Method of producing high-strength cold-rolled steel sheet suitable for working

A method of producing a high-strength cold-rolled steel sheet suitable for working uses which utilizes a steel material having the following composition: not more than 0.006 wt % of C, not more than 0.5 wt % of Si, not more than 2.0 wt % of Mn, and not less than 0.01 wt % but not more than 0.10 wt % of Ti, the Ti, C and N contents being determined to meet the condition of Ti>(48/12) C wt %+(48/14) N wt %, the steel also consisting essentially of not less than 0.0010 wt % but not more than 0.0100 wt % of Nb, not less than 0.0002 wt % but not more than 0.0020 wt % of B, not less than 0.03 wt % but not more than 0.20 wt % of P, not more than 0.03 wt % of S, not less than 0.010 wt % but not more than 0.100 wt % of Al, not more than 0.008 wt % of N, not more than 0.0045 wt % of O, and the balance substantially Fe and incidental inclusions. The steel material is cast and hot-rolled and then subjected to a cold rolling conducted at a sheet temperature not higher than 300.degree. C. under such a condition that the sum of the rolling reductions of passes which meet the following condition between said sheet temperature (T .degree.C.) and the strain rate .epsilon. (S-1) is 50% or greater: EQU T.times..epsilon..gtoreq.50,000.degree. C. S.sup.-1 The steel sheet is then continuously annealed or galvannealed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of producing a high-strength 
cold-rolled steel sheet which excels in workability and which is free from 
the problem of P segregation zone which is produced when a large amount of 
P is added for the purpose of enhancing the strength of the steel sheet. 
In recent years, there is an increasing demand for high-strength steel 
sheets in the field of automobile production, in order to meet current 
requirements for reduction in the weight of automobiles to attain a higher 
fuel economy and for ensuring safety of drivers and passengers. 
In modern automobile production, high-strength cold-rolled steel sheets are 
used not only for the inner panels but also for outer panels such as 
engine hoods, trunk lid and fenders. As a consequence, high-strength 
cold-rolled steel sheet is required to have an excellent workability. 
Description of the Related Art 
Hitherto, an art has been proposed in which, in order to improve 
workability of cold-rolled steel sheet, the carbon content of the steel is 
reduced and a carbonitride formers are added to the steel. For instance, 
Japanese Patent Laid-Open Publication No. 63-317648 discloses a 
cold-rolled steel sheet in which Ti, Nb and B are added to a low-carbon 
steel for the purpose of improving press-workability and spot-weldability. 
It has also been proposed to add strengthening elements such as P and Mn 
to the above-mentioned steel system. For instance, Japanese Patent 
Publication No. 61-11294 discloses a method of producing a high-strength 
steel sheet having a superior workability in which a steel enriched with P 
is continuously annealed after a cold rolling. Similarly, Japanese patent 
Publication No. 1-28817 discloses a method in which a steel enriched with 
P and Mn is continuously annealed to form a high-strength cold-rolled 
steel sheet. 
These known methods exhibit disadvantages. The method disclosed in Japanese 
Patent Laid-Open No. 63-317648 cannot provide required strength, while the 
methods disclosed in Japanese Patent Publication Nos. 61-11294 and 1-28817 
inevitably reduce workability although they exhibit improved strength. 
Under these circumstances, steel sheets superior both in strength and 
workability are strongly demanded. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a method of producing, 
from a low-carbon steel having an extremely small carbon content, a 
high-strength cold-rolled steel sheet suitable for working, and more 
particularly a steel sheet having a superior workability, specifically a 
Lankford value (r) of 1.8 or greater, a tensile strength T.S.) of 40 
kgf/mm.sup.2 or greater, an elongation (El) of 40% or greater, and a 
truncated-cone height of 40 mm or greater in the conical cup test. 
To this end, according to the present invention, there is provided a method 
of producing a high-strength cold-rolled steel sheet suitable for working, 
comprising the steps of: 
preparing a steel consisting essentially of not more than 0.02 wt % of C, 
not more than 1.0 wt % of Si, not more than 2.0 wt % of Mn, and not less 
than 0.01 wt % but not more than 0.10 wt % of Ti, the Ti, C and N contents 
being determined to meet the condition of Ti&gt;(48/12) C wt %+(48/14) N wt 
%, said steel also consisting essentially of not less than 0.0010 wt % but 
not more than 0.0100 wt % of Nb, not less than 0.0002 wt % but not more 
than 0.0020 wt % of B, not less than 0.03 wt % but not more than 0.20 wt % 
of P, not more than 0.03 wt % of S, not less than 0.010 wt % but not more 
than 0.100 wt % of Al, not more than 0.008 wt % of N, not more than 0.0045 
wt % of O, and the balance substantially Fe and incidental inclusions; 
subjecting said steel to an ordinary casting and a subsequent hot-rolling; 
subjecting the hot rolled steel to a cold rolling conducted at a sheet 
temperature not higher than 300.degree. C. under such a condition that the 
sum of the rolling reductions of passes which meet the following condition 
between said sheet temperature (T .degree. C.) and the strain rate 
.epsilon. (S.sup.-1) is 50% or greater: 
EQU T.times..epsilon..gtoreq.50,000.degree. C. S.sup.-1 
and 
subjecting the cold-rolled steel to a continuous annealing. 
The sheet temperature T (.degree.C.) is the temperature of the steel sheet 
at positions immediately downstream from the cold-rolling stands as 
measured by an infrared pyrometer, while the strain rate is calculated in 
accordance with the following formula: 
##EQU1## 
where, n represents the roll peripheral speed (rpm), H.sub.0 represents 
the sheet thickness at inlet side, r represents the rolling reduction and 
R represents the radius of the roll.

DETAILED DESCRIPTION OF THE INVENTION 
Through an intense study on improvement in workability of high strength 
cold-rolled steel sheet, the inventors have found that a high-strength 
cold-rolled steel sheet having a superior workability, specifically a 
Lankford value (r) of 1.8 or greater, a tensile strength T.S.) of 40 
kgf/mm.sup.2 or greater, an elongation (El) of 40% or greater and a 
truncated-cone height of 40 mm or greater, can be obtained by selecting 
the strain-imparting condition in the cold rolling of a very-low-carbon 
steel which is rich in P and small in oxygen content. 
The present invention is based upon the above-described discovery. A 
description will be given first of the reason why the condition is posed 
that the sum of the rolling reductions of passes which meet the condition 
of T.times..epsilon..gtoreq.50,000.degree. C. S.sup.-1 between the sheet 
temperature (T .degree.C.) and the strain rate .epsilon. (S.sup.-1) is 50% 
or greater. 
Three types of continuous-cast steel slabs A,B and C having the 
compositions shown in Table 1 were prepared by a converter. 
TABLE 1 
__________________________________________________________________________ 
Steel 
type Contents (wt %) 
Symbols 
C Si Mn P S Al N Ti Nb B O Ti* 
__________________________________________________________________________ 
A 0.0025 
0.01 
0.25 
0.075 
0.008 
0.055 
0.0022 
0.032 
0.004 
0.0012 
0.0031 
0.014 
B 0.0025 
0.01 
0.22 
0.015 
0.007 
0.048 
0.0028 
0.033 
0.004 
0.0011 
0.0033 
0.013 
C 0.0024 
0.01 
0.25 
0.077 
0.008 
0.067 
0.0023 
0.033 
0.004 
0.0011 
0.0078 
0.016 
__________________________________________________________________________ 
Ti* = Ti(48/12) C(48/14) N 
Each slab was heated to 1250.degree. C. and rough-rolled at a rolling 
reduction of 88%, followed by a hot finish-rolling at a rolling reduction 
of 88% (hot-rolling finish temperature: 880.degree. C., coiling 
temperature: 500.degree. C.) so as to be formed into a hot coil of 4.0 mm 
thick. Then, an ordinary cold rolling was effected at a rolling reduction 
of 82.5% so that the steel was formed into a sheet 0.7 mm thick. 
Subsequently, a continuous annealing was conducted at 810.degree. C. 
followed by a temper rolling at a rolling reduction of 0.8% thereby 
producing a rolled steel sheet. 
The cold rolling was conducted while varying the sheet temperature within 
the range of 30.degree. C. to 300.degree. C., while varying the reduction 
rate, i.e., the strain rate .epsilon. within the range between 10 S.sup.-1 
to 2,000 S.sup.-1. The sheet temperature was controlled by varying the 
initial sheet temperature for the cold rolling and the flow rate of the 
cooling water. 
The Lankford value (r), elongation, tensile strength and truncated-cone 
height were measured for each of the sample steel sheets. The 
truncated-cone height, which is an index indicative of the workability 
approximating that in actual working was measured by a conical cup test 
conducted under the following conditions: 
punch diameter: 80 mm .PHI. 
die diameter: 140 mm .PHI. 
wrinkle pressing force: 10 t 
FIG. 1 shows the relationship between these measured values and the sum of 
the rolling reductions of the passes which meet the condition of the 
product of the cold rolling sheet temperature and the strain rate being 
not smaller than 50,000.degree. C. S.sup.-1. 
As will be clearly understood from FIG. 1, the low-oxygen steel material A 
rich in P exhibited a tensile strength (T.S.) which is smaller than that 
of the steel B which has a small P content. In addition, when the sum of 
the rolling reductions of the passes having the product of the sheet 
temperature and the strain rate being 50,000 .degree. C. S.sup.-1 or 
greater is 50% or above, the truncated-cone height indicative of the 
workability approximating that of actual working is remarkably improved to 
a value approximating that of the steel B which has a large tensile 
strength, while the elongation (El) and the Lankford value (r) increase 
only slightly. 
The steel C which is rich both in P and C does not show remarkable 
improvement in the properties indicative of the workability such as the 
Lankford value (r), elongation (El) and the truncated-cone height. 
In order to produce a high-strength cold-rolled steel sheet having superior 
workability, therefore, it is necessary to use a low-oxygen material 
having a large P content and that the cold rolling is conducted under a 
condition which meets the condition of the sum of the rolling reductions 
of the passes having the product of the sheet temperature and the strain 
rate being 50,000 .degree. C. S.sup.-1 or greater is 50% or greater. 
In conventional cold rolling of steel sheets, the sum of the rolling 
reductions of passes which meet the condition of the product of the sheet 
temperature and the strain rate being 50,000 .degree. C. S.sup.-1 or 
greater is generally around 30%. In order to raise the value of the sum of 
the rolling reductions, it is necessary to take suitable measures such as 
an increase in the rolling speed, control of flow rate of cooling water, 
or elevation of the initial cold rolling temperature through a continuous 
change from the preceding step, which is usually pickling. 
According to the invention, it is possible to obtain a high-strength 
cold-rolled steel having high workability by using a low-oxygen steel rich 
in P as the material and by conducting the cold rolling under the specific 
condition mentioned above. The reason why such superior workability is 
obtained has not been clarified yet. 
The reason, however, is considered to reside in the following fact. In 
general, a microscopic observation of structure of a steel sheet rich in P 
exhibits a segregation zone in the thicknesswise central region of the 
sheet. In contrast, the steel produced by the method of the present 
invention does not exhibit such a degradation zone. This suggests that a 
certain effect which could not be produced by the conventional methods is 
caused on the segregation zone by the cold rolling condition peculiar to 
the invention. Although the reason is still unknown, it is considered that 
the cold rolling condition peculiar to the invention produces a uniform 
working effect in the thicknesswise direction so that a greater rolling 
effect is produced on the segregation zone as compared to known methods. 
The segregation zone does not produce any substantial unfavorable effect on 
the elongation Lankford value (r) which is measured in tensile test. In 
the actual use of the material, however, the segregation zone reduces the 
uniformity of the steel sheet in the thicknesswise direction and, hence, 
is considered to cause a reduction in the workability. 
According to the method of the present invention, however, the cold rolling 
conducted under the specified condition produces a working effect which 
serves to break the segregation zone, so that the uniformity of the 
structure in the thicknesswise direction of the steel sheet is improved so 
as to improve the workability as confirmed through the conical cup test 
which simulates the actual condition of use. When the oxygen content in 
the steel is large, however, the large quantity of the inclusions impedes 
the cold-rolling straining of P in the segregation zone so as to reduce 
the effect of improving the workability. 
A description will now be given of the reason for limitation of the 
chemical composition of the steel. C: C serves, when added to the steel 
material together with Ti, to strengthens the steel without impairing 
workability. In order to obtain an excellent workability, therefore, the C 
content is preferably below 0.006 wt %. 
Si: The upper limit of Si content is set to be 1.0 wt %, since the drawing 
characteristic of the steel is impaired when the Si content exceeds 1.0 wt 
%. 
Mn: This element is effective in raising the strength without impairing the 
drawing characteristic. Addition of this element in an excessive amount 
reduces the drawing characteristic so that the Mn content is limited to be 
not more than 2.0 wt %. 
Ti: This element serves to fix C and N in the steel so as to prevent 
deterioration of the material caused by solid solution of C. In addition, 
this element impedes formation of BN so as to prevent reduction in the 
amount of solid solution of B. In order to obtain an appreciable effect, 
therefore, this element should be added in an amount exceeding the sum of 
the C equivalent [(48/12) C wt %] and N equivalent [(48/14) N wt %]. 
However, Ti content below 0.01 wt % is too low to enable Ti to produce any 
appreciable effect. On the other hand, addition of Ti in excess of 0.10 wt 
% reduces the strength. Therefore, the Ti content should be not less than 
0.01 wt % and not more that 0.10 wt % and be determined to exceed the 
value of [(48/12) C wt %+(48/14) N wt %]. 
Nb: This element is essential since it improves the Lankford value (r) and 
strengthens the steel when added together with B. Nb content below 0.0010 
wt %, however, does not produce any remarkable effect. On the other hand, 
addition of Nb in excess of 0.0100 wt % reduces the workability so as to 
impair the balance between strength and workability. The Nb content, 
therefore, is determined to be not less than 0.0010 wt % but not more than 
0.0100 wt %. When the steel is bound to be a deep drawing, however, the Nb 
content is preferably not less than 0.0075 wt %. 
B: This element is indispensable since it improves the strength when added 
together with Nb. B content below 0.0002 wt % does not produce any 
remarkable effect, while addition of B in excess of 0.002 wt % seriously 
degrades the material. The B content, therefore, is determined to be not 
less than 0.0002 wt % but not more than 0.002 wt %. Preferably, B content 
is determined to be not more than 0.0012 wt %. 
P: This element is an important strengthening element. The effect of this 
element is remarkable particularly when the content is 0.03 wt % or more. 
However, addition of P in excess of 0.20 wt % deteriorates the balance 
between strength and workability and, in addition, causes an undesirable 
effect on the brittleness of the steel. The content of P, therefore, is 
determined to be not less than 0.03 wt % but not more than 0.20 wt %, more 
preferably not less than 0.04 wt % but not more than 0.15 wt %. 
S: A reduction is S content in the steel is necessary for improving deep 
drawability. However, the undesirable effect on the workability produced 
by S is not so serious when the S content is reduced down below 0.03 wt %. 
The upper limit of the S content is therefore set to be 0.03 wt %. 
Al: This element is necessary for improving yield of carbonitride formers 
through deoxidation and for eliminating generation of surface defects 
caused by formation of TiO.sub.2. The effect of addition of this element, 
however, is not appreciable when the content is below 0.010 wt %. In 
addition, the deoxidation effect is saturated when the Al content is 
increased beyond 0.10 wt %. In addition, increase in the Al content tends 
to cause surface defect due to generation of Al.sub.2 O.sub.3. The Al 
content, therefore, is determined to be not less than 0.01 wt % but not 
more than 0.10 wt %. 
N: This element degrades deep drawability of the steel and, in addition, 
reduces anti-secondary working embrittlement due to bonding with B, unless 
it is fixed by Ti. Thus, a greater N content uneconomically requires 
greater amount of Ti. The N content, therefore, should be not more than 
0.0008 wt %, preferably not more than 0.0006 wt %. 
O: In order to improve workability which is the critical requirement in the 
present invention, it is necessary to reduce O concentration. When the O 
content exceeds 0.0045 wt %, the cold-rolling straining to the segregation 
zone is impeded by a large amount of inclusions as explained before. As a 
consequence, the effect of improving workability produced by the cold 
straining is impaired and, in addition, an effect which is not negligible 
is caused on the brittleness. For this reason, the upper limit of O 
content is set to be 0.0045 wt %, preferably to 0.004 wt %. Reduction in 
the oxygen content in the steel is effected by controlling the length of 
time of killed treatment in degassing step in ordinary steel making 
process. 
A description will now be given of the preferred condition for the 
preparation of the starting steel material having the above-described 
composition and preferred condition for the production of a steel sheet 
from the starting steel material. 
The steel making process and a subsequent hot rolling can be carried out in 
the same manner as the known process, except that the oxygen content is 
reduced by the method described above. 
A material having satisfactory properties can be obtained when the coiling 
temperature of the steel after the hot rolling falls within the range of 
ordinary process, e.g., between 400.degree. C. and 700.degree. C. Thus, it 
is not necessary to employ a specifically high coiling temperature. 
Rather, it is preferred that the coiling temperature is comparatively low, 
e.g., 550.degree. C. or less, in order to avoid any deterioration in 
pickling property caused by the thickening of scale and to prevent 
excessive softening of the product. 
The cold rolling may be conducted by using an ordinary cold rolling mill, 
provided that the aforementioned cold rolling condition is met. Namely, it 
is necessary that the sum of the rolling reduction of passes which meets 
the condition of the product of the sheet temperature and the strain rate 
being not smaller than 50,000.degree. C. S.sup.-1 is 50% or greater. There 
is no restriction in the total rolling reduction, i.e., the sum of the 
reductions of all passes employed, provided that the above-described 
condition is met. 
As stated before, the cold rolling sheet temperature has to be not higher 
than 300.degree. C. because a cold rolling at higher temperature causes 
concentration of shear deformation to the surface region of the steel 
sheet, making it difficult to work the central segregation zone. 
When the steel having the described composition is annealed by batch-type 
box annealing method, the steel tends to be come brittle due to grain 
boundary segregation of P due to high P content, particularly when the 
cooing rate is small. In order to obviate this problem, according to the 
present invention, a continuous annealing method which enables rapid 
heating and cooling. The annealing temperature, however, may be not lower 
than recrystallization temperature but not higher than A.sub.3 
transformation temperature, as in the case of ordinary steel annealing 
process. 
The temper rolling subsequent to the annealing may be effected under 
ordinary steel tempering condition with a rolling reduction corresponding 
to the sheet thickness (mm), for the purpose of, for example, obtaining 
optimum shape of the sheet. 
EXAMPLE 
Ten types of steels, including 7 types meeting the composition condition of 
the invention and 3 types as reference examples, were prepared in a 
converter and were continuously cast into slabs. Each slab was hot-rolled 
to form a hot coil of 3,0 mm thick and cold-rolled to a thickness of 0.72 
mm. Subsequently, a continuous annealing was conducted under ordinary 
condition. Then, the steel sheets other than the type No. 3 were subjected 
to a temper rolling with a rolling reduction of 0.7%, whereby 10 types of 
steel sheets including one which has not been subjected to temper rolling 
were prepared. 
The roll used in the cold rolling had a diameter of 600 mm. The cold 
rolling speed was 1500 to 2500 m/min at the outlet side of the cold 
rolling stand. 
Among ten types of steel, each of type Nos. 1 and 2 were subjected to three 
different production conditions with different cold-rolling and continuous 
annealing conditions, so that three samples were produced for each of the 
steel type Nos. 1 and 2. Similarly, two samples were prepared from the 
steel type No. 1 through different production conditions. Only one sample 
was prepared for each of the remainder steel types. 
Table 3 shows the hot-rolling and continuous annealing conditions, Table 4 
shows the cold rolling conditions and Table 5 shows the result of 
examination of the properties of the cold-rolled sample steel sheets. 
TABLE 2 
__________________________________________________________________________ 
Steel 
type Contents (wt %) 
No. 
Class C Si Mn P S Al N Ti Nb B O Ti* 
__________________________________________________________________________ 
1 Invention 
0.0021 
0.01 
0.11 
0.055 
0.008 
0.040 
0.0025 
0.032 
0.0034 
0.0008 
0.0025 
0.015 
2 Invention 
0.0026 
0.02 
0.45 
0.073 
0.012 
0.039 
0.0027 
0.042 
0.0024 
0.0007 
0.0019 
0.022 
3 Invention 
0.0020 
0.03 
0.09 
0.130 
0.006 
0.081 
0.0031 
0.072 
0.0044 
0.0010 
0.0037 
0.053 
4 Invention 
0.0029 
0.02 
0.33 
0.084 
0.005 
0.036 
0.0015 
0.036 
0.0070 
0.0009 
0.0033 
0.019 
5 Invention 
0.0056 
0.25 
0.29 
0.085 
0.018 
0.024 
0.0043 
0.051 
0.0020 
0.0006 
0.0028 
0.014 
6 Comp. Ex. 
0.0080 
0.02 
0.34 
0.062 
0.027 
0.065 
0.0051 
0.057 
0.0099 
0.0016 
0.0036 
0.008 
7 Comp. Ex. 
0.0035 
0.76 
1.54 
0.042 
0.017 
0.035 
0.0021 
0.061 
0.0048 
0.0011 
0.0030 
0.040 
8 Comp. Ex. 
0.0034 
0.01 
0.34 
0.060 
0.015 
0.050 
0.0022 
0.045 
0.0032 
0.0012 
0.0054 
0.024 
9 Comp. Ex. 
0.0030 
0.02 
0.24 
0.088 
0.010 
0.060 
0.0019 
0.015 
0.0025 
0.0010 
0.0034 
-0.004 
10 Comp. Ex. 
0.0021 
0.05 
0.33 
0.068 
0.022 
0.061 
0.0034 
0.038 
0.0250 
0.0005 
0.0037 
0.018 
__________________________________________________________________________ 
Comp. Ex. = Comparative Example 
Ti* = Ti(48/12) C(48/14) N 
TABLE 3 
__________________________________________________________________________ 
Continuous 
annealing condition 
Steel Slab Hot-roll Re- 
Sample 
type heating 
finishing 
Coiling crystallization 
Max. heating 
No. No. 
Class temp. (.degree.C.) 
temp. (.degree.C.) 
temp. (.degree.C.) 
CR* temp. (.degree.C.) 
temp. (.degree.C.) 
__________________________________________________________________________ 
1 1 Invention 
1200 920 480 77 770 790 
2 1 Comp. Ex. 
1200 920 480 34 770 790 
3 1 Invention 
1200 920 480 68 770 *1 790 
4 2 Invention 
1150 910 500 61 780 810 
5 2 Comp. Ex. 
1150 910 500 40 780 810 
6 2 Comp. Ex. 
1150 910 500 *2 118 
780 810 
7 3 Invention 
1100 900 550 62 800 850 
8 4 Invention 
1250 900 550 62 770 780 
9 4 Comp. Ex. 
1250 900 550 26 770 780 
10 5 Invention 
1200 880 600 55 750 880 
11 6 Comp. Ex. 
1200 850 650 65 730 850 
12 7 Comp. Ex. 
1250 890 550 51 760 850 
13 8 Comp. Ex. 
1200 900 550 63 770 800 
14 9 Comp. Ex. 
1200 900 550 65 770 800 
15 10 Comp. Ex. 
1200 900 550 63 770 800 
__________________________________________________________________________ 
Comp. Ex. = Comparative Example 
CR*: Sum of rolling reductions of paths which meets condition of sheet 
temp. (T) .times. strain rate(..epsilon.) .gtoreq. 50,000.degree. 
C.s.sup.-1 
*1: Continuous hotdip galvanizing line used 
*2: Sheet temp. in coldrolling exceeded 300.degree. C. 
TABLE 4 (1) 
__________________________________________________________________________ 
Steel 
Sample 
type Stand No. 
No. No. 
Class Items 1 2 3 4 5 6 CR* (%) 
__________________________________________________________________________ 
1 1 Invention 
Rolling re- 
37 47 24 5 -- -- 
76 
duction (%) 
T (.degree.C.) 
50 100 130 140 -- -- 
-- 
..epsilon. (s.sup.-1) 
400 1,170 
1,280 
650 -- -- 
-- 
T .times. ..epsilon. (.degree.C.s.sup.-1) 
20,000 
117,000 
166,000 
91,000 
-- -- 
-- 
2 1 Comp. Ex. 
Rolling re- 
57 19 19 15 -- -- 
34 
duction (%) 
T (.degree.C.) 
45 75 100 120 -- -- 
-- 
..epsilon. (s.sup.-1) 
750 620 850 980 -- -- 
-- 
T .times. ..epsilon. (.degree.C.s.sup.-1) 
34,000 
47,000 
85,000 
117,000 
-- -- 
-- 
3 1 Invention 
Rolling re- 
45 42 18 8 -- -- 
68 
duction (%) 
T (.degree.C.) 
55 90 115 130 -- -- 
-- 
..epsilon. (s.sup.-1) 
430 960 850 630 -- -- 
-- 
T .times. ..epsilon. (.degree.C.s.sup.-1) 
24,000 
87,000 
98,000 
82,000 
-- -- 
-- 
4 2 Invention 
Rolling re- 
17 40 40 17 4 -- 
61 
duction (%) 
T (.degree.C.) 
50 80 100 120 130 -- 
-- 
..epsilon. (s.sup.-1) 
160 520 1,120 
960 500 -- 
-- 
T .times. ..epsilon. (.degree.C.s.sup.-1) 
8,000 
42,000 
112,000 
115,000 
65,000 
-- 
-- 
5 2 Comp. Ex. 
Rolling re- 
48 29 14 14 12 -- 
40 
duction (%) 
T (.degree.C.) 
30 60 90 120 140 -- 
-- 
..epsilon. (s.sup.-1) 
610 810 690 860 990 -- 
-- 
T .times. ..epsilon. (.degree.C.s.sup.-1) 
18,000 
48,000 
62,000 
103,000 
139,000 
-- 
-- 
6 2 Comp. Ex. 
Rolling re- 
43 35 18 11 10 -- 
117* 
duction (%) 
T (.degree.C.) 
350 350 350 360 360 -- 
-- 
..epsilon. (s.sup.-1) 
310 530 520 480 540 -- 
-- 
T .times. ..epsilon. (.degree.C.s.sup.-1) 
109,000 
186,000 
181,000 
174,000 
194,000 
-- 
-- 
7 3 Invention 
Rolling re- 
47 44 18 3 -- -- 
62 
duction (%) 
T (.degree.C.) 
55 90 110 120 -- -- 
-- 
..epsilon. (s.sup.-1) 
480 1,110 
960 390 -- -- 
-- 
T .times. ..epsilon. (.degree.C.s.sup.-1) 
27,000 
100,000 
106,000 
47,000 
-- -- 
-- 
__________________________________________________________________________ 
Comp. Ex. = Comparative Example 
*Sheet temp. 300.degree. C. or above. 
TABLE 4 (2) 
__________________________________________________________________________ 
Steel 
Sample 
type Stand No. 
No. No. 
Class Items 1 2 3 4 5 6 CR* (%) 
__________________________________________________________________________ 
8 4 Invention 
Rolling re- 
33 28 28 19 12 4 63 
duction (%) 
T (.degree.C.) 
40 70 70 120 130 140 -- 
..epsilon. (s.sup.-1) 
350 520 840 960 910 570 -- 
T .times. ..epsilon. (.degree.C.s.sup.-1) 
14,000 
36,000 
59,000 
116,000 
119,000 
79,000 
-- 
9 4 Comp. Ex. 
Rolling re- 
33 25 23 17 17 9 26 
duction (%) 
T (.degree.C.) 
30 50 70 80 90 100 -- 
..epsilon. (s.sup.-1) 
250 340 490 560 730 610 -- 
T .times. ..epsilon. (.degree.C.s.sup.- 1) 
8,000 
17,000 
34,000 
45,000 
66,000 
61,000 
-- 
10 5 Invention 
Rolling re- 
33 33 30 18 8 -- 56 
duction (%) 
T (.degree.C.) 
40 80 120 140 150 -- -- 
..epsilon. (s.sup.-1) 
340 600 970 1020 
760 -- -- 
T .times. ..epsilon. (.degree.C.s.sup.-1) 
13,000 
48,000 
117,000 
143,000 
113,000 
-- -- 
11 6 Comp. Ex. 
Rolling re- 
48 39 13 8 5 -- 65 
duction (%) 
T (.degree.C.) 
35 70 100 110 120 -- -- 
..epsilon. (s.sup.-1) 
490 920 650 610 520 -- -- 
T .times. ..epsilon. (.degree.C.s.sup.-1) 
17,000 
65,000 
65,000 
67,000 
62,000 
-- -- 
12 7 Comp. Ex. 
Rolling re- 
33 35 35 9 6 -- 50 
duction (%) 
T (.degree.C.) 
40 70 100 130 140 -- -- 
..epsilon. (s.sup.-1) 
350 690 1290 
790 720 -- -- 
T .times. ..epsilon. (.degree.C.s.sup.-1) 
14,000 
48,000 
129,000 
102,000 
101,000 
-- -- 
13 8 Comp. Ex. 
Rolling re- 
33 28 24 21 14 4 63 
duction (%) 
T (.degree.C.) 
40 60 80 100 120 130 -- 
..epsilon. (s.sup.-1) 
310 460 650 860 870 500 -- 
T .times. ..epsilon. (.degree.C.s.sup.-1) 
12,000 
27,000 
52,000 
86,000 
104,000 
65,000 
-- 
14 9 Comp. Ex. 
Rolling re- 
48 39 13 8 5 -- 65 
duction (%) 
T (.degree.C.) 
35 70 100 110 120 -- -- 
..epsilon. (s.sup.-1) 
490 920 650 610 520 -- -- 
T .times. ..epsilon. (.degree.C.s.sup.-1 ) 
17,000 
65,000 
65,000 
67,000 
62,000 
-- -- 
15 10 Comp. Ex. 
Rolling re- 
33 28 24 21 14 4 63 
duction (%) 
T (.degree.C.) 
40 60 80 100 120 130 -- 
..epsilon. (s.sup.-1) 
310 460 650 860 870 500 -- 
T .times. ..epsilon. (.degree.C.s.sup.-1) 
12,000 
27,000 
52,000 
86,000 
104,000 
65,000 
-- 
__________________________________________________________________________ 
CR*: Sum of rolling reductions of paths which meets condition of sheet 
temp. (T) .times. strain rate (..epsilon.) .gtoreq. 50,000.degree. 
C.s.sup.-1 
Comp. Ex. = Comparative Example 
TABLE 5 
__________________________________________________________________________ 
Steel Truncated- 
Sample 
type Y.S. T.S. El. cone height 
No. No. 
Class (kgf/mm.sup.2) 
(kgf/mm.sup.2) 
(%) 
T.S. + El. 
-r value 
(mm) 
__________________________________________________________________________ 
1 1 Invention 
20.0 35.4 50.3 
85.7 2.2 55 
2 1 Comp. Ex. 
20.4 35.4 50.5 
85.9 2.2 30 
3 1 Invention 
20.6 36.2 49.6 
85.8 2.1 51 
4 2 Invention 
21.2 38.6 47.5 
86.1 2.2 55 
5 2 Comp. Ex. 
22.5 38.5 47.5 
86.0 2.2 25 
6 2 Comp. Ex. 
22.7 38.8 45.5 
84.3 2.0 20 
7 3 Invention 
25.8 45.2 41.2 
86.4 2.1 55 
8 4 Invention 
20.7 36.5 49.2 
85.7 2.3 50 
9 4 Comp. Ex. 
20.9 36.1 49.1 
85.2 2.2 33 
10 5 Invention 
23.3 40.5 45.3 
85.8 2.1 52 
11 6 Comp. Ex. 
28.1 48.5 36.4 
85.1 2.0 45 
12 7 Comp. Ex. 
24.9 54.3 33.4 
87.7 2.0 53 
13 8 Comp. Ex. 
21.5 35.4 49.4 
84.8 2.0 35 
14 9 Comp. Ex. 
26.4 34.8 42.1 
76.9 1.6 20 
15 10 Comp. Ex. 
22.0 36.1 43.1 
79.2 2.0 30 
__________________________________________________________________________ 
Comp. Ex. = Comparative Example 
From Table 5, it will be understood that the sample Nos. 2, 5, 6, 9, 13, 14 
and 15 as reference examples showed comparatively small values of 
truncated-cone height ranging from 20 mm to 35 mm. In contrast, other 
samples which meet the condition of the invention showed large values of 
truncated-cone height ranging from 45 mm to 55 mm, thus proving superior 
workability. 
Sample No. 3 was subjected to a galvannealing instead of the continuous 
annealing. This galvannealed steel sheet also showed excellent workability 
as in the cases of other samples meeting the conditions of the invention. 
Sample No. 6 was cold-rolled at a cold-rolling sheet temperature exceeding 
300.degree. C., although the sum of the rolling reductions of the passes 
having the product of the sheet temperature and the strain rate exceeding 
50,000.degree. C. S.sup.-1 was greater than 50%. Consequently, this sample 
showed a too small workability which was 20 mm in terms of truncated-cone 
height. 
As will be understood from the foregoing description, a method has been 
established by the present invention which enables production of a 
high-strength cold-rolled steel sheet having superior workability by 
processing a low-oxygen low-carbon steel rich in P under specific 
cold-rolling conditions. The cold-rolled steel sheet produced by the 
method of the invention is suitable for use as a material of products 
which are produced through press-forming, bulging, deep-drawing and other 
plastic works.