 
containing about 0.2 to about 3% of cobalt as impurities. (b) Over the 
base surface is applied a first layer of nickel, cobalt or alloy thereof 
over which is formed a second layer of an alloy comprising 3 to 20% by 
weight of phosphorus and/or 2 to 15% by weight of boron, and nickel and/or 
cobalt as the balance. When containing phosphorus and/or boron in lesser 
amounts, the second layer is prone to have lower heat resistance and 
hardness. But the use thereof in larger amounts leads to economical 
disadvantage. The second alloy plating, although applicable by 
electrodeposition, is preferably formed by an electroless plating 
procedure because the procedure usually produces fine crystals and easily 
affords a plating of uniform thickness whether over the planar or curved 
base surface or over the base surface of a mold in the form of a 
quadrilaterally fabricated tube or a cylinder. The thicknesses of the 
first layer and the second layer, although variable with the casting 
temperature, kind of steel, dimensions of the mold, etc. are usually about 
30 to about 1900 .mu.m and about 10 to 100 .mu.m, respectively, the 
desired total thickness being about 40 to 2000 .mu.m, and more preferably 
about 100 to about 1000 .mu.m and about 20 to about 60 .mu.m, 
respectively, the combined thickness being about 120 to about 1060 .mu.m. 
The first layer is interposed between the copper material and the second 
layer different in the properties from the copper and can support the 
second layer against thermal, mechanical and other various loads and serve 
as a buffering layer to permit the second layer to satisfactorily 
function. The first layer with less than 30 .mu.m thickness fails to meet 
the requirements. The first layer more than 1900 .mu.m thick likely 
produces cracks upon receipt of high heat and to lead to insufficient 
cooling of the mold in high-speed casting operation. The second layer less 
than 10 .mu.m thick is low in abrasion resistance, while the one over 100 
.mu.m thick likely creates cracks and causes damage to the mold because of 
insufficient cooling of the mold resulting from the low thermal 
conductivity of the alloy of the second layer. (c) A third layer of 
chromium plating formed over the second layer stated above in (b) can 
provide prolonged mold life. The chromium plating can be applied by the 
usual electroplating. The chromium plating is extremely effective in 
preventing the adhesion of splash of molten steel which otherwise would 
occur on initial influx of molten steel. The third layer is usually about 
5 to about 100 .mu.m thick, preferably about 10 to about 30 .mu.m thick. 
(d) An oxidized layer is formed by oxidizing the surface of the second 
layer described above in (b). This layer is also markedly effective in 
precluding the adhesion of splash of molten steel taking place on initial 
influx of molten steel. The oxidatively surfaced layer can be formed by 
conventional oxidizing methods such as those in which the second layer of 
the alloy as the anode is oxidized by electrolysis in an aqueous solution 
of sodium hydroxide or like alkaline material or those in which the 
surface of the alloy layer is heated in an atmosphere by a gas burner 
(flame oxidation method). The oxidized layer is at least about 0.001 .mu.m 
thick, preferably up to about 0.5 .mu.m thick. 
The mold of this invention has the feature in the combination of forming an 
irregular base surface and applying specific protective layers, which can 
achieve remarkable results: the extension of mold life, improvements in 
the quality of steel slab and reduction in the amount of a lubricant to be 
used. 
The following examples illustrate this invention in more detail.

EXAMPLE 1 
A copper mold (300 mm wide.times.1300 mm long.times.800 .mu.m high) for 
continuously casting steel slabs was masked with a vinyl chloride coating 
composition over a portion of the base surface other than a portion 
thereof to be brought into contact with molten steel. The mold was 
degreased by being immersed at 55.degree. C. for 30 minutes in an aqueous 
solution containing 55 g/l of sodium hydroxide, 30 g/l of sodium carbonate 
and 5 g/l of an anion surfactant and then was washed with water. 
Subsequently it was electrolytically degreased in an aqueous solution 
containing 35 g/l of sodium hydroxide, 160 g/l of sodium orthosilicate and 
10 g/l of an anion surfactant and having a temperature of 55.degree. C. at 
a cathode current density of 10 A/dm.sup.2 for 3 minutes. The mold body 
thus degreased was washed with water and then was activated by being 
immersed at an ordinary temperature for 15 minutes in a 5% aqueous 
solution of sulfuric acid. After washing with water, the mold was 
electroplated by being dipped in a bath containing 450 g/l of nickel 
sulfamate, 40 g/l of nickel chloride, 20 g/l of boric acid and 3 g/l of 
sodium naphthalene trisulfonate and having a temperature of 50.degree. C. 
and a pH of 4.5 at a cathode current density of 1.5 A/dm.sup.2 for 30 
hours while continuously filtering the bath, whereby a 550 .mu.m-thick 
nickel plating was formed on the mold body. Then the mold body was 
electroplated in a bath containing 320 g/l of anhydrous chromic acid, 0.8 
g/l of sulfuric acid and 5 g/l of potassium silicofluoride and having a 
temperature of 50.degree. C. at a cathode current density of 25 A/dm.sup.2 
for 40 minutes to form a 10 .mu.m thick chromium plating over the nickel 
plating. 
Five other molds were treated over the base surface in the same manner as 
above. The molds were tested by continuously casting low-carbon steel 
slabs at a casting rate of 0.8 m/min to check how the uneven surface of 
each mold affected the formation of crack and separation of the chromium 
layer, mold life and appearance of the surface of steel slabs. Table 1 
shows the results. Before electroplating, the base surface of the mold had 
been machined by a shaper to give a specific surface roughness so that the 
series of infinitesimal ridges and valleys of the irregular surface run in 
the direction of flow of the molten steel being poured. 
TABLE 1 
__________________________________________________________________________ 
Appearance of 
Amount of vitreous 
Appearance of chromium 
Mold life 
slab surface 
powder used 
Surface layer after 100 charges* 
(number of 
(after 100 
(kg/t of molten 
No. 
roughness 
Crack Separation 
charges) 
charges) 
steel) 
__________________________________________________________________________ 
1 Less than 
Found Locally found 
150 Normal 0.50 
10S 
2 25S None None 300 Good 0.45 
3 70S None None 350 Excellent 
0.35 
4 150S None None 550 Excellent 
0.35 
5 200S None** 
None 600 Good 0.30 
6 250S None None 300 Good 0.40 
__________________________________________________________________________ 
*Amount of molten steel per charge = 250 t 
**Abraded in corners 
As apparent from Table 1, the molds of this invention were outstanding in 
the durability and gave steel slabs with improved quality and markedly 
reduces the amounts of vitreous lubricating material to be used. The tests 
show that when using the rough surfaced molds, the amounts of vitreous 
powder were reduced by about 20 to about 30% compared with the amounts of 
0.45 to 0.5 kg/t in conventional molds. 
EXAMPLE 2 
A roll with a minutely arranged wavelike pattern was moved over the base 
surface of a copper mold dimensionally identical with the molds used in 
Example 1 to form a surface roughness of 70S. The same procedure as above 
was repeated to obtain nine other molds similarly surfaced. Over the base 
surfaces of the molds were formed first layers having the compositions and 
thicknesses as indicated in Table 2 and second layers of 20 .mu.m-thick 
chromium. The molds were used to continuously cast low-carbon steels in 
the same manner as in Example 1. Table 2 shows the results. 
TABLE 2 
__________________________________________________________________________ 
First layer Appearance of 
Composition (wt. %) 
Thickness 
Mold life slab surface 
Ni Co (m) (number of charges) 
(after 200 charges) 
__________________________________________________________________________ 
1 85 15 250 280 Good 
2 85 15 1000 380 Excellent 
3 85 15 2500 450 Good 
4 50 50 200 250 Normal 
5 50 50 1500 350 Good 
6 50 50 2700 410 Good 
7 30 70 500 280 Normal 
8 30 70 1000 320 Good 
9 -- 100 350 150 Normal 
10 -- 100 800 180 Normal 
__________________________________________________________________________ 
EXAMPLE 3 
(i) Formation of a rough surface 
A mold (300 mm wide.times.1300 mm long.times.800 mm high) for continuously 
casting steel slabs was machined by a shaper over a portion of the base 
surface to be brought into contact with molten steel so that the series of 
infinitesimal ridges and valleys of the rough surface run in the direction 
of flow of molten steel being poured. 
(ii) Pretreatment 
The base surface of the mold was masked with a vinyl chloride coating 
composition over the portion thereof other than that to be in contact with 
molten steel. The mold body was degreased by being immersed at 50.degree. 
C. for 40 minutes in an aqueous solution containing 50 g/l of sodium 
hydroxide, 25 g/l of sodium carbonate and 5 g/l of an anion surfactant. 
The mold was washed with water and was electrolytically degreased in an 
aqueous solution containing 30 g/l of sodium hydroxide, 150 g/l of sodium 
orthosilicate and 10 g/l of an anion surfactant and having a temperature 
of 60.degree. C. at a cathode current density of 10 A/dm.sup.2 for 2 
minutes. The mold body was then washed again with water and was activated 
by being dipped in a 5% aqueous solution of sulfuric acid having an 
ordinary temperature for 10 minutes. 
(iii) Formation of a nickel plating 
The mold body thus activated was washed with water and was electroplated in 
a bath containing 500 g/l of nickel sulfamate, 30 g/l of nickel chloride, 
10 g/l of boric acid, and 3 g/l of sodium naphthalene trisulfonate and 
having a temperature of 45.degree. C. and a pH of 4.8 at a cathode current 
density of 1 A/dm.sup.2 for 10 hours while filtering the bath to form a 
120 .mu.m-thick nickel plating. 
(iv) Formation of an alloy plating 
The mold with the nickel plating formed over the base surface was washed 
with water and was subjected to an electroless plating procedure by being 
immersed in a bath containing 30 g/l of nickel sulfate, 180 g/l of sodium 
citrate and 18 g/l of sodium hydrophosphite and having a temperature of 
90.degree. C. and a pH of 12 for 8 hours to form a 23 .mu.m-thick plating 
of nickel-phosphorus alloy containing 88% by weight of nickel and 12% by 
weight of phosphorus. The mold body was then washed with water and dried. 
The coating composition was removed from the masked area. 
Six other molds were treated in the same manner as above. 
The molds were used to continuously casting medium-carbon steel at a 
casting rate of 0.8 m/min and checked to find how the uneven surface of 
the mold affected the formation of crack and separation of the alloy 
plating, mold life and appearance of surface of slabs. Table 3 shows the 
results. 
TABLE 3 
__________________________________________________________________________ 
Appearance of topcoat 
Appearance of 
Surface after 100 charges* 
Mold life slab surface 
No. 
roughness 
Crack 
Separation 
(number of charges) 
(after 100 charges) 
__________________________________________________________________________ 
1 Less than 10S 
A** None 350 Normal 
2 20S A** None 400 Good 
3 50S None None 400 Good 
4 100S None None 550 Excellent 
5 150S None None 600 Excellent 
6 200S None None 500 Good 
7 250S B*** None 400 Normal 
__________________________________________________________________________ 
*Amount of molten steel per charge = 250 t 
**The mark "A" in Table 3 (and in other tables given hereinafter) shows 
that small cracks were formed but caused no trouble to the casting 
operation. 
***The mark "B" in Table 3 (and in other tables appearing later) indicate 
that there were small cracks and abraded crests of ridges hindering the 
casting operation. 
As seen from Table 3, the molds of this invention are excellent in the 
durability and give steel slabs with improved quality. Further the molds 
of this invention with the irregular surfaces reduced the amount of 
vitreous powder approximately by 20 to 30%, compared with the amounts of 
0.45 to 0.5 kg/t in conventional molds. 
EXAMPLE 4 
The similar copper mold as used in Example 3 was machined by a shaper over 
the base surface in the same manner as in Example 3 to give a surface 
roughness of 100S. Nine other molds were treated in the same manner as 
above. Then over the base surfaces of the molds were formed first layers 
and then second layers respectively having the compositions and 
thicknesses as indicated in Table 4 below. The molds thus surfaced were 
used to continuously cast medium-carbon steels in the same manner as in 
Example 3. Table 4 shows the results. 
TABLE 4 
__________________________________________________________________________ 
Appearance of 
First layer Second layer Mold life 
slab surface 
Composition (wt. %) 
Thickness 
Composition (wt. %) 
Thickness 
(number of 
(after 200 
No. 
Ni Co (.mu.m) 
Ni Co 
P B (.mu.m) 
charges) 
charges) 
__________________________________________________________________________ 
1 100 -- 500 95 -- 
5 -- 60 900 Good 
2 100 -- 500 86 -- 
14 -- 30 500 Good 
3 100 -- 1000 96 -- 
-- 4 30 800 Good 
4 -- 100 500 -- 95 
5 -- 30 500 Normal 
5 -- 100 500 -- 91 
9 -- 30 500 Normal 
6 -- 100 500 -- 97 
-- 3 20 450 Normal 
7 60 40 500 80 12 
8 -- 30 500 Normal 
8 80 20 500 90 7 
3 -- 30 600 Normal 
9 80 20 1000 60 34 
-- 6 30 700 Normal 
10 80 20 1000 60 30 
6 4 30 700 Normal 
__________________________________________________________________________ 
EXAMPLE 5 
(i) Formation of a rough surface 
A mold made of copper alloy containing 1% of chromium (200 mm 
wide.times.1300 mm long.times.700 mm high) for continuously casting steel 
was machined in the same manner as in Example 3 to provide a non-flat 
surface. 
(ii) Pretreatment 
The same pretreatment as in Example 3 was effected. 
(iii) Formation of a cobalt plating 
After activation, the mold body was washed with water and was electroplated 
by being immersed at 70.degree. C. for 15 hours in a bath containing 260 
g/l of cobalt chloride and 30 g/l of boric acid and having a pH of 4.5 at 
a cathode current density of 1 A/dm.sup.2 to form a 170 .mu.m-thick cobalt 
plating. 
(iv) Formation of an alloy plating 
The mold with the cobalt plating formed over the base surface was washed 
with water and was subjected to an electroless plating procedure by being 
immersed in a bath containing 30 g/l of nickel sulfate, 140 g/l of sodium 
citrate, 18 g/l of sodium hypophosphite and having a temperature of 
90.degree. C. and a pH of 10 for 10 hours to form a 30 .mu.m-thick plating 
of nickel-phosphorus alloy consisting of 93 wt. % of Ni and 7 wt. % of P. 
(v) Formation of a chromium plating 
The mold body with the alloy plating formed was washed with water and was 
electroplated over the alloy plating by being dipped in a bath containing 
320 g/l of anhydrous chromic acid, 0.8 g/l of sulfuric acid and 5 g/l of 
potassium silicofluoride and having a temperature of 50.degree. C. at a 
cathode current density of 25 A/dm.sup.2 for 60 minutes to form a 15 
.mu.m-thick chromium plating. 
The mold was washed with water and dried. The coating composition was 
removed from the masked area. Then the mold was used to continuously cast 
stainless steels at a casting rate of 0.8 m/min. 
Six other molds were subjected to the same procedure as above and used for 
the same casting operation. Table 5 shows the results. 
TABLE 5 
__________________________________________________________________________ 
Appearance of topcoat 
Appearance of 
Surface after 100 charges* 
Mold life slab surface 
No. 
roughness 
Crack 
Separation 
(number of charges) 
(after 100 charges) 
__________________________________________________________________________ 
1 Less than 10S 
A None 400 Normal 
2 20S A None 500 Good 
3 50S A None 500 Good 
4 100S None None 600 Excellent 
5 150S None None 600 Excellent 
6 200S None None 600 Good 
7 250S B None 500 Normal 
__________________________________________________________________________ 
*Amount of molten steel per charge = 250 t 
The molds used in this example were found remarkable in the durability and 
gave steel slabs with improved quality. The amounts of the vitreous powder 
used were reduced by 20 to 30% compared with the amounts in conventional 
molds. 
EXAMPLE 6 
(i) Formation of a rough surface 
A copper mold similar to that of Example 3 was machined by a shaper to 
provide an uneven base surface. 
(ii) Pretreatment 
The same procedure of Example 3 was repeated. 
(iii) Formation of a nickel-cobalt plating 
After the activation, the mold body was washed with water and was 
electroplated by being immersed in a bath containing 300 g/l of cobalt 
chloride, 40 g/l of nickel chloride and 20 g/l of boric acid and having a 
temperature of 70.degree. C. and a pH of 4.5 at a cathode current density 
of 1 A/dm.sup.2 for 10 hours while continuously filtering the bath, 
whereby a 130 .mu.m-thick plating containing 15% by weight of nickel and 
85% by weight of cobalt was formed. 
(iv) Formation of an alloy plating 
The mold body having the nickel-cobalt plating over the base surface was 
washed with water and was subjected to an electroless plating procedure by 
being dipped at 85.degree. C. for 7 hours in a bath containing 28 g/l of 
nickel chloride, 30 g/l of sodium citrate and 3 g/l of sodium borohydride 
having a pH of 9 to form a 32 .mu.m-thick alloy plating consisting of 97% 
by weight of nickel and 3% by weight of boron. 
(v) Formation of a chromium plating 
A 20 .mu.m-thick chromium plating was formed in the similar manner as in 
Example 1. 
The mold was washed with water and dried. Then the coating composition was 
removed from the masked area, giving the mold of this invention. 
Six other molds were treated in the same manner as above. 
The molds were used to continuously cast low-carbon steels at a casting 
rate of 1.0 m/min. 
Table 6 below shows the results. 
TABLE 6 
__________________________________________________________________________ 
Appearance of topcoat 
Appearance of 
Surface after 100 charges* 
Mold life slab surface 
No. 
roughness 
Crack 
Separation 
(number of charges) 
(after 100 charges) 
__________________________________________________________________________ 
1 Less than 10S 
A None 350 Normal 
2 20S A None 400 Good 
3 50S None None 600 Good 
4 100S None None 800 Excellent 
5 150S None None 800 Excellent 
6 200S B None 700 Good 
7 250S B None 500 Good 
__________________________________________________________________________ 
*Amount of molten steel per charge = 250 t 
As apparent from Table 6, the molds used in this example were found 
outstanding in the durability and gave steel slabs with improved quality. 
The amounts of vitreous powder used were reduced by 20 to 30% on an 
average. 
EXAMPLE 7 
(i) Formation of a rough surface 
A copper mold (400 mm wide.times.1500 mm long.times.700 mm high) for 
continuously casting steel slabs was machined in the same manner as in 
Example 3 to provide an uneven base surface. 
(ii) Pretreatment 
The similar procedure as in Example 3 was repeated. 
(iii) Formation of a nickel plating 
After the activation, the mold body was washed with water and was 
electroplated by being immersed in a bath containing 450 g/l of nickel 
sulfamate and 25 g/l of boric acid and having a temperature of 55.degree. 
C. and a pH of 3.1 at a cathode current density of 2 A/dm.sup.2 for 26 
hours to form a 500 .mu.m-thick nickel plating. 
(iv) Formation of an alloy plating 
The mold body having the nickel plating formed over the rough surface was 
washed with water and was subjected to an electroless plating procedure by 
being dipped in a bath containing 20 g/l of nickel sulfate, 10 g/l of 
cobalt chloride, 60 g/l of sodium citrate and 20 g/l of sodium 
hypophosphite and having a temperature of 85.degree. C. and a pH of 4.8 
for 20 hours to form a 67 .mu.m-thick alloy plating containing 62% by 
weight of nickel, 26% by weight of cobalt and 12% by weight of phosphorus. 
(v) Formation of a chromium plating 
A 25 .mu.m-thick chromium plating was formed in the similar manner as in 
Example 3. 
The mold body was washed with water and dried. The coating composition was 
removed from the masked area, giving the mold of this invention. 
Six other molds were treated in the similar manner as above. 
These molds were used to continuously cast high-carbon steels at a casting 
rate of 1.5 m/min. 
Table 7 shows the durability of the molds and the appearance of surface of 
slabs. 
TABLE 7 
__________________________________________________________________________ 
Appearance of topcoat 
Appearance of 
Surface after 100 charges* 
Mold life slab surface 
No. 
roughness 
Crack 
Separation 
(number of charges) 
(after 100 charges) 
__________________________________________________________________________ 
1 Less than 10S 
B Found along 
250 Normal 
minute cracks 
2 20S A None 350 Normal 
3 50S A None 550 Good 
4 100S None 
None 800 Excellent 
5 150S None 
None 800 Excellent 
6 200S None 
None 750 Excellent 
7 250S B None 550 Good 
__________________________________________________________________________ 
*Amount of molten steel per charge = 250 t 
The molds used in this example produced remarkable results as indicated in 
Table 7. The amounts of the vitreous powder were decreased by 20 to 30% 
compared with those involved in conventional molds. 
EXAMPLE 8 
A copper mold dimensionally identical with the mold used in Example 7 was 
machined by a shaper over the portion of the base surface to be brought in 
contact with molten steel to give a surface roughness of 70S so that the 
ridges and valleys of the irregular surface extend in the direction of 
flow of molten steel. Nine other molds were treated in the same manner as 
above to give a similar surface roughness. There were formed first layers 
over the base surfaces and second layers over the first layers, each layer 
having the compositions and thicknesses as shown in Table 8. Over each 
second layer was applied a 10 .mu.m-thick chromium layer. 
These molds were used to continuously cast high-carbon steels in the same 
manner as in Example 7. Table 8 below shows the results. 
TABLE 8 
__________________________________________________________________________ 
Appearance of 
First layer Second layer Mold life 
slab surface 
Composition (wt. %) 
Thickness 
Composition (wt. %) 
Thickness 
(number of 
(after 200 
No. 
Ni Co (.mu.m) 
Ni Co P B (.mu.m) 
charges) 
charges) 
__________________________________________________________________________ 
1 95 5 500 95 4 -- 
1 30 800 Excellent 
2 95 5 500 89 3 8 
-- 30 800 Excellent 
3 95 5 200 91 4 -- 
5 30 700 Good 
4 70 30 500 62 27 11 
-- 30 500 Normal 
5 70 30 500 85 4 11 
-- 30 600 Good 
6 80 20 500 91 4 -- 
5 30 800 Excellent 
7 95 5 1000 86 3 11 
-- 30 700 Good 
8 95 5 1000 90 4 6 
-- 30 800 Excellent 
9 95 5 2000 90 4 6 
-- 60 800 Good 
10 60 40 300 53 35 10 
2 30 500 Normal 
__________________________________________________________________________ 
EXAMPLE 9 
(i) Formation of a rough surface 
A mold made of copper alloy containing 1% by weight of silver (280 mm 
wide.times.1000 mm long.times.700 mm high) was machined in the same manner 
as in Example 3 to provide an uneven surface. 
(ii) Pretreatment 
The same procedure as in Example 3 was repeated. 
(iii) Formation of a nickel plating 
After the activation, the mold body was washed with water and was 
electroplated by being immersed at 55.degree. C. for 11 hours in a bath 
containing 450 g/l of nickel sulfamate and 25 g/l of boric acid having a 
pH of 3.1 at a cathode current density of 2 A/dm.sup.2 to form a 200 
.mu.m-thick nickel plating. 
(iv) Formation of an alloy plating 
The mold body electroplated above was washed with water and was submerged 
in an electroless plating bath containing 40 g/l of cobalt chloride, 15 
cc/l of ethylenediamine, 10 g/l of sodium citrate, 15 g/l of sodium 
hypophosphite and 3 g/l of sodium borohydride and having a temperature of 
80.degree. C. and a pH of 12.0 for 10 hours to form a 37 .mu.m-thick alloy 
plating consisting of 86% by weight of cobalt, 9% by weight of phosphorus 
and 5% by weight of boron. Then electrolysis was continued for 10 minutes 
at room temperature and an anode current density of 20 A/dm.sup.2 by 
passing current through an aqueous solution containing 100 g/l of sodium 
hydroxide to form about 0.1 .mu.m-thick oxidized layer. 
Then the mold body was washed with water and dried. The coating composition 
was removed from the masked area. The outer surface of the mold was cooled 
with water while the inner surface of the mold was uniformly heated for 
about 40 minutes by oxy-propane burner flame. Six other molds were treated 
in the same manner as above. The molds were used to continuously casting 
medium-carbon steels at a casting rate 1.2 m/min. Table 9 below indicates 
the durability of the test molds and the appearance of surface of slabs. 
TABLE 9 
__________________________________________________________________________ 
Appearance of topcoat 
Appearance of 
Surface after 100 charges* 
Mold life slab surface 
No. 
roughness 
Crack 
Separation 
(number of charges) 
(after 100 charges) 
__________________________________________________________________________ 
1 Less than 10S 
A None 250 Normal 
2 20S A None 350 Normal 
3 50S None None 600 Good 
4 100S None None 750 Excellent 
5 150S None None 800 Excellent 
6 200S None None 750 Good 
7 250S B None 600 Normal 
__________________________________________________________________________ 
*Amount of molten steel per charge = 250 t 
EXAMPLE 10 
(i) Formation of a rough surface 
A copper mold (320 mm wide.times.1500 mm long.times.700 mm high) for 
continuously casting steel slabs was machined in the same manner as in 
Example 3 to provide an uneven surface. 
(ii) Pretreatment 
The same procedure as in Example 3 was repeated. 
(iii) Formation of a nickel plating 
After the activation, the mold body was washed with water and was 
electroplated by being immersed in a bath containing 320 g/l of nickel 
sulfate, 30 g/l of nickel chloride, 10 g/l of boric acid and 3 g/l of 
sodium naphthalene trisulfonate and having a temperature of 55.degree. C. 
and a pH of 4.5 at a cathode current density of 2 A/dm.sup.2 while 
continuously filtering the bath, whereby a 210 .mu.m-thick nickel plating 
was formed. 
(iv) Formation of an alloy plating 
The mold thus plated was washed with water and was subjected to an 
electroless plating procedure by being dipped at 72.degree. C. and for 9 
hours in a bath containing 30 g/l of nickel chloride, 15 g/l of cobalt 
sulfate, 10 g/l of sodium hypophosphite, 5 g/l of sodium borohydride and 
65 g/l of sodium citrate having a pH of 10, whereby over the first layer 
was formed a 23 .mu.m-thick alloy plating consisting of 84% by weight of 
nickel, 11% by weight of cobalt, 3% by weight of phosphorus and 2% by 
weight of boron. Then an oxidized layer was applied over the alloy plating 
in the same manner as in Example 9. 
The mold was washed with water and dried. The coating composition was 
removed from the masked area. 
Six other molds were similarly treated. 
These molds were tested for the properties and durability and appearance of 
the slab surface by continuously casting high-carbon steels at a casting 
rate of 1.2 m/min. Table 10 shows the results. 
TABLE 10 
__________________________________________________________________________ 
Appearance of topcoat 
Appearance of 
Surface after 100 charges* 
Mold life slab surface 
No. 
roughness 
Crack 
Separation 
(number of charges) 
(after 100 charges) 
__________________________________________________________________________ 
1 Less than 10S 
A None 400 Normal 
2 20S A None 500 Good 
3 50S A None 550 Good 
4 100S None None 750 Excellent 
5 150S None None 800 Excellent 
6 200S None None 750 Good 
7 250S B None 600 Normal 
__________________________________________________________________________ 
*Amount of molten steel per charge = 250 t 
EXAMPLE 11 
Eight molds similar to those used in Example 10 were machined and 
pretreated in the same manner as therein. There were formed over the base 
surfaces first layers and then second layers having the compositions and 
thicknesses as listed in Table 11. Over each second layer was forced an 
oxidized layer about 0.1 .mu.m thick by electrolysis. 
These molds were used for continuously casting high-carbon steels in the 
same manner as in Example 10. Table 11 below indicates the results. 
TABLE 11 
__________________________________________________________________________ 
Appearance of 
First layer Second layer Mold life 
slab surface 
Composition (wt. %) 
Thickness 
Composition (wt. %) 
Thickness 
(number of 
(after 200 
No. 
Ni Co (.mu.m) 
Ni 
Co P B (.mu.m) 
charges) 
charges) 
__________________________________________________________________________ 
1 100 -- 200 85 
4 11 -- 30 750 Excellent 
2 100 -- 300 95 
4 -- 1 30 800 Excellent 
3 100 -- 500 91 
4 -- 5 30 750 Good 
4 100 -- 1000 62 
27 11 -- 30 700 Normal 
5 70 30 500 62 
27 8 3 30 550 Normal 
6 70 30 500 62 
27 10 1 60 500 Normal 
7 70 30 300 53 
35 10 2 30 450 Normal 
8 60 40 300 53 
35 10 2 60 400 Normal 
__________________________________________________________________________