Process for producing deep-drawing, non-ageing, cold rolled steel strips having excellent paint bake-hardenability by continuous annealing

Al-killed steels containing 0.001-0.01% C., not larger than 1.5% Mn, 0.005-0.20% Al, not larger than 0.007% N and B in amounts determined by the ratio of B/N ranging from 0.5 to 2.5, and optionally containing not less than 1% Si and 0.04 to 0.12% P are subjected to ordinary hot and cold rolling operations, then soaked in a temperature range of from 730.degree. C. to A.sub.3 point by a continuous annealing process, and rapidly cooled from a temperature between the soaking temperature to 450.degree. C. down to a temperature not higher than 250.degree. C. at an average cooling rate not less than 60.degree. C./second, without subsequent overageing treatments, to obtain deep-drawing, non-ageing cold rolled steel sheets and strips having excellent press formability and paint bake-hardenability.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to processes for producing deep-drawing, 
non-ageing cold rolled steel sheets having excellent press formability and 
paint bake-hardenability. 
2. Description of the Prior Art 
Press-forming cold rolled steel sheets and strips (hereinafter called 
"strips") used in automobile cars are required to have excellent 
deep-drawability, stretchability, shape quality and non-ageing property, 
and these requirements are particularly important for use in the outer 
skin applications, such as doors, roofs and quater pannels. 
Moreover in recent years, for the purpose of obtaining a high 
dent-resistance of pannels due to the car vibration, increasing demands 
have been made on the strips for an additional property, called "paint 
bake-hardenability" that the yield point of the steel strips can rise 
remarkably during the heat treatment for paint baking on the steel strips 
in the automobile car production. 
Cold rolled steel strips having such paint bake-hardenability are known, as 
disclosed in Japanese Patent Application Laid-Open No. Sho 54-107419, 
according to which Al-killed steels are subjected to hot and cold 
rollings, then subjected to an open coil annealing wherein the strips are 
soaked at a temperature ranging from A.sub.1 point to A.sub.3 point, and 
cooled at a cooling rate of 30.degree. to 200.degree. C./hour, or 
Al-killed steels having a lowered carbon content of about 0.01% are 
subjected to a tight-coil box-type annealing so as to increase the solid 
solution carbon. However, the paint bake-hardening degree obtained by this 
prior art is still far below 5 kg/mm.sup.2 which is an ordinary standard 
for the purpose. Moreover, the annealing in the prior art is done by the 
box-type annealing process which comprises slow cooling, long-time 
soaking, and slow cooling, so that a considerably long time is required, 
thus causing problems with respect to the productivity. 
Meanwhile, several proposals have been made as disclosed in Japanese Patent 
Publications No. Sho 47-33409 and No. Sho 49-1969 for production of cold 
rolled steel strips having an excellent press formability, such as 
deep-drawing and stretchability, and their production has limitedly been 
made on a commercial scale. 
However, according to these prior arts, it is essential to heat and soak 
the steel in a continuous annealing furnace, then rapidly cool the steel 
to about 400.degree. C., for example, and overage the steel near this 
temperature, or to cool the steel to the room temperature, then reheat the 
steel to about 400.degree. C. and overage the steel near this temperature. 
These conventional cold rolled steel strips obtained by continuous 
annealing have a problem that the yield point elongation appears so far as 
they are in "as non-skinpassed state", namely they are ageing, even if 
they have been overaged, or even if they have lowered C and N contents and 
contain additional elements, such as Al and B. 
Therefore, these prior arts cannot satisfactorily produce a deep-drawing, 
non-ageing, cold rolled steel strip having an excellent paint 
bake-hardenability as desired by the present invention. 
Also according to the conventional continuous annealing process, it is 
essential to perform the overageing treatment as mentioned before in order 
to reduce the solute C and N, so that the production cycle can be 
shortened only limitedly and the continuous annealing line must be 
considerably long. 
SUMMARY OF THE INVENTION 
Therefore, one of the objects of the present invention is to provide a 
process for producing deep-drawing, non-ageing cold rolled steel strips 
having excellent press formability and paint bake-hardenability, and the 
present inventors have been extensive studies for this object, 
particularly with respect to the steel composition and the continuous 
annealing cycle and have found that the above object can be achieved by 
soaking B-containing Al-killed steels with a lowered carbon content 
ranging from 0.001 to 0.010% in a temperature range of from 730.degree. C. 
to Ar.sub.3 point in a continuous annealing process and then rapidly 
cooling the steels from a temperature between the soaking temperature and 
450.degree. C. 
The process according to the present invention comprises hot and cold 
rolling a steel containing 0.001 to 0.01% C, not larger than 1.5% Mn, 
0.005 to 0.20% Al, not larger than 0.007% N and B in an amount determined 
by the ratio of B/N ranging from 0.5 to 2.5, and optionally containing not 
larger than 1.0% Si and 0.04 to 0.12% P in an ordinary way, then soaking 
the strip thus obtained in a temperature range of from 730.degree. C. to 
A.sub.3 point by continuous annealing, and rapidly cooling the strip from 
a temperature between the soaking temperature and 450.degree. C. to a 
temperature not higher than 250.degree. C. at an average cooling rate not 
less than 60.degree. C./second without a subsequent overageing treatment. 
The present invention has been completed after various extensive tries and 
studies for the purpose of meeting with apparently contradictory demands 
to provide a very small degree of ageing property and at the same time 
excellent paint bake-hardening property. 
The steel strips obtained by the present invention can restrict the 
occurrence of yield point elongation in the as-annealed condition prior to 
skinpass rolling and are less ageing but have an excellent paint 
bake-hardening property, and further can maintain these excellent 
qualities even after they are subjected to skin-pass rolling or levelling 
for shape correction and surface roughness adjustment. 
The non-ageing quality desired and obtainable by the present invention 
means such that the occurrence of the yield point elongation (YPEL) of the 
strip after artificial ageing at 100.degree. C. for 30 minutes is not more 
than 0.3%. 
No theoretical clarification has not yet been made why the excellent 
properties of the strips according to the present invention can be 
obtained, but most probably they are related to the grain boundary 
strength and the behaviour of the solid solution carbon. 
Generally speaking, the production of cold rolled steel strips by 
continuous annealing requires a cycle comprising short-time heat 
treatments, namely a rapid heating, a short-time heat treatment and a 
rapid cooling, so that the carbon in the steel remains in an 
over-saturated state. Therefore, it is a common practice to perform an 
overageing treatment in order to provide non-ageing quality or to soften 
the steel. 
In this case, it has been proposed that the steel is exceedingly rapidly 
cooled directly from the soaking temperature or from a relatively high 
temperature zone during the slow cooling so as to intentionally increase 
the over-saturated solid solution carbon, and then precipitation of the 
carbon is promoted by a subsequent overageing treatment. The present 
invention is based on a technical thought completely different from the 
prior art and does not require the overageing treatment. Contrary to the 
prior art, the overageing treatment is rather harmful in the present 
invention because it tends to increase the yield point elongation in the 
as-annealed condition as mentioned hereinbefore and increase the ageing 
degree, thus failing to achieve the objects of the present invention. 
Meanwhile, dual-phase cold rolled steel strips are known as a steel strip 
similar to the steel strip according to the present invention, which are 
produced by continuous annealing without an overageing treatment and are 
restricted in the occurrence of yield point elongation in the as-annealed 
conditions prior to skinpass rolling and show less ageing and an excellent 
paint bake-hardening property. However, these dual-phase cold rolled steel 
strips have the mixed structures of ferrite and martensite which is 
transformed during rapid cooling from the .alpha.-.gamma. temperature 
region, while the steel structure produced by the present invention 
consists of ferrite as cooled rapidly from mainly the .alpha. single phase 
condition. 
Therefore, the steep strips according to the present invention are 
completely different from the dual-phase steel strips with respect to the 
metallography as well as the steel composition and the resultant strength 
level. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention will be described in more details hereinbelow. 
The essential features of the present invention and various limitations 
made in the present invention will be explained. 
Regarding the chemical composition of the steel strips according to the 
present invention, carbon is one of the most important elements and must 
be limited to the range of from 0.001 to 0.01% in order to restrict the 
occurrence of yield point elongation in the as-annealed condition when the 
steel is rapidly cooled from a temperature between the soaking temperature 
and 450.degree. C. and to provide less ageing and excellent paint 
bake-hardening property. 
When the carbon content is less than 0.001%, no enough paint bake-hardening 
can be obtained, but when it exceeds 0.010% a significant yield point 
elongation develops under the as-annealed condition, and the ageing 
property increases and the elongation greatly deteriorates. A preferable 
carbon range is from 0.002 to 0.006%. 
Manganese is essential for preventing the hot embrittlement of the steel, 
but excessive manganese contents will produce excessive hardness of the 
steel. Therefore, in the present invention, the upper limit of the 
manganese content is 1.5%, and the manganese may be contained in various 
amounts within the defined range depending on the desired strength of the 
products. For example, when low strength deep-drawing cold rolled steel 
strips are desired the manganese content is maintained at about 0.6% or 
less, and for special applications it may be maintained less than about 
0.3%. Naturally larger manganese contents are maintained for obtaining 
high strength steel sheets. 
Aluminum must be contained in amounts not less than 0.005% as soluble 
aluminum for desired deoxidation of the steel, but aluminum contents of 
0.2% or larger will very often cause surface defects. Therefore, the 
aluminum content should be desirably maintained not more than 0.06%. 
Nitrogen, when contained in excessive amounts, is harmful to the object of 
the present invention to restrict the occurrence of yield point elongation 
in the as-annealed condition and assure less ageing. In the present 
invention, the nitrogen contents within the defined range are combined 
with boron to form BN, thus rendering the nitrogen content harmless. 
However, excessive nitrogen contents will necessitate considerable 
wasteful consumption of ferro-boron alloy. Therefore, the upper limit of 
the nitrogen content in the present invention is 0.007%, and preferably 
0.004%. 
Boron is one of the important features of the present invention, and in 
order to eliminate the harm of the nitrogen content, the boron content 
must be in amounts equivalent to the B/N ratio (weight %) of 0.5 or 
larger. On the other hand, if the B/N ratio exceeds 2.5, boron in solid 
solution will harden the steel. A preferable range of the B/N ratio is 
from 0.7 to 1.0. 
Within the scope of the present invention, silicon and phosphorus are 
additionally contained when a higher strength level of the products is 
required. 
Silicon is effective for strengthening the steel, but excessive silicon 
contents will tend to cause deterioration of the corrosion resistance of 
the steel after paint coating. Therefore, the upper limit of the silicon 
content in the present invention is 1.0%. 
In this connection, it should be noted that in conventionally known 
Al-killed steels, when Si and Mn are contained and extra rapid cooling is 
performed, remarkable temper colors develop so that Si and Mn are limited 
to very small contents, while in the present invention, Si and Mn contents 
can be increased without danger of temper color development due to a 
secondary effect of the limitation of the carbon content to 0.01% or less. 
This is a significant advantage of the present invention. 
Phosphorus is most effective to strengthen the steel and at least 0.004% 
phorphorus is required for this purpose. 
Excessive phosphorus contents will deteriorate weldability of the steel and 
the upper limit should be placed at 0.12%. It is worthy to note that 
satisfactory non-embrittled fracture during press stamping which is the 
most important concern when phosphorus is contained in extra-low carbon 
Al-killed steels can be maintained. 
With the above steel composition in combination with the effects of the 
continuous annealing process details of which will be described 
hereinbelow, non-ageing steel strips having excellent press formability 
with respect to deep-drawability and stretchability in particular and 
excellent paint bake-hardening property can be produced. 
Now according to the present invention, no special limitations are imposed 
on the hot and cold rolling operations. However, in the hot rolling 
operation, it is desirable to maintain the finishing temperature not lower 
than Ar.sub.3 point and the cooling temperature not higher than 
650.degree. C. for the desired deep-drawability. Meanwhile, in the cold 
rolling operation, a rolling reduction rate not less than 75% is 
desirable. 
In the present invention, the continuous annealing conditions after the 
cold rolling step are most important. 
The reasons for soaking the steel in the temperature range of from 
730.degree. C. to A.sub.3 point in the continuous annealing process are 
that when the soaking temperature is too low, only incomplete grain growth 
can be produced, which is considered to be hinderous to the restriction of 
occurrence of yield point elongation in the as-annealed condition and the 
less ageing property, and the deep-drawability is deteriorated by the too 
low soaking temperature. On the other hand, when the soaking temperature 
exceeds Ar.sub.3 point, the deep-drawability is again extremely damaged. A 
preferable soaking temperature range is from 750.degree. C. to 850.degree. 
C. Regarding the soaking time, about 10 to 180 seconds is most 
practicable, but may be longer or shorter as cases require. 
After the soaking, the steel is rapidly cooled from any desired temperature 
within the range of from the soaking temperature to 450.degree. C. to a 
temperature not higher than 250.degree. C. at an average cooling rate not 
less than about 60.degree. C./second. This soaking condition, as well as 
the carbon content limitation, is one of the most important features of 
the present invention, and if this condition is not satisfied, it is 
impossible to restrict the occurrence of yield point elongation in the 
as-annealed condition prior skinpass rolling and provide the less-ageing 
property. 
Although theoretical clarification of the above phenomenon has not yet been 
made, it is considered to be related with the the fact that the 
precipitation of carbon into cementites etc. can be practically prevented 
by the rapid cooling as defined above. 
As described above, the rapid cooling is done directly from the soaking 
temperature or is started when the steel is slowly cooled to a temperature 
not lower than 450.degree. C. This slow cooling to 450.degree. C. may be 
practically performed at a cooling rate of about 10.degree. C./second. 
Meanwhile, the starting temperature for the rapid cooling should 
preferably be between 775.degree. C. and 600.degree. C. and the average 
cooling rate for the rapid cooling should preferably be not lower than 
200.degree. C./second. 
It is also essential in the present invention to avoid an overageing 
treatment after the rapid cooling, quite contrary to the conventional 
arts. Thus in the present invention, when an overageing treatment is done 
around 400.degree. C., the yield point elongation restores after the 
annealing and it is difficult to reduce the ageing property even if a 
temper rolling is performed. However, the conventional continuous 
annealing apparatus is generally annexed with an overageing furnace after 
the annealing furnace, so that if it is unavoidable to pass the strip 
through the overageing furnace, the passage must be made at a temperature 
not higher than 250.degree. C., for example. Further, in the present 
invention as the occurrence of yield point elongation in the as-annealed 
condition is restricted, it is generally unnecessary to perform the temper 
rolling, but it may be done for shapeness correction and surface roughness 
adjustment of the strip production. However, it is desirable to perform 
the temper rolling with a slight reduction so as to avoid lowering of the 
ductility. 
Further within the scope of the present invention, the steel strips may be 
coated by hot dipping during the cooling step of the continuous annealing 
but before the rapid cooling so as to obtain surface treated deep-drawing 
steel strips such as Zn coated and Al coated steel strips which are 
non-ageing and have an excellent paint bake-hardening property.

The present invention will be more clearly understood from the embodiments 
described hereinbelow. 
EXAMPLE 1 
Steels having chemical compositions shown in Table 1 are prepared by means 
of a converter and a vacuum degassing vessel, continuously cast into 
slabs, hot rolled into hot coils of 3.0 mm in thickness, with a finishing 
temperature at 910.degree. C. and a coiling temperature at 625.degree. C., 
then subjected to descaling and cold rolling into strips of 0.8 mm in 
thickness, and continuous annealing under the following conditions. The 
soaking is done at 830.degree. C., and the strips are held at the 
temperature for 60 seconds, then slowly cooled to 700.degree. C. at an 
average cooling rate of 10.degree. C./second, and rapidly cooled from this 
temperature to 200.degree. C. at an average cooling rate of 1000.degree. 
C./second with or without a subsequent skinpass rolling with reduction 
rates as shown in the Table 1. 
Steel Nos. 1-5 shown in Table 1 are produced according to the present 
invention are practically non-ageing and show a significantly high level 
of paint bake-hardening with excellent deep-drawability, while comparative 
steels Nos. 6 and 7 which are outside the scope of the present invention 
with respect to the carbon content show substantial occurrence of yield 
point elongation in the as-annealed condition prior to skinpass rolling, a 
high degree of ageing, and are considerably inferior to those obtained 
according to the present invention with respect to the elongation. 
Comparative steels Nos. 8 and 9 which are outside the scope of the present 
invention with respect to the B/N ratio show restricted occurrence of 
yield point elongation in the as-annealed condition, but show considerably 
large ageing as compared with the steels according to the present 
invention. 
EXAMPLE 2 
This example is intended to illustrate the criticalities of the continuous 
annealing conditions. 
Steels having the same chemical compositions as steel No. 1 and No. 2 in 
Table 1 are subjected to various soaking temperatures, starting 
temperatures for the rapid cooling, average cooling rates in the rapid 
cooling to 250.degree. C. and steel G only was subjected to overageing at 
400.degree. C. for 2 minutes. 
Steels A to D are within the scope of the present invention and practically 
non-ageing, and show a high level of paint bake-hardening with excellent 
deep-drawability. 
Steels E and F are outside the scope of the present invention with respect 
to the average cooling rate in the rapid cooling to 250.degree. C., and 
steel G is outside the scope of the present invention with respect to the 
overageing, and steels H and I are outside the scope of the present 
invention with respect to the starting temperature of the rapid cooling 
and steels J and K are outside the scope of the present invention with 
respect to the soaking temperature. All of these comparative steels show a 
considerable yield point elongation in the as-annealed condition prior to 
skinpass rolling, and a high degree of ageing, thus unsuitable for 
applications where the non-ageing property is required. 
EXAMPLE 3 
Steels having chemical compositions as shown in Table 3 are prepared by 
means of a converter and a vacuum degassing vessel, continuously cast into 
slabs, hot rolled into hot coils of 4.0 mm in thickness with a finishing 
temperature at 910.degree. C. and a coiling temperature at 600.degree. C., 
then subjected to descaling, cold rolling into strips of 0.8 mm in 
thickness, and continuous annealing under the following conditions. 
The strips are soaked at 800.degree. C. for 60 seconds, and then cooled to 
250.degree. C. under the conditions shown in Table 1. The cooling after 
the soaking to the starting temperature of the rapid cooling is done at a 
cooling rate of 10.degree. C./sec. The tensile test was performed in the 
as-annealed condition, and the ageing was evaluated at 100.degree. C. for 
30 minutes, but the test pieces which showed yield point elongation in the 
as-annealed condition were subjected to 0.8% temper rolling reduction and 
then artificial ageing. The paint bake-hardening was expressed by the 
increase in yield stress of 2% prestrained specimen after the heat 
treatment simulated to paint baking at 170.degree. C. for 20 minutes. 
The test results are shown in Table 3, from which it is clearly 
demonstrated that the test pieces No. 1, No. 2, No. 5, No. 8 and No. 9 
which are within the scope of the present invention show no yield point 
elongation in the as-annealed condition and are non-ageing with excellent 
paint bake-hardenability and deep-drawability as well as high strength. 
Meanwhile, the comparative test pieces No. 4 which is outside the scope of 
the present invention with respect to the starting temperature of the 
rapid cooling, and No. 6 and No. 7 which are outside the scope of the 
present invention with respect to the chemical composition show a 
considerable yield point elongation or a considerably high degree of 
ageing or further remarkable tendency of embrittlement during stamping, 
thus failing to suit for outer skin applications of automobile cars. 
As clearly understood from the foregoing descriptions, the present 
invention has significant industrial advantages because it can produce 
deep-drawing, high strength cold rolled steel strips having excellent 
paint bake-hardening property by continuous annealing with a very high 
production efficiency without overageing, and can well meet with the 
increasing demands of such steel strips. 
TABLE 1-1 
__________________________________________________________________________ 
Chemical Composition 
Steel 
Production 
C Si Mn P S Sol. Al 
N B 
No. 
Process 
(%) 
(%) 
(%) 
(%) 
(%) 
(%) (%) (%) B/N 
__________________________________________________________________________ 
1 Present 
0.003 
0.03 
0.22 
0.012 
0.013 
0.028 
0.0020 
0.0032 
1.60 
Invention 
2 Present 
" " " " " " " " " 
Invention 
3 Present 
0.005 
0.05 
0.34 
0.007 
0.008 
0.054 
0.0046 
0.0035 
0.76 
Invention 
4 Present 
" " " " " " " " " 
Invention 
5 Present 
0.002 
0.02 
0.11 
0.007 
0.005 
0.012 
0.0015 
0.0020 
1.3 
Invention 
6 Comparative 
0.012 
0.01 
0.26 
0.010 
0.012 
0.073 
0.0021 
0.0019 
0.90 
7 Comparative 
" " " " " " " " " 
8 Comparative 
0.004 
0.03 
0.11 
0.015 
0.011 
0.069 
0.0020 
-- 0 
9 Comparative 
" " " " " " " -- " 
__________________________________________________________________________ 
TABLE 1-2 
__________________________________________________________________________ 
Yield 
Skinpass 
Mechanical Properties Increase 
Point 
Rolling Yield in Yield 
Elonga- 
Paint 
and Yield Tensile 
Elon- 
Point Stress due 
tion after 
bake- 
Steel 
Production 
Reduction 
Stress 
Strength 
gation 
Elonga- 
-r to Ageing 
Ageing 
hardening 
No. 
Process 
Rate (kg/mm.sup.2) 
(kg/mm.sup.2) 
(%) tion (%) 
Value 
(kg/mm.sup.2) 
(%) (kg/mm.sup.2) 
__________________________________________________________________________ 
1 Present 
None 18.0 29.5 5.06 
0 1.8 0 0.1 5.8 
Invention 
2 Present 
0.8% 16.3 30.1 49.8 
0 1.8 0 0 6.0 
Invention 
3 Present 
None 18.4 30.4 49.7 
0.1 1.7 0.2 0.1 6.0 
Invention 
4 Present 
0.4% 17.1 30.7 49.2 
0 1.7 0.1 0 6.5 
Invention 
5 Present 
None 15.1 28.8 52.0 
0 2.0 0 0 5.0 
Invention 
6 Comparative 
None 21.1 33.1 43.2 
2.3 1.4 1.7 2.8 5.6 
7 Comparative 
0.8% 19.4 35.0 39.8 
0 1.4 4.1 1.2 5.8 
8 Comparative 
None 20.8 31.3 48.2 
0.1 1.6 0.8 0.6 6.2 
9 Comparative 
0.8% 19.4 32.2 47.1 
0 1.6 1.2 0.4 5.9 
__________________________________________________________________________ 
Note: 
Ageing Condition: Artificial Ageing at 100.degree. C. for 30 minutes. The 
paint bakehardening is expressed by the increase in yield stress by a hea 
treatment simulated to the paint baking at 170.degree. C. for 20 minutes 
after 2% prestrain. 
TABLE 2-1 
__________________________________________________________________________ 
Continuous Annealing Condition 
Average Skinpass 
Starting 
Cooling Rate 
Rolling 
Temp. of 
from Start of 
and 
Production 
Soaking Rapid 
Rapid Cooling 
Reduction 
Steel 
Process 
Temp. .times. seconds 
Cooling 
to 250.degree. C. 
Rate 
__________________________________________________________________________ 
A Present 
830.degree. C. .times. 60 sec 
700.degree. C. 
1000.degree. C./sec 
-- 
Invention 
B Present 
" " " 0.8% 
Invention 
C Present 
" " 300.degree. C./sec 
-- 
Invention 
D Present 
" " " 0.8% 
Invention 
E Comparative 
" " 30.degree. C./sec 
-- 
F Comparative 
" " " 0.8% 
G Comparative 
" " 1000.degree. C./sec 
-- 
H Comparative 
" 400.degree. C. 
" -- 
I Comparative 
" " " 0.8% 
J Comparative 
700.degree. C. .times. 60 sec 
700.degree. C. 
" -- 
K Comparative 
" " " 0.8% 
__________________________________________________________________________ 
TABLE 2-2 
__________________________________________________________________________ 
Yield 
Mechanical Properties Increase 
Point 
Yield in Yield 
Elonga- 
Paint 
Yield Tensile 
Elon- 
Point Stress due 
tion after 
bake- 
Production 
Stress 
Strength 
gation 
Elonga- 
-r to Ageing 
Ageing 
hardening 
Steel 
Process 
(kg/mm.sup.2) 
(kg/mm.sup.2) 
(%) tion (%) 
Value 
(kg/mm.sup.2) 
(%) (kg/mm.sup.2) 
__________________________________________________________________________ 
A Present 
18.0 29.5 50.6 
0 1.8 0 0.1 5.8 
Invention 
B Present 
16.3 30.1 49.8 
0 1.8 0 0 6.0 
Invention 
C Present 
18.3 29.6 50.8 
0 1.8 0.3 0.2 6.0 
Invention 
D Present 
17.0 30.1 50.2 
0 1.8 0 0.1 5.9 
Invention 
E Comparative 
20.8 29.4 50.9 
4.3 1.8 0.2 4.4 5.7 
F Comparative 
17.9 30.0 48.2 
0 1.8 2.0 1.2 6.1 
G Comparative 
23.3 29.2 49.0 
5.2 1.8 0.6 5.5 4.8 
H Comparative 
20.6 29.4 50.7 
3.9 1.8 0.3 4.0 5.6 
I Comparative 
18.0 30.1 48.6 
0 1.8 1.6 1.1 5.8 
J Comparative 
21.3 31.9 46.2 
1.8 1.4 1.6 2.5 5.5 
K Comparative 
20.2 33.0 44.6 
0 1.4 0.7 1.2 5.7 
__________________________________________________________________________ 
Steel G was reheated and overaged at 400.degree. C. for 2 minutes after 
rapid cooling. 
TABLE 3-1 
__________________________________________________________________________ 
Steel 
Chemical Composition (wt %) 
No. C Si Mn P S Sol. Al 
N B B/N 
__________________________________________________________________________ 
1.circle. 
0.003 
0.03 
0.23 
0.065 
0.010 
0.030 
0.0034 
0.0028 
0.8 
2.circle. 
" " " " " " " " " 
3 " " " " " " " " " 
4 " " " " " " " " " 
5.circle. 
0.005 
0.32 
0.89 
0.085 
0.012 
0.036 
0.0024 
0.0035 
1.4 
6 0.004 
0.02 
0.22 
0.068 
0.011 
0.043 
0.0032 
-- -- 
7 0.016 
0.02 
0.50 
0.058 
0.010 
0.052 
0.0040 
0.0035 
0.9 
8.circle. 
0.002 
0.02 
1.2 
0.041 
0.005 
0.012 
0.0018 
0.0020 
1.1 
9.circle. 
0.004 
0.81 
0.14 
0.046 
0.007 
0.009 
0.0046 
0.0040 
0.9 
__________________________________________________________________________ 
.circle. Present Invention 
TABLE 3-2 
__________________________________________________________________________ 
Cooling Condition Yield 
Starting Average Cooling 
Mechanical Properties Point 
Temp. of 
Rate from Start Yield Elongation 
Paint 
Rapid 
of Rapid Cooling 
Yield Tensile 
Elon- 
Point after bake- 
Steel 
Cooling 
to 250.degree. C. 
Stress 
Strength 
gation 
Elonga- 
-r Ageing 
hardening 
No. (.degree.C.) 
(.degree.C./sec) 
(kg/mm.sup.2) 
(kg/mm.sup.2) 
(%) tion (%) 
Value 
(%) (kg/mm.sup.2) 
__________________________________________________________________________ 
1.circle. 
700 1000 23.4 36.4 44 0 1.8 0 5.5 
2.circle. 
" 300 23.8 36.1 45 0 1.8 0.3 5.8 
3 " 20 25.7 36.0 45 4.2 1.8 *1.2 5.0 
4 400 1000 25.5 36.0 45 3.6 1.8 *1.0 5.1 
5.circle. 
700 " 27.0 41.7 39 0 1.7 0 6.2 
6 " " 26.1 37.6 43 0.2 1.5 *0.8 5.9 
7 " " 31.2 43.8 32 2.8 1.4 *1.2 5.4 
8.circle. 
" " 25.8 39.0 44 0 1.7 0 5.0 
9.circle. 
" " 30.2 43.0 38 0 1.7 0 5.1 
__________________________________________________________________________ 
Note: 
.circle. Present Invention 
(1) Ageing Condition: 100.degree. C. for 30 minutes. 
*Artificial ageing is done after 0.8% skinpass rolling following the 
annealing. 
(2) The paint bakehardening is expressed by the increase in yield stress 
by a heat treatment simulated to the paint baking at 170.degree. C. for 2 
minutes after 2% prestrain.