Molds for continuous casting of steel

This invention provides a copper or copper alloy mold for continuous casting of steel characterized in that a nickel-boron alloy plating layer containing 0.06 to 0.3 wt. % of boron is formed on its interior surface.

TECHNICAL FIELD 
The present invention relates to a mold for continuous casting of steel, 
such as low carbon steel, high carbon steel, stainless steel, special 
steel, etc. and more particularly to a mold for continuous casting of 
steel which has an extended useful life. 
BACKGROUND ART 
The mold for continuous casting of steel is so designed that molten steel 
poured from its top end is solidified by cooling and the resulting product 
is withdrawn from its lower end in a continuous sequence. As such, from 
productivity points of view, the mold is required to have a long service 
life. The long-life continuous casting mold heretofore known is the one 
disclosed in Japanese Examined Patent Publication No. 40341/1980. This 
mold comprises a copper or copper alloy body and as formed on the internal 
surface thereof which is to be exposed to molten steel, (A) an 
intermediate plating layer comprising at least one member selected from 
the group consisting of nickel and cobalt and (B) a surface alloy plating 
layer formed from either 3 to 20 weight % of phosphorus or 2 to 15 weight 
% of boron or both and the balance of at least one member selected from 
the group consisting of nickel and cobalt. The reasons why this mold has a 
long life are allegedly as follows. One of the reasons is that the 
provision of said intermediate layer (A) serves to flatten the gradient of 
hardness between the copper or copper alloy mold body which is very low in 
hardness and the alloy layer (B) which has a high hardness to thereby 
increase the bond between the three members, viz the body metal, 
intermediate layer and alloy layer. The other reason is that the alloy 
layer has high resistances to heat and wear at high temperature. 
As improved versions of the above-mentioned mold, there also are known the 
mold carrying a chromium plating layer in superimposition on said alloy 
layer (B) (Japanese Examined Patent Publication No. 50734/1977) and the 
mold carrying an oxide layer as formed by oxidizing said alloy layer (B) 
(Japanese Examined Patent Publication No. 50733/1977). The chromium 
plating layer and the oxide layer in these molds serve to preclude 
deposition of molten steel splashes evolved at the start of casting on the 
mold surface and eliminates chances for breakout troubles. Thanks to this 
feature and the above-mentioned increased intimacy of the three members, 
namely the mold body, intermediate layer and alloy layer and the high wear 
resistance of the alloy layer at high temperature, these molds have 
serviceable lives even longer than the life of the first-mentioned mold 
described in Japanese Examined Patent Publication No. 40341/1980. 
The above-mentioned molds carrying two or three protective layers 
essentially have an intermediate layer comprising at least one member of 
the group consisting of nickel and cobalt and, as disposed thereon, an 
alloy layer and, in the case of three-layer molds, further a chromium 
plating layer or an oxide layer, and, as such, require complicated 
manufacturing procedures and high production costs. 
DISCLOSURE OF THE INVENTION 
In connection with a mold wherein molten steel is charged at its top and 
solidified product is withdrawn from its bottom, the present inventor 
conducted an intensive research to develop a protective layer which is 
structurally simple and easy to manufacture and which is to be formed over 
a substrate mold body (a plate or tube of copper or copper alloy which 
constitutes the internal surface of a mold). As a consequence, the 
inventor surprisingly discovered that notwithstanding the widely-accepted 
notion that nickel-boron alloy plating in general are poorly bonded to 
substrate copper or copper alloy, a nickel-boron alloy plating layer with 
a low boron content in a certain specific range has a good ability to bond 
to the substrate copper or copper alloy and serves on its own as an 
excellent protective layer even without the provision of an intermediate 
layer used in the foregoing prior art mold, thus providing for the 
manufacture of a mold having a life at least equal to or even longer than 
the lives of the above-mentioned mold having two or three superposed 
protective layers. The further research by the inventor led to the finding 
that a mold having a long life, which is unexpected from the conventional 
knowledge, can also be produced by forming an under plating layer 
consisting essentially of at least one of nickel and cobalt and, in 
superimposition thereon, a nickel-boron alloy plating layer with a low 
boron content. The present invention is predicated on the above findings.

The present invention provides a mold for continuous casting of steel which 
is characterized in that the mold has a nickel-boron alloy plating layer 
containing 0.05 to 1.5 weight % of boron on its inner surface. 
In accordance with the present invention, the simple structure of a 
substrate mold body and a nickel-boron alloy plating layer with a boron 
content in the above specific range as formed over the substrate mold body 
assures a mold life which is at least comparable or even longer than the 
lives of the conventional molds having two or three superposed protective 
layers. This is quite unexpected in view of the facts that a nickel-boron 
alloy plating layer was believed to have a poor ability to bond to 
substrate copper or copper alloy, that a boron content not more than 2 
weight % was considered to be inadequate in terms of heat resistance and 
hardness, and that it was considered essential to form a chromium plating 
layer on the alloy layer or to oxidize the alloy layer to form an oxide 
layer in order that the deposition of splashes may be positively 
precluded. 
While the detailed reason why the mold of the invention has such an 
extended life is not fully clear, it is presumably based on the following: 
the nickel-boron alloy layer containing 0.05 to 1.5 weight % of boron has 
a high ability to bond to the substrate copper or copper alloy of the mold 
and has a coefficient of thermal expansion similar to that of the 
substrate copper or copper alloy, and this alloy layer has a microvickers 
hardness of about 500 to 800 HV, high wear resistance at high temperature, 
high lubricating property at high temperature, remarkably high heat 
conductivity to allow a rapid dissipation of heat which prevents formation 
of a major temperature gradient, and a low affinity for molten steel which 
tends to preclude deposition of splashes. 
In addition to the extended life of the mold, the following advantages are 
achieved by the present invention. 
(a) In the prior art mold disclosed in Japanese Examined Patent Publication 
No. 40341/1980, the alloy layer has a high boron content of 2 to 15 weight 
% and is so hard as to give rise to a strain by stress. Moreover, it has a 
low thermal conductivity and therefore may cause a large temperature 
gradient. Therefore, there was a likelihood that cracks are formed. In 
contrast, the alloy layer according to the invention has a low risk of 
cracking and assures a high reliability of the mold. 
(b) Since the alloy layer according to the invention has a very high 
thermal conductivity, it achieves a very high cooling efficiency. 
In the mold for continuous casting of steel according to the invention, the 
substrate body of the mold is made of copper or copper alloy. This copper 
alloy may be virtually any of the alloys heretofore used in the art. For 
example, alloys of copper with small amounts, particularly about 0.02 to 
0.12 weight %, of at least one element selected from the group consisting 
of silver, iron, tin, zirconium, phosphorus, etc. can be mentioned. 
Particularly preferred copper alloys are deoxidized coppers containing 
small amounts of phosphorus and copper alloys containing 0.1 weight % of 
iron, 0.04 weight % of tin and 0.03 weight % of phosphorus. 
In the present invention, the foregoing specific nickel-boron alloy layer 
is formed on the above-mentioned substrate mold body. The method usable 
for this purpose is not limited but includes the following as an example. 
First, the surface of the mold body is pretreated in the conventional 
manner. This pretreatment may, for example, be conducted by serially 
conducting electrolytic degreasing for 30 minutes at 10A/dm.sup.2 using an 
iron plate as the cathode, rinsing with water, rinsing with 50% 
hydrochloric acid, rinsing with water and rinsing with 3% sulfamic acid. 
After the above pretreatment, the above-mentioned nickel-boron alloy 
plating layer with a specified low boron content is formed. If the boron 
content of the alloy layer is less than 0.05 weight %, the microvickers 
hardness of the layer is reduced and the wear resistance and lubricating 
property at high temperature also tend to be lowered. Conversely if the 
boron content exceeds 1.5 weight %, the coefficient of thermal expansion 
tends to be decreased to cause an inadequate bond to the substrate metal, 
and the resulting decreased thermal conductivity and poor dissipation of 
heat tends to increase internal stress of the alloy layer and consequent 
likelihood of cracking. From the standpoints of high temperature wear 
resistance, lubricating property, thermal conductivity and resistance to 
cracking, the boron content is preferably in the range of about 0.05 to 
0.7 weight % and more preferably in the range of about 0.06 to 0.3 weight 
%. 
The thickness of this alloy layer can be chosen from a broad range 
according to the particular application of the mold, and the like. 
Generally, it is about 50 .mu.m to 2 mm, preferably about 50 .mu.m to 1.5 
mm, and more preferably about 100 .mu.m to 2 mm uniformly throughout the 
whole surface area of the substrate mold body. If the thickness of the 
alloy layer is less than 50 .mu.m, local wear may develop due to 
operational damage to adversely affect the mold life. On the other hand, 
increasing the thickness beyond 2 mm is not rewarded with further improved 
effect but is uneconomical. 
According to the research by the inventor, the thickness of said 
nickel-boron alloy plating layer of the mold of the invention may be about 
50 .mu.m to about 2 mm, preferably about 50 .mu.m to about 1.5 mm, and 
more preferably about 100 .mu.m to about 1 mm in the lower half of the 
inner surface of the mold body. In the area corresponding to the upper 
half of the mold, the thickness of the alloy plating layer may be less 
than 50 .mu.m or even there may be no alloy layer with the substrate 
copper or copper alloy remaining exposed. In the present invention, 
therefore, it is possible to finish the mold body (1) in such a manner 
that its thickness decreases continually from its top end to its bottom 
end and to deposit the alloy layer (2) in a tapered fashion such that its 
thickness increases continually from said top end to said bottom end as 
illustrated in FIG. 1. In this connection, the gradient of the taper can 
be chosen from a broad range but it is generally preferable to assure that 
the thickness of the alloy layer of the invention is about 0 to about 100 
.mu.m at the top end, and about 150 .mu.m to about 2 mm and preferably 
about 200 .mu.m to about 1 mm at the bottom end. More desirably, the alloy 
plating layer has a taper such that the difference between its top end 
thickness and its bottom end thickness is about 500 to 1000 .mu.m. 
Alternatively, as illustrated in FIGS. 2 and 3, the alloy layer (2) may be 
formed in such a fashion that it is then in the upper half and thick in 
the lower half of the mold. Furthermore, as shown in FIGS. 4 and 5, the 
alloy layer (2) may be formed only in the area corresponding to the lower 
half of the mold body. In any case, the alloy layer (2) may be formed in 
such a manner that as in the case illustrated in FIG. 1, its thickness is 
about 50 .mu.m to 2 mm in the area corresponding to the lower half of the 
mold body. 
The formation of the above nickel-boron alloy plating layer may be effected 
by the conventional electroplating technique or the conventional 
non-electrolytic plating technique. When the thickness of the alloy layer 
is to be large, the electroplating process is more advantageous. For the 
formation of said alloy layer by the non-electrolytic plating technique, 
the following plating bath may, for example, be employed. 
______________________________________ 
Nickel sulfate 20-30 g/l 
Sodium potassium tartarate 
30-40 g/l 
Sodium borohydride 2.0-2.5 g/l 
pH 12.0-12.5 
Temperature 45-50.degree. C. 
______________________________________ 
For the formation of the alloy layer by the electroplating technique, the 
following plating bath, for instance, may be employed. 
______________________________________ 
Nickel sulfate 250-300 g/l 
Nickel chloride 20-25 g/l 
Boric acid 30-40 g/l 
Dimethylamineborane 0.01-0.3 g/l 
Stress reducing agent 0-suitable 
amount 
Surfactant 0-1.5 g/l 
pH 3.0-4.0 
Bath temperature 40-45.degree. C. 
Current density 1-3 .ANG./dm.sup.2 
______________________________________ 
In addition to the above plating baths, any other plating bath capable of 
yielding a nickel-boron alloy plating layer with the specified boron 
content can also be employed. 
The above nickel-boron alloy plating layer varying in thickness from the 
top end to the bottom end can be formed, for example, by carrying out the 
plating procedure with the anode inclined and, then, finishing the 
resulting plating layer by machining if necessary. 
As already mentioned, the inventor of the present invention further 
discovered that a long-life mold to which splashes are difficult to adhere 
can also be obtained by forming a plating layer consisting essentially of 
at least one of nickel and cobalt on the copper or copper alloy substrate 
and, then, forming on said plating layer a nickel-boron alloy plating 
layer with a boron content of about 0.05 to about 0.5 weight %, preferably 
about 0.05 to about 0.30 weight %. 
Thus, the present invention provides a copper- or copper alloy-based mold 
for continuous casting of steel which has a plating layer formed on the 
inner surface of said mold and consisting essentially of at least one of 
nickel and cobalt, and a nickel-boron alloy plating layer formed on said 
plating layer and containing about 0.05 to 0.5 weight %, preferably about 
0.05 to 0.30 weight %, of boron. 
In this connection, the plating layer comprising at least one of nickel and 
cobalt (hereinafter referred to as "under layer") has a good ability to 
bond to both of the above-mentioned alloy plating layer and the copper or 
copper alloy substrate, thus serving as a protective film of good bonding 
ability as a whole, and this good bonding ability coupled with the 
excellent high-temperature wear resistance and lubricating property, good 
heat-dissipating property and good splash-repellency of the alloy plating 
layer appears to synergistically assure an extended life of the resulting 
mold. 
The above result is totally unexpected in view of the facts that Japanese 
Examined Patent Publication No. 40341/1980 referred to hereinbefore, for 
instance, mentions that a boron content of less than 2 weight % is 
unsatisfactory in terms of heat resistance and hardness and that Japanese 
Examined Patent Publication No. 50733/1977 and other literature recommend 
the provision of a chromium plating layer or an oxide layer for positive 
prevention of the adherence of splashes. 
Furthermore, the mold of the present invention which has the 
above-mentioned under layer and alloy layer has not only the 
above-mentioned advantage of extended mold life but also the advantage 
that even if the alloy layer is damaged to a certain extent by external 
physical forces, etc., the mold can still be serviceable because of the 
presence of the under layer. 
The above-mentioned under layer consisting essentially of at least one of 
nickel and cobalt can be easily provided by pretreating the surface of the 
mold body in the conventional manner and, then, electroplating the surface 
in the usual manner. As to the alloy layer, it is exactly the same as the 
nickel-boron alloy layer mentioned hereinbefore and formed, after the 
formation of said under layer, by the aforementioned electroplating or 
non-electrolytic plating technique. The thickness of the under layer and 
that of the alloy layer can also be chosen from broad ranges as mentioned 
hereinbefore. Generally, the minimum thickness of the alloy layer is about 
50 .mu.m and the total thickness of the under layer and alloy layer is 
about 100 .mu.m to about 3 mm and preferably about 100 .mu.m to about 2 
mm. 
As in the direct formation of the nickel-boron alloy plating layer alone on 
the substrate mold body, the thickness of the boron alloy plating layer 
may be at least about 50 .mu.m in the lower half of the mold body, with 
the total thickness of the under layer and alloy layer being about 100 
.mu.m to about 3 mm and preferably about 100 .mu.m to about 2 mm. In the 
upper half of the mold body, the total thickness may be less than 100 
.mu.m and there may be neither the under layer nor the alloy layer, with 
the substrate copper or copper alloy being exposed. Therefore, in the 
present invention, the above-mentioned under layer (3) and alloy layer (2) 
may be formed in a tapered fashion as illustrated in FIG. 6. In this 
connection, the gradient of the taper may be chosen from a broad range. 
Generally, however, it is desirable that the total thickness of the under 
layer and alloy layer is about 50 .mu.m to about 300 .mu.m at the top end 
and about 150 .mu.m to about 2 mm and preferably about 200 .mu.m to about 
1.5 mm at the bottom end. As illustrated in FIGS. 7 and 8, the under layer 
(3) and alloy layer (2) may be thin in the area corresponding to the upper 
half of the mold body and thick in the area corresponding to the lower 
half of the mold body. Alternatively, as illustrated in FIGS. 9 and 10, 
the under layer (3) and alloy layer (2) may be formed only in the area 
corresponding to the lower half of the mold body. In any case, in the area 
corresponding to the lower half of the mold body, the thickness of the 
alloy layer (2) should be at least about 50 .mu.m and the total thickness 
of the under layer (3) and alloy layer (2) be about 100 .mu.m to about 3 
mm. 
The mold of the invention having, on its substrate copper or copper alloy 
mold body, either a nickel-boron alloy plating layer alone or an under 
layer consisting essentially of at least one of nickel and cobalt and a 
nickel-boron alloy plating layer can be used in the continuous casting of 
steel into slabs, blooms, billets and other products and invariably 
assures an extended life. 
The following examples are further illustrative of the present invention. 
EXAMPLE 1 
A short side mold body (250 mm wide.times.900 mm high) made of pure copper 
for continuously casting steel whose section perpendicular to its 
horizontal axis is substantially rectangular and having a tapered 
configuration with the thickness at the bottom end thereof being smaller 
than that at the top end by 300 .mu.m was masked over the surface thereof 
except the area to be exposed to molten steel and then subjected to 
30-minute electrolytic degreasing at 10 A/dm.sup.2 using an iron plate as 
the anode. The degreased mold body was rinsed with water, 50% hydrochloric 
acid, water, and 3% sulfamic acid in the order mentioned for pretreatment. 
The mold body was finally rinsed with water and, then, using the following 
plating bath, a tapered nickel-boron alloy plating layer with a boron 
content of 0.3 weight % was formed on the mold body at a current density 
of 1 to 3 A/dm.sup.2, pH 3.0-4.0 and a temperature of 40.degree. to 
45.degree. C. 
______________________________________ 
Nickel sulfate 250 g/l 
Nickel chloride 20 g/l 
Boric acid 30 g/l 
Dimethylamineborane 0.2 g/l 
______________________________________ 
The thickness of the alloy layer was 100 .mu.m at the top end and 400 .mu.m 
at the bottom end (See FIG. 1). Then, the masking was removed. 
On the other hand, a long side mold body (2200 mm wide.times.900 mm high) 
for continuously casting steel whose section perpendicular to its 
horizontal axis is substantially rectangular and having a tapered 
configuration with the thickness at its bottom end being smaller than that 
at the top end by 150 .mu.m was masked over the surface thereof except the 
area to be exposed to molten steel and, then, using the following nickel 
plating bath, a nickel plating layer having a thickness of 300 .mu.m was 
formed as an under layer over the entire surface of the mold body at a 
bath temperature of 50.degree. C., pH 3.0 and a cathode current density of 
2.0 A/dm.sup.2. 
______________________________________ 
Nickel sulfamate 250 g/l 
Nickel bromide (50%) 10 cc/l 
Boric acid 20 g/l 
______________________________________ 
Then, on this under layer, a tapered nickel-boron alloy plating layer with 
a boron content of 0.3 weight % was formed in a tapered fashion using the 
same nickel-boron alloy plating bath as used for the plating of the short 
side mold body above. The thickness of this alloy plating layer was 50 
.mu.m at the top end and 200 .mu.m at the bottom end. The masking was then 
removed. 
By using the mold comprising the thus plated short sides and long sides, 
1300 charges of steel slabs free of any defect were produced without 
breakout. The mold appeared to be further usable but the production was 
discontinued for safety's sake. The condition of the alloy layers on the 
short and long sides of the above molds after use showed slight scratch 
marks but the mold was still useful. 
EXAMPLE 2 
A continuous steel casting mold bodies made of pure copper whose section 
perpendicular to its horizontal axis is substantially rectangular and 
having a tapered configuration with the thickness at the bottom end being 
smaller than that at the top end by 150 .mu.m (short side: 250 mm 
wide.times.700 mm high; long side: 2200 mm wide.times.700 mm high) were 
pretreated in the same manner as described in Example 1. 
After the final aqueous rinse, a 300 .mu.m-thick nickel plating layer was 
formed by electroplating using a nickel sulfamate plating bath of the 
following composition at a temperature of 50.degree. C., pH 3.0 and a 
cathode current density of 2.0 A/dm.sup.2 for 18 hours. 
______________________________________ 
Nickel sulfamate 250 g/l 
Nickel bromide (50%) 10 cc/l 
Boric acid 20 g/l 
______________________________________ 
After aqueous rinse and cooling, the nickel plating surface was finished so 
as to adjust its degree of precision by means of a stretch gauge, filler 
gauge and disk grinder. 
After electrolytic degreasing and activation, a tapered nickel-boron alloy 
plating layer with a boron content of 0.3 weight % was formed using a 
plating bath of the following composition under the conditions of pH 
3.0-4.0, bath temperature 40.degree.-45.degree. C. and current density 1.5 
A/dm.sup.2. The thickness of the alloy plating layer was 50 .mu.m at the 
top end and 200 .mu.m at the bottom end. The masking was then removed. 
______________________________________ 
Nickel sulfate 250 g/l 
Nickel chloride 20 g/l 
Boric acid 30 g/l 
Dimethylamineborane 0.2 g/l 
______________________________________ 
Using the mold thus obtained, 1000 charges of slabs free of any defect were 
produced without breakout. The mold appeared to be still useful but the 
production was discontinued for safety's sake. The condition of the alloy 
layers of the above molds showed slight scratch marks but the mold was 
still useful. 
EXAMPLE 3 
The mold used in this example was a continuous bloom casting mold (inside 
dimension: 612 mm.times.392 mm, 900 mm high) which was made of copper 
alloy containing 0.1 weight % of iron, 0.04 weight % of tin and 0.03 
weight % of phosphorus and which had substantially a rectangular section 
perpendicular to its horizontal axis and had a taper with the thickness at 
the bottom end being smaller than that at the top end by 400 .mu.m. 
The inside cavity of the mold was filled with an electrolytic degreasing 
solution and electrolytic degreasing was carried out in the same manner as 
Example 1. The degreased mold was rinsed with water, 50% hydrochloric 
acid, water and 3% sulfamic acid in the order mentioned for pretreatment. 
Then, from an external service tank, a plating bath of the following 
composition was circulated into the cavity of the mold and electroplating 
was carried out at a current density of 3.0 A/dm.sup.2, bath temperature 
of 40.degree. C. and pH 4.0. 
______________________________________ 
Nickel sulfate 250 g/l 
Nickel chloride 20 g/l 
Boric acid 30 g/l 
Dimethylamineborane 0.1 g/l 
______________________________________ 
By gradually lowering the liquid level of the plating bath, a nickel-boron 
alloy layer with a boron content of 0.06 weight % was formed in a tapered 
fashion with the thickness increasing from the top end to the lower end. 
Then, the surface was finished by machining to provide a tapered 
nickel-boron alloy layer with an evenly increasing thickness from 100 
.mu.m at the top end to 500 .mu.m at the bottom end. 
By using the mold thus obtained at a casting speed of 0.6 to 0.7 m/min, 
1000 charges of blooms free of any defect were produced without breakout. 
While the mold appeared to be still useful, the production was discontinued 
to be on the safe side. The internal surface of the mold after use 
revealed only slight scratch marks and no exfoliation or cracking of the 
nickel-boron alloy plating layer was observed, indicating that the mold 
was still useful. 
EXAMPLE 4 
In a round tubular mold made of deoxidized copper containing a trace amount 
of phosphorus (213 mm.phi. inside diameter.times.900 mm high; wall 
thicknesses: 14.02 mm at the top end and 15.17 mm at the bottom end), a 
plating bath of the following composition was circulated and 
electroplating was carried out at a current density of 2.0 A/dm.sup.2, 
bath temperature of 40.degree. C. and pH 4.0. 
______________________________________ 
Nickel sulfate 250 g/l 
Nickel chloride 20 g/l 
Boric acid 30 g/l 
Dimethylamineborane 0.2 g/l 
______________________________________ 
In this manner, a nickel-boron alloy plating layer (boron content 0.18 
weight %) with a uniform thickness of 75 .mu.m from the top end to the 
bottom end was formed. 
By using the above mold at a casting speed of 1.9 m/min, 300 charges of 
carbon steel billets free of any defect were produced without breakout. 
While the mold appeared to be further usable, the production was 
discontinued to be on the safe side. 
Observation of the internal side of the mold revealed only slight scratch 
marks and no exfoliation or cracking of the nickel-boron alloy layer was 
observed, indicating that the mold was still useful. 
With a mold fabricated as above except that a nickel plating layer was used 
in lieu of the above nickel-boron alloy layer, only 120 charges of carbon 
steel billets could be produced and the mold after production revealed a 
wear of the nickel layer, with local exposure of the substrate copper, and 
could not be further usable.