Process for producing deep-drawing cold rolled steel strip by continuous annealing

Process for producing a deep-drawing cold rolled steel strip by continuous annealing which comprises hot rolling a steel containing not more than 0.06% C, not more than 0.40% Mn, 0.0005 to 0.0020% B, the ratio of B/N being in the range from 0.5 to 1.5 and the balance being iron and unavoidable impurities, coiling the hot rolled steel strip at a temperature not higher than 680.degree. C., cold rolling and continuously annealing the hot rolled strip. The process produces super deep-drawing cold rolled steel strips without overageing treatment.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for producing deep-drawing cold 
rolled steel strips by continuous annealing. 
Cold rolled steel strips are very often used in the manufacture of 
cold-formed articles, such as press-formed automobile parts, and the 
strips are thus required to be soft and to have an excellent press-forming 
property. 
Conventionally, aluminum-killed steels have been usually treated by a box 
annealing process for production of the deep-drawing cold rolled steel 
strips. However, the batch annealing process has the critical disadvantage 
that the process takes a long period of time to perform, and hence 
considerably lowers the production efficiency. 
Therefore, much attention has been paid to new arts such as continuous 
annealing process, aiming at the production of deep-drawing cold rolled 
steel strips, and in recent years some continuous annealing processes have 
been in actual practice for production of cold rolled steel strips. 
In the conventional continuous annealing arts, the steel is subjected to 
rapid heating, a short time of soaking and then rapid cooling. Therefore, 
when an Al-killed steel or an ordinary low carbon steel is treated by a 
conventional continuous annealing, the resultant steel has a small grain 
size and is hard, showing an inferior r value which is a parameter of the 
deep-drawability of steels as compared to that obtained by a box annealing 
process, hence failing to provide a deep-drawing cold rolled steel strip 
which can be satisfactorily press formed. 
For eliminating the above disadvantages, it has been proposed that the 
steel strip after hot rolling is coiled at a high temperature not lower 
than 700.degree. C. and subjected to cold rolling and then a continuous 
annealing process. However, the high-temperature coiling causes 
difficulties in acid pickling, and surface defects, such as ridging, which 
appear when the resultant cold rolled steel strip is worked, as well as 
deterioration of ductility due to formation of massive carbides. 
Then for eliminating the disadvantages of the high temperature coiling, an 
art has been proposed for producing a deep-drawing cold rolled steel strip 
by a continuous annealing process without adopting a high-temperature 
coiling step, as disclosed in Japanese Patent Publication No. Sho 
51-29696, according to which a soft cold rolled steel strip can be 
produced from an Al-killed steel containing boron by continuous annealing 
even with a low temperature coiling at about 650.degree. C. 
However, the B-containing Al-killed steel, as disclosed in the above 
mentioned Japanese Patent Publication, usually contains nitrogen in an 
amount as about 0.005 to 0.0065% and therefore, it is essential that the 
steel contains at least 0.0020% B. According to the disclosure of this 
prior art, the addition of boron has a subsidiary harmful effect to 
degrade the r value. This has been the critical problem of the 
conventional B-containing Al-killed steel. 
Deep-drawing steel strips for press forming are required to have material 
qualities, in addition to a high r value, such that an excellent shape can 
be obtained by the press forming and that they have excellent 
stretchability. For these qualities they are required to have desirably a 
low yield point and a large elongation. 
However, when the B-containing Al-killed steels are subjected to continuous 
annealing, they are often found to be hard and have a high yield point and 
a low elongation, although the steel can be softened to some degrees by 
the continuous annealing. Therefore, it has been found to be difficult to 
produce a satisfactory deep-drawing steel strip with consistency from the 
B-containing Al-killed steel by continuous annealing. 
SUMMARY OF THE INVENTION 
Therefore, the present inventors have made extensive studies and 
experiments for consistently producing cold rolled steel strips having 
excellent press formability by continuous annealing of B-containing 
Al-killed steels, and found that the reason why the r value of 
continuously annealed B-containing Al-killed steels is inferior is that in 
the prior art boron is added irrespective of the nitrogen content so that 
the softening effect of boron has not been fully developed, and that boron 
carbides which precipitate by the reaction between boron and carbon are 
harmful to the r value. This tendency becomes more apparent in steels 
containing nitrogen in an amount not less than 0.0050% or more and such 
steels show considerable variation in their yield point ranging from high 
to low values, and also in their elongation. 
The present invention has been made in view of the above discoveries and 
the present invention is to provide a process for consistently producing a 
deep-drawing cold rolled steel strip by continuous annealing which 
comprises hot rolling B-containing Al-killed steel stock containing not 
more than 0.0040% N, preferably not more than 0.002% N, 0.0005 to 0.0020% 
B with the ratio of B/N being from 0.5 to 1.5, coiling the hot rolled 
steel strip at a temperature not higher than 680.degree. C. so as to 
precipitate BN by reaction between the boron and the nitrogen thus 
prohibiting precipitation of boron carbides. The present invention has a 
technical advantage that the resultant steel provides satisfactory growth 
of grains inspite of the rapid heating and the short time of soaking 
inherent to continuous annealing.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is applicable to a steel composition containing not 
more than 0.06% C, not more than 0.40% Mn, 0.005 to 0.05% acid soluble Al, 
not more than 0.0040%, preferably not more than 0.002% N, 0.0005 to 
0.0020% B, with the ratio of B/N being from 0.5 to 1.5, the balance being 
iron and unavoidable impurities. 
Carbon hardens the steel and the boron carbide precipitating from the 
reaction between boron and carbon lowers the r value. Therefore, the upper 
limit of the carbon content is set at 0.06%. When the carbon content is 
lowered to an amount not more than 0.01% by a vacuum degassing treatment 
etc. further improved deep-drawability can be obtained. 
Manganese is essential for preventing brittleness fracture induced by 
sulfur during the hot rolling, but an excessive addition of manganese 
tends to lower the deep-drawability. Therefore, the upper limit of the 
manganese content is set at 0.40%. 
Aluminum is required only for deoxidation of the steel applicable to the 
present invention, so at least 0.005% acid soluble-Al is necessary for 
performing a stable deoxidation treatment and for reducing the surface 
defects of the resultant steel strip. On the other hand, when an excessive 
amount of aluminum is added, the harmful boron carbide precipitation is 
caused becuase the precipitation of AlN predominates the precipitation of 
BN, so that the object of the present invention cannot be achieved. 
Therefore, the upper limit of the acid soluble-Al is set to 0.05%. 
Nitrogen hardens the steel, lowers the deep-drawability, increases the 
yield point and lowers the elongation with considerable variation of the 
yield point and the elongation as above mentioned. Therefore, the nitrogen 
content should be maintained at 0.0040% or less, preferably at 0.002% or 
less, and should be further limited with respect to the ratio of B/N as 
described hereinafter. 
Conventional steels normally contain about 0.005% N, and in order to lower 
the nitrogen content to the range defined in the present invention, it is 
necessary during the steel refining step in a converter to employ both the 
top blowing and the bottom blowing so as to lower the blown-off nitrogen 
content, or to prevent the pick-up of nitrogen from the air during the 
pouring of the heat. 
Boron is one of the most important elements for the steel applicable to the 
present invention. In order to precipitate BN with reaction between boron 
and nitrogen in in the steel, and to prevent the boron carbide formation 
at the stage when the steel strip is coiled as a hot coil after the hot 
rolling, 0.0005 to 0.0020% B within the B/N range of from 0.5 to 1.5 
should be added. 
The boron range from 0.0005 to 0.0020% has been defined in view of the fact 
that even when the nitrogen content is lowerd as much as possible during 
the steel preparation, the resultant steel contains about 0.0008% 
nitrogen, and in order to react with this lowest limit of the nitrogen 
content and to soften the steel, at least 0.0005% boron is required. On 
the other hand, as the boron content increases the deep-drawability 
lowers, and thus the boron content should be limited to the upper limit of 
0.0020%. 
The lower limit 0.5 of the ratio of B/N has been defined from the fact that 
at a ratio less than 0.5 a fine AlN precipitation is caused, resulting in 
excessive refinement of grains after the cold rolling and annealing, hence 
an increased yield point, a lowered elongation and a lowered 
deep-drawability. On the other hand, when the ratio exceeds 1.5, the 
excessive boron reacts with the carbon to precipitate boron carbide which 
lowers the deep-drawability, and at the same time the grain size after the 
cold rolling and annealing becomes excessively fine, thus causing the 
hardening of the steel. The most desirable B/N ratio is from 0.8 to 1.0. 
The effects of the B/N ratio on the r value, the yield point and the 
elongation of the steel after the continuous annealing are shown in FIG. 
1. The steels used for the test contained 0.02 to 0.04% C, 0.0015 to 
0.025% Si, 0.10 to 0.15% Mn, 0.010 to 0.020% acid soluble Al and contained 
different nitrogen contents in a range of from 0.0013 to 0.0020% which is 
within the scope of the present invention and in another range of from 
0.0045 to 0.005% which is outside the scope of the present invention, and 
the steels were hot rolled at a temperature not lower than the Ar.sub.3 
point, coiled at a temperature between 600.degree. and 650.degree. C., 
cold rolled, then subjected to continuous annealing at 850.degree. C. for 
one minute, and further subjected to an overageing treatment at 
400.degree. C. for three minutes. 
As clearly understood from the results shown in FIG. 1, the steels 
containing a lowered nitrogen content and containing boron within the B/N 
range of from 0.5 to 1.5 according to the present invention show a high r 
value, a large elongation value, and a low yield point value. It should be 
further noted that the tendencies of these properties are flat. This 
indicates that these properties can be obtained with a high degree of 
consistency. On the other hand, the steels containing an excessive 
nitrogen content which is outside the scope of the present invention show 
a sharp variation in the above properties and are inferior with respect to 
these properties in spite of their B/N ratio being within the range of 
from 0.5 to 1.5. 
As illustrated above, deep-drawing strips having excellent 
deep-drawability, sharp fixability and stretchability can be produced with 
a high degree of consistency according to the present invention. 
Phosphorus, sulfur, silicon and so on which unavoidably come into the steel 
as impurity should be preferably lowered as much as possible. 
According to the present invention molten steel having the above defined 
composition is made into steel slabs which are then subjected to finishing 
hot rolling and coiled at a temperature not higher than 680.degree. C. 
The steel slab may be prepared either by a continuous casting process or an 
ingot-making process, and also the steel slab may be hot rolled as a hot 
slab or a cold slab. Regarding the heating temperature for the hot 
rolling, a lower temperature is more desirable for the purpose of 
promoting the BN precipitation. 
The finishing hot rolling temperature is preferably not lower than the 
Ar.sub.3 point for the purpose of obtaining the desired deep-drawability. 
If the coiling temperature is excessively high, a large amount of boron 
carbide is formed in the hot rolled steel strip, thus causing 
deterioration of the deep-drawability. Therefore, the coiling temperature 
should be not higher than 680.degree. C. 
The hot rolled coil thus obtained is subjected to acid pickling and cold 
rolling with a cold reduction ranging from 60 to 90% ordinarily, then 
subjected to continuous annealing including overageing treatment, and 
further, if necessary, subjected to temper rolling. It should be noted 
that the conditions of these steps are not specifically limited. It is 
desirable, however, that the annealing is done not lower than the 
recrystallization temperature, but not higher than the Ar.sub.3 point, and 
for obtaining excellent deep-drawability in particular, an annealing 
temperature not lower than 800.degree. C. is preferable. 
DESCRIPTION OF PREFERRED EMBODIMENT 
The present invention will be better understood from the following 
embodiments. 
EXAMPLE 1 
The steel having a chemical composition shown in Table 1 was prepared in a 
converter, continuously cast into slabs, which were hot rolled into hot 
coils of 4.0 mm in thickness under the conditions shown in Table 1, 
acid-pickled, cold rolled into 0.8 mm thickness, subjected to 
recrystallization annealing at 750.degree. C. for one minute, subjected to 
overageing treatment at 400.degree. C. for three minutes, and temper 
rolled with 1.0% reduction. The mechanical properties of the steel strips 
thus obtained are shown in Table 2. 
As clearly understood from the results shown in Table 2, the steels A, B, C 
and D within the scope of the present invention show a higher r value and 
softer than the steels E, F and G and thus provide a better deep-drawing 
steel sheet. 
EXAMPLE 2 
An extremely low-carbon steel having a chemical composition shown in Table 
3 was continuously cast into slabs, which were hot rolled at various 
temperatures shown in Table 3 into 4.0 mm thickness, acid-pickled, cold 
rolled into 0.8 mm thickness, soaked at 850.degree. C. for one minute, 
cooled in the air, and temper rolled with 0.8% reduction. The mechanical 
properties of the steel sheets thus obtained are shown in Table 4. 
As clearly shown by the above results, the cold rolled steel strips 
obtained according to the present invention show a very high r value not 
lower than 1.9 and much better deep drawability and stretchability as 
compared with the comparative steels. Also it should be noted that the 
steel strips obtained by the present invention show very excellent 
elongation as high as 50% or more without an overageing treatment. 
As clearly shown by the foregoing examples, the B-containing super low 
carbon Al-killed steel produced according to the method of the present 
invention provides very excellent deep-drawability and stretchability with 
the low-temperature coiling but without the overageing treatment. Thus the 
present invention has great industrial advantage that super deep-drawing 
cold rolled steel strip can be produced by low temperature coiling and 
without the overageing treatment. 
The present invention has a further advantage that the cold rolled steel 
sheet produced by the method of the present invention may be 
surface-coated with zinc, tin, chromium, aluminum etc., and thus surface 
treated steel sheets having excellent deep-drawability can be produced by 
continuous annealing. 
TABLE 1 
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Hot Rolling 
Temperature (.degree.C.) 
Desig- 
Chemical Composition (wt. %) Heating 
Finishing 
Coiling 
nation 
C Si Mn P S sol. Al 
N B B/N 
Temp. 
Temp. 
Temp. 
__________________________________________________________________________ 
Present 
A 0.041 
0.022 
0.21 
0.013 
0.010 
0.020 
0.0021 
0.0019 
0.90 
1250 885 630 
Invention 
B 0.045 
0.023 
0.23 
0.010 
0.009 
0.035 
0.0014 
0.0010 
0.71 
1250 890 650 
C 0.043 
0.021 
0.20 
0.012 
0.012 
0.027 
0.0030 
0.0020 
0.67 
1250 895 620 
D 0.022 
0.025 
0.22 
0.013 
0.011 
0.031 
0.0010 
0.0008 
0.80 
1250 895 635 
Comparative 
E 0.048 
0.024 
0.25 
0.014 
0.010 
0.030 
0.0040 
-- -- 1250 880 620 
Steels F 0.044 
0.021 
0.26 
0.012 
0.012 
0.083 
0.0034 
0.0058 
1.71 
1250 890 620 
G 0.045 
0.018 
0.29 
0.013 
0.011 
0.033 
0.0066 
0.0029 
0.44 
1250 880 645 
__________________________________________________________________________ 
TABLE 2 
______________________________________ 
Present Invention 
Comparative Steels 
Designation 
A B C D E F G 
______________________________________ 
Y.P. (kg/mm.sup.2) 
19.6 19.4 20.7 18.5 24.8 24.9 23.5 
T.S. (kg/mm.sup.2) 
32.0 31.8 32.4 31.4 34.0 33.8 34.6 
Elongation (%) 
46.5 46.5 45.5 48.0 43.0 43.5 45.0 
-rValue 1.52 1.56 1.44 1.63 1.26 1.30 1.24 
______________________________________ 
TABLE 3 
__________________________________________________________________________ 
Hot Rolling 
Temperature (.degree.C.) 
Desig- 
Chemical Composition (wt. %) Heating 
Finishing 
Coiling 
nation 
C Si Mn P S sol. Al 
N B B/N 
Temp. 
Temp. 
Temp. 
__________________________________________________________________________ 
Present 
H 0.004 
0.026 
0.22 
0.013 
0.010 
0.033 
0.0029 
0.0017 
0.59 
1250 895 630 
Invention 
I 0.009 
0.024 
0.30 
0.014 
0.012 
0.042 
0.0036 
0.0020 
0.56 
1250 900 620 
J 0.003 
0.023 
0.12 
0.012 
0.010 
0.022 
0.0011 
0.0009 
0.81 
1250 910 640 
K 0.005 
0.020 
0.20 
0.013 
0.011 
0.035 
0.0020 
0.0018 
0.90 
1250 900 635 
Comparative 
L 0.008 
0.025 
0.27 
0.013 
0.013 
0.020 
0.0042 
-- -- 1250 895 610 
Steels M 0.005 
0.026 
0.21 
0.014 
0.010 
0.077 
0.0062 
0.0028 
0.46 
1250 900 620 
__________________________________________________________________________ 
TABLE 4 
______________________________________ 
Comparative 
Present Invention 
Steels 
Designation H I J K L M 
______________________________________ 
Y.P. (kg/mm.sup.2) 
16.3 16.4 15.2 15.7 20.6 19.0 
T.S. (kg/mm.sup.2) 
30.6 30.8 29.8 30.7 33.0 32.7 
Elongation (%) 
53 53 54 53 45 47 
-rvalue 1.94 1.90 2.05 
2.00 1.56 1.64 
______________________________________