Process of annealing for preventing temper colors on a steel sheet

A process of annealing for preventing temper colors on a steel sheet in a box annealing furnace, which comprises placing an oxidizable iron-base material at a position of low temperature in a circulating path of a furnace gas.

BACKGROUND OF THE INVENTION 
FIELD OF THE INVENTION 
The present invention relates to a bright annealing process in a box 
annealing furnace for preventing temper colors which are produced on the 
outermost layer of a cold rolled steel sheet during its annealing. 
In the bright annealing, the annealing is done without oxidation of the 
cold rolled steel sheet so as to maintain its surface brightness, and for 
this purpose, it is necessary to enclose the cold rolled steel sheet in a 
furnace atmosphere of such as DX, NX and HNX all through the process 
including the heating step and the cooling step. 
However, the furnace atmosphere of such as DX and NX etc. varies in its 
equilibrium composition depending on the gas concentration and produces 
temper colors on a cold steel sheet which is heated and cooled in the 
atmosphere, thus damaging a considerable portion of the cold rolled steel 
sheet surface. Not only the temper colors damage the surface appearance of 
the sheet, but the surface on which the temper colors are produced gives 
only a rougher surface even after acid-pickling as compared with the 
surface which is not affected by the temper color and when it is subjected 
to a phosphate treatment it gives surface irregularlities due to its 
difference in reactivity from that of the surface which is not affected by 
the temper color. 
Particularly, a high strength cold rolled steel sheet (Si-Mn-Cr steel), a 
low-grade silicon steel (ZNC-3), and a Riband steel which have been 
developed with the advent of safety automobiles in recent years have far 
much larger susceptibility to the temper colors as compared with an 
ordinary low carbon steel (SPC steel). 
Thus, a steel sheet containing a large amount of Si and Mn has large 
susceptibility to the temper colors during the annealing because Si and Mn 
themselves are far more easily oxidized than Fe, and temper colors which 
are produced at a relatively high temperature between 650.degree. and 
750.degree. C. are oxides of Mn and Si such as MnSi0.sub.3, which are 
oxidized under the presence of a small amount of water vapor and become 
milky white. Further, in a cooling furnace with the same dew point of 
-60.degree. C., the steel sheet containing a large amount of Mn and Si has 
a higher temperature range in which the temper color takes place as 
compared with the ordinary carbon steel sheet, and the temper colors thus 
produced are brown. 
The present inventors have conducted extensive studies to find technical 
means for minimizing the thickness of the oxide film which causes the 
temper colors on an assumption that it would be possible to reduce the 
oxide film thickness to a thickness of no visual problem, although it 
would be impossible to prevent completely the oxidation of Mn and Si, and 
have found that the visual oxides, namely the temper colors, which are 
produced when the cold rolled steel sheet of an ordinary composition or 
containing Si, Mn and Cr, etc. is annealed under the presence of dew or 
H.sub.2 O can be prevented by placing an oxidizable material in the 
annealing process, particularly in a lower temperature zone in the cooling 
step. 
The feature of the present invention lies in that an oxidizable material is 
placed at a position of a low temperature zone in a circulating path of a 
furnace gas in a box annealing furnace.

As understood from FIG. 1, in case of a high-Mn-Si steel material, which 
has been annealed in a gas mixture with a dew point of -25.degree. C., for 
example, the oxidation is caused at the temperature Ta during the cooling 
step and the temper colors are caused. In order to prevent the temper 
colors, it is necessary to maintain a dew point of lower than -60.degree. 
C. 
However, when the annealing is done with the oxidizable material such as 
lathe turnings provided on the inside wall of the annealing furnace, the 
oxidizable material reaches the temperature Tb before the steel sheet 
reaches the temperature Ta during the cooling step so that the oxidizable 
material can absorb and eliminate H.sub.2 O contained in the furnace 
atmosphere just before the oxidation of the steel sheet, thus preventing 
the temper colors from occurring on the steel sheet. 
The present invention is based on the above discovery and one of the 
objects of the present invention is to provide a process for bright 
annealing in a box type furnace. 
In FIG. 2, 1 represents a cold rolled steel sheet to be annealed in the 
form of a coil or a cut sheet. The cold rolled steel sheet is of an 
ordinary steel composition with or without a small amount of Si, Mn, Cr, 
etc. and has been hot rolled and cold rolled. 2 is a carrier for the cold 
rolled steel sheets, 3 is an inner cover which covers spacedly the cold 
rolled sheets. Inside the outer cover 4, a heating member such as a gas 
burner, a radient tube and an electric heating wire is provided so as to 
heat the cold rolled steel sheet through the inner cover 3. 5 and 6 are 
respectively a supply opening and an exhaust opening, both provided on the 
furnace bottom for the furnace gas, 7 is a fan provided on the furnace 
bottom for circulating the supplied furnace gas. These members constitute 
the box annealing furnace. 
8 is an oxidizable material, which is arranged at a place which is at a 
temperature lower (preferably 50.degree. to 100.degree. C.) than the 
temperature retained by the cold rolled steel sheet after heating near the 
inside of the inner cover 3 or near the furnace bottom. As for the 
oxidizable material 8, a low cost iron-base material which is easily 
oxidized at low temperatures, and has a high melting point, such as lathe 
turnings of cast iron and steel scraps may be used, or steel fins may be 
provided on the inner cover 3. 
Now, when the annealing of the cold rolled steel sheet 1 is done in the box 
annealing furnace while supplying and circulating the furnace gas within 
the inner cover 3, with a part thereof being exhausted from the exhaust 
opening 5 or 6, the cold rolled steel sheet 1 is oxidized together with 
the oxidizable iron-base material 8 during the heating step, but is 
reduced while it is maintained at high temperatures without causing any 
problem, and during the cooling step, the oxidizing reaction of the 
furnace gas due to the temperature change oxidizes only the oxidizable 
material positively, so that the furnace gas can maintain a reducing 
atmosphere to the cold rolled steel sheet, and thus the temper color can 
be prevented. 
According to the present invention, as no temper color is produced on the 
cold rolled steel sheet, not only a beautiful appearance of the cold 
rolled steel sheet can be assured, but also surface treatments such as a 
phosphate treatment can be performed without a problem of irregular 
surface appearance. 
An example of the present invention will be described hereinunder. 
EXAMPLE 
A coil of cold rolled steel strip containing 0.08% C, 0.05% Si, 0.3% Mn and 
0.05% Cr was annealed in a furnace atmosphere (N.sub.2 : 95%, H.sub.2 : 
5%, dew point: -50.degree. C.) at 700.degree. C. within an inner cover 
provided with steel scrap on its inside wall, covered further with an 
outer cover. 
For comparison, a similar annealing was done without the steel scrap. The 
results revealed that no temper color was observed on the sheet annealed 
according to the present invention, but temper colors of 50 to 100 mm 
width were observed on edge portions of the coil annealed without the 
steel scrap. 
As understood from the above, the temper color can be eliminated or reduced 
to a negligible degree of no practical problem when a high-Si or a high-Mn 
steel sheet such as a high strength steel sheet, a low-grade electrical 
steel sheet and Riband steels is annealed by the present invention, and 
thus a high degree of production yield, and a high degree of efficiency in 
subsequent surface treatment can be obtained.