Metal treatment – Stock – Ferrous
Patent
1997-04-28
1998-12-29
Yee, Deborah
Metal treatment
Stock
Ferrous
148330, 148602, 148661, C21C 800, C22C 3804, C22C 3806
Patent
active
058535039
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to hot rolled steel sheets, particularly steel sheets as-rolled alone or further cold rolled and a method of producing the same, and more particularly to a hot rolled steel sheet having such a thin scale that the peeling of scale is less in the working as a mill scale (as-rolled), while the pickling efficiency is good in applications after the pickling and a surface roughness Ra is not more than 0.8 .mu.m and an average scale thickness is not more than 4 .mu.m, and a method of producing the same.
BACKGROUND ART
In general, the hot rolled steel sheets are produced by hot rolling a slab of steel obtained through a continuous casting method or a blooming method. In a surface layer of the thus obtained hot rolled steel sheet is created so-called secondary scale produced during the hot rolling and comprised of three layers of FeO--Fe.sub.3 O.sub.4 --Fe.sub.2 O.sub.3 having a thickness of about 5 .mu.m.about.15 .mu.m.
When the secondary scale created on the surface of the hot rolled steel sheet is subjected to a shaping work at a mill scale state (at a state of holding the mill scale on the surface of the hot rolled steel sheet), a part of the scale is peeled off to contaminate the working line, or the peeled scale induces a surface defect of a product after the work as an indentation flaw. For this end, a slightly light degree of the working has hitherto been conducted to the hot rolled steel sheet having the mill scale.
Under the above circumstance, when such the hot rolled steel sheet is subjected to a work at a large strain amount or is used as a starting material for cold rolled steel sheet, it is necessary to attempt the removal of the scale through a pickling step. Even in this case, when a coiling temperature after the hot rolling is rendered into a high temperature above 550.degree. C. from the reason of the material properties in the conventional technique, there are problems that the scale existing on the edge of the steel sheet thickly grows, and the transformation from FeO to Fe.sub.3 O.sub.4 +Fe is caused to densify the scale and hence the pickling efficiency is lowered to considerably increase the load to the work line.
In order to mitigate the aforementioned problems exerting on the scale, therefore, there have been attempted some efforts for thinning the scale.
For example, JP-B-6-104853 discloses a method wherein steel containing Si: 0.02-0.2% and Cr: 0.02-0.2% is soaked to 1150.degree. C. and the rolling at a rolling reduction of not less than 90% is started at not higher than 1000.degree. C. and terminated at not higher than 860.degree. C. and then the coiling is carried out at not higher than 500.degree. C.
As a method of removing scale in the course of the hot rolling, for example, JP-A-4-238620 discloses a method wherein when hot rolled steel sheets are manufactured by subjecting a kind of steels creating hardly peelable scale to hot rolling, descaling is carried out by jetting a high-pressure spraying water onto the surface of the steel sheet at a jetting pressure per unit area of 20-40 g/mm.sup.2 and a flowing quantity of 0.1-0.2 liter/min.multidot.mm.sup.2 prior to a finish rolling.
However, there is a problem that the above method of JP-B-6-104853 is not applicable to a kind of steel requiring a coiling temperature of higher than 500.degree. C. from a viewpoint of the material because it restricts the coiling temperature after the hot rolling to not higher than 500.degree. C.
In the method of JP-A-4-238620, a greater part of scale is removed, but there is a problem that in case of a kind of steel containing a great amount of Si, scale of a structure entering into matrix is created and can not be removed and hence scale flaw called as red scale is caused after the rolling. And also, this method has a problem that it is not necessarily enough to provide the thin scale.
Moreover, only the steel sheets having a surface roughness Ra of about 1-3 .mu.m are obtained by these conventional techniques, so that when they are subjected to forming w
Furukimi Osamu
Obara Takashi
Sakata Kei
Seto Kazuhiro
Kawasaki Steel Corporation
Yee Deborah
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