CA1043592A - Drawability of deoxidized steels by the addition of phosphorus and silicon - Google Patents

Abstract

SPECIFICATION
DAVID C. LUDWIGSON
IMPROVING THE DRAWABILITY OF
DEOXIDIZED STEELS BY THE ADDITION
OF PHOSPHORUS AND SILICON

ABSTRACT OF THE DISCLOSURE
The high drawability of deoxidized steels with Mn contents below about 0.22% is enhanced by the addition of from about 0.015 to 0.05% P. The beneficial effect of such phosphorus additions on drawability is further enhanced by employing from about 0.02 to 0,05% Si.

Description

104;~592 ~1 _ ! This invention relates to an economical sheet steel Il product with exceptionally high deep drawability, as evidenced by j ~ value, and is more particularly related to a speciflc range o~
¦¦ steel compositions which will provide such deep drawability.
¦¦ It ls generally recognized that the perrormance Oe a sheet steel during forming o~erations known as deep drawing is I closely associated with the ratio, r, Or true width strain to ¦ true thickness strain when the steel ls strained in tension in the 15 ! length dlrection. Therefore, the suitability of a steel ~or deep drawing may be assessed by measuring r in the laboratory and the j~ greater di~rlculty o~ ~ull scale drawing trials can o~ten be averted. It is normal to measure r in the plane of the sheet in three directlons, parallel to the rolling dlrection (rO), diagonal ~0 1 to the rolling directlon (r45), and perpendicular to the rolling dlrectlon (r90). From these three oomponents, two summary jl ` 1 9~ ```

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.
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¦characteristic~ are usually derived:

r = (rO ~ 2r4s + r90)/4 (1) r = (rO ~ 2r45+ rgO)/2 (2) l High values o~ r are assdciated with a hlgh capabllity to undergo 1 deep drawing without fracture; and values o~ r near zero are !associated with a low tendency toward a detrimental directional ¦nonuniformity in deep drawn item~ known as earing.
¦ Isotroplc steels have been produced with r and ~ r values lnear 1.0 and 0.0, respectively Such steels have limited deep ¦¦ drawability but excellent resistance to earlng. Cold-rolled I rimmed sheet steels generally exhlbit r and ~ r of about 1.2 and ~ 0.4, respectively. Such steels can be subjected to mild deep drawing operations, but develop detrimental earlng. Drawing-~quality spe&ial-killed (DQSK) steels are often characterized by r l¦and ~r values near 1.5 and ~ 0.5, respectively. Although these ¦steels can withstand severe draws, they too tend to su~rer from earing; and they are more costly to produce than rimmed steels.
Recently sheet steels containing columbium or titanium to combine with interstitial elements have appeared. These steels have r ¦values o~ 2.0 or more and thus can with~tand very severe deep Idrawing. Earing tendency may be small in some instances, as ¦ indicated by ~ r values near -0.1, or high in other instances, as ¦ indicated by ~ r values near + 0.5. However, because Or their ¦¢olumblum or titanium contents, these steels are very costly to 25 1I produce. A signirlcantly more economical method ror improvln~

! deep drawability ls through the addition o~ phosphorus to unkilled l steels. Thus, U. S. Patents 3,215,567 and 3,244,565 show that the ¦~addition o~ greater than about 0.03% phosphorus to an unkilled ¦I steel, followed by a decarburizatlon o~ the sheet product, can Il . ' `'`' , Il - 2 - ,'Ludwigson ~043592 significantly improve deep drawability. However, the r values of these steels only approach those of the above noted l~QSK steels.
Therefore this invention provides a phosphorus containlng steel composition which can yield sheet products with r values of the order of 2.0 and greater.
This invention also provides a phosphorus containing steel composi-tion which can provide superior drawability without the need for decarburiza-tion, i.e. in which the carbon content of the final product is greater than 0.010 percent.
Thus, in accordance with the present invention there is provided a deep drawable killed-steel sheet product exhibiting an r value of at least about 2.0, the composition of which consists essentially of, in weight percent, C O . 010 - O . 1 Mn 0.22 max.
P 0.03 - 0.05 Si 0.02 - 0.06 oxygen 0.02 max.
balance Fe and incidental steelmaking impurities, said product having been produced from a steel melt which was killed in order to reduce the oxygen content thereof to said maximum percentage.
These and other advantages of the invention will be more readily understood from a reading of the following description when read in conjunction with the appended claims and drawings, in which:
Figure 1 shows the interacting effects of Mn and P on r value, and Figure 2 shows the beneficial effect, on r value, of Si additions to a phosphorus containing steel.
The anisotropy that results in favorable r and ~r values is developed through metallurgical reactions in the steel that favor certain orientations of individual grains in the steel with respect to the steel surface and rolling direction. In particular, when a high fraction of grains are oriented with their < 111) axes perpendicular to the face of the sheet, high r values are observed. To determine the effect on such orientations, of those residual elements which are normally found in sheet steels, a series of heats were prepared with planned variations of the following elements:
~ _ 3 _ .~

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~; manganese, phosphorus, sulfur, silicon, copper, nickel, oxygen and nitrogen.
Each heat was finish hot-rolled to a nominal thickness of 0.09 inch, at a temperature of about 1650F and the resultant band slow coolod from 1150F to simulate the - 3a -i i~., ~j . 1043S92 thermal history of commercial coiled product. After descaling, each band was cold reduced to a nominal thickness of 0.030 inch, i.e. to an aim cold reduction of about 67%. The resultant cold-reduced strips were recrystallization-annealed in conventional manner and subsequently evaluated for both r and ~ r values. From the data on the experimental steel compositions, and with the use of regression analysis; the relationship of r and ~ r as a func-tion of the levels of the above noted alloy elements was derived.
It was found that within the ranges tested, that the elements sulfur copper, nickel, oxygen and nitrogen were all detrimental to the achievement of high r values. Stated another way, in-creasing the level of any of the foregoing five elements had an inverse effect on r value. In viéw thereof, it is desirable that the aforementioned steelmaking impurities be maintained at levels less than the following specified maximums: 0.04% S, preferably 0.03~ max; 0.05% Cu; 0.12~ Ni; 0.015%N, preferably 0.01 max.;
and 0.02~ oxygen,~preferably 0.015~ max. ~owever, in distinction to the teachings of the aforementioned U. S. Patents (3,215,567 and 3,244,565) it was found that phosphorus can enhance the drawability of killçd (deoxidized) steels-, as well. However, with killed steels P will only be beneficial if the Mn level is main-tained below about 0.22 wt. percent. The rather surprising in-teraction between Mn and P in a deoxidized steel will be more readily understood by reference to Figure 1. It may be seen, in accord with the teaching of U.S. Patent 3,215,567 (e~. compare therein steels 17 vs 18) that for a "killed steel" in which the level of Mn is, for example, 0.35%, that an increase in the level of phosphorus will have a significant, detrimental effect on drawability.

~ 104359Z
with compositions containing from 0.22 to 0.28, the level of P
has only a slight or negligible effect on r value. More im-portantly, however, with Mn levels of about 0.22% or less, the r value materially increases with increasing phosphorus contents;
this beneficial effect of phosphorus becoming even more pronounced with progressively decreasing manganese contents. -In view of this "flip-flop" effect of P in "killed steels" depending on Mn content, it may therefore be surmised why the above noted prior art teachings as to the beneficial effects of P are limited to "unkilled" steels. The reduction of Mn to levels below about 0.2~ is not only expensive, but may be detrimental in that there generally would be an insufficient amount to bind up the S present in steel; thereby rendering the steel subject to edge-cracking thot-shortness). Thus, the art has only employed low Mn levels when it was absolutely required, i.e. for improving the sag-resistance of enameling steels.
Equally important, until recently, such low Mn steels were not capable of being recrystallized by a subcritical anneal. Thus, it was necessary that such low Mn steels be recrystallized by normallizing; a treatment which would destroy the attainment of high r values. It is, of course, now known (e.g. U.S. Patents 3,668,016 and 3,709,744) that such low Mn steels can be sub-critically annealed if they are deoxidized to a requisite extent and that such lower levels of Mn are in fact beneficial for the production of high r values. Therefore, the investigations of U.S. Patents 3,215,567 and 3,244,565 were probably limited to the effect of P in steels with Mn levels above about 0.25%. However, in (deoxidized) steels with such higher Mn levels, it may be seen that increasing the P

~ 104359Z
~¦content will actually-decrease the r value attained, i.e. Figure 1, ¦¦and U. S. Patent 3,215,567, noted above.
Additionally, it was ~ound that higher levels Or Si al50 'lenhanced drawabillty; with the bene~;clal e~ect increasing llprogressively as the P content was increased. This latter relationship is sh~ n in Figure 2. Referring to that ~igure, it imay be seen that a P level Or 0.01%, an lncrease in Si content llprovides some increase in r value. However, at a P level of 0.02 ¦lor greater, a similar increase in Si provides a rather dramatic 0 Ijincrease in r value.
I! Three steel heats Or varying compositions were processed ¦jto sheet, in the manner outlined above. The speciric compositions !or the resultant sheet products are shown in Table I, while ITable II presents both the measured and calculated strain-ratio ¦~alues. Example 1, provided ~or comparison, is a control sample iwherein P is outside the scope Or this invention Examples~2 and 3 are compositions within the scope of this invention, ~ .
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jl Aside from~their somewhat lower Ni contents, it may be seen that examples 2 and 3 are essentially the same composition as the control, except for their higher P and Si contents The ¦ decidedly higher r values of the latter two examples, is clearly ; ¦ evident. It is also noteworthy that such high r values have been : ¦ achieved without any sacrifice in directional uniformity, i.e.
. I Q r's very near to zero ' I

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Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A deep drawable killed-steel sheet product exhibiting an ? value of at least about 2.0, the composition of which consists essentially of, in weight percent, balance Fe and incidental steelmaking impurities, said product having been produced from a steel melt which was filled in order to reduce the oxygen content thereof to said maximum percentage.

2. The sheet product of claim 1 wherein oxygen is below about 0.015 percent.