Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2000-12-07
2003-07-01
Yee, Deborah (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C075S520000, C075S545000, C148S542000, C148S541000
Reexamination Certificate
active
06585799
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a cast steel excellent in workability and quality with few surface flaws and internal defects, having a solidification structure of a uniform grain size, and to a steel material obtained by processing the cast steel.
Further, the present invention relates to a method for processing molten steel capable of improving quality and workability by enhancing the growth of solidification nuclei and fining a solidification structure when producing an ingot or a cast steel from the molten steel after it is subjected to decarbonization refining using a ingot casting method or a continuous casting method.
Yet further, the present invention relates to a method for casting a chromium-containing steel with few surface flaws and internal defects having a fine solidification structure, and to a seamless steel pipe produced using the steel.
BACKGROUND ART
Until now, cast steels have been produced by casting molten steel into slabs, blooms, billets and cast strips, etc. through ingot casting methods using fixed molds and through continuous casting methods using oscillation molds, belt casters and strip casters, etc. and by cutting them into prescribed sizes.
Said cast steels are heated in reheating furnaces, etc., and then processed to produce steel sheets and sections, etc. through rough rolling and finish rolling, etc.
Likewise, cast steels for seamless steel pipes are produced by casting molten steel into blooms and billets using ingot casting methods and continuous casting methods. Said cast steels are heated in reheating furnaces, etc., are then subjected to rough rolling, and are sent to pipe manufacturing processes as steel materials for pipe manufacturing. Further, the steel materials are formed into rectangular or round shapes after being heated again, and then are pierced with plugs to produce seamless pipes.
Solidification structures of cast steels before processing, as well as the conditions of processing such as rolling, etc., have a great influence on the properties and quality of the steel materials.
In general, the structure of a cast steel is, as shown in
FIG. 7
, composed of relatively fine chilled crystals in the surface layer cooled and solidified rapidly by a mold, large columnar crystals formed at the inside of the surface layer, and equiaxed crystals formed at the center portion. In some cases, the columnar crystals may reach the center portion.
When coarse columnar crystals exist in the surface layer of a cast steel as mentioned above, tramp elements of Cu, etc. and their chemical compounds segregate at the grain boundaries of the large columnar crystals, resulting in the brittleness of the segregated portions and the generation of surface flaws in the surface layer of the cast steel, such as cracks and dents caused by uneven cooling, etc. As a result, the yield deteriorates due to the increase of reconditioning work such as grinding and scrapping of the cast steel.
When processing the above-mentioned cast steel by rolling etc., since anisotropy of deformation caused by uneven crystal grain size becomes large, deformation behavior in the transverse direction becomes different from that in the longitudinal direction and the defects such as scabs and cracks, etc., are apt to arise. Further, forming properties such as the r-value (drawing index) deteriorate, and/or surface flaws such as wrinkles (in particular, ridging and roping in stainless steel sheets) appear.
In particular, in a stainless steel material in which the appearance is important, surface flaws such as edge seam defects and roping arise, leading to poor appearance and an increase in the edge trimming amount.
Further, when a seamless steel pipe is produced from the above-mentioned cast steel, surface flaws such as scabs and cracks or internal defects such as internal cracks, voids and center segregation caused by the cast steel remain in the steel pipe. Moreover, during pipe manufacturing, the above-mentioned defects are promoted by forming and piercing and defects such as cracks and scabs are generated on the inner surface of the steel pipe. This leads to the lowering of the yield due to the increase of reconditioning such as grinding or the frequent occurrence of scrapping.
This tendency appears markedly in ferritic stainless seamless pipes containing chromium.
When coarse columnar crystals and large equiaxed crystals exist at the interior of a cast steel, internal defects, such as internal cracks resulted from strain imposed by bulging and straightening, etc., center porosity resulted from the solidification contraction of molten steel and center segregation caused by the flow of unsolidified molten steel at the last stage of solidification, are generated in the cast steel.
Thus the surface flaws generated on a cast steel cause the deterioration of yield caused by an increase in reconditioning work such as grinding and the frequent occurrence of scrapping. If this cast steel is used as it is for processing such as rough rolling and finish rolling, etc., in addition to the surface flaws generated on the cast steel, internal defects such as internal cracks, center porosity and center segregation, etc., remain in the steel material, resulting in the rejection by UST (Ultrasonic Test), the degradation of strength or the deterioration of appearance, and consequent increase of reconditioning work and frequent occurrence of scrapping of the steel material.
Surface flaws and internal defects in a cast steel can be suppressed by improving the solidification structure of the cast steel.
Further, the generation of surface flaws such as surface cracks and dents caused by uneven cooling and uneven solidification contraction arising in a cast steel can be suppressed by making the solidification structure of the cast steel uniform and fine.
Moreover, the generation of internal defects such as internal cracks, center porosity and center segregation, etc., caused by the solidification contraction and the flow of unsolidified molten steel, etc. at the interior of the cast steel can be suppressed by raising the equiaxed crystal ratio at the interior of the cast steel.
Therefore, to suppress the occurrence of surface flaws and internal defects of a cast steel and a steel material produced therefrom and improve the workability and quality such as toughness, etc., of the cast steel, it is important to suppress the coarsening of columnar crystals at the surface layer of the cast steel, to raise the equiaxed crystal ratio at the interior of the cast steel, and to make a uniform and fine solidification structure as a whole.
To cope with these problems, various measures for preventing the occurrence of surface flaws and internal defects in a cast steel and a steel material produced therefrom, such as to devise the form of inclusions in molten steel and to make a solidification structure into fine equiaxed crystal structure by controlling solidification process, have been attempted.
By the way, as measures to raise an equiaxed crystal ratio in the solidification structure of a cast steel, known are (1) a method for casting at a low temperature by lowering the temperature of molten steel, (2) a method for electromagnetically stirring molten steel in solidification process, and (3) a method for generating oxides and inclusions in molten steel by adding themselves or other components in molten steel to act as solidification nuclei at the time of the solidification of molten steel, or a method combining the above methods (1) to (3).
As an embodiment related to low temperature casting by the above method (1), for example, disclosed is a method in Japanese Examined Patent Publication No. 7-84617 for preventing ridging from occurring on a ferritic stainless steel sheet by extracting a cast steel while cooling it in a mold and maintaining the superheat temperature (a temperature obtained by subtracting liquidus temperature of molten steel from actual temperature of molten steel) at not more than 40° C. while continuously casting molten steel, and by maintaining the equiaxed crystal ratio of t
Abe Masayuki
Kinari Yasuhiro
Koyama Yuji
Kusunoki Shintaro
Miura Ryusuke
Kenyon & Kenyon
Nippon Steel Corporation
Yee Deborah
LandOfFree
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