Patent Application
20120321501
An aspect of the present invention relates to a high-carbon martensitic stainless steel and a production method therefor, and more particularly, to a high-carbon martensitic stainless steel and a production method therefor, which decreases the size of primary carbides by producing the high-carbon martensitic stainless steel, which contains, by percentages by weight, 0.4 to 0.8% carbon and 11 to 16% chromium, using a strip-casting process.
In general, a high-carbon martensitic stainless steel containing, as percentages by weight, more than 0.40% carbon has excellent corrosion resistance, hardness and abrasion resistance, and thus is used for razors, knives and the like. When a razor produced using the high-carbon martensitic stainless steel is used, the razor comes in contact with moisture in a shaving process.
Since the razor is kept under a humid atmosphere, its corrosion resistance is required. Such an atmosphere is very severe when a high-carbon steel is used, and therefore, a martensitic stainless steel containing, as percentages by weight, about 13% chromium, is frequently used. In a razor produced using the martensitic stainless steel, martensite that is a base structure of the martensitic stainless steel contains, as percentages by weight, about more than 12% chromium. As a result, a thin chromium oxide is densely formed on the surface of the razor so as to suppress the corrosion of the base structure of the razor from moisture.
Meanwhile, a shaving process is a process in which a beard is cut by allowing a razor to be adhered closely to a material. In order to cut a high-strength beard, high hardness of the razor is required more than anything else. The high hardness required by the razor is implemented by a martensite base structure of steel. The martensite structure is a very light microstructure formed when high-temperature austenite is quickly cooled down. As the content of carbon contained in the high-temperature austenite increases, the amount of carbon contained in the martensite is large, and thus the hardness of the martensite increases. Therefore, carbon should be added to the steel as much as possible in order to produce steel for razor having high hardness.
420 series martensitic stainless steels are frequently used as a material for razor, which satisfies the requirements described above in terms of corrosion resistance and hardness. Each of the steels contains, as percentages by weight, 0.45 to 0.70% carbon, maximum 1% manganese, maximum 1% silicon and 12 to 15% chromium. Among the steels, a steel containing about 0.65% chromium and a steel containing about 13% chromium are frequently used.
Meanwhile, the thickness of a razor is generally 0.2 mm or less. Therefore, a very thin high-carbon martensitic stainless steel having a thickness of 0.2 mm or less is used as an initial material so as to produce the razor. The initial material has a microstructure configured with a base structure and fine chromium carbides uniformly distributed in the base structure. In this case, the distribution of the fine chromium carbides is a main factor that enables carbon to be quickly re-contained in high-temperature austenite in a subsequent hardening process, and controls the martensite deformed by the cooling to have hardness enough to be used as the razor.
The size of the chromium carbides contained in the base material may be defined by the number of chromic carbides per unit area. When the initial material is observed in high magnification of ×10000, 50 or more chromium carbides having a size of 0.1 μm or more should exist per area of 100 μm2. The initial material is slit to an appropriate width, coiled and then goes through several subsequent processes, thereby producing the razor. The subsequent processes include a hardening process of heating and maintaining the razor as a high-temperature austenite region and then cooling the razor so as to provided high hardness of the razor, a sharpening process of sharpening the razor, a coating process for providing corrosion resistance and lubrication, a welding process for mounting the razor to a shaver, and the like.
In an initial material of a thin substance (with a thickness of 0.2 mm or less) used to produce the razor, coarse chromium carbides should not exist in the microstructure of the initial material, and the reason is as follows. In case where coarse chromium carbides exist in the microstructure of the initial material, the coarse chromium carbides are torn out from an edge portion of the razor during the sharpening process, and therefore, the edge portion of the razor becomes blunt. Such a phenomenon is called as edge tear-out, and the edge tear-out is a main factor that causes a wound on person's skin during shaving. In addition to the coarse chromium carbides, a coarse intervenient acts as a factor that causes the edge tear-out. The maximum size of the chromium carbides allowed in terms of the edge tear-out is 10 μm. The coarse chromium carbides having a size of 10 μm or more, which exist in the initial material and acts as a main factor causing the edge tear-out, are coarse primary carbides formed in a casting process of an alloy. The coarse primary carbides are distinguished from fine secondary carbides formed during a hot-rolling process or heat treatment process of the alloy. The coarse primary carbides are formed by segregation formed between dendrite arms during a solidification process of the high-carbon martensitic stainless steel. Since the segregation of carbon and chromium is a natural phenomenon occurring in the solidification of the high-carbon martensitic stainless steel, the primary carbides are unavoidably formed, but the size of the primary carbides should be minimized during the solidification process so as to prevent the edge tear-out.
The edge tear-out is an important quality factor for determining the blade-end quality of the razor for use in cutting implements. As described above, a large amount of carbon should be added to the steel as much as possible so as to produce the high-hardness razor. However, the content of carbon increases, the primary carbides are coarsely formed during the solidification process, and therefore, it is difficult to produce a high-quality razor.
For this reason, Japanese Patent No. 61034161 has disclosed an alloy in which the content of carbon is lowered to 0.40 to 0.55% so as to minimize the edge tear-out caused by primary carbides. Particularly, serious segregation occurs in an ingot-casting method generally used to produce a material for razor, and therefore, primary carbides are coarsely formed. Because of such a disadvantage, additional heat treatment and hot-rolling such as forging are essentially applied to an ingot so as to re-contain the primary carbides or to decrease its size.
Therefore, in order to produce a high-quality razor, it is required to develop a method for suppressing the formation of coarse primary carbides in a casting process. Particularly, it is necessary to develop an economical casting method capable of effective decreasing the size of primary carbides in the microstructure of stainless steel without lowering the content of carbon as compared with ordinary steel for razor.
Accordingly, an object of the present invention is to provide a high-carbon martensitic stainless steel using a strip-casting method for the purpose of replacing an ingot-casting method frequently used in the production of the convention high-carbon martensitic stainless steel.
Another object of the present invention is to provide a method of producing an economical high-carbon martensitic stainless steel while remarkably suppressing coarse primary carbides formed in a solidification process, which is the most serious disadvantage of the conventional ingot-casting method.
According to an aspect of the present invention, there is provided a production method for a high-carbon martensitic stainless steel, wherein, in a strip-casting device comprising a pair of rolls rotating in opposite directions, edge dams respectively provided to both sides of the rolls so as to form a molten steel pool, and a meniscus shield for supplying inert nitrogen gas to the upper surface of the molten steel pool, a stainless-steel thin sheet is cast by supplying a stainless molten steel containing, as percentages by weight, 0.40 to 0.80% carbon and from 11 to 16% chromium to a molten steel pool from a tundish via a nozzle, and the cast stainless-steel thin sheet is made into a hot-rolled annealed strip using in-line rollers to a rolling reduction of 5 to 40% immediately just after the casting so that the size of primary carbides within the microstructure of the hot-rolled annealed strip is 10 μm or less.
The martensitic stainless sheet may contain, as percentages by weight, 0.1 to 1.0% silicon (Si), 0.1 to 1.0% manganese (Mn), over 0 to 0.1% nickel (Ni), over 0 to 0.04 sulfur (S), and over 0 to 0.05 phosphorus (P), and Fe and other unavoidable impurities as remnants.
A hot-rolled annealed sheet may be produced by performing batch annealing on the hot-rolled annealed strip at a temperature of 700 to 950° C. under a reducing gas atmosphere.
The batch annealing may be performed in the range of once to three times.
In the sectional microstructure of the hot-rolled annealed strip, the batch annealing may be performed so that the number chromium carbides having a size of 0.1 μm or more is more than 50 EA/100 μm2.
Pickling treatment may be performed on the hot-rolled annealed strip subjected to the batch annealing after shot blasting.
In the hot-rolled annealed strip before the pickling treatment, the depth of a decarburized layer may be 20 μm or less directly under a surface layer scale.
Cold rolling may be performed on the hot-rolled annealed strip, and the one-time cold rolling rate may be maximum 70%.
Annealing may be performed on the cold-rolled strip five times or less under a reducing atmosphere.
Cold-rolled annealing may be performed on the cold-rolled strip at a temperature of 650 to 800° C.
According to another aspect of the present invention, there is provided a high-carbon martensitic stainless steel produced by means of a production method, wherein, in a strip-casting device comprising a pair of rolls rotating in opposite directions, edge dams respectively provided to both sides of the rolls so as to form a molten steel pool, and a meniscus shield for supplying inert nitrogen gas to the upper surface of the molten steel pool, a stainless-steel thin sheet is cast by supplying a stainless molten steel containing, as percentages by weight, 0.40 to 0.80% carbon and from 11 to 16% chromium to a molten steel pool from a tundish via a nozzle, and the cast stainless-steel thin sheet is made into a hot-rolled annealed strip using in-line rollers to a rolling reduction of 5 to 40% immediately just after the casting so that the size of primary carbides within the microstructure of the hot-rolled annealed strip is 10 μm or less.
As described above, according to the present invention, it is possible to apply a strip-casting method for directly producing a hot-rolled coil from a molten steel produced through a steelmaking process. Since the strip-casting method can remarkably decrease the size of primary carbides formed in a solidified structure, the strip-casting method can be usefully applied to the production of a high-quality razor. Particularly, since the hot-rolled coil is directly produced from the molten steel, the production process of the hot-rolled coil is simpler than the conventional ingot-casting method. Thus, it is possible to improve the quality of a razor and to considerably reduce production cost.
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments but may be implemented into different forms. These embodiments are provided only for illustrative purposes and for full understanding of the scope of the present invention by those skilled in the art. Throughout the drawings, like elements are designated by like reference numerals. In the drawings, the thickness or size of layers are exaggerated for clarity and not necessarily drawn to scale.
In this case, the important technique in a twin roll strip casting process of directly producing a thin sheet with a thickness of 10 mm or less from a molten steel is to produce a thin sheet with a desired thickness, which has no crack and an improved real yield by supplying the molten steel through an injection nozzle between internal air-cooled twin rolls rotating in opposite direction at a high speed.
The present invention relates to a production method of a high-carbon martensitic stainless steel using a strip-casting process. Particularly, since the high-carbon martensitic stainless steel containing, as percentages by weight, 0.40 to 0.80 carbon and 11 to 16% chromium as main components is produced using a strip-casting method, the size of primary carbides formed in the cast structure is decreased to 10 μm or less, so that it is possible to produce the high-carbon martensitic stainless steel for razor having outstanding blade-end quality.
The strip-casting process is a process of minimizing segregation occurring in casting by directly casting a liquid steel into a sheet with a thickness of 1 to 5 mm and applying a vary fast cooling speed to the cast sheet. In the present invention, a hot-rolled coil is produced using the twin roll strip caster. The twin roll strip caster supplies a molten steel between twin-drum rolls rotating in opposite directions and between side dams, and casts the molten steel while discharging a large amount of heat through surfaces of the water-cooled rolls. In this case, a solidified cell was formed on the surfaces of the rolls at a high cooling speed, and a hot-rolled thin sheet with a thickness of about 1 to 5 mm was finally produced through in-line rolling continuously performed at a high temperature after the casting.
Hereinafter, embodiments of the present invention will be described.
A base material used in the present invention is a high-carbon martensitic stainless steel containing, as percentages by weight, 0.4 to 0.8% carbon (C) and 11 to 16% chromium (Cr). In case where the content of the carbon is set to 0.4% or less in the present invention, a large quantity of primary carbides are not formed in a strip or ingot, but the hardness of the martensitic stainless steel is not preferable. In case where the content of the carbon is set to 0.8% or more, it may be difficult to suppress the formation of coarse primary carbides even though the martensitic stainless steel is produced using the strip-casting method. Therefore, in the present invention, the martensitic stainless steel containing, as percentages by weight, 0.4 to 0.8% carbon (C) and 11 to 16% chromium (Cr) is proposed as the optical range of content.
The martensitic stainless steel according to the embodiments of the present invention is an alloy that contains compositions comprising, as percentages by weight, 0.1 to 1.0% silicon (Si), 0.1 to 1.0% manganese (Mn), over 0 to 0.1% nickel (Ni), over 0 to 0.04 sulfur (S), and over 0 to 0.05 phosphorus (P), and Fe and other unavoidable impurities as remnants.
In these embodiments, microstructural Characteristics of a hot-rolled annealed sheet produced using the conventional ingot-casting method and a steel produced using the strip-casting method were compared. Table 1 shows compositions of steels respectively produced by means of the ingot-casting method and the strip-casting method. First, an ordinary razor steel was produced as an ingot so as to compare the microstructure of a material cast by means of the strip-casting method with that of a material cast by means of the ingot-casting method, and compositions of the ingot were shown as Comparative example #1 of Table 1. The ingot was produced to have a weight of 50 kg using a vacuum induction melting method. The ingot was reheated at a temperature of 1200° C. and then hot-rolled as a sheet with a thickness of 3.5 mm, and the sheet was water-cooled just after the hot rolling. Then, steels containing various compositions were produced as hot-rolled sheets using the twin roll strip caster. Each of the steels was cast to have a weight of 100 tons, and compositions of each of the steels were shown in Table 2. The material with the weight of 100 tons, cast between the hot-watered rolls, was continuously produced as a hot-rolled coil with a thickness of 1 to 5 mm by being hot-rolled between in-line rollers in a high-temperature state just after the casting.
In case where the high-carbon martensitic stainless steel is cast using the strip-casting method, the casting process can be simplified, and thus production cost is reduced, as compared with the conventional ingot-casting method. When a high-carbon martensitic hot-rolled coil is produced by means of the ingot-casting method, a subsequent hot-rolling processes such as ingot-division and hot-rolling processes are essentially required. The addition processes are main factors that increase production cost of the ingot-casting method. A heat treatment process comprising the cooling and heating of a material, which is essentially required in the subsequent hot-rolling processes such as ingot-division and hot-rolling processes, should be very slowly performed due to the occurrence of cracks caused by thermal shock. Since an operation for transferring the material between processes should also be carefully performed at a high temperature, it is very disadvantageous in terms of productivity. On the other hand, in the strip-casting method, the hot-rolled coil is directly produced without going through the separate hot-rolling processes comprising the ingot-division process described above. Thus, the high-carbon martensitic stainless steel can be produced at low price.
The inventive steel 6 in #6 of Table 2 as a hot-rolled coil with a thickness of 2.1 mm, produced through the strip-casting process, was batch-annealed in a batch-type heat-treating furnace for a long period of time. In this case, the hot-rolled coil was slowly heated at an annealing temperature of 700 to 950° C. under a reducing atmosphere. The hot-rolled coil was maintained at the annealing temperature for a long period of time and then slowly cooled down in the furnace. The batch annealing may be performed from once to three times. As the number of times of performing the batch annealing increases, the material can be more homogenized, but production cost may be additionally increased. The heat treatment in the strip-casting process functions to convert martensite and remaining austenite constituting the microstructure of the hot-rolled coil into ferrite and chromium carbide. The hardness of the batch-annealed structure was about 220 Hv. The annealed hot-rolled coil was subjected to shot blasting, and scale and decarburized layers on the surface of the hot-rolled coil were removed at a temperature of about 70° C. using a pickling solution configured as a mixture of sulfuric acid and sulfuric acid/nitric acid. In this case, the depth of the decarburized layer is formed to 20 μm or less directly under a surface layer scale, so that the decarburized layer can be easily removed the pickling solution. Generally, a heat treatment process is unavoidably performed on the ingot produced by means of the ingot-casting method at a high temperature so as to reduce segregation of an alloy element generated in casting. Since serious decarburization occurs in the heat treatment process, it may be required to perform an additional operation for removing the decarburized layer after the production of the hot-rolled coil. The decarburized layer may also exist in the coil produced by means of the strip-casting method, but the time at which the coil is exposed at a temperature of 1000° C. or more until the coil is cooled after the casting is merely within 5 minutes, and therefore, the decarburized layer is slightly formed. Thus, since the decarburized layer can be easily removed through a pickling process in the hot-rolled coil produced by means of the strip-casting method, an additional coil grinding process can be omitted to remove the decarburized layer, which is economical.
Meanwhile, a cold-rolling process was performed on the inventive steel in #6 of Table 2 as the hot-rolled coil on which the pickling process has been finished. As described above, since an initial material for producing a razor has a thickness of 0.2 mm, a considerable cold-rolling process is required to decrease the thickness of the initial material to a target thickness from the hot-rolled annealed coil with a thickness of 2.1 mm. Particularly, the hardening of the steel material for razor is fast in the cold rolling, and the softening of the steel material is considerably lowered, due to fine carbides existing in the microstructure. The cold rolling of maximum 70% or less was performed during one-time cold rolling so as to prevent the fracture of the sheet, caused by the occurrence of edge cracks during the cold rolling, and to perform the cold rolling to the target thickness. Then, edge trimming and intermediate annealing was performed. In this case, the intermediate annealing was performed at a temperature of about 750° C. within 5 minutes. The cold rolling and the intermediate annealing were repeatedly performed several times so as to perform the rolling to the final target thickness. In such a manner, a cold-rolled thin coil with a thickness of 0.075 was produced. In this case, the total number of times of performing the annealing so as to obtain the cold-rolled sheet is limited to five times or less, including the number of times of performing the annealing in the hot-rolled annealed strip, which is economical. In the present invention, the economical efficiency in the same quality can be further improved using the number of times of performing the annealing within five times. Further, the cold-rolled annealing can be performed on the cold-rolled strip at a temperature of 650 to 800° C.
As described above, according to the present invention, the high-carbon martensitic stainless steel produced using the strip-casting method remarkably suppresses the formation of coarse primary carbides, as compared with the steel for razor, produced using the ingot-casting method, so that it is possible to economically produced a high-quality razor. Although a specific embodiment produced for the use of razors has been described, the scope of the present invention is not limited to the use of razors, and includes the scope defined by the appended claims.
While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2009-0128110 | Dec 2009 | KR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/KR2010/009108 | 12/20/2010 | WO | 00 | 8/28/2012 |
