CA1216499A - Method of producing cold rolled steel sheets for deep drawing - Google Patents

Abstract

Abstract of the Disclosure The present invention relates to a method of producing a cold rolled steel sheet for deep drawing.
In order to improve the deep drawability (r-value), ductility (yield strength and elongation E?) and the like of ultra-low carbon aluminum killed steel, the chemical composition (% by weight) of the steel is adjusted such that the steel contains C : ?0. 015%, Mn : ?0.4%, P : ?0.03%, sol. A? : 0.005-0.100%, N : ?0.010%, Ti (exclusive of Ti present in the form of oxide) : in an amount satisfying a formula of the hot rolling of the steel slab is effected at a soaking temperature lower than 1,100°C and at a finishing temperature of 600-780°C.
As illustrated in the drawings, a low temperature rolling can be carried out, and the resulting cold rolled steel sheet is excellent in the deep drawability and ductility and further excellent in the treatable property of the surface and in the surface property.
Therefore, the cold rolled steel sheet is particularly suitable for the production of automotive exterior plate.

Description

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The present -invention re~Lates to a method of producing cold rolled steeL sheets used for a-utomotive exterior plate and the like and adapted for deep drawing.
Co:Ld rolled steel sheets used for the produc-tion of shaped articles through deep drawing, such as cold rolled steel sheets for deep drawing and the like, are required to be low in the yield strength (YS) and high in the elongation (EQ), that is, to be excellent in the ductility, and further to have a high Lankford value (r-value) as important mechanical properties.
Moreover, shaped articles produced through deep drawing are often used in the outer surface of mechanical products, such as automotive exterior plate and the like, and therefore it is an impor-tant property for the cold rolled steel sheet to have an excellent surface property.
Cold rolled steel sheets for deep drawing have hi-ther-to been produced from a low-carbon aluminum killed steel through a box annealing. However, recently a continwous annealing method is widely used for the production of the cold rolled steel sheets for deep drawing from the low-carbon aluminum killed steel in view of the improvement of produc-tivity and the energy saving. However, the use of commonly used low-carbon aluminum killed steel as a starting material can not give satisfactorily excellent mechanical properties to the resulting cold rolled steel sheets for deep drawing.
Accordingly, it was proposed to use ultra-low carbon

- 2 -steel having a C content o~` as low as 0.020% or less as a starting material for the production of cold rolled steel sheet for deep drawing. However, it was difficult to secu-re, in the conventional method, satisfactorily high r-value and ductility enough to bear the deep ~rawing even in the use of such ultra-low carbon s-teel.

Under these circumferences, there have been proposed vario-us methods, wherein carbide- and nitride-forming elements of Nb, Ti, ~r and the like are added to ultra-low carbon steel. Among the prior arts disclosing these methods, Japanese Patent Application Publication No. 18,066/69 and Japanese Patent Laid-open Specification No. 137,021/78 disclose cold rolled steel sheets containing Ti and having deep drawability, and methods of prod-ucing the steel sheets.
However, in -these methods, it is necessary -to carry o-ut a hot rolling at a high finishing temperature, and a high temperature heating of slab and a hi~h temperature hot rolling rrlust 'be carried C)Ut:. However, the high temperature heating of slab has such draw'backs that cost for heating energy is high, yield is low due to the oxidation of slab surface, quali~y of the result-ing cold rolled s-teel sheet is poor d-ue to t'he increase of interna:Lly oxidized procluct, and trou'bles occur during the cold rolling. While, the high temperature hot rolling is apt to cause breakage and other troubles of roll and to deteriorate the quality of the s-urface of the res-ulting cold rolled steel sheet.

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Further, .Japanese Patent Lai.d-open Specifica-tion No. 1.3,123/82 discloses a method of p-roducing a cold rolled steel sheet for deep drawing from a steel containing C : 0.002-0.05% and Ti : 0.070-0.210% through a low-temperature hot rolling. However, in this method, a large amount of Ti is used, and therefore the resulting cold rolled steel sheet is very e~pensive, and further the cold rolled steel sheet is poor in the surface property and in the treatable property of surface due to the increase of Ti series inclusions.
The object of the present invention is to provide a method o:E producing cold rolled steel sheets adapted for deep drawing and having excellent ductility and surface property from a Ti-containing steel through a low-temperature hot rolling.
The feature of the present invention lies in a method of producing cold rolled steel. sheets fo-r deep drawing, comprising soaking at a temperat-ure lower than 1,100C a steel slab having a composition consisting of, in % by we.ight, not more than 0. 015% of C, not more than 0.40% of Mn~ not more than 0.03% of P, 0.005-0.100%
of sol. AQ, not more than 0.010% N, Ti in an amount within the range satisfying the following formula -O.020% < Ti - (~S -t ~TN) < O.004%

provided that the Ti in the formula does no-t include Ti present in the slab in the form of oxide, and the remainder being Fe and inciden-tal impuri.ties; finishing ~2~

a hot rolling of the soaked slab at a temperat-lre of 600-780C; cold rolling the hot rollecl sheet, and annealing the cold rolled sheet.
For a better understanding of the invention, reference is taken to -the accompanying drawings, wherein:
Fig. 1 is a graph illustra-ting the influences of -the M-value and soaking temperature of a steel slab upon the properties of the resulting cold rolled steel shee-t; and Fig. 2 is a graph il:Lustrating the influence of the finishing temperature of hot rolling of a steel slab upon the properties of -the resulting cold rolled s-teel sheet.
The inventors have made vario~ls basic experi-ments and ascertained that, when an ultra-low carbon steel having an M-value within a specifically limited range, which M-value is def'ined by -the for~lula M = Ti ~ S -~ T~N~

with respect to the amount of Ti present in the steel in the form other than oxide, is soaked at a temperature lower than the ordinary soaking temperature, a cold rolled steel sheet having excellent deep drawability can be obtained. This fact will be explained hereinafter.
In a basic e~periment, molten steels of ultra-low carbon steels containing carbon in two different levels and having a widely ranging M-value (M=Ti-(38S+41-N)) as shown in the following 'I'a'ble 1 were ~2~

produced through a converter and an RH degassing apparatus. In the steels substantially all of the oxide was aluminum series oxide, and therefore total amoun~ of Ti was used as the Ti content of the steels in the calculation of the M-value.

Table 1 Chemical composition of ~teels used in the basic e~periment (% by weight) _ C Si Mn P S N 0 AQ Ti M
_ __ _ 0.003 0.0013 0.002 0.03 0.008 -0.034 A ~ ~ ~ ~, ~ ~ ~ ~ ~
_ 0.004 0.0l 0.15 0.01 0.020 0.0086 0.004 0.05 0.065 0.~30 0.004 0.0015 0.002 ~.02 0.012 -0.038 B ~ ~ ~ ~ ~ ~ ~ ~ ~
_ 0.009 0.01 0 15 0.01 0 021 0.0072 0.005 0 06 0 055 L__ Each of molten steels shown in I'able 1 was cast into a steel slab by means oE a continwous casting apparatus, and the slab was cooled to about room temper-ature. The slab was soaked at 1,260C, which is a commonly used soaking temperature, or at l,080C or 940C, which is lower tha-n the commonly used soaking temperature, and then subjected to hot rolling. The hot rolling was carried o-ut by means of a hot strip mill comprising 4 stands of roughing mills and 7 stands of finishing mills to produce a hot rolled s-teel sheet having a thickness of 3.2 mm. The finishing tempera-ture 6~

in the hot rolling was about 730C, ancl the coil.ing temperature was a'bout 580C in all steel samples.
To hot rolled steel sheet was picklecl, cold rolled (final gauge : 0.7 mm), and then subjected to a cont:inuous annealing at a constan-t temperature of 810C
for 30 seconds and to temper-rolling at a recluction rate of 0.5%. The elongation EQ and r-value of the above treated cold rolled steel sheet were platted in Fig. 1.
It can be seen from Table 1 that (1) the properties of the resulting cold rolled steel shee-t can be determined 'by -the M-value lC
independen-tly of C content, and when the M-value is within -the range of from 0.020% to less -t'han 0.00~%, a cold rolled s-teel sheet having excellent deep drawability can be obtained; and (2) When the soaking temperatwre of slab is hi~h, the properties of col.d rolled s-teel sheet are very poor independently of the :M-val-le.
The deep drawa'bi.lity of a Ti-containing ul.tra-low carbon steel has hitherto been determine~ 'by the ratio of the Ti con-tent to the C content. The reason has hi.therto been metallographically explained as follows.
C is bonded with Ti to form a carbide TiC and -to decrease the amoun-t of free state C or solute C, ~-hereby a (lll) recrystallization texture, which acts favorably on the improvement of deep drawability~ is developed in a large amount duri.ng the recrystal]iza-tion anneali.ng.
However, the inventors have found o-ut a novel fact as described above that, w'hen a Ti-containing wltra-low carbon steel slab :is soaked at low temperat-ure a-nd then hot rolled, the deep drawabili-ty of the resulting cold roll.ed steel sheet is not determined by the ra-tio of the Ti con-tent to the C content, but is determined by the ratio of the Ti content to the (S+N) content.
Based on the above described basic experiments, -the inventors have repeated experiments by changing hot rolling condition and other conditions with respect to steels having a chemical composition different from that shown in Table 1, and ascertained -that a cold rolled steel sheet having excellent cold drawability can be obtai.ned by limi-ting the chemical composition of the steel and -the production condi-tion of the cold rolled steel sheet.
The present invention is based on the above described cliscovery, ancl provides a methocl of producing a cold rolled steel sheet for deep drawing, comprising soaking at a tempe-rature lower than ]~100C a steel slab havi.ng a composition consisting of, in % 'by weight, not more than 0.015% of C, not more than 0.40% of Mn, not more than 0.03% of P, 0.005-0.100% of sol. AQ, not mo:re than 0.010% of N, Ti in an amount within -the range satisfying the ~ollowing formula -0.020% < Ti (3~8S + ~N) < O.004%

provided that the Ti in the formula does not include Ti present in the sla'b in -the form of oxide, and the 6~
remainder being Fe ancl incidenta'L imT)urities; finishing a hot rolling of the soaked sla'b at a temperatwre of 600-700C; cold rolling -the hot rollecl sheet~ and annealing the cold rolled sheet.
An explanation will be made wi-th respect to the reason for the limitation of the chemical composition of the steel to be used in the presen-~ invention.
When the C content is high, the resulting cold rolled steel sheet is high in the yield strength and is poor in the elongation EQ, and fur-ther is un-satisfactory in the r-value. Accordingly, -the C content is limited to -not higher than 0.015%.
P acts to embrittle a cold rol]ed steel sheet, particularly causes troubles, such as crack in the secondary working and the like, after deep drawing.
Therefore, the P content is limited to not higher than 0.03%-AQ is effec-tive for decreasing the oxygen con-tent in a steel, and must 'be addecl to a steel sheet in an amount of at least 0.005% in an acicl-soluble form. However, when the amount of sol. AQ exceeds 0.100%, the surface property of the resulting cold rolled steel sheet is poor. Therefore, the conten-t of sol. AQ is limited to not higher than 0.100%.
When the N content is higher than 0.010%~
satisfactorily high ductility and ageing resistance can not 'be obtained in the resulting cold rolled steel sheet. Therefore, the N conten-t is limited to not ~L2~6~
higher than 0.010%.
Ti is an important element in -the present invention. As already e~plained in the above described basic experiments, it is necessary -to ad~ Ti to a star-ting steel such that the M-value (=Ti-(~S+I~N)) is within the range of from -0.020% to less than C).00~%, preferably from 0.015% -to less than 0.00~%. However, Ti may be bonded with oxygen depending upon the produc-tion condition of the starting steel. Accordingly, in the definition formula for the M-value, the amount of Ti present in -the steel in the form of oxide is excluded.
Then, an explanation will be made with respect to the production steps of a cold rolled steel shee-t according to the present invention. The steel making method is not particularly limited. However, in order to decrease the C content to not higher than 0.015%, a combination system of a converter and a degassing apparatus is effective. A steel sla'b can be produced by an optional method. ~[owever, the continuous casting method or an ingot making-sla'b'bi.ng method is advan-tageously used.
In the present invention, a step for producinga hot rolled steel strip from a steel sla'b is particu-larly important. ~hen a steel slab after cooled to about room temperature is soaked or a steel slab still having a high temperat-ure is directly soaked~ a low temperature soaking is necessary, wherein the steel slab is soaked at an average temperature of less than 6~9 1,100C, preferably less than l,000C, as clear~ly understovd from the basic experiments illustrated in Fig~ 1.
Then, in the hot rol.ling, the ho-t rolling :finishing temperature must be within the range of 600-7~0C, and is preferably within the range :Erom 600C to less than 700C, in order to obtain excellent deep drawability as illustrated in Fig. 2.
The coiling -temperature after hot rolling is not particularly limited. However, in order to improve the pickling efficiency, the coiling temperature is preferably not higher than 600C. The cold rolling step is not particularly limited as well. However, in order to obtain high r-value and to obtain low planer anisotropicity, the cold rolling reduction rate is preferably 50-95%.
The final annealing can be carried out by either a box annealing by means of a bell furnace or a continuous anneali.ng through a rapicl heating-short time heating cycle. However, the continuous annealing is superior to the box annealing in view of the productivity.

The annealing temperature is preferably within the range of 650-900C. As to the heat cycle in the continuous annealing, the cooling rate after constant-temperature heating or the addition of overageing treatment and the overageing condition have no-t an ~5 essential infl-uence upon the properties of the resulti.ng cold rolled steel sheet. However, a grad-ual cooling at ~6~
a rate of 10C/sec or less, or an overageing treatrnent at a temperature of a'bout 3S0C is effective for t'he improvement of the properties, particularly the ductility, of the prodwct.
After completion of the annealing, the cold rolled steel sheet may be subjected to a temper-rolling at a reduction rate of not higher than 1.5% in order to correct its shape and for other purposes.
The following example is given for the purpose of illustration of this invention and is not intended as a limitation thereof.
Example Molten steels having a chemical composition shown in the following Table 2 were produced. In -the steel shown in Table 2, Steels 1-~ are those of the present inven-tion, and Steels 5-7 are conventional steels. Each of the molten s-teels was rnacle into a sla'b through a converter-degassing-continuous casting method.
The slab was cooled to room temperatwre ancl-then heated to a soaking temperature shown in Table 2 in a heating furnace. However, only in Steel 2, the cooling of the 2~
slab was stopped at about 500C, and the slab was changed into the heating -furnace and heated to a soaking temperature shown in Table 2.
The soaked slab was hot rolled into a 'ho-t rolled shee-t of 3.2-3.8 mm thickness under a hot rolling condition shown in Table 2, pickled, and then cold rolled into a cold rolled sheet having a final ga-uge of 6~

0.7-0.8 m~ thickness. The cold rollecl s'heet of Steel 4 was subjected to a continuous annealing (constant temperature : 800C) and successively to a ho-t-dip zinc plating in a con-tinuous hot-clip zinc plating line.
Cold rolled sheets other than S-teel ~ were annealed at a constant temperature of 820C in a con-tinuous annealing line. After the annealing, Steels 3 and 5 were rapidly cooled at a rate of not less than ~0C/sec, and subjected to an overageing treatment at 350-~00C for 150 seconds.
~ll the above trea-ted steel sheets were subjected to a temper-rolling at a reduc-tion rate of 0.3-0.8%, and the surface properties and mechanical proper-ties of -the resulting products were tested. The obtained results are shown in the following Table 3.
It can be seen from Ta'ble 3 that the cold rolled steel sheet accordi-ng to the present invention is high in the ductility and r-value and has excellent deep drawa'bility. Part;c~llarly, the hot-dip zinc plated steel sheet (Steel ~l) is excellent in the throwing power and adhesion, and the s-lrface properties of the all resulting cold rolled steel sheets are excellent.

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this invention 3 18 48 1.8 4 - 13 ~8 - - 53 - 2.1 ...... _ . _ 3~ 43 1.4 Comparati.ve 6 18 30 45 1.4 steel 7 19 31 44 1.5 _. _ _ (Note~ Test specimen : JIS No. 5 Pulling direction :
Directions at inclination angles of 0, 45~ and 90 with respect to the rolling direc-tion. The property is a mean value of the values in these directions~

As described above, according to the present invention~ a cold rolled s-teel sheet having high r-value and deep drawa'bility and f'wrther having low yield strength and high elongation, t'hat is, having excellent ductility, can be produced. Therefore, the present invention can be applied to the production of a cold rolled steel sheet which will be formed into mechanical parts through deep drawing. Particularly, the res-ulting cold rold steel sheet has high throwing power in the plating, and is excellent in the adhesion and surface property. Therefore, the present inven-tion is suitable for the production of a cold rolled steel sheet to be - :L5 -used for the production of automotive exterior p~ate through deep drawing.

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined follows:-

1. A method of producing cold rolled steel sheets for deep drawing, comprising soaking at a temperature lower than 1,100°C a steel slab having a composition consisting of, in % by weight, not more than 0.015% of C, not more than 0.410% of Mn, not more than 0.03% of P, 0.005-0.100% of sol. A?, not more than 0.010% of N, Ti in an amount within the range satisfying the following formula provided that the Ti in the formula does not include Ti present in the slab in the form of oxide, and the remainder being Fe and incidental impurites; finishing a hot rolling of the soaked slab at a temperature of 600-780°C; cold rolling the hot rolled sheet; and annealing the cold rolled sheet.

2. A method according to claim 1, wherein the steel slab contains Ti in an amount within the range satisfying the following formula provided that the Ti in the formula does not include Ti present in the slab in the form of oxide.

3. A method according to claim 2, wherein the steel slab is soaked at a temperature lower than 1,000°C
and the hot rolling is finished at a temperature of not lower than 600°C and lower than 700°C.