Method for making shadow masks

Al killed cold rolled sheet is manufactured. The sheet is composed of PA0 C: less than 0.01%, Mn: 0.10-1.00%, S: less than 0.025%, Sol.Al: 0.010-0.120%, N: less than 100 ppm, balance being Fe and impurities. The above N and Sol. Al are in relation of Sol.Al/N.times.14/27.gtoreq.1.5 and most of the N is further fixed as AlN. The Al killed cold rolled sheet is re-rolled to a thickness below 0.2 mm and bored by photo-etching. Then the sheet is finally annealed in a decarburizing atmosphere at temperatures in the range of 650-850.degree. C. for 1-30 minutes, preferably 1-10 minutes. Subsequently the sheet is subjected to a leveling process and press forming to form a shadow mask for color TV.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for making shadow masks to be 
incorporated in the cathode ray tube for color TV. 
2. Description of the Prior Art 
The shadow mask is manufactured by the following process. The normally 
produced cold-rolled steel sheet is subjected to re-cold roll to be less 
than 0.2 mm in thickness. Such re-rolled sheet is bored through in a 
photo-etching process and is then annealed at 650.degree.-950.degree. C. 
for a short period of time (hereinafter referred to as "final annealing"). 
The leveller process follower to decrease yield point elongation (Y.P.El) 
which causes stretcher strain, and to correct the shape of the steel 
sheet. Finally, the sheet is pressed into a shape corresponding to a front 
curve of the cathode ray tube. 
In relation to the above mentioned shadow mask making process, the 
inventors have found that Al-killed ultralow C and cold rolled sheet 
including C and solute N in minimal amounts the is superior in 
photo-etching characteristics and press-formability, and is as a whole 
more suitable as the material for the shadow masks in comparison with the 
rimmed steel. The inventors have already proposed in Japanes patent 
application No. 53-133245 the method for making shadow maskes using a 
material of such Al killed ultra-low C cold rolled sheet. 
SUMMARY OF THE INVENTION 
The required characteristics with respect to the press formability are to 
be low in yield point stress (Y.P. Stress) and yield point elongation 
(Y.P.El). It is known experientially that Y.P.Stress is to be below 17 
kg/mm2, and Y.P.El is to be below 3%. 
However even such treated Al-killed steel still often shows high Y.P.El 
after the final annealing so that it is unstable in the press-forming. 
The object of the present invention is to develop and improve the said 
proposed invention in this regard for the purpose of providing further 
superiority in the press-formability characteristics. 
Method of the present invention comprises, 
making Al killed steel, re-cold rolling, photoetching, finally annealing 
and pressing the Al-killed steel. 
The Al killed steel is produced through hot rolling, cold rolling and 
annealing a usual slab or, in addition, temper-rolling it, to contain 
C: less than 0.01%, Mn: 0.10-1.00%, S: less than 0.025%, Sol. Al: 
0.010-0.120%, N: less than 100 ppm, balance being Fe and unavoidable 
impurities, and, wherein 
the above-mentioned N and Sol. Al satisfy the relationship of 
Sol.Al/N.times.14/27.gtoreq.1.5, and also most of the N is fixed as AlN. 
Then the final annealing is carried out in a decarburizing atmosphere at 
650.degree.-850.degree. C. for 1-30 minutes, preferably for 1-10 minutes.

DETAILED DESCRIPTION OF THE INVENTION 
Al-killed cold rolled steel sheet is manufactured by hot-rolling a usually 
produced slab (the finishing temperaures of above 820.degree. C. is 
preferred), coiling (the coiling temperatures in the range of 
480.degree.-800.degree. C. is preferable), and cold rolling (reduction is 
above 40%) followed by pickling and finally annealing the slab at 
540.degree.-850.degree. C. of steel temperatures. 
The chemical composition of the Al killed cold rolled sheet is defined as 
follows. 
C: C is less than 0.01% because high C content take a long time for 
decarbonization in the final annealing. To reduce the C content to less 
than 0.01%, a vacuum degasing treatment may be utilized for the molten 
steel. A decarbonizing annealing also may be employed as the annealing for 
making the cold-rolled sheet. 
Mn: Mn is in the range of 0.10-1.00% in consideration of deoxidation, 
avoiding hot brittleness by S, satisfying Mn/S&gt;15 and further cost saving. 
S: A minimum content of S is preferred, as S above 0.025% causes an 
increase in the Mn S-inclusions and affects the shape of holes made in the 
shadow maskes. 
Al: Al is set so as to extremely decrease oxide-inclusions by strong 
deoxidizing action in the steel making, to improve photo-etching 
characteristics and further to fix solute N as AlN on account of causing 
Y.P.El to be large. 
According to the inventors experiments, and investigations, Al addition is 
needed to satisfy the relation of Sol.Al/N.times.14/27.gtoreq.1.5. 
To satisfy the above relation, Al is added above 0.010% as Sol.Al with 
regard to the N content, but the maximum Al content is limited to 0.120% 
because Al content higher than desired caused higher production cost, and 
makes the flowing of molten steel worse and makes non-metallic inclusions 
difficult to float up. 
N: N content is preferred to be as low as possible, since it increases 
Y.P.El. N is limited to less than 100 ppm, because N does not exceed 100 
ppm except being added in the ordinary process. In the present invention, 
most of the N shall be fixed as AlN. 
Two ways are known to fix N as AlN. One is to adjust the coiling 
temperatures in hot-rolling and the other is to perform the box annealing 
at temperatures of 650.degree.-800.degree. C. When the decarburizing 
treatment is carried out in the box annealing process to make the cold 
rolled sheet of C content less than 0.1%, it is sufficient to fix solute N 
as AlN by the annealing in above mentioned temperature range. In this 
case, the said coiling at high temperatures in the hot rolling process is 
not always needed and the coiling is well enough when carried out at low 
temperatures. 
When the vacuum degassing treatment is employed to decrease C to less than 
0.01%, the coiling is preferred to be carried out at high temperatures. In 
this case, the subsequent annealing is sufficient to only provide 
recrystallization, and therefore it may depend upon the continuous 
annealing or the box annealing. The annealing temperatures therefore have 
to be selected in the range of 540.degree.-800.degree. C. of the steel, 
because if below 540.degree. C. the recrystallization does not take place, 
and if above 800.degree. C. AlN precipitated at coiling can be subject to 
re-solution. 
Since denitronization in the box annealing decreases solute N in steel, the 
amount of Al which is added can be decreased accordingly. 
The ultra low C cold rolled sheet (steel band) controlled to be with the 
above composition is again cold-rolled (above 40% of reduction to a 
thickness below 0.2 mm and bored in the photo-etching in the usual shadow 
mask making process. The final annealing and the press-forming process 
follow it. It is necessary to employ the substantially decarburized Al 
killed ultra low C cold rolled sheet for improving the press formability. 
However, insufficiently or non-uninformly decarburized ultra low C cold 
rolled steel sheet happens to be manufactured in the ordinary process. 
That is because even the steel sufficiently decarbonized by the vacuum 
degassing treatment will be carburized by ferromanganese or the like in 
the subsequent controlling process of adding ferro-alloy. The 
decarburization by the open coil annealing process sometimes causes 
partially insufficient decarburization by dispersion of the decarburizing 
strength, or contact of strips with each other. 
The inventors repeated many experiments and investigations and found out 
that it is substantially effective to finally anneal the sheet in a 
decarburizing atmosphere at 650.degree.-850.degree. C. of annealing 
temperatures for 1-30 minutes, preferably for 1-10 minutes to exactly 
obtain the superiority in the press formability. 
Such annealing in a decarburizing atmosphere is to compensate the 
inefficiency of decarburization which often arise in the usual 
decarburizing treatment, and to avoid the phenomenon of the carburization 
which will appear in the final annealing process. 
The decarburizing atmosphere referred to herein is not defined beyond the 
meaning of the non-oxidizing atmosphere wherein decarburization can occur. 
For example, the atmosphere used in the open-coil annealing furnace 
utilized in the general process for making decarburized cold rolled sheets 
are suitable therefor. A vacuum is also suitable because the thickness of 
the sheets is small. The above range of temperatures are set because 
efficient decarburizing is not performed below 650.degree. C. for the 
above defined time, and above 850.degree. C. solute N can increase. The 
above annealing time is defined because no effect of annealing appears in 
a shorter time then 1 minute, and resolution of AlN increases greatly to 
make the press formability worse in the annealing at temperatures above 
750.degree. C. for time more than 30 minutes. Such re-solution of AlN 
starts to increase in the annealing after 10 minutes, and annealing for 
1-10 minutes is preferred. 
Reference will be made to example of the present invention hereunder. 
EXAMPLE 
Test pieces were Al-killed ultra low C cold rolled steel manufactured with 
sufficiently treating the continuously cast slab, of finish rolling at 
850.degree. C., coiling at 520.degree. C., cold rolling of 77% after 
pickling, and annealing at 750.degree. C. 
The composition of the test pieces are shown in the below table. 
__________________________________________________________________________ 
COMPOSITION OF TEST PIECES 
Wt % 
Test Pieces 
C Si 
Mn 
P S Sol.Al 
N O 
##STR1## 
__________________________________________________________________________ 
A 0.002 
0.03 
0.27 
0.012 
0.010 
0.045 
0.0057 
0.0079 
4.09 
B 0.003 
0.03 
0.28 
0.013 
0.014 
0.037 
0.0045 
0.0045 
4.26 
C 0.005 
0.02 
0.30 
0.011 
0.009 
0.047 
0.0043 
0.0027 
5.67 
D 0.009 
0.02 
0.28 
0.016 
0.016 
0.044 
0.0038 
0.0036 
6.00 
__________________________________________________________________________ 
Such test pieces were further cold rolled 75% to 0.18 mm in thickness. 
After that, some of test pieces were finally annealed at 600.degree. to 
900.degree. C. for 10 minutes in a decarburizing atmosphere, the remainder 
of the test pieces were finally annealed for 10 minutes in a 
non-decarbonizing atmosphere. The test pieces were subject to the tension 
test at the room temperature, the aging test and the measurement of grain 
size, the results of which are shown in FIG. 1-FIG. 3. Marks in FIGS. 1-3 
respectively indicate as follows. 
______________________________________ 
Test 
Atmosphere in final annealing 
pieces Mark 
______________________________________ 
.circle.A .circle. 
Non-oxidizing and decar- 
.circle.B .quadrature. 
burizing atmosphere 
.circle.C .DELTA. 
(The present invention) 
.circle.D .gradient. 
Non-oxidizing and .circle.A 
Non-decarburizing .circle.B 
atmosphere 
(The conventional method) 
.circle.C 
.circle.D 
______________________________________ 
Marks A , B and C represent Al-killed ultra low C cold rolled sheet 
manufactured by the open coil annealing, the C content of which are 
0.002%, 0.003% and 0.005% respectively. 
Mark D represents Al killed ultra low C cold rolled sheet decarbonized by 
the vacuum degassing treatment, the C content of which is 0.009%. 
It is seen from FIGS. 1 and 2 that the test pieces annealed in 
non-decarburizing atmosphere show wide variation of Y.P.Stress and Y.P.El. 
It is apparent from FIG. 3 that the inventive test pieces show superior 
characteristics of lower Y.P.El, Y.P.Stress and more stability of grain 
size than such conventional test pieces despite that they are respectively 
made of variable C cold rolled sheets. This is due to the final annealing 
in a decarburizing atmosphere at the range of 650.degree.-850.degree. C. 
for the short period of time. 
The test piece B annealed in non-decarburizing atmosphere show high 
Y.P.El and Y.P.Stress despite the C content of the material is low level 
of 0.003%. This is assumed due to the carburization by the extremely small 
amount of carbon and chemical compound of carbon remained in the surface 
of the sheet. 
The aging index of steel generally depends on the amount of solute N and C, 
i.e. the index is high for a large amount of solute C and N, and low for a 
small amount of solute C and N. The aging index of the steel of the 
present invention depends on the amount of solute C since solute N is 
almost entirely fixed as AlN. 
Thus the low aging index means that the steel is sufficiently decarburized 
and the C content is stable at low level, consequently the press 
formability is excellent. FIG. 3 shows that the aging index in the present 
invention is stable below 1.0 The method of the present invention can 
exactly provide excellent press formability and may make it possible to 
omit the leveller process to be carried out before the press forming, 
since on the steel as a low Y.P.El value.