1. Field of the Invention
The present invention relates to a process for hot rolling stainless steel that does not damage the surface of the rolled product and thus improves product yield when a steel slab is heated in a heating furnace and is then hot-rolled.
2. Description of the Related Art
Oxide scale formed during a rolling process functions as a lubricant between a working roll and a workpiece to be rolled. However, during hot rolling of stainless steel, oxide scale generally forms to a lesser extent on the steel slab surface, such that ductility of the oxide scale is inferior to that of plain steel. Thus, seizing more readily occurs between the working roll and the workpiece during hot rolling of stainless steel. The seizing increases the roughness of the working roll surface due to heat scratches, and that roughness is transferred to the surface of the workpiece. As a result, the hot-rolled product has surface defects (sometimes called "surface deterioration").
In stainless steel containing at least one element selected from the group consisting of Al, Mo, Ti, and Nb in a total amount of at least about 0.2 weight percent, and in stainless steel containing Cr in an amount of about 16 weight percent, the thickness of the oxide scale before hot rolling is only several microns and is thus especially small. Since the oxide scale has poor ductility due to a high Cr content, seizing between the working roll and the workpiece occurs often.
Since stainless steel sheets used in exterior materials must have a beautiful surface finish, the above-described surface deterioration is corrected by surface grinding or the like of the steel sheets. Such additional treatment, however, incurs high production cost and causes a significantly decreased yield.
Japanese Patent Application Laid-Open No. 2-132190 discloses a method for preventing seizing between a working roll and a workpiece to be rolled using a hot rolling lubricant in rolling of such types of steel.
The present inventors have discovered a method for suppressing seizing between a working roll and a workpiece to be rolled, and thus preventing surface deterioration of the steel sheet. In this method, oxidation is moderately enhanced to form a relatively thick, low-chromium surface layer (a surface layer having a decreased chromium content due to enhanced oxidation of chromium) of the workpiece during heating in a heating furnace before hot rolling.
A method for promoting oxidation of the steel slab which relates to the present method is disclosed in Japanese Patent Application Laid-Open No. 58-138501, in which the defects on the surface of a steel slab are removed as scales by enhanced oxidation so that a steel sheet having superior surface quality is obtained without the need for refinishing by grinding. In this method, a melt of CaCl.sub.2, NaCl, or V.sub.2 O.sub.5 is adhered to the slab surface of heated plain steel, or to parts of the slab surface to be refinished, in order to remove surface defects by enhanced oxidation. Furthermore, Japanese Patent Application Laid-Open No. 8-49018 discloses a method for removing surface defects on a steel slab by enhanced oxidation, in which oxides and/or inorganic and organic salts of alkaline metals and alkaline earth metals are applied to the slab surface at a rate of 100 g/m.sup.2 or more before a high-alloy steel containing 18 weight percent or more of chromium is placed in a heating furnace prior to hot rolling, and the steel is then heated at a temperature of at least 1,200.degree. C. for at least 30 minutes in an oxidizing atmosphere so as to remove surface defects on the steel slab by enhanced oxidation.
The above-mentioned methods, however, have the following problems.
(1) Problems in Use of a Hot-rolling Lubricant
Failure in biting may occur during the actual hot rolling operation when there is a large biting angle for the workpiece to be rolled by a working roll, as occurs in a rough mill and in a preceding stage of a finishing mill; hence, use of the hot-rolling lubricant is generally suspended during biting. As a result, seizing occurs between the working roll and the workpiece at portions where the hot-rolling lubricant is not used. Accordingly, the surface roughness of the resulting steel sheet increases due to roughening of the rolled surface.
(2) Problems in Conventional Processes for Promoting Oxidation of Steel Slabs
In the conventional processes disclosed in Japanese Patent Application Laid-Open Nos. 58-138501 and 8-49018, a surface treatment composition is used for promoting oxidation on a steel slab, but no attention whatsoever is given to maintaining adhesion of the surface treatment composition to the steel slab until oxidation of the steel slab by the surface treatment composition is completed in a heating furnace. Thus, the surface treatment composition is scraped off by a transfer roll or a steel slab support or becomes detached therefrom by vibration during transfer when the slab is moved after coating and is placed into the heating furnace. Accordingly, sufficient oxidation effects are not achieved at the corresponding portions.
When these conventional processes are used for preventing surface deterioration in a hot rolling process, seizing occurs between a working roll and insufficiently oxidized portions of a workpiece. Since the working roll surface is rapidly damaged, surface deterioration of the steel sheet cannot be prevented.
When the surface treatment composition disclosed in Japanese Patent Application Laid-Open No. 58-138501 is used for preventing surface deterioration of stainless steel containing 10 weight percent or more of chromium, unlike in plain steel, the chromium content in the low-chromium layer does not substantially decrease, due to insufficient oxidation, regardless of the adhesion of a typical melt such as NaCl or V.sub.2 O.sub.5. When CaCl.sub.2 is applied, the thickness of the low-chromium layer on the workpiece surface is small in spite of the progress of oxidation, and thus formation of scale during hot rolling is insufficient to effectively prevent surface deterioration (details will be described below).
When the surface treatment composition disclosed in Japanese Patent Application Laid-Open No. 8-49018 is used, surface treatment compositions other than Ca-based surface treatment compositions and Ba-based surface treatment compositions do not cause sufficient oxidation. Furthermore, application of the Ca-based surface treatment compositions and Ba-based surface treatment compositions also does not sufficiently prevent surface deterioration due to an insufficient thickness of the low-chromium layer.
Furthermore, in these conventional processes, oxidation continues rapidly in the heating furnace so that the thickness of the formed scale reaches 1 mm or more; hence, a decreased product yield causes increased production cost. Since these conventional surface treatment compositions cause vigorous oxidation of a steel slab support that bears the steel slab in the heating furnace during heating, rapid damage to the steel slab support results in a decreased rate of operation in the hot-rolling facility.