Method for reuse of steel sheet scrap

Abstract

A recycling method of scrap steel plate that can recycle scrap steel plates including scrap of high-tensile steel plate to produce good quality of cast iron efficiently, while avoiding unnecessary cost increase. In the recycling method, scrap of high-tensile steel plate and scrap of ordinary steel plate are collected together in a batch collecting process and, then, in a preparing process, the scrap steel plates collected are melted and if not less than 0.7 weight % manganese is contained in the hot metal, then the hot metal is prepared so that a sulfur content in the range of 0.02 to 0.2 weight % in the hot metal and rare earth element or misch metal which is two times as much as the sulfur content is added to the hot metal. To obtain spheroidal graphite cast iron, spheroidizing agent (which preferably comprises no rare-earth element but comprises bismuth) is added further to the hot metal after that.

Description

TECHNICAL FIELD [0001] The present invention relates to a recycling method of a scrap steel plate and, more particularly, to a recycling method of a scrap steel plate including scrap of high-tensile steel plate. BACKGROUND ART [0002] In recent years, a high-tensile steel plate (high-tensile plate) has been increasingly employed for a shell body or a shell plate of an automotive vehicle, for the purpose of improvement in collision safety and weight saving of the vehicle. [0003] On the other hand, scrap steel plates made when the steel plate of the automotive vehicle is pressed are generally used as main raw material of cast iron. In the collecting of the scrap steel plates, as a proportion of the scrap of high-tensile steel plate to an existing ordinary steel plate (mild steel plate) increases, the cast iron's properties reduce disadvantageously. [0004] The high-tensile steel plate contains more manganese, phosphorous and chromium than the ordinary steel plate contains. On the other ...

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Benefits of technology

[0008] It is an object of the present invention to provide a recycling method of scrap steel plate that can use scrap steel plates including scrap of high-tensile steel plate to produce good quality of cast iron efficiently, while avoiding unnecessary cost increase.

[0010] According to this method, since the scrap of high-tensile steel plate and the scrap of ordinary steel plate can be collected together, the specific facilities and tasks for separation and collection of these scrap steel plates are necessary, thus achieving cost saving and improvement in workability.

[0011] On the other hand, when the scrap of high-tensile steel plate is collected together with the scrap of ordinary steel plate in a batch process, the content of manganese, phosphorous and chromium in hot metal increases. If not less than 0.7 weight % manganese is contained in the hot metal of the scrap steel plate obtained, then the hot metal is prepared so that a sulfur content is in the range of 0.02 to 0.2 weight % in the hot metal, and a rare earth metal or a misch metal which is two times as much as the sulfur is added to the hot metal. Then, a complex sulfide comprising manganese, and rare earth metal or misch metal is produced. The complex sulfide serves as an effective base for crystallization of graphite, promoting the crystallization of graphite and thus suppressing the chilling (production of cementite (Fe3C)) at the solidification of the hot metal inexpensively without adding the high-purity pig iron. Therefore, the cast iron excellent in easiness for casting, machinability and mechanical nature and suitably useable for structural parts can be produced easily. Further, since the cast iron thus produced is improved in tensile strength by manganese, phosphorus and chromium dispersed in the iron base, as compared with a conventional cast iron, the improvement in maturity can also be achieved.

[0013] In detail, when the hot metal contains not less than 0.7 weight % manganese, the cast iron obtained therefrom usually reduces in elongation significantly. However, when a complex sulfide comprising manganese, and rare earth metal or misch metal is formed in the preparing process, first, the influence of manganese can be suppressed. Thereafter, when the spheroidizing agent is added to it, the production of complex sulfide comprising the added spheroidizing agent, and rare earth metal or misch metal can be suppressed effectively, as compared with the process of adding the spheroidizing agent before or at the same time as the addition of rare-earth metal or misch metal to produce spheroidal graphite cast iron. As a result of this, the spheroidal graphite cast iron of large in elongation and excellent in mechanical strength can be produced efficiently.

[0015] When spheroidizing agent comprises bismuth, the spheroidal graphite can be granulated finely, and as such can provide further enhanced mechanical strengths such as fatigue strength, tensile strength and elongation. The spheroidizing agent usually comprises rare-earth elements. As compared with the spheroidal graphite cast iron produced by mixing bismuth in the spheroidizing agent comprising these rare-earth elements and then adding the spheroidizing agent comprising rare-earth element and bismuth, the spheroidal graphite cast iron produced by adding spheroidizing agent comprising no rare-earth element but comprising bismuth can be granulated efficiently, thus providing further enhanced mechanical strengths.

Problems solved by technology

On the other hand, the cast iron has an iron base and a graphite structure, so that when the content of manganese, phosphorous and chromium increases in the cast iron, the cast iron has a hard, brittle structure with cementite (Fe3C), which is called chill, being mixed, around a thin-walled area thereof, thus causing the problem that the cast iron reduces in easiness for casting and in machinability and mechanical nature, so that the cast iron cannot be used for structural parts.

However, the former measure of adding the high-purity pig iron to the scrap steel plate to dilute the content of manganese, phosphorous and chromium involves the trouble to buy the high-purity pig iron and add it to the scrap steel plate, thus causing cost increase inevitably.

The latter measure of separating the scrap of high-tensile steel plate from the scrap of ordinary steel plate when collecting scrap steels requires the facilities for separation and collection of the respective scrap steel plates, thus not only causing cost increase but also taking a lot of troublesome separation tasks and thus causing significant reduction in workability.

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