MOLD FOR CONTINUOUSLY CASTING STEEL

Abstract: Mould is provided on the inside surface with an Ni-B alloy coating layer contg. 0.05-1.5 wt. % B (pref. 0.05-0.7 wt. %, esp. 0.06-0.3 wt. %) The thickness of the Ni-B alloy coating layer is 50 microns-2mm (pref. 50 microns, -1.5mm, esp. 100 microns-1mm) over at least half of the inside surface. A coating comprising Ni and/or Co can be formed on the inside surfacee of the copper or copper alloy mould and on this layer can be formed an Ni-B alloy coating contg. 0.05-0.5 wt. % B (pref. 0.05-0.3 wt. %). In this case the thickness of the Ni-B alloy coating layer is at least 50 microns and the total thickness of the two coating layers is 100 microns-3mm. These coating layers can be formed on at least the lower part of the inside wall of the mould. The layers can be coated using an electrolytic or or a nonelectrolytic plating method.

Description: 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of steel is so designed that molten steel poured from its top end is solidified by cooling and the resulting product is withdrawn from its lower end in a continuous sequence.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 has 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. JP-A-55 100 851, upon which the preamble of claim 1 is based, discloses a mold formed from a substrate of copper or copper alloy having a first layer formed on said substrate and composed of at least one of nickel and cobalt, and a second layer formed on the first and composed of an alloy containing 1 to 2 wt.% of phosphorus, or 0.7 to 1.5 wt.% of boron, and at least one of nickel and cobalt forming the balance. JP-A-5 954 444 describes a mold having a plating layer of Ni-W-B wherein the tungsten content is significant being in the range of 3 to 10 wt.% FR-A-2 314 001 (US-A-4 037 646) also describes a mold protected by a bilayer, the first layer of which consists essentially of at least one of Ni and Co, whilst the second layer formed on the first again consists of at least one of Ni and Co, and at least one of P and B. Considering molds of the type wherein the molten metal is charged at the top of the mold and solidified product is withdrawn from its bottom, the inventor carried out intensive research to improve such molds, particularly with a view to developing a structurally simple and easy to manufacture protective layer for a mold substrate body. As a consequence the inventor discovered that contrary to the widely accepted notion that nickel-boron alloy plating in general is poorly bonded to substrates of copper or copper alloy, a nickel-boron alloy plating layer with a low boron content within a certain specific range has a good ability to bond to substrates of copper or copper alloy and serves on its own as an excellent protective layer even without the provision of an intermediate layer as used in the prior art molds. Accordingly this invention provides a copper or copper alloy mold for continuous casting of steel comprising a copper or copper alloy mold having formed on its internal surface a plating layer, characterised in that the plating layer is a nickel-boron alloy plating layer having a boron content of from 0.06 to 0.3 weight % and having a thickness of from 50 µm to 2 mm. Manufacture of such a mold provides for a service life at least equal to or longer than that of a prior art mold having two or three superposed protective layers. Referring, now, to the accompanying drawings, Fig. 1 is a vertical section view showing an example of the mold having a nickel-boron alloy plating layer in a tapered fashion according to the invention; Fig. 2 to 5 are vertical section views showing other examples of the mold having a nickel-boron plating layer according to the invention; 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/dm² 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 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 µm to 2 mm, preferably about 50 µm to 1.5 mm, and more preferably about 100 µm to 1 mm uniformly throughout the whole surface area of the substrate mold body. If the thickness of the alloy layer is less than 50 µm, 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 µm to about 2 mm, preferably about 50 µm to about 1.5 mm, and more preferably about 100 µm 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 µm 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 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 µm at the top end, and about 150 µm to about 2 mm and preferably about 200 µm 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 µm. Alternatively, as illustrated in Figs. 2 and 3, the alloy layer (2) may be formed in such a fashion that it is thin 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 µm 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°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°C Current density 1 - 3A/dm² 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. The mold of the invention having, on its substrate copper or copper alloy mold body, 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 x 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 µm 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/dm² 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/dm², pH 3.0 - 4.0 and a temperature of 40 to 45°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 µm at the top end and 400 µm at the bottom end (See Fig. 1). Then, the masking was removed. On the other hand, a long side mold body (2200 mm wide x 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 µm 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 µm was formed as an under layer over the entire surface of the mold body at a bath temperature of 50°C, pH 3.0 and a cathode current density of 2.0 A/dm². 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 µm at the top end and 200 µm 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 µm (short side: 250 mm wide x 700 mm high; long side: 2200 mm wide x 700 mm high) were pretreated in the same manner as described in Example 1. After the final aqueous rinse, a 300 µm-thick nickel plating layer was formed by electroplating using a nickel sulfamate plating bath of the following composition at a temperature of 50°C, pH 3.0 and a cathode current density of 2.0 A/dm² 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 - 45°C and current density 1.5 A/dm². The thickness of the alloy plating layer was 50 µm at the top end and 200 µm 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 x 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 µm. 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/dm², bath temperature of 40°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 µm at the top end to 500 µm 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⊘ inside diameter x 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/dm², bath temperature of 40°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 µm 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.

Claims: A copper or copper alloy mold for continuous casting of steel comprising a copper or copper alloy mold having formed on its internal surface a plating layer, characterised in that the plating layer is a nickel-boron alloy plating layer having a boron content of from 0.06 to 0.3 weight % and having a thickness of from 50 µm to 2 mm. A copper or copper alloy mold according to claim 1 wherein the nickel-boron alloy plating layer has a thickness of from 50 µm to 2 mm in the area corresponding to the lower half of the mold.

Inventors: SATO, TAKAYUKI
Applicants: SATOSEN CO., LTD.
Application year: 1989
Main classification: B22D11
IPC codes: B22D11