Flexible hot rolled steel sheets having improved deep drawability

A flexible hot rolled steel sheet having an improved deep drawability comprises not more than 0.10 wt % of C, less than 0.20 wt % of Mn, 0.10-2.0 wt % of Cr, 0.002-0.100 wt % of Al, and the balance being Fe and inevitable impurities or further contains particular amounts of at least one of Ti, Nb and Zr and/or B. Such a hot rolled steel sheet has a tensile strength of not more than 35 kgf/mm.sup.2 and a total elongation of not less than 50%.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to flexible hot rolled steel sheets having a tensile 
strength of not more than 35 kgf/mm.sup.2 and a total elongation of not 
less than 50% and an improved deep drawability and good bending property 
and bulging property. 
2. Related Art Statement 
Lately, manufacturers for automobiles, domestic electrical articles and the 
like tend to use thin hot rolled steel sheets instead of cold rolled steel 
sheets from a viewpoint of the reduction of cost. However, conventional 
hot rolled steel sheet is poor in deep drawability as compared with cold 
rolled steel sheet, so that their application is restricted at the 
present. 
The deep drawability of the steel sheet is dependent upon the elongation 
property of the starting material and r-value. As a cause which the 
conventional hot rolled steel sheet is hardly applied to the deep drawing, 
in the hot rolled steel sheet produced by the usual method, the level of 
r-value is not more than 1.0 and it is difficult to obtain the r-value of 
not less than 1.5 as in the cold rolled steel sheet. 
A countermeasure for improving the drawback on the deep drawability in hot 
rolled steel sheets is roughly divided into two methods, one being a 
method of approaching the r-value to that of the cold rolled steel sheet 
as far as possible and the other being a method of largely improving the 
elongation property. 
As to the first method, Japanese Patent laid open No. 55-97431, No. 
60-77927 and the like have proposed a method wherein steel having the 
conventionally used chemical composition is used and hot rolled under such 
a finish hot rolling condition that the reduction is high or the strain 
rate is high or the reduction is carried out within a particular low 
temperature range to thereby improve the r-value. 
However, even if the above method is adopted, the resulting r-value is 
1.0-1.3 at most and does not reach to the level of the cold rolled steel 
sheet, so that the deep drawability is not yet satisfied at the present. 
Furthermore, if it is intended to obtain a higher r-value by the above 
method, it is necessary to largely change the hot rolling condition from 
the usually used range toward high reduction side and high speed rolling 
side, which finally exceeds over the range of rolling conditions 
selectable in the existing hot rolling mill. 
On the other hand, as a proposal for the improvement of elongation property 
on the latter deep drawability, there are mentioned hot rolled steel 
sheets obtained by adding B to the chemical composition in the usual 
flexible hot rolled steel sheet and regulating the weight ratios of B/N 
and Mn/S within given ranges, respectively, as disclosed in Japanese 
Patent laid open No. 62-139849. However, the total elongation of such a 
hot rolled steel sheet is not more than 48%, and even when the best 
properties of the hot rolled steel sheet are compared with the properties 
of the cold rolled steel sheet, they are only equal to the level of the 
cold rolled steel sheet at most. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the invention to provide flexible hot rolled 
steel sheets having a very excellent elongation property, concretely 
flexible hot rolled steel sheets having a tensile strength of not more 
than 35 kgf/mm.sup.2 and a total elongation of not less than 50% and an 
improved deep drawability. 
The term "tensile strength" used herein means a tensile strength using a 
tensile test specimen of JIS No. 5, while the elongation property is a 
value of total elongation using a tensile test specimen of JIS No. 5 with 
a thickness of 1.4 mm. Moreover, when the thickness of the specimen is 
different, the total elongation is represented by the value of El* 
corrected according to the following equation: 
EQU El*=(1.4/t)El.sub.t (1) 
wherein t is the thickness of the tensile test specimen (mm) and El.sub.t 
is the total elongation of the tensile test specimen at the thickness t 
(%). 
It is known that in the case of steel sheets having a thickness of not more 
than 1.0 mm as in cold rolled steel sheets, the reduction of the thickness 
in the press forming depends on the forming limit, so that the deep 
drawability becomes high in the depending ratio to the r-value rather than 
the elongation property, while in the case of steel sheets having a 
thickness of not less than 1 mm as in hot rolled steel sheet, the 
influence by the reduction of the thickness is mitigated, so that the deep 
drawability is small in the depending ratio to r-value and becomes 
relatively high in the depending ratio to the elongation property. 
Under the above circumstances, the inventors have developed a means for 
considerably improving the deep drawability of hot rolled steel sheet 
through the improvement of the elongation property and made various 
studies, and found hot rolled steel sheets having an r-value equal to that 
of the conventional steel and elongation property considerably higher than 
that of the conventional steel, and consequently the invention has been 
accomplished. 
That is, a first embodiment of the invention lies in a flexible hot rolled 
steel sheet having an improved deep drawability, comprising not more than 
0.10 wt % (hereinafter shown by simply) of C, less than 0.20% of Mn, 
0.10-2.0% of Cr, 0.002-0.100% of Al, and the balance being Fe and 
inevitable impurities. 
Further, a second embodiment of the invention lies in a flexible hot rolled 
steel sheet having an improved deep drawability, comprising not more than 
0.10% of C, less than 0.20% of Mn, 0.10-2.0% of Cr, 0.002-0.100% of Al, 
0.005-0.10% in total of at least one of Ti, Nb and Zr, and the balance 
being Fe and inevitable impurities. 
And also, a third embodiment of the invention lies in a flexible hot rolled 
steel sheet having an improved deep drawability, comprising not more than 
0.10% of C, less than 0.20% of Mn, 0.10-2.0% of Cr, 0.002-0.100% of Al, 
0.0004-0.0100% of B, and the balance being Fe and inevitable impurities. 
Moreover, a fourth embodiment of the invention lies in a flexible hot 
rolled steel sheet having an improved deep drawability, comprising not 
more than 0.10% of C, less than 0.20% of Mn, 0.10-2.0% of Cr, 0.002-0.100% 
of Al, 0.005-0.10% in total of at least one of Ti, Nb and Zr, 
0.0004-0.0100% of B, and the balance being Fe and inevitable impurities.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The flexible hot rolled steel sheets according to the invention will 
concretely be described below. 
The flexible hot rolled steel sheet according to the invention is a low 
carbon aluminum killed steel containing not more than 0.10% of C, wherein 
Cr is included in an amount of 0.10-2.0% while controlling the amount of 
Mn to a low range of less than 0.20%. The chemical composition range of 
the flexible hot rolled steel sheet according to the invention is 
determined by the following alloy formulation to ensure a tensile strength 
of not more than 35 kgf/mm.sup.2 and total elongation of not less than 
50%. 
That is, when the hot rolling is carried out under usual rolling 
conditions, in order to render the sheet into flexible material as far as 
possible, it is required to completely eliminate strength raising factors 
such as texture reinforcement, solid solution reinforcement, precipitation 
hardening and the like and to regulate crystal grains as a microstructure 
so as to have a ferrite crystal grain size of 20-100 .mu.m. For this 
purpose, according to the invention, the chemical composition range is 
specified as mentioned above. The reason on the limitation of the chemical 
composition range will be described below. 
C: not more than 0.10% 
The C amount should be not more than 0.10% in order to obtain a flexible 
hot rolled steel sheet. When the C amount exceeds 0.10%, the tensile 
strength also exceeds 35 kgf/mm.sup.2, and consequently the total 
elongation of not less than 50% aiming at the invention can not be 
obtained. Moreover, as the C amount becomes small, the steel sheet becomes 
more flexible and the high elongation property is easy to be obtained, so 
that the C amount is desirable to be not more than 0.025%. However, the 
lower limit of the C amount is about 0.0010% from a viewpoint of 
steel-making restriction. Even when the amount is within a range of 
0.025-0.10%, the tensile strength aiming at the invention is obtained by 
properly selecting amounts of Mn and Cr as mentioned later. 
Mn: less than 0.20% 
The Mn amount is necessary to be controlled to be less than 0.20% for 
providing the high elongation property for the following two reasons. The 
first reason is to suppress the rising of the tensile strength due to the 
solid solution reinforcement with Mn, and the second reason is to prevent 
the fine division of ferrite crystal grains produced from the lowering of 
ferrite transformation starting temperature by the action of Mn decreasing 
A.sub.r3 point to thereby suppress the rising of tensile strength due to 
such a fine division. When the Mn amount exceeds 0.20%, the above Mn 
action can not be suppressed, and consequently the tensile strength of not 
more than 35 kgf/mm.sup.2 and total elongation of not less than 50% aiming 
at the invention can not be obtained. Moreover, when the Mn amount is less 
than 0.01%, the action for fixing S is reduced to cause a fear of creating 
hot shortness, so that the amount of about 0.01% may be added at minimum. 
Cr: 0.10-2.0% 
The Cr amount is necessary to be within a range of 0.10-2.0% for preventing 
the bad influence of solid solute C and optimizing the microstructure. 
According to the invention, the point that the Cr amount is adjusted to 
the above range is an essential point. In this connection, the knowledge 
found from the inventors' studies will be described in detail below. 
As a mean adopted for producing the flexible hot rolled steel sheet, there 
are generally well-known a method of reducing the strength raising element 
such as C, Mn or the like as far as possible, and a method of raising the 
hot finish temperature or coiling temperature to enlarge the ferrite 
crystal grain size. However, the inventors have found that even if these 
methods are conducted, the level of the resulting elongation property is 
critical due to the remaining solid solute C or formation of film-like 
cementite at grain boundary, abnormal increase of ferrite crystal grain 
size and the like and consequently the high elongation property of not 
less than 50% aiming at the invention is never obtained only by these 
methods. 
The reason why the solid solute C is easy to remain as the C amount is 
decreased in the conventional hot rolled steel sheet is due to the fact 
that the supersaturation degree of C soluted in steel is reduced to make 
the driving force for precipitation as a carbide weak and also the 
precipitation nucleus is decreased by the formation of coarse crystal 
grain. Particularly, when the C amount is as low as 0.02%, if the 
precipitation of cementite at grain boundary is promoted, the film-like 
cementite is produced in the boundary of ferrite crystal grains to degrade 
the elongation property. 
As a means for preventing the remaining of solid solute C or the formation 
of film-like cementite, there is also known a method of adding a 
carbide-forming element such as Ti, Nb, Zr or the like. When this method 
is particularly applied to extremely low carbon steel containing not more 
than 0.0050% of C, fairly effective results are obtained, but considerable 
improvement of the elongation property is not achieved. Because, in order 
to sufficiently eliminate the bad influence of solid solute C by the 
addition of the carbide-forming element, it is necessary that the ratio of 
carbide-forming element to the amount of C is at least 10 as an atomic %, 
but the effect of raising the strength by the addition of the 
carbide-forming element itself is caused to finally disappear the 
improving effect through the reduction of solid solute C. 
The inventors have found that the above inconvenience can advantageously be 
improved by the effect of Cr. 
At first, the addition of Cr eliminates the bad influence of solid solute C 
and prevents the formation of the film-like cementite. Although the 
detailed mechanism of this phenomenon is not clear, the following is 
believed to occur. That is, Cr is not a strong carbide-forming element as 
in Ti, Nb and Zr, but strongly tends to form a carbide as compared with Fe 
or Mn, so that when Cr is existent together with solid solute C, it is 
considered to hold a state of locally existing solid solute C atom around 
a Cr atom by the interaction between both the atoms. In this case, C 
exists at the same state as precipitated as a carbide, i.e. a state that 
the amount of solid solute C is reduced at a greater part of a matrix to 
cause no solid solution reinforcement on one hand and at a state of 
forming no carbide to prevent the formation of the film-like cementite o 
the other hand. By such an addition of Cr, the existing state of C is very 
advantageously held in view of the mechanical properties. As is 
well-known, the solid solution reinforcing amount of Cr itself is very 
small as compared with that of Mn or the like, so that increasing strength 
by the Cr addition is small to bring about the favorable result. 
The second effect by Cr addition that the regulated structure of ferrite 
grains having good grain size is easy to be obtained. In order to promote 
the flexible material of the hot rolled steel sheet and the improvement of 
the elongation property, it is required that the ferrite crystal grain 
size is rendered into a grain regulated structure sufficiently grown 
within a range of causing no orange peel. In order to obtain such a 
structure, it is desirable that the hot rolling finish temperature and the 
coiling temperature are higher, but in the production of the conventional 
steel there is a problem that the range of the optimum coiling temperature 
is very narrow. FIG. 1 schematically shows the difference of the relation 
between the coiling temperature and the ferrite crystal grain size in the 
conventional steel and the Cr-containing steel according to the invention. 
As seen from FIG. 1, when the coiling temperature exceeds a certain value 
in the conventional steel, the phenomenon of abnormal grain growth rapidly 
occurs to exhibit the mixed grain structure and hence the coarse grain 
structure being orange peel. Therefore, in order to avoid such a risk to 
provide the regulated grain structure, the range selectable as the coiling 
temperature is relatively low and narrow, and consequently the resulting 
ferrite crystal grain size is insufficient. On the other hand, in the 
Cr-containing steel according to the invention, the abnormal grain growth 
is suppressed and the crystal grain growth gently proceeds together with 
the rising of the coiling temperature, and consequently the regulated 
grain structure having an optimum grain size is easily obtained. 
In order to develop the above effect, Cr is necessary to be added in an 
amount of at least 0.10%, while when the Cr amount exceeds 2.0%, the 
tensile strength rises and it is difficult to attain the tensile strength 
of not more than 35 kgf/mm.sup.2 aiming at the invention, so that the 
upper limit is 2.0%. 
Al: 0.002-0.100% 
Al improves the cleanness of steel as a deoxidizing component and fixes N 
to prevent the degradation of the elongation property at strain aging, so 
that it is an essential component in the invention. 
In order to develop the above effect, Al is necessary to be added in an 
amount of at least 0.002%, while when the Al amount exceeds 0.100%, the 
effect is saturated and also AlN rather increases to obstruct the growth 
of ferrite crystal grains. For this end, the Al amount should be within a 
range of 0.002-0.100%. 
In the invention, the object can be fundamentally achieved by regulating 
the C, Mn and Cr amounts, but the deep drawability is further improved by 
adjusting other chemical components to the range as mentioned later. 
In the second and fourth embodiments, at least one of Ti, Nb and Zr is 
included in an amount of 0.005-0 10% in total. Ti, Nb and Zr form a 
carbide to reduce solid solute C, so that when they are added in a proper 
amount, the deep drawability is improved. In order to develop this effect, 
after the C amount is limited to not more than 0.020%, the ratio as atomic 
% of the total amount of the above components to C amount 7 is sufficient 
to be within a range of 1-5. However, when the ratio is less than 1, the 
addition effect of these components is not developed. Moreover, when the 
ratio exceeds 5, the tensile strength inversely rises to degrade the 
elongation property. Therefore, at least one of Ti, Nb and Zr is added in 
an amount of 0.005-0.1000% in total. 
B: 0.0004-0.0100% 
In the third and fourth embodiments, B is 
included in an amount of 0.0004-0.0100%. B advantageously serves to prevent 
the strain aging through N and acts as a nucleus for precipitating 
supersaturated solid solute C when BN is precipitated, so that when the B 
amount is a proper range, the deep drawability is improved. When the B 
amount is less than 0.0004%, the above effect is not developed, while when 
it exceeds 0.0100%, there is a fear of degrading the elongation property. 
Preferably, B is added in an amount of not more than 0.0050%. 
N causes the solid solution reinforcement and degradation through strain 
aging likewise C and also forms AlN to obstruct the growth of ferrite 
crystal grain, so that the N amount is desirable to be reduced as far as 
possible. Preferably, the N amount is not more than 20 ppm. 
P easily segregates into grain boundary to cause secondary work brittleness 
and raises the strength of matrix through solid solution reinforcement, so 
that the P amount is desirable to be reduced as far as possible. 
Preferably, the P amount is not more than 0.012%. 
S badly affects the elongation property as a non-metallic inclusion and 
acts to promote hot shortness and secondary work brittleness, so that the 
S amount is desirably not more than 0.010%. 
The flexible hot rolled steel sheets according to the invention ca be 
produced by hot rolling steels having the above chemical composition 
according to the usual manner. The production conditions are not 
particularly restricted, but the finish rolling at a temperature of not 
lower than A.sub.r3 point is most general, and in this case good 
mechanical properties can be obtained as the finish rolling temperature 
becomes higher. In the flexible hot rolled steel sheet according to the 
invention, even if the finish rolling temperature is as low as a range of 
750.degree. C.-A.sub.r3 point, the recrystallization ferrite structure of 
regulated grains can be obtained by adjusting the reduction at final pass 
to not less than 20%, and consequently good mechanical properties ar 
obtained likewise in the case of high temperature finish rolling. Because, 
the recrystallization of the worked ferrite crystal grains is easily 
promoted by the addition of Cr, so that the recrystallization is caused at 
a final pass reduction of not less than 20%. Thus, the point that the good 
mechanical properties are obtained even at a low finish rolling 
temperature of not higher than A.sub.r3 point is suitable for the 
production of thin hot rolled steel sheets having a thickness of about 
1.0-2.0 mm which tend to increase the demand lately. 
Furthermore, the coiling temperature is desirable to be not lower than 
550.degree. C for making the hot rolled steel sheet flexible and improving 
the elongation property. The upper limit of the coiling temperature is not 
particularly restricted from a viewpoint of the mechanical properties, but 
it is desirable to be not higher than 750.degree. C. from a viewpoint of 
the pickling. 
The following example is given in illustration of the invention and is not 
intended as limitation thereof. 
In the hot rolled steel sheets according to the invention, the rolling 
conditions are not particularly restricted to those described in this 
example. 
EXAMPLE 
Steels having a chemical composition as shown in the following Table 1 were 
rolled to a thickness of 1.4 mm under hot rolling conditions as shown in 
the following Table 2 to obtain hot rolled steel sheets. Then, the tensile 
properties, elongation property and limit drawing ratio (L.D.R.) were 
measured with respect to these hot rolled steel sheets to obtain results 
as shown in Table 2. 
TABLE 1 
__________________________________________________________________________ 
(wt %) 
Steel No. C Si Mn Cr Ti Nb Zr B Al N P S 
__________________________________________________________________________ 
Comparative steel 1 
0.0250 
0.01 
0.25 
0.02 
-- -- -- -- 0.035 
0.0026 
0.008 
0.005 
Comparative steel 2 
0.0105 
0.01 
0.25 
0.01 
-- -- -- -- 0.038 
0.0022 
0.011 
0.008 
Comparative steel 3 
0.0022 
.ltoreq.0.01 
0.16 
0.01 
0.036 
0.013 
-- -- 0.053 
0.0031 
0.006 
0.007 
Invention steel 4 
0.0025 
.ltoreq.0.01 
0.06 
0.51 
-- -- -- -- 0.015 
0.0017 
0.012 
0.005 
Invention steel 5 
0.0098 
.ltoreq.0.01 
0.07 
0.50 
-- -- -- -- 0.015 
0.0021 
0.009 
0.006 
Invention steel 6 
0.0352 
.ltoreq.0.01 
0.07 
0.30 
-- -- -- -- 0.012 
0.0018 
0.011 
0.006 
Invention steel 7 
0.0951 
.ltoreq.0.01 
0.08 
0.51 
-- -- -- -- 0.013 
0.0018 
0.006 
0.007 
Comparative steel 8 
0.1200 
.ltoreq.0.01 
0.08 
0.50 
-- -- -- -- 0.016 
0.0017 
0.009 
0.005 
Invention steel 9 
0.0100 
.ltoreq.0.01 
0.15 
0.51 
-- -- -- -- 0.012 
0.0016 
0.013 
0.005 
Comparative steel 10 
0.0110 
.ltoreq.0.01 
0.25 
0.50 
-- -- -- -- 0.011 
0.0019 
0.011 
0.005 
Invention steel 11 
0.0021 
.ltoreq.0.01 
0.06 
1.05 
-- -- -- -- 0.012 
0.0026 
0.009 
0.005 
Invention steel 12 
0.0022 
.ltoreq.0.01 
0.06 
1.52 
-- -- -- -- 0.008 
0.0018 
0.013 
0.008 
Invention steel 13 
0.0020 
.ltoreq.0.01 
0.07 
1.90 
-- -- -- -- 0.013 
0.0015 
0.009 
0.007 
Comparative steel 14 
0.0031 
.ltoreq.0.01 
0.08 
2.20 
-- -- -- -- 0.014 
0.0017 
0.011 
0.005 
Invention steel 15 
0.0029 
.ltoreq.0.01 
0.05 
0.50 
-- 0.031 
-- -- 0.015 
0.0035 
0.006 
0.006 
Invention steel 16 
0.0022 
.ltoreq.0.01 
0.06 
0.52 
0.030 
-- -- -- 0.012 
0.0019 
0.009 
0.006 
Invention steel 17 
0.0022 
.ltoreq.0.01 
0.06 
0.50 
-- -- 0.033 
-- 0.013 
0.0019 
0.013 
0.007 
Invention steel 18 
0.0034 
.ltoreq.0.01 
0.07 
0.50 
0.045 
0.015 
-- -- 0.016 
0.0017 
0.008 
0.005 
Invention steel 19 
0.0032 
.ltoreq.0.01 
0.06 
0.50 
0.020 
-- 0.022 
-- 0.012 
0.0018 
0.007 
0.005 
Invention steel 20 
0.0035 
.ltoreq.0.01 
0.06 
0.50 
-- 0.023 
0.012 
-- 0.003 
0.0022 
0.007 
0.006 
Invention steel 21 
0.0032 
.ltoreq.0.01 
0.06 
0.52 
0.022 
0.015 
0.014 
-- 0.015 
0.0015 
0.010 
0.005 
Invention steel 22 
0.0035 
.ltoreq.0.01 
0.11 
0.55 
-- -- -- 0.0022 
0.015 
0.0015 
0.009 
0.003 
Invention steel 23 
0.0112 
.ltoreq.0.01 
0.09 
0.42 
-- -- -- 0.0050 
0.014 
0.0019 
0.009 
0.004 
Invention steel 24 
0.0304 
.ltoreq.0.01 
0.07 
0.44 
-- -- -- 0.0009 
0.009 
0.0018 
0.009 
0.002 
Invention steel 25 
0.0033 
.ltoreq.0.01 
0.06 
0.52 
-- 0.021 
-- 0.0015 
0.011 
0.0018 
0.007 
0.002 
Invention steel 26 
0.0029 
.ltoreq.0.01 
0.07 
0.40 
0.036 
-- -- 0.0034 
0.013 
0.0018 
0.006 
0.003 
Invention steel 27 
0.0032 
.ltoreq.0.01 
0.08 
0.45 
-- -- 0.041 
0.0020 
0.010 
0.0017 
0.012 
0.003 
Invention steel 28 
0.0032 
.ltoreq.0.01 
0.05 
0.43 
0.030 
0.011 
-- 0.0025 
0.010 
0.0018 
0.010 
0.008 
Invention steel 29 
0.0031 
.ltoreq.0.01 
0.06 
0.43 
0.024 
-- 0.020 
0.0024 
0.011 
0.0015 
0.010 
0.007 
Invention steel 30 
0.0035 
.ltoreq.0.01 
0.06 
0.45 
0.022 
-- 0.015 
0.0020 
0.012 
0.0015 
0.008 
0.007 
Invention steel 31 
0.0034 
.ltoreq.0.01 
0.07 
0.48 
-- 0.025 
0.010 
0.0012 
0.005 
0.0018 
0.009 
0.005 
Invention steel 32 
0.0032 
.ltoreq.0.01 
0.07 
0.41 
0.030 
0.010 
0.020 
0.0010 
0.010 
0.0019 
0.007 
0.005 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
Finish rolling 
Coiling 
Final gauge 
temperature 
temperature 
YS TS EL 
Steel No. (mm) (.degree.C.) 
(.degree.C.) 
(kgf/mm.sup.2) 
(kgf/mm.sup.2) 
(%) 
L.D.R. 
__________________________________________________________________________ 
Comparative steel 1 
1.4 900 600 27.0 37.5 38 1.91 
Comparative steel 2 
" " " 25.5 33.1 43 1.94 
Comparative steel 3 
" " " 19.8 31.5 47 1.97 
Invention steel 4 
" " 650 18.7 27.5 53 2.06 
Invention steel 5 
" " " 20.1 27.9 52 2.06 
Invention steel 6 
" " " 20.9 29.8 51 2.02 
Invention steel 7 
" " " 22.0 30.6 50 2.03 
Comparative steel 8 
" " " 26.4 36.7 45 1.94 
Invention steel 9 
" " " 20.7 30.9 52 2.06 
Comparative steel 10 
" " " 24.2 34.1 44 1.94 
Invention steel 11 
" " " 20.3 29.8 53 2.06 
Invention steel 12 
" " " 20.8 31.0 52 2.06 
Invention steel 13 
" " " 22.1 32.0 51 2.03 
Comparative steel 14 
" " 700 25.3 34.6 47 1.94 
Invention steel 15 
1.4 900 700 18.0 27.7 55 2.08 
Invention steel 16 
" " " 17.5 26.5 57 2.12 
Invention steel 17 
" " " 18.1 27.4 56 2.12 
Invention steel 18 
" " " 18.7 28.3 58 2.12 
Invention steel 19 
" " " 19.0 28.3 58 2.12 
Invention steel 20 
" " " 18.2 28.1 58 2.12 
Invention steel 21 
" " " 19.1 29.0 56 2.12 
Invention steel 22 
" 780 650 18.4 27.1 56 2.12 
Invention steel 23 
" " " 19.6 28.6 55 2.09 
Invention steel 24 
" " " 21.6 30.8 54 2.09 
Invention steel 25 
" 900 " 17.3 26.6 58 2.15 
Invention steel 26 
" " " 17.3 27.1 58 2.15 
Invention steel 27 
" " " 17.8 28.2 57 2.12 
Invention steel 28 
" " " 17.2 27.3 59 2.15 
Invention steel 29 
" " " 17.4 27.6 58 2.15 
Invention steel 30 
" " " 16.4 26.9 58 2.15 
Invention steel 31 
" " " 15.8 26.2 59 2.15 
Invention steel 32 
" " " 16.6 26.8 59 2.15 
__________________________________________________________________________ 
Moreover, the tensile properties and elongation property were measured with 
respect to a specimen of JIS No. 5 obtained by subjecting the hot rolled 
steel sheet of 1.4 mm in thickness to a skin pass rolling of 1.0% and 
cutting out therefrom in a direction parallel to the rolling direction. 
The measurement of the limit drawing ratio was carried out by subjecting a 
specimen cut out from the hot rolled steel sheet of 1.4 mm in thickness 
after the pickling to cupping as shown in FIG. 2 under conditions as shown 
in the following Table 3. 
TABLE 3 
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Automatic Erichsen testing machine 
(made by Erichsen AG in West Germany) 
______________________________________ 
ponch diameter dP 33 mm 
ponch shoulder .rho.p 
5 mm 
die shoulder R .rho.d 
3 mm 
blank holder 1 ton 
Lubrication condition 
Nippon Kosaku oil #720 
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As seen from Table 2, all steels according to the invention exhibit the 
tensile strength of not more than 35 kgf/mm.sup.2 and total elongation of 
not less than 50%. 
In order to clarify the difference between the steel according to the 
invention and the comparative steel, the relations between tensile 
strength and total elongation and between tensile strength and limit 
drawing ratio (L.D.R.) in the invention steels and comparative steels 
shown in Table 2 are shown in FIG. 3, from which the difference between 
the invention steel and the comparative steel becomes obvious. 
In the hot rolled steel sheet according to the first embodiment, the bad 
influence of solid solute C is disappears and the ferrite crystal grains 
ca be made into an optimum regulated grain structure by deleting Mn from 
the chemical composition range of the conventional flexible hot rolled 
steel sheet and adding a proper amount of Cr thereto, so that the 
resulting steel sheet is flexible and excellent in the elongation property 
as compared with the conventional steel sheet. Therefore, such a hot 
rolled steel sheet is considerably suitable for deep drawing. 
In the hot rolled steel sheet according to the second embodiment, at least 
one of Ti, Nb and Zr is further contained in addition to the chemical 
components of the first invention, so that the deep drawability is further 
improved. 
In the hot rolled steel sheet according to the third embodiment, B is 
contained in addition to the chemical components of the first invention, 
so that the deep drawability is further improved. 
In the hot rolled steel sheet according to the fourth embodiment, at least 
one of Ti, Nb and Zr and B are included in addition to the chemical 
components of the first invention, so that the deep drawability is more 
improved.