US2381435A - Grain shape control in killed deep drawing materials - Google Patents

Description

I Patented Aug. 7, 1945 GRAIN- snArr: CONTROL IN KILLED DEEP Y DRAWING MATERIALS;

Robert s. Burns, Middletown, an Arthur s. y

McCabe, Seven Mile, hio, assig'norsto The j "AmericanRolling'MillCompany, Middletown,

I Ohio, a corporation or Ohio v No Drawing; appucaaonnugust3,1940.

j Serial No. 351,130 v a sc i s. (onus-12) "inratent' b. 2,069,758, issued February 9, 1937,

Hayesfand qrifilsihavedescribed the preparation of a "deep drawing iron or steel which is non a ing, particularlyin that it is free from the tendency to redevelop a definite yield point after, the yield point has once been destroyed thereinJ The iron orsteel is offa killed character, thoroughly deoxidized, and containing residual de; oxidizer of the natureqof aluminum or titanium.

It isfalso free from blue brittleness. The non- I aging 'ieature'of the material is attained through the characteristics mentioned; above and a stabilizing heat treatment. r

TIt willbeunderstood that ,theprovision of a deep drawing sheet in which the yield point, having once been destroyed by cold straining, will not returm sq thatth'e sheet Imaybe stored indeiinltelyand still retain its deep drawing characteristics fhas' provedto be. of tremendous im- D rtance in the art. The fundamental objectof j the"- present invention is toenha'nce the ,deep' v drawing characteristics 'of iron orfmild steels of the Hayes and Grifils type,j wh1le retaining the other advantageous characteristics of their sheet. Particular objects oi'our'invention are the pro vision of a :killed, non-aging,- deep. drawing iron or-mild steel having'not onlyunusual drawing properties, 'butiireedom from urface 'diflficulties, the provision o'f'a'material of the classreferred 1 to having the excellent drawing properties or, ma-v terial characterized by large grainsfand-the freedom from surface" coarsening characteristicfof materials having small grains.

These and other'obiects of our invention v a will. be set'ior'th hereinafter or will be apparent toione skilledin the art upon reading these specifications, we. accomplish by those treatments and fcertain exemplary embodiments, v v

. stabilized; non-aging steelsascurrently inanin those products of which We'shall now describe *are not limiting but are given 'as'an example of successful commercial practice. For'example,

ufactured have an analysis including components, of the following order: carbon up to .08% ,1' man- .08%. It. willbe'understoodthat these'analyses' innthe light of the teachings of, the Hayes and I 'Grifllsypatent, vtitaniumor otherma'terials may be employedjin the place of the'aluminum.

nd is box; annealed, at temperatures ranging The killed steelis hot rolled to a' gaugeof the generaLorder 01.10 inch; dependingfupon the' flnalgauge, and is then cold reduced about the resultant. sheets. ar'e non-aging. we. have discovered thatthe' deep high 'Qlsenductility test,- these tests being standby a large grain size isin general capableof taktic. to the same, extentof materials having a smallfgrain size; but a fine grained material is In other words the grain inour materials-has annealing. This latter material. is characterized from'l200 to 1400-F., whence it is slowly cooled and temper rolled. Thebox annealing and 0001- ing constitute a thermal stabilizig treatment, and

drawing characteristics of killed, non-aging iron or mild steels'may be improvedby treatments which tend to. produce a .peculiargrain structure. Deep drawing materials v shouldhave a relatively low yieldpoint, arelatively high elongation, a rela-, tlvely jlowRockwell hardness value and relatively ard tests or the A. S. T. M. Thesetests taken togetherare indicative. of deep drawing equality L but arenot conclusive, nor is the composition, taken alo Among other factors, the grain size of deg drawing metals has beenfound to be of great importance. A material characterized ing .deep draws; but such materials are susceptible to an undesirable coarsening of the surface when drawn; These defectsare not characterisin generalnot as drawable.

We have stated that by certain treatment and manipulationsin the process of making materials of the Hayes and Griflis type we can attain an. unusual grain shape. This shape may be roughly described as spheroidal or pancake shape, the projection of the grains on 'planes longitudinal and transverse to the'rolling-direction showing. the grain to be flattened. The flattened grains areiso orientated that their dimensions in the plane .of the rolled sheet aregreater than their dimension normal to the plane of the sheet.

assumed the shape .of :an oblate spheroid whose vertical axis" (perpendicular to the sheet surface) is shorter than either of its horizontal axes. It is to be understood, of course, that as in any aggregate of metallic grainsthe grain, surfaces are not regularly formed and smoothly curved.

The type. of grainstructure to which we have referred is sharply distinguished on the one hand from a rounded grain st z'ucture or a structure in whichthe axes of thegrains in all directions are 1 substantiallyequal, which previously has been considered to be the ideal condition, but which material is; of inferiordrawin'g quality. It is to be distinguishedion the other hand from that elongation of grains in the rolling direction only' which results from coldv rolling withinsuflicien't by having only the grain; axes parallel tothe rolling direction elongated so that the grains are roughly cigar shaped. It is a material of very poor drawing quality. The appearance of the flattened or spheroidal grain characteristic of our products is associated'with the treatment of l the hot rolled material or thin bar ahead of the cold reduction and is attained as we shall now set forth.

The essential condition in securing our new type of grain is a pre-treatment, prior to the cold rolling in which the material has a controlled cooling insteps from a temperature range of 1525 to 1650 F. or higher. 7 with respect to which the controlled cooling is practiced may be attained asthe finishing. temattained as will be set forth hereinafter.

The temperature,

.15 perature of hot rolling, or it may otherwise be As an exemplary practice'under our invention,

composition arethen hot rolledto some thickness in the neighborhood say of .10 inch, finishing the rolling operation in the range of 1525 to 1650 F. or higher.

perature substantially betweenj1100-to -1375 F. and then is coiled, whence it cools more slowly to room temperature. If the material is in sheet rather than in strip sheet'form, substantially between 1100 to 1375 F. inv the cooling cycle the cooling'rate may be retarded in any way desired,

as for example by stacking the sheets. Instead The material, if in strip sheet form, is cooled in air, or more rapidly, to a tem-' ming steel. The action is peculiar to killed steel; and the same finishing temperature and cooling treatment applied to rimming steels will not produce it. The amount of distortion or flattening of the grains varies from aratio of one for the vertical axis to one and one-half for both horizontal axes, to one for thevertical axis to four for both horizontal axes. Although there appears to be a variation in grain size within the steel, eachgrain has the characteristic flattened spheroidal shape. Moreover, though the grain sizein the plane of the sheet is relatively coarse, the surface of the sheet after stretching or drawing is definitely superior in smoothness to materials otherwise produced, and is not by any means as rough as the grain size in the plane of the sheet would indicate from experience with steels processed by conventional preferred ratio.

of coiling or stacking, the cooling of the. material may be retarded to a similar extent by any other procedure. i V

The material is next cold reduced, the reduction being ofthe order of and may vary f10II130 to 70% so that the material is of the desired gauge. r

It is then box or close annealed at 1250, to

1400 F. for four or more hours and is cooled to room temperature at a relatively slow rate which for best results will be of the order of about 25 per hour, whence it is temper rolled to prevent stretcher straining.

It will be noted that of these factors, the com- F position of the metal and the deoxidation resulting in killing the steel and the leaving ofresidual deoxidizer therein, together with hot and cold reduction and the final annealing, are characteristic of the. teachings of Hayes and Griflis; but the finishing of the'hot rolling in thetemperature range of 1525 to 1650 F. or higher, and the controlled cooling from that point preparatory to the cold rolling constitute-additional features,

which, when combined with the other steps, produce the peculiar grain condition to which we have referred.

The finishing of the materialat a temperature of from 1525 to 1650 F. or higher, and the coiling of it at a temperature substantially between 1100 to 1375 F. has the effect of giving to the material a rapid cool simulating a quench from the higher temperature to substantially between 1100 to 1375 F. and a slow c001 from that temperature to room temperature.

We have foundthatkilled steels made as described, and having the flattened or spheroidal graincharacteristic of our materials, are not-only non-aging but have .deep drawing qualities which are superior to those of freshly tempered rim- The effects characteristic of our invention are not,so far as we have been able to ascertain, attainable by substantially different temperatures. For example, finishing the ,hot rolling at too low a temperature, namely .a temperature somewhatbelow 1400" F., gives a grain structure elongated in the rolling direction only, which material. has very poor drawing .qualities. Material identical in formula to the first mentioned example when so treated gave a grain in which the-ratio of the vertical axis to the horizontal axis transverse to the direction of rolling was one to one while the ratio of the vertical axis to the horizontal axis in the direction of rolling was one to four. While such material had a satisfactoryyield point and a very low Rockwell value, it had a low percentage of elongation and poor drawing quality. Regardless of .hot mill finishing temperature, a fast normalize (cooling rate approximately 300/min.) of the thin bar ahead of the cold reduction and stabilizing heat treatment gave a material in which the ratio of the vertical axis to both horizontal axes was one to one. This material had a lower elongation, a higher yield point, anddefinitely poorer drawing qualities thanour new material. It was a material finished at. or slightly under 1400" F. from the hot rolling; but the same fast normalizing treatment appliedto a material on which the hot'rolling was finished at 1600 F. gave the same grain axis ratio, an even higher yield point, and an even lower elongation, and had very poor drawing qualities.

Having thus described our invention, what we claim as new and desire to secureby Letters Patent,"is:

1. A process of producing a deep drawing iron or mild steel sheet or stripstock, which comprises hot rolling a killed iron or mild'steel stock, finishing the hot rolling at a temperature in the range of substantially 1525 to 1650 F., rapidly cooling the stock in air from said temperature range to substantially between 1100 to 1375 F. and then more slowly cooling it to a cold rolling temperature, cold rolling the stock with a re duction ofthe order of 50%, and thereafter giving the stock a. thermal stabilizing treatment, by heating said stock to a temperature below the As point and preferably in'the neighborhood of the A1 point and then cooling said stock sufllciently slowly to establish freedom from the unstable condition in the ferrite which gives rise to overstrain aging.

2. A killed iron or mild steel for deep drawing characterized by a grain structure in which the grain axis normal to the plane of the stock is less in dimension than both grain axes in the plane of the stock;

3. A deep drawing iron or mild steel strip or sheet stock containing carbon not to exceed substantially .08%, manganese substantially of the order of .15 to 40%, copper and sulphur being low, with metallic aluminum substantially of the order of .02 to .08%, balance substantially all iron, said stock being free from a tendencyto re-develop a yield point when the yield point therein has been destroyed by cold straining, and further characterized by a grain shape in which the grain axis'normal to the plane of the stock is less in dimension than both axes in the plane of the stock.

4. A deep drawing iron or mild steel strip or sheet stock containing carbonnot to exceed substantially-.08,% manganese substantially of the order of .15 to .40%, copper and sulphur being low, with metallic aluminum substantially of the order of .02 to -.08%, balance all iron, said stock being free from a tendency to re-develop a yield point when the yield point therein has been destroyed by cold straining, and further characterized-by a grain shape in which thegrain axis normal to the planeof the stock is less in dimension than both axes in the plane of the stock, said material having deep drawing characteristics with a drawability characteristic of coarse grained material and surface effects characteristic of a fine grained material.

5. Killed'iron or mild steel strip or sheet stock for deep drawing characterized by high ductility and excellent surface when drawn and further characterized by grain dimensions which, measured normal to the plane of the stock are characteristic of fine grain materials and measured in either direction in the plane of the stock are characteristic of coarse grained materials.

6. A process of producing a deep drawing iron or mild steel sheet or strip stock, which comprises thoroughly deoxidizing iron or mild steel with the employment of sufllcient deoxidizing metal of the character of titanium and aluminum, to leave residual deoxidizer in the metal, hot rolling the metal to thin bar, finishing the hot rolling of the thin bar at a temperature of substantially 1525 to 1650 F., cooling the hot rolled material at least as rapidly as in air to a temperature of the order of 1100 to 1375 F., retarding the cooling thereof by juxtaposing cooling portions of the metal to each other by coiling or stacking so as more slowly to cool the metal to cold rolling temperature, then cold rolling the metal to gauge with a reduction of the order of and thereafter annealing the material at a temperature in the range of 1250 to 1400 F., and slowly cooling it.

7. A process as set forth in claim 6 in which the metal as hot rolled contains a maximum carbon of the order of .08%, manganese of the order of .15 to .40% is low in copper and sulphur and contains residual deoxidizer of the order of .02% to .08%, balance being iron.

8. A killed iron or mild steel sheet or strip stock for deep drawing, characterized by grain structure in which a preponderance of the grains are of a flattened shape, such that a typical grain has its two axes in the plane of the stock of such a length that each is from one and onehalf to four times as long as the axis of such grain normal to the plane of the stock.

ROBERT S. BURNS. ARTHUR S. MCCABE.