US3666569A - Production of deep drawing steel - Google Patents

Abstract

DEEP-DRAWABLE, ALUMINUM-KILLED STEEL SHEET IS PRODUCED TO BE SUBSTANTIALLY NON-EARING BY PROCEDURE WHEREIN AFTER HOT ROLLING TO A FINISH TEMPERATURE OF AT LEAST ABOUT 1500* F. AND COILING AT A TEMPERATURE NOT HIGHER THAN ABOUT 1100*F., THE STEEL IS COLD REDUCED BY AN AMOUNT BETWEEN ABOUT 50% AND 80% AND THEN ANNEALEDX BY A FIRST STEP OF HEATING TO A TEMPERATURE IN THE RANGE BETWEEN ABOUT 1030* F. AND 1090*F. FOR AT LEAST TWO HOURS, ADVANTAGEOUSLY SIX HOURS, AND THEN HEATING THE SHEET TO A TEMPERATURE BELOW THE LOWER CRITICAL POINT, FOR SOFTENING.

Description

May 30, 1972 F. A. HULTGREN 3,666,569

- PRODUCTION OI DEEP DRAWING STEEL Filed June 18, 1969 R D E10. 3. f

RTD

n 0.30 I 0.2q

INVENTOR.

fizz/w A HUNG/25M United States Patent 3,666,569 PRODUCTION OF DEEP DRAWING STEEL Frank A. Hultgren, North Royalton, Ohio, assignor to Republic Steel Corporation, Cleveland, Ohio Filed June 18, 1969, Ser. No. 834,369 Int. Cl. CZld 9/48; C22c 39/00 US. Cl. 14812 17 Claims ABSTRACT OF THE DISCLOSURE Deep-drawable, aluminum-killed steel sheet is produced to be substantially non-earing by procedure wherein after hot rolling to a finish temperature of at least about 1500 F. and coiling at a temperature not higher than about 1100 F, the steel is cold reduced by an amount between about 50% and 80% and then annealed by a first step of heating to a temperature in the range between about 1030 F. and 1090 F. for at least two hours, advantageously six hours, and then heating the sheet to a temperature below the lower critical point, for softening.

BACKGROUND OF THE INVENTION This invention relates to deep drawing steel, particularly steel in sheet form which has deep drawability such as required for manufacture of automobile fenders, oil pans, containers and many other contoured articles. conventionally, aluminum killed steel, being generally in such cases an ordinary steel as distinguished from special alloy compositions, has been employed and processing techniques have been developed to achieve the deep drawing property. For example, employing an aluminum killed, low carbon steel, a present practice has involved high finishing and low coiling temperatures in the hot rolling mill, followed by about 60% reduction in thickness in the cold rolling mill, and a continuous heating to the soaking temperature in the box or single stack annealing rocess.

Whereas the product resulting from the above treatments, and generally from others heretofore used is capable of being deep drawn, it displays extensive eating in such operations. As will be understood caring is the creation of a scalloped or deep wave-like contour along the edge of a deep-drawn shape, involving a waste of metal and even leading to weakness or cracking at localities between the projecting ears. There may also be ditficulty in removing the articles from the die or dies employed in the deep draw operation. In general, the extent of caring has appeared to be proportional to the deep drawability and thus it has been generally assumed that considerable earing must be tolerated in connection with deep drawing.

The present invention is accordingly directed to method of producing deep drawing steel sheet which is essentially non-caring, e.g. in that the caring tednency or the percent earing, as determined by appropriate measurement is acceptably low. A further object is to achieve these results, and to provide a corresponding deep drawing, substantially non-eating sheet, as a new product, by procedure that involves no great modification of the steel-treating operations and no special or costly changes in composition of the metal, the results of the invention being that the desired, non-caring product is obtained from aluminum killed metal which, although of selected or controlled chemistry, is not radically different and furthermore achieves, in the end product, full mechanical properties, including deep draw characteristics, that have heretofore been attained.

Patented May 30, 1972 Ice To these and other ends, it has been discovered that an aluminum killed steel having deep drawability and essential absence of caring may be produced by novel procedure which not only follows certain advantageous prior practice designed for the development of deep drawing properties, such practice including preferred hot rolling and coiling operations relative to the hot rolled band, but also embraces, in at least preferred aspects, selection of certain chemical or compositional characteristics of the steel, and most particularly involves selected features of cold rolling operation and special treatment following such operation, all as now found to develop a critical crystal orientation or texture, i.e. a crystallographic preferred orientation, which appears to be requisite for a deep drawing, non-earing sheet. The preferred chemical characteristics extend to a content of aluminum and nitrogen, preferably selected within ranges heretofore recognized as appropriate to deep drawing steel, as will be set forth in greater detail below.

The novel rolling and annealing or further heat-treating operation, now discovered to be of critically cooperating advantage for attainment of the desired results, comprises cold rolling the hot rolled band to a total reduction of the range of about 50% to about and then as a special part or stage of annealing, subjecting the cold rolled strip to a temperature in the range of about 1040 F. to about 1090" F. for a period of at least about two hours, and advantageously six hours or upwards, followed by heating to a temperature slightly below the lower critical temperature, i.e. advantageously a little below about 1350 F. for a short interval, to effect softening as necessary for the desired working, e.g. drawing, capability.

The resulting product is found to have achieved a recrystallization which is unusually appropriate in character, indeed such as has now been discovered to be requisite, for providing a value of average-r or F, suitably great for deep drawing, and also a value of a parameter, further described below and identified as Ar/F that is significantly low and that is correspondingly indicative of absence of serious caring. 'In particular, the new product of substantially non-caring sheet of aluminum killed steel is found to have not only an arrangement or orientation of the socalled [111] crystal planes substantially aligned with the plane of the sheet, but also orientation of such crystals, considering each to have a predetermined axis for its [111] plane, as to be essentially random in all directions of the sheet plane, i.e. the axial orientation of these crystal planes being fairly uniformly distributed in respect to the rolled strip, e.g. as distinguished from a large predominance in the rolling direction, or transversely or otherwise. The structure of the product, including particularly the stated distribution of crystal axes around different directions of the sheet, i.e. as considered with respect to the rolling direction and the transverse direction and intermediate angles, is recognizable by appropriate examination of the metal, notably X-ray examination as described below. Examples of structural determination, representing the deep drawing, non-caring properties of the steel are also illustrated in the drawings. Additional features of the procedure of the invention and of the product are further set forth in the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are polygonal graphs respectively related to two examples of steel sheet and illustrating R values in difierent directions in each, FIG. 1 representing a sheet processed outside certain limits of the invention and FIG. 2 a sheet processed in accordance with the invention.

FIGS. 3 and 4 are (222) pole figures derived from X- ray diffractometer data and respectively related to the steel sheet specimens of FIGS. '1 and 2.

DETAILED DESCRIPTION The steel to which the present invention is applicable is identifiable as in general a carbon steel of aluminum killed type, advantageously a so-called low carbon steel, e.g. from zero to about 0.18 Weight percent carbon, or more usually 0.03 to 0.12%, although it is conceived that the present improvements are applicable to steels having a substantially higher carbon content and indeed are not closely dependent on a selected amount of car bon. The production of aluminum killed steel suitable for deep drawing is generally well known, including the addition of aluminum metal to the melt, usually in the ladle, and therefore need not be set forth here.

It has been observed that the controlled presence of aluminum nitride appears to be a factor of special significance in the production of deep drawing metal, as for example in that premature re-precipitation of aluminum nitride (as by hot coiling after hot rolling) is undesirable, whereas by relatively cold coiling of the hot band the aluminum nitride appears, with advantage, to remain unstable, the further indication being that preferred results are attained, with the desired deep drawing condition of predominantly coplanar appearance of the [111] crystal planes, where the aluminum nitride then precipitates during the first part of recrystallization in the annealing stage. In consequence the chemistry, so to speak, of the selected steel is preferably of such aluminum and nitrogen content as to accommodate desired effects of aluminum nitride precipitation, or at least, to accord with present understanding with respect to such compositional preference, and indeed regardless of any particular theory as to the occurrence and nature of a precipitate of aluminum nitride.

As will be understood, the introduction of aluminum metal for the killing operation and the normally unavoidable presence of nitrogen in the melt, commonly lead to a compositional situation that agrees with the above desiderata respecting aluminum nitride, at least providing that there is not an undue excess of aluminum. It presently appears that for thoroughly satisfactory results the final, solidified ingot should contain from about 0.02 to 0.08 weight percent aluminum, considered, of course, as free aluminum and thus excluding such aluminum as is consumed in a primary function of combining with oxygen. A substantial excess of free aluminum is believed to be undesirable, as tending to tie up the nitrogen during hot rolling and thus prevent or impair the desired delay in precipitation of aluminum nitride, i.e. the delay until recrystallization after cold reduction, as explained above. A nitrogen content in the finished ingot, of about 0.002 to about 0.01 weight percent, is presently preferred, a convenient and usual range being up to 0.008 weight percent. Such nitrogen concentration may be considered to appear normally in carbon steels of the character here contemplated, although if necessary some addition of nitrogen may be effected.

The described conditions of aluminum and nitrogen content are known in steel production as appropriate for deep drawing properties, and control of composition in these respects is not ordinarily difiicult; in most instances it is only necessary to control the amount of metallic aluminum added (as in the ladle), having regard for the primary function of aluminum addition in the particular type or melt of steel and utilizing conventional testing for the effect of such addition. Indeed it may be fairly stated that reference to an aluminum killed deep drawing steel necessarily means a steel having suitable aluminum and nitrogen content as explained above and is so understood in the art.

It is normally desirable that there be a modest content of manganese, as for example from 0.02 weight percent upwards, even to 1% or more, although normally 0.2% to 0.4% is sufficient. This content of manganese is included for its conventional function of tying up sulfur and thus insuring that the steel is rollable in hot condition. As will be appreciated, if the steel is otherwise compositionally suitable for hot rolling or if other situations prevail, specific attention to manganese content may be unnecessary. It does not appear that the presence of manganese is of consequence for the specific procedure herein described, i.e. as to a novel cold rolling and annealing sequence that yields a non-earing deep drawing sheet.

For the present invention, the hot rolling operation advantageously follows accepted practice in the manufacture of deep drawing steel, particularly in that the hot reduction should finish at a relatively high temperature, with cooling to a substantially lower temperature before the band is coiled. Specifically the steel is preferably finish rolled in the hot mill at a metal temperature of at least about 1500" F., usually at 1550 F. or above. The selected temperature in this range will depend on convenience and efficiency, as will be readily understood, a practical maximum, for example, being of the order of 1700 F. It is important that the hot rolled band be cooled, preferably in a rapid manner as with water sprays, to a temperature not higher than about 1100 F. (advantageously below 1050 F.), while advancing to be coiled. Again as a matter of practicality a range of about 900 F. to 1100 F., and preferably a temperature in the lower half of such range, is appropriate as a value to be reached in the metal by the described quench-like cooling, before the band goes into the coiler. The total extent of hot mill reduction can be as is conventional or appropriate, usual values of finish thickness being up to about 0.175 inch, also having in mind that the maximum thickness that can be handled in a cold mill is ordinarily one-fourth inch.

As stated, the foregoing practice of finish hot rolling at as high a temperature as convenient, with coiling at the prescribed low temperature is known in the preparation of hot band for supply to the cold mill to produce deep drawing steel, the effect being to avoid premature or permanent precipitation of aluminum nitride and to facilitate maintenance of desired internal texture of the metal in subsequent processing. That is to say, aluminum nitride presumably remains in unstable condition, whereby the desired reversion or precipitation of the stable nitride will occur during annealing after cold reduction.

As explained above, the present invention involves suitably and critically selected characteristics of cold rolling, specifically as to extent of reduction, together with a novel annealing operation including a specific heating stage of criticality in temperature and time. In particular, it is found that if the hot band is cold reduced by an amount in the range of about 50% to about or perhaps a little more, and the finish-gauge cold rolled strip is then subjected to a heating operation, as at a temperature of about 1030 F. to about 1090 F. for a minimum of about two hours, preferably followed by a softening treatment at a higher temperature (e.g. just below the lower critical point, which is about 1350 F.), the result is a steel in sheet form which not only has good deep drawing characteristics, as well as other mechanical properties similarly equal to previous deep drawing steel, but also is characterized by essential absence of caring. This valuable property of the finished product has been determined experimentally by measured indicators as well as by practical tests or standard drawing operations, absence of caring being generally here considered to be less than about 5% earing, and preferably less than 4%, on standard cup tests. In such tests producing straightsided, cylindrical, deep-drawn cups, percent earing is determined as the quotient of the difference between maximum and minimum heights, from the cup bottom, of the eared or wavy upper edge, divided by the average of such heights.

Characteristics of deep drawing are generally determinable from r values in various directions of the plane of the finished metal sheet. The term r is the coefficient of normal plastic anisotropy, being a measure of the resistance of a sheet to thinning during stretching, and specifically being the following ratio of natural logarithms of dimensional ratios of a specimen:

r is measured with respect to a selected direction along the specimen defined as length L, the values W W and T T; being the original and final width and thickness of the specimen, relative to a linear draw that effects a selected elongation (in L), for example 15% or 20%. As will be understood, the term T /T can also be expressed as the product of W /W multiplied by a factor derived from the elongation, e.g. 1.15 where the latter is 15%.

Conventionally r is determined, in separate specimens of a given strip, for directions parallel to the rolling direction (r transverse of the latter (r and at 45 to it (r one 45 measurement being indicative of both such. The average-r value, F, is a criterion of deep drawability, being simply one-fourth of the sum of r r and twice r Ar, which is an indication of earing, is a measure of the variations of r which occur as a function of orientation relative to the rolling direction. It is defined as:

Since it is found that the percent earing, i.e. in a cup test as explained above and through a range up to an undesirably excessive value of 18%, is directly proportional to the ratio Arfi, the latter expression is taken as a best measurement of expected earing.

In general, useful deep drawability requires sheet having F well over 1.3, indeed ordinarily at least equal to about 1.4, and advantageously more, as 1.5 and above. For a useful non-caring attribute, the numerical value of Ar/? should be significantly less than 0.3, and may advantageously be defined as not more than about 0.2, e.g. for caring less than about 5%. The value of this ratio is taken in an absolute numerical sense, as it can sometimes be a negative quantity, so that references here to a Ar/F value of not more than about 0.2 mean any value in the calculated approximate range of -0.2 to +0.2. Indeed for superior results in freedom of earing, the value of Arfi should be not greater than about 0.1 As will be apparent below, the method of the present invention alfords a cold rolled steel sheet product having good properties of deep drawability without earing, evidenced by measurement under the foregoing standards.

As will be further explained below, the product is believed to be characterized by a novel crystal structure, readily identifiable, as by X-ray techniques, and involving a crystallographic preferred orientation which results in a remarkable uniformity in r values, as measured in substantially all directions. Since earing in drawn parts may be traced to substantial differences between r values at adjacent positions and cars will thus form at positions of high r value when the adjacent angular locations display low r values, the attained substantial uniformity of r value and the determined absence of significant earing confirm the effectiveness of the observed crystal orientation for such results.

With the percent cold reduction selected in the range noted above, the special heating or anneal operation eifectuates a recrystallization, i.e. of the ferritic structure of the steel, that provides deep drawability and is also of novel character in eliminating earing, the crystal texture thus developed being distinguished by directional uniformity in the sheet. The parameters of the process appear to be inter-related in a critical manner: where the cold reduction is carried too far, e.g well above earing is likely to be high, and the deep draw indicator F is too low, the latter being also the result when the stated heating operation is effected at temperatures substantially above 1090 F.; and where the anneal is at too low a value, e.g. substantially below 1030" F., fairly good 7 value may sometimes be reached, and Ar/F is then high and there is objectionable earing.

The nature of the invention will be further understood from the following example, including a series of tests at various values of cold reduction and annealing temperature, from which the selected operating ranges become apparent. A slab of aluminum killed steel containing 0.056 weight percent aluminum was hot rolled to a finish thickness of 0.085 inch, the finish hot rolling being at 1550" F. and the final band being rapidly chilled and coiled at 90097 5 F. This steel had the following chemical composition, in weight percent, balance Fe:

Portions of the hot band were respectively reduced by cold rolling (i.e. subjected to the necessary number of passes in a cold mill in conventional manner) by 53%, 63%, 73%, 83% and 88% in thickness, and separate portions of the respective, different finish gauge strips (as above) were subjected to different annealing treatments. In particular, these heating treatments respectively involved annealing at 975, 1025", 1045", 1065 1085 and 1125 F. for various times. In each case the steel was then heated to 1325 F. for thirty minutmnd cooled. The exact thermal treatments (prior to the softening at 1325 F.) are given in Table II, with the resultant 1 and Ar/? values as found by test (of suitable, known type, with r values determined for 20% elongation) and calculation:

TABLE IL-COLD ROLLING, RECRYSTALLIZATION Cold Annealed time and reduction temp. (hrs.F.) (percent) r Ar/r 1, 1% hours 53 1. 04 0. 2O 63 0.97 0. 24

1,085 6 hours 53 1. 42 0. 10 63 1. 40 0.07

1, 065 16 hours 53 1. 57 0 025 63 1. 55 0. 05

1, 045 6 hours 53 1. 31 0. 04 63 1. 54 0. 045

1, 025 56 hrs. 40 min- 3 1. 45 0- 31 63 1. 49 0. 36

As presently understood, the mechanism of the invention is the control of crystallographic preferred orientation in the sheet by forcing prompt precipitation of aluminum nitride during recrystallization and by eifectuating the recrystallization under what appear to be critical limitations of temperature, for a time having a minimum of about two hours, the extent of cold reduction being correspondingly selected (at least to the extent of being limited to not more than about 80% and preferably somewhat less) to favor the development of the desired texture of crystallized ferrite during the special anneal. Finally, the product is given a second, short treatment, which may be considered as of the nature of the anneal but is expressly for the purpose of softening in the sense of enhancing ductility, and which can effectively be performed at a temperature below the lower critical point of the steel, so as not to impair the desired texture that is attained during the special or step anneal.

Table II clearly indicates that the desired results of a high F value (which is average-r), e.g. at least approach ing and very preferably exceeding 1.4, and of a low Ar/F value, e.g. 0.2 or preferably less, were substantially uniformly and essentially only achieved, among these specific tests, at the cold reductions of 53%, 63% and 73% and with the special anneal at 1085", 1065 or 1045 F. The plastic strain characteristics of the steel, for deep drawability (high T") and absence or earing (very low Ar/F) were generally unsatisfactory in one or the other respects, or were at least erratic, at the higher or lower annealing temperatures tested and at the 83% and 88% cold reductions. The strips annealed at 975 F. are omitted from the table, for brevity, but at reductions other than the higher ones these strips showed an excessive Ar/i and gave results that indicated questionable reliability of this lower temperature treatment for attaining a truly non-caring deep-draw steel in any case.

The times of heating were varied considerably (in Table II), largely to ascertain whether greatly prolonged treatment would be beneficial, it being also understood that consideration of kinetics made it clear that at 1125 F. recrystallization would be easily completed in 1 /2 hours. Further tests and experience have indicated that within the range of temperature effective for the invention, a minimum time at heat is about 2 hours or possibly somewhat more at the lower temperatures; present preference and indeed special advantage is noted for an interval of about 6 hours, with little reason, although no harm, in substantially larger periods.

In summary, it appears that the special anneal, which determines the preferred orientation, should be at a temperature in the range of about 1030 (preferably at least about 1040") to about 1090 F., and the cold reduction, which governs the desired texture, should not be more than about 80%, and most advantageously not greater than about 75%. In a specific sense, excellent results and optimum reliability in attainment thereof appear to reside in the cold reduction range of about 55% to about 70%, and in a recrystallization treatment temperature between about 1040 and 1070 F.

The second or softening step of heating, which may conceivably be omitted in some cases, is dependent on the degree of ductility required. As explained it should not be at a temperature greater than the so-called lower critical point, which is about 1350 F. for most steel compositions here contemplated. Preferred temperatures are slightly below such point, e.g. 1325 F., with the metal held at this value for an interval from minutes upward, the heat-up and cooling times not being critical. At lower temperatures, e.g. l250, longer treatment will be needed to get the same results, and still lesser values of temperature, down to 1175 may be used, as where full ductility may not be needed The basic drawability of the steel is, of course, independent of ductility (softness), which is a measure of the amount of force, or how little force, is required for a given draw. It is, moreover, an extremely simple matter of test to find appropriate conditions of temperature and time to achieve a desired degree of ductility in a given case, the only requirement of the present invention being that the lower critical point of the steel (which is always known) be not exceeded.

FIGS. 1 and 2, by way of example, show the development of r values in a graphical fashion, wherein such values in the longitudinal, 45 and transverse directions (relative to rolling) are plotted as a function of angle around the rolling direction. The magnitude of r is the distance from the center, in the directions of the corners of the polygon, and the reference circle in each plot is for an r value of one (1), theoretically uniform in all directions. Although the values would be fully shown by plotting a single quarter they are duplicated around 360 for ready understanding.

Specifically, FIG. 1 is such a plot for the steel that was cold rolled to 88% and then annealed at 1065 F. for 16 hours, as set forth in line 15 of the tabulated data in Table II, whereas FIG. 2 represents the steel cold rolled to 63%, subjected to the same annealing treatment, as in line 12 of the tabulated data. These figures thus in effect show the values of r used for computation of average-r, and thus also for Ar and the computation of Ar/? The determined values mark the strip of FIG. 1 as unsatisfactory for deep drawing, and of no better than a borderline nature as to absence of caring (5 to 6% but characterize the strip of FIG. 2 as having good deep drawability, with substantially no eating at all, e.g. 0 to 2%.

That the non-earing steel product is characterized by a new crystal structure or orientation has been effectively demonstrated by X-ray examination, particularly in the ploting of pole figures such as depicted in FIGS. 3 and 4, which represent the crystal texture of the same samples to which FIGS. 1 and 2 respectively relate. As will be understood, a pole figure of the sort here shown is in effect a stereographic projection representing, so to speak, the positions (on a reference sphere) of the poles of selected crystal planes, the data being obtained by known X-ray dilfractometer techniques, i.e. as commonly used for the determination of pole figures. Specifically, these are (222) pole figures, which can also be described as (1-11) pole figures, and which represent X-ray examination of the [111] crystal planes in the steel sheet.

It is recognized that in cold rolled-annealed, aluminum killed steel, the [111] planes of the ferrite crystals should be predominantly, or at least should have a so-called preferred orientation which is parallel or approximately parallel to the sheet surface. In general, this orientation appears upon diifractometer examination of prior cold rolled sheet of deep drawing capability, It has been found that in past steel of this character, the orientation of the grains as noted above, i.e. with a predominance or preference of the [111] plane for parallelism with the surface, is accompanied by a like predominance or preference of orientation of a specific direction of that crystal plane, Le. a dominance (in a specific direction) of a directional characteristic of the crystal structure, as for example a dominant directional alignment usually in the rolling direction. Thus in the case of the sheet examined in FIG. 3, which is characterized by a dominant directional alignment of this sort, the dominant direction in the rolling direction is the '2'll It has now been discovered that in sheet wherein the [111] planes have a preferred orientation parallel with the surface, the accompanying presence of a dominant directional alignment as described above (eg in the rolling or transverse direction) is speci fically related to substantial earing and to high values of Ar/F.

In contrast the steel products of the present invention are characterized by a crystal orientation or texture whereby the [111] planes are still preferentially parallel to the surface, but there is an absence of directional alignment. Thus in the case of FIG. 4, there is no dominant directional alignment, but only the planar parallelism to the sheet surface. Although this condition can conceivably be described as a random positioning or angularly distributed situation of the directions of the crystal grains or structure, or in effect of the directions of the planes that have the stated planar orientation, it is believed that the metal structure can be identified most simply and yet positively and definitely as being characterized by the absence of directional alignment. That is to say, the crystals have no dominant directional alignment of appreciable sort, e.g. as determined by suitable pole figures (i.e. of a known kind, recognizable as appropriate) that are designed to reveal whether the [111] planes have a preferred orientation parallel to the sheet surface and necessarily also 'whether there is a dominant directional alignment.

It will now be understood that X-ray dilfractometer techniques are capable of determining, as for representation in pole figures, the orientation characteristics of the [111] planes taken selectively, i.e. the existence or absence of a predominance of parallelism with the sheet, and also whether or not there is a dominant directional alignment (and in which direction, if present), this last determination being alternatively considered as finding, in effect, the presence or absence of directional .alignment (in an angular sense, in the sheet plane) of the stated planes. Such data are determined and plotted in the pole figures by position of the poles of the planes, and are measured by pole density, which may be set forth in arbitrary units since the inquiry is concerned with preference or predominance of pole orientation.

Turning now to FIGS. 3 and 4, it will be understood that the several contour lines represent points of the same pole density, significance here being achieved by densities designated (with arithmetic rather than absolute significance) by the numbers 300 and 400. Compared with FIG. 4, the steel of FIG. 3 showed relatively small areas of high pole density outside the central region of the figure, and also a rather meager distribution of these high-numbered contour areas circumferentially around the figure. In FIG. 4, the presence of relatively large areas of high pole density at substantial departure from the center (which is an angular departure on the reference sphere), e.g. at a and b, indicate a substantial predominance or preference for parallelism of the [111] planes with the sheet surface.

Moreover, the fact that these last-mentioned areas in FIG. 4 are circularly elongated or otherwise distributed in a circumferential direction in the figure, and not restricted to either of the rolling or transverse directions and indeed extend, together, through more than half of a circle roughly passing through them-shows that in effeet the directions or any individual directional characteristics of the [111] planes in the sheet plane are randomly distributed, instead of being concentrated, for example, in either the rolling or transverse directions. More specifically, FIG. 4 reveals no dominant directional alignment, but only the planar parallelism with the sheet surface; in accordance with the present invention, the steel is characterized by an absence of directional aligment. It will be understood that the contours at the center of each figure are not of very great significance, since the nature of stereographic projection is such that the poles of randomly oriented planes tend to have an apparent high density in this region of the plot.

The correlation of FIGS. 3 and 4 with FIGS. 1 and 2 is extremely close, and likewise with the determined numerical indicia for deep drawing and earing characteristics of the specimens (a linear relationship through 'a significant range having been found, moreover, between percent earing and values of AH?) to the extent that measured values of F and Ar/F may be taken to represent an effective measurement of the actual crystal orientation in the sheet. In other words, whereas numerical evaluation of the pole figures may be difficult to relate in terms (of extent or degree) of predominance of the desired coplanar crystal position and the desired absence of dominance of crystal plane direction, an F value of 1.4 and above signifies a sheet wherein the [111] planes have a sufiicient preferred orientation in parallelism with the surface plane to aiford good deep drawing, and a Ar/F value of 0.2 or less (measured absolutely, regardless of sign) means a sufficient absence of dominant directional alignment, e.g. in the sense of appearance in (111) pole figures and in respect to angular position relative to the rolling direction, to provide usefully minimal characteristics of earing. In other words, pole figures such as shown in FIGS. 3 and 4 can be taken as sufiicient evidence-as has been established by other and like pole figures for other specimens of steel processed within and outside the present invention-that the steel of the invention does in fact have a new crystallographic texture in the respects noted and that in effect such crystallographic texture can itself be determined or measured in a numerical manner by the values of r and Ar/?.

The foregoing has nothing to do with the presence or absence in the finished sheet of a so-called elongated grain structure; the described crystallographic orientation or texture properties are understood to be achieved by the present process, whether or not the grains are elongated as is often the case in deep drawing steel. Although such elongated grain was at one time deemed significant for deep drawing, present information of the art is that it is not and that equi-axed grain structure is at least as good.

In summary, the present, novel process has been demonstrated to produce an aluminum killed steel sheet product which has excellent deep drawability, which retains at least substantially the same mechanical properties (of strength and otherwise) as previous deep drawing steel, and which has a remarkable absence of caring when deep drawn, the product being thus new and characterized by a new crystal structure or texture.

It is to be understood that the invention is not limited to the particular compositions and steps hereinabove described but may be carried out in other ways without departure from its spirit.

I claim:

1. A method of producing deep-drawable, substantial- 1y non-earing sheet from aluminum-killed steel compositionally constituted and suitably hot rolled and coiled to provide a hot rolled band capable of cold reduction to form deep drawing steel sheet, comprising cold reducing said hot band by not more than about and subjecting the resulting cold-reduced steel to a first annealing step consisting essentially of heating to a temperature in the range of 1040 F. to 1090 F. for at least about two hours, said annealing step being followed by heating the sheet to a temperature between about 1175 F. and the lower critical point of the steel, to soften the sheet.

2. A method as defined in claim 1 in which said cold reduction is effected to an amount in the range of about 50% to about 75%.

3. A method as defined in claim 2 in which the temperature of said last-mentioned heating is in the range between 1175 F. and 1350 F.

4. A method as defined in claim 1 in which said cold reduction is effected in the range of about 55% to about 70% and said heating of the first annealing step is to a temperature of 1040 F. to 1070" F.

5. A method as defined in claim 1 in which said steel is a low-carbon steel containing up to about 0.12% C, and containing aluminum and nitrogen to provide precipitation of aluminum nitride in the cold-reduced steel during annealing.

6. A method as defined in claim I which includes, prior to cold reduction, finish hot rolling the steel at a temperature of at least about 1500 F., cooling the resulting hot band and coiling said hot band at a temperature not higher than about 1100 F.

7. A method of producing deep-drawable, substantially non-earing sheet from aluminum-killed carbon steel that contains about 0.02% to about 0.08% aluminum, about 0.002% to about 0.01% nitrogen, and manganese as required for hot rolling said steel including finish hot rolling at a temperature of at least about 1500 F., cooling the finish hot rolled band and coiling same at a temperature not higher than about 1100" F.; and thereafter cold rolling said hot band to a reduction which is selected in the range of about 50% to about 80% and annealing the resulting cold-reduced sheet by a first step and a second, softening step, said first step consisting essentially of heating to a temperature which is selected in the range of 1040 F. to 1090 F. for a time which is selected in the range of about two hours and upwards, to provide deep-drawable, substantially non-caring steel sheet having a value of F of at least about 1.4 and a value of Ar/F of not more than 0.2, said second step consisting essentially of heating the sheet to a temperature in the range of about 1175 F. to about 1350 F., to soften the sheet.

8. A method as defined in claim 7 in which said finish hot rolling is at a temperature of at least about 1550 F.

9. A method as defined in claim 8 in which said coiling is at a temperature not higher than about 1000 F.

10. A method as defined in claim 7 in which said cold rolling is to a reduction of not more than about 75%.

11. A method as defined in claim 10 in which said first annealing step is effected for a time of at least about 6 hours at the selected temperature, and said second step of heating the sheet is effected to a temperature not higher than the lower critical point of the steel, to soften the sheet.

12. A method as defined in claim 8 in which said first cold reduction is about 55% to about 75% and said first annealing step is at 1040" F. to 1085 n F.

13. Deep-drawable steel sheet which is substantially non-eating, said sheet being cold-rolled, aluminum-killed, carbon steel compositionally and structurally constituted for deep drawing, said steel sheet being annealed and softened and having a value of F of at least about 1.4 and a value of Ar/F of not more than 0.2, and consisting essentially of crystal structure having a texture which provides deep drawability without substantial caring and in which the [111] planes of the crystals have a predominant orientation parallel to the plane of the sheet sufiicient to provide said value of F, and in which there is such absence of dominant directional alignment in crystal structure, considered with reference to said crystal planes, as to provide said value of Ar/F.

14. Steel sheet as defined in claim 13 in which said value of Ar/F is not more than about 0.1.

15. Deep-drawable, cold-rolled steel sheet which is substantially non-caring, said sheet being formed of aluminum-killed carbon steel containing about 0.02% to about 008% aluminum, about 0.002% to about 0.01% nitrogen and up to about 1% manganese, said sheet being in annealed and softened state and characterized by cold reduction of not more than about said sheet having a value of 7 of at least about 1.4 and a value of Arfi of not more than 0.2 and consisting essentially of cubic ferritic crystal structure wherein the [111] planes of the crystals have a predominant orientation parallel to the plane of the sheet sufiicient to provide said value of F, and in which there is such absence of dominant directional alignment in crystal structure, considered with reference to said crystal planes, as to provide said value of Ar/F, said sheet including a content of precipitated aluminum nitride and having a texture of said crystal structure including said orientations of said crystal planes, for providing both deep drawability and absence of substantial caring upon deep drawing.

16. Steel sheet as defined in claim 15 in which said value of Ar/F is not more than about 0.1 and said steel has a carbon content of not more than about 0.12%.

17. Steel sheet as defined in claim 16 in which said value of F is at least about 1.5.

References Cited UNITED STATES PATENTS 3,248,270 4/1966 Laidman et a1 148-l2 3,259,488 7/1966 Nazkamura 14836 3,320,099 5/ 1967 Weber.

OTHER REFERENCES Richards, P. N.: The Effects of Heating Rates and Subrecrystallization [Heat Treatments on Aluminum Killed Deep Drawing Steel, Recent Developments in Annealing, Special Report 79, The Iron and Steel Institute, London, 1963, pp. 22-33.

L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner US. Cl. X.R. l4836 my UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIQN Patent N 3 ,6665569 Dated May 30 1972 k FRANK) A;, HULTGREN Inventofls) It is certified that error appea rs in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col 1, line 59, '-'tec 1ne-ncy"- should read- --tendency- Col. 6, line 9, for "and" read, "but"- Col. 11, line 31, for "8" read "11- end of line, delete I "first" N v 1 e Signed and sealed this 26th day of December 1972. I

a s (SEAL) Attest:

EDWARD MQFLETCHERJR. ROBERT GOTTSQHALK Attesting Officer 4 Commissioner of Patents

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