IL24356A - High strength steel sheet or strip and process of making same - Google Patents

Description

HIGH STRENGTH STEEL SHEET OR STRIP AND PROCESS ©F MAKING IT onj^n I'Vnm mm ηπ η¾: πι¾ s^w-na This invention relates to novel high strength steel sheet and strip products and to a novel continuous process for making the same. The Invention also relates to novel coated products of the aforementioned character, especially tin plate.

High strength thin gauge steel strip is widely used for steel strapping. Coated steel products, such as tin plate, galvanized steel, and aluminum coated steel, in thin gauges with high strength are also highly desirable. It has been customary to obtain the desired strength in suoh materials by cold reduction or cold rolling.

For example, in the manufacture of conventional black plate a low carbon steel strip is hot rolled to intermediate gauge, pickled, and then cold rolled to desired gauge which is on the order of .007 "to .015 inch. However, the extent of cold rolling required impairs the ductility of the steel strip so that an annealing step Is necessary to soften the steel before it is temper rolled and tin plated. The annealing step improves the ductility but reduces the tensile strength and yield strength. Conventional tin plate has a tensile strength of about 45 , 000 to 65, 000 pel (pounds per square inch) with an elongation in two Inches of about 1 to 25 . To meet the requirements for the higher temper grades of conventional tin plate, it Is usually necessary to resort to the use of rephosphorized or nitrogenized steel.

Higher strength tin plate known as double reduced tin plate has also been developed. In the manufacture of this product, the steel strip after annealing is given a second cold reduction either before or after the tin plating step.

Usually the extent of reduction in the second cold reduction may have a thiokness on the order of .005 to .015 inch with a tensile strength of from about 80,000 to about 110,000 psi which is appreciably higher than the strength level of conventional tin plate, but the ductility of the product is poor, e.g. less than about 1% elongation in two inches. To obtain the higher strength levels in double reduced tin plate, it is considered necessary to nitrogenize the steel. Much thinner grades of double reduced tin plate with a thickness as low as .002 inch have also been manufactured commercially with gen-erally the same properties as the heavier gauges.

The reliance on work hardening by cold reduction to obtain the desired strength in thin gauge steel sheet and strip products has serious disadvantages which are particularly acute in the case of double reduced tin plate. In addition to having poor ductility or formability, as mentioned above, double reduced tin plate is also characterized by a high degree of directionality or anlsotropy, i.e. it has significant ly different mechanioal properties in the transverse and longi tudinal directions with respect to the direction of rolling.

The present invention avoids the aforementioned disadvantages and relies on the production of a high-strength microstructure to obtain thin gauge steel products having the desired properties.

Accordingly, the primary object of the present in-vention is to provide novel and improved means for obtaining thin gauge steel sheet and strip products having high strength and good ductility.

A further object of the Invention is to provide novel high strength steel sheet and strip products of the aforementioned character which have greater ductility and less valent strength level is obtained by other means.

A specific object of the invention is to provide a novel low carbon unalloyed steel strip or sheet product having a mlcrostructure consisting essentially of tempered martensite and a tensile strength of at least about 130,000 pel with an elongation in two inches of at least about 1.5$· Another specific object of the Invention is to provide a novel low carbon unalloyed steel strip or sheet product having a duplex mlcrostructure consisting essentially of fer-rite and martensite and having a tensile strength of from about 90,000 to about 130,000 psi with an elongation in two inches of at least about 2.5$.

Another object of the invention is to provide novel coated steel produots of the aforementioned character, partl-cularly tin plate, galvanized steel, and aluminum coated steel.

An additional object of the invention is to provide a novel process for making thin gauge low carbon steel strip of the aforementioned character.

Other objects and advantages of the Invention will become apparent from the subsequent detailed description taken in conjunction with the accompanying drawing, wherein: Figure 1 is a schematic diagram of a continuous heat treating and quenching line for the production of high strength steel strip in accordance with the present invention; Figure 2 is an enlarged schematic view of the quenching apparatus of the line shown in Figure 1; and Figure 3 is an enlarged cross-sectional view taken along the line 3-3 of Figure 2.

The principal constituents of steel which determine high temperature, which is dependent upon the carbon content, steel exists in the form known as austenlte which is a solid solution of carbon or cementite in ferrlte. When steel is cooled slowly from a high temperature at which austenlte is stable, the ferrlte and cementite precipitate together in a characteristic lamellar structure known as pearlite. However, dependent upon the rate of quenching and other factors, the transformation from austenlte to pearlite proceeds through a series of different microstructures. The low temperature de- composition product in the transformation of austenlte upon cooling is martensite which is a body-centered tetragonal structure in which the carbon atoms are thoroughly dispersed. Martensltic steels are characterized by high tensile and yield strengths .

As is well known, very rapid quenching of the austenlte phase is required to obtain a fully martensltic mlcso-structure. Plain carbon steels of relatively high carbon content and certain alloy steels, particularly those containing hardenabllity agents such as boron, are more easily quenched to martensite, but plain carbon steels of relatively low carbon content (e.g. from about .05 wt.$ to about .25 wt.% carbon) are considerably more difficult to quench to martensite. A tempering step may be required following the quench In order to restore a desired amount of ductility. If the steel is heated to an austenltlzlng temperature above the A^ critical point but is quenched at a rate slower than the critical cooling rate, a mixed or duplex microstructure is obtained which comprises one or more transforma ion products in addition to or instead of martensite. This type of quench is referred to as slack quenching and the resultant products are referred to also be required In the case of slack quenched steels. In general, It has been considered desirable heretofore to avoid duplex microstructures in order to obtain the most desirable combination of mechanical properties. In particular, the literature indicates that slack quenched and tempered steels are inferior to steels fully quenched to martenslte and tempered back to an equivalent hardness.

In accordance with one main embodiment of the present invention, low carbon work hardened steel strip in thin gauges and commercial widths is heat treated and quenched on a continuous line to obtain a microstructure consisting essentially of tempered martenslte. Thus, an exceptionally high tensile strength is obtained because of the microstructure and without the development of poor ductility and a high degree of anlsotropy which are characteristic of severely cold worked products. Moreover, by appropriate choice of quench techniques, the resultant product has acceptable flatness or can easily be rolled to the desired flatness. Quite unexpectedel , it has been found that even though the tensile strength of this product is in excess of about 1^0,000 psi (typically, from about 150,000 to about 250,000 psl) and increases with increasing carbon content, nevertheless, within a carbon range of from about .05 wt,# to about .25 wt.# the ductility of the product is uniformly good, i.e. an elongation in two inches of at least about 1.5 and generally from about 1. $ to about 10$.

However, the fully martensitic product may sometimes have greater tensile strength and hardness than are needed or desirable for a given end use. In accordance with a second main embodiment of the invention, a different combination of unalloyed steel of low carbon content in thin gauge sheet or strip form from an intermediate temperature between the lower Ai critical point and the upper A^ critical point at a rate in excess of the critical cooling rate, so that substantially all of the austenite is transformed to martensite. The resultant duplex structure has a very fine grain size and possesses exceptionally good ductility for a given strength level as compared with the products obtained by fully quenching to martensite and tempering or by slack quenching and tempering. More specifically, this product will have a tensile strength of from about 90, 000 to about 1^0, 000 psl and an elongation in two Inches of at least 2.5%, e.g. from about 2 .5$ to about 1 .

The steel sheet or strip employed as the starting material in the invention is plain carbon alloy-free steel having the following composition range (wt. ^) j carbon .05- .25 , manganese .20- 5o , phosphorus .05 max., sulfur, .05 max., and the balance iron with residual elements In the usual amounts. The steel sheet or strip will ordinarily, though not necessarily, be in "work hardened" condition. The term "work hardened", as used herein, refers to a steel strip which has been relatively severely cold reduced and is still In its as-cold reduced condition (sometimes referred to as "full hard"), i.e. without having been subjected to a sub-sequent annealing or tempering treatment. More specifically, a hot rolled strip of Intermediate gauge is pickled and then cold rolled in one or more stages to effect a reduction of at least about θ and preferably at least about 6θ . The cold rolled strip in work hardened condition may have any desired commercial width, e.g. from about l8 to about 72 of about 100,000 pel with poor ductility, e.g. less than 1% elongation in two inches.

Although the gauge of the steel strip starting material will usually and preferably be within the range of from about .002 to about .050 inch, the invention in Its broadest aspect is also applicable to steel strip having a thickness as low as about .0002 inch and as high as about .100 inch. It should be understood, however, that for the very thin or foil gauges ranging from about .0002 to about .002 inch and for the heavier gauges ranging from about .050 to about .100 inch, it may be necessary to employ different quench media or to modify the heating, tension control, or quench systems as compared with the corresponding process system employed for the preferred thickness range of .002 to .050 inch.

As seen in Figure 1, a work hardened or as-cold reduced strip 10 is fed from a payoff reel 11 through a bridle 12 and a looper 15 to a conventional cleaning and rinsing step 14 in which the residual rolling oil is removed. For e ample, an alkaline cleaning medium may be used either with or without electrolytic means. The cleaned strip then passes through the usual roll system and downwardly through a furnace 15 where the strip is heated to a uniform temperature. In the case where a fully martensltic product is being made the strip is heated above the ^ critical point so that the steel is fully austenit ized. This temperature may range from about 1525°F. to as high as about 2100°F., dependent upon the carbon content, but from a practical standpoint effective results may be obtained within the range of from about 1 25° . to about 1750°F. In the case where the product of duplex micro- critical point but below the A3 critical point so that the steel le only partially austenltized. This temperature may range from about 1330°F. to about l670°F. dependent upon the carbon content, but again from a practical standpoint effec-tive results may be obtained within the range of from about l400°F. to about l600°F.

Immediately upon leaving the furnace 15 the heated strip passes into a quench system l6 (more fully described below) where the strip is rapidly quenched to ambient or room temperature at a rate in excess of the critical cooling rate required to transform substantially all of the austenite present to martensite. In general, this requires quenching from the temperature of the strip as it leaves the furnace 15 to below the temperature for the start of martensite formation in from about .1 to about .8 seconds, dependent upon the quench temperature. In the oase of a fully martensitlc product the corresponding quench time is from about .1 to about .4 seconds. The oxide scale formed during quenching is removed from the surface of the strip by pickling in an acid dip 17, and after passage through another looper l8 and bridle 19 the strip is recoiled on a take-up reel 20.

As shown in Figures 2 and 3» "the quench system l6 comprises a tank 30 containing a sinker roll 31 an Extending upwardly from the tank 0 is an elongated conduit section 3^ of rectangular cross-section which provides a restricted quench channel 35. Quenching water flows upwardly through the conduit 34 and spills over the upper edge into a trough 36 having an upright weir 37· Extending downwardly section 58 the lower end of which is disposed in the trough 56 below the upper edge of the weir 57. Effluent water is discharged from the trough 56 through a drain line 39. Since the water level in the space between the weir 57 and the con-duit 5 is determined by the height of the weir 57, it will be recognized that the lower end of the connecting or seal section 58 is sealed by the water confined in the rectangular weir 57 so as to prevent infiltration of air into the furnace 15. If desired, a reducing or other non-oxidizing gas may be supplied to the connecting section (by means not shown) for passage upwardly through the furnace 15, thereby preventing oxidation of the strip.

As the heated strip 10 moves downwardly from the furnace 15 Is passes quickly through the section 58 and enters the upper end of the quench ohannel 35 where it is immediately immersed in the upwardly flowing stream of water. Preferably, the quench section j is also provided Just below its upper end with a plurality of submerged spray units designated schematically at 4θ and having elongated slit orifices (not shown) for directing high velocity streams of water against opposite sides of the strip 10. As the strip 10 leaves the lower end of the quench section $4 it enters the tank 30, passes beneath the roll Jl, and emerges from the exit chute 32.

Uniformity of quenching is essential not only for the sake of obtaining a strip having uniform micro structure and uniform physical properties but also to avoid warpage and distortion of the strip. Irregular vaporization of the water or other quenching medium in contact with the strip can result in substantial differentials in heat transfer rates between and other portions in contact with water vapor. These dlf- ferentials cause different rates of contraction in the steel strip and result in quenching stresses and deformation. However, in the illustrated quench system the necessary high cool- ing rate and the desired uniformity of quenching are realized as a result of the high degree of turbulence and the high volume rate of flow of the water through the restricted quench channel 55 and as a result of the action of the submerged . sprays 4θ. Consequently, a quenched strip of fully or partial-ly martensitlc mlcrostructure is obtained which is either flat enough for its intended use or can easily be rolled to flatness .

Although water is the preferred quenching medium, other media may be used including brine or other aqueous salt solutions, oil, liquid nitrogen, etc. Regardless of the quenching liquid used, however, the volume rate of flow of the quench liquid must be high enough to provide a cooling rate in excess of the critical rate required to convert austenite to martensite, and the turbulence of the quench liquid relative to the strip must be great enough to prevent the accumulation of vapor film which would lead to non-uniformity of quenching and consequent distortion of the strip.

Typical line speeds may range from about 100 ft./min. to about 2000 ft./min. dependent upon the gauge of the strip and the carbon content. The water introduced tc the quench system may be at the ordinary available temperature, e.g. from about 55°F. to about 65°F., and the strip will normally be cooled to approximately the water temperature before leaving the quench tank. If desired, the water or other quench medium may be recirculated through a heat exchanger for temperature When the fully martensltic product Is being made, in situ tempering or self-tempering will take place during the quenching step because the martensite transformation, temperature is quite high for plain carbon steels at the relative ly low carbon levels contemplated by the present invention. For example, for plain carbon steel containing .05 wt. carbon and .40 wt. manganese, the martensite start temperature is estimated to be about 990°F. and the martensite finish tempera ture is estimated as about 6lO°F. For plain carbon steel of .25 wt. carbon and .40 wt. manganese the respective martensite start and finish temperatures are estimated as about 805°F. and about 470°F. Consequently, sufficient tempering of the strip will take place during the short time required to cool from the martensite transformation temperature range down to the ambient temperature or water temperature. Moreover, in those instances where the martensltic strip is subsequently hot dip coated, as in galvanizing or aluminum coating, further tempering will take place during the coating step.

When the duplex mlcrostructure or partially martensltic product is being made, the ductility of the product at a given level of tensile strength is equal to or better than the ductility obtained by fully quenching to martensite from a temperature above the point and subsequently tempering. In addition, the ductility is also better than can be obtained by slack quenching. The duplex mlcrostructure of the product consists essentially of ferrite and martensite with a carbon content corresponding to approximately the equilibrium amount present in austenlte at the temperature from which the steel is quenched. Dependent upon the quench temperature, the car martensite is auto-tempered to a greater or lesser degree during the quenching. It is "believed that the superior ductility of this product is accounted for by the fact that the ferrite does not appear to form envelopes around the austenite as is the case when steel is slack quenched.

In order to obtain a duplex mlcrostructure product having consistently the same desired combination of mechanical properties, it Is necessary to control carefully the temperature from which the steel is quenched. However, a reasonable temperature variation, e.g. from about 10°F. to about 50°F. , can be tolerated without any significant departure from the Intended combination of strength and ductility in the final product. It will be recognized that within the intermediate quench temperature range, the amount of ferrite present is a function of the temperature. However, the carbon content of the ferrite and austenite increase as the temperature from which the steel is quenched Is lowered from the point to the point, thereby producing when quenched a stronger ferrite and a higher carbon content martensite which is also stronger and more resistant to auto-tempering than a lower carbon martensite. Thus, any variation in the relative a-mounts of ferrite and martensite tends to be counteracted by a compensating variation in the carbon content and strength of the ferrite and martensite.

Assuming that the quenching operation has been carried out under optimum conditions, as discussed above, so as to achieve uniformity of quenching across the full width of the strip, the final quenched product will have acceptable flatness for many end uses, as mentioned above. However, the quenched strip can readily be rolled, as on a temper mill, to For example, successful flattening is usually obtained by a single pass through a twin-stand four-high temper mill, each stand having two work rolls and two back-up rolls. Because of the unusual hardness of martenslte, the work rolls may have a high degree of roughness without impairing the surface of the strip, thereby providing adequate flattening in a single pass. This is a particularly advantageous feature in the case where the steel strip is to be tin plated since the conventional black plate practice requires smooth or only slightly roughened work rolls. Although wet rolling may be used, dry rolling is entirely adequate and is preferred because it effects less reduction of the strip and therefore has a less detrimental effect on ductility. In general, the extent of reduction in the temper rolling step should not exceed about % and pref-erably should not exceed about Following the step of rolling for flatness the strip may then be tin plated in a conventional electrolytic tinning operation, the details of which are well known to those skilled in the art. Alternatively, the strip may be galvanized or aluminum coated.

For illustrative purposes, the invention is hereafter described specifically in connection with the production of tin plate. However, It is to be understood that the invention also embraces other coated strip or sheet products, such as galvanized steel and aluminum coated steel, as well as the steel strip per se which is useful, for example, in the manufacture of steel strapping.

EXAMPLE I A series of continuous runs were made on commercial scale equipment of the type Illustrated in Figures 1-3 using starting material in each instance was cold rolled full hard strip of 52-5/4 inch width having a tensile strength of about 100,000 psi and an elongation in two inches of less than 1%. The chemical analyses and gauges of the test coils are shown in the following table · Table I Run Gauge Chemical Analysis No. (inch) C n IP S Si Cu As Nl 1 .0065 .08 .45 .009 .054 .006 .04 .016 .05 2 .ΟΟ65 .14 .48 .009 .028 .005 .05 .015 .02 .0068 .12 .47 .009 .025 .009 .05 .015 .02 4 .ΟΟ65 .08 .41 .016 .050 .001 .08 .018 - 5 .ΟΟ65 .09 .41 .016 .050 .014 .08 .018 — The processing data for the five runs are shown Table II, as follows: Table II Line Water Temp. Water Flow Run Speed Steel Quench (°F.) (gal./min. ) No. (ft. /tain. Temp.*^.) Entry Exit Inlet 33 Sprays 40 1 750-775 1450-1 0 65 84 720 650 2 80-70 l44o-l450 64 84 700 450 75Ο-7 Ο 1420 65 85 790 640 4 650-720 l420-l48o 54 74 740 700 680 1460-1550 54 74 740 700 ♦Temperature of heated strip just before contact with the quench water.

Photomicrographs of test specimens from the quenched strips showed that in each case a duplex micr ostructure was obtained consisting of ferrite and martensite.

The coils from each run were rolled for flatness with rough blasted rolls on a twin-stand or three-stand four-high temper mill. Adequate flattening of the strip was ob tin plated In a conventional acid electrolytic tinning line.

The average properties of the final tin plate product are shown in Table HI below, the carbon contents and gauges being repeated for convenience.

Table III Run No. 1 2 5 4 5 Carbon, vt.% .08 .14 .12 ,o8 -.09 Gauge, in. .0065 .0065 .ΟΟ68 .ΟΟ65 .0065 Yield strength 90,000 124,100 96,8θΟ 79,200 106,200 (.2% offset), psi Tensile strength, 102, 600 136,100 108,000 101,200 12^,800 psi Elongation, % in 3.0 2.5 3.0 6.5 4.1 2 inches Olsen Ductility, .123 .1^1 .137 .176 Inch Comparing the results of Runs 2 and J, it will be seen that at the higher carbon content and higher quench temperature of Run 2 the resultant product had greater yield and tensile strengths. On the other hand, the products from Runs 1 and 4 which were conducted at the same carbon level and substantially the same quench temperature had similar yield and tensile strengths.

The effect of quench temperature Is shown by com-paring Runs 4 and 5 at substantially the same carbon level. The product from Run 5 at the higher quench temperature had appreciably greater yield and tensile strengths indicating a greater amount of martensite in the microstructure .

The compensating effects of carbon content and quench temperature are illustrated in Runs 1 and 3. Thus, the product properties obtained in Run 1 at a lower carbon content and higher quench temperature were substantially the same as and a lower quench temperature.

EXAMPLE II The results obtained In Example I may be compared with the results of quenching fully to martensite and temper- ing. For the latter purpose, a plain carbon steel strip having substantially the same analysis shown for Run 2 in Example I was heated to 1700°F., i.e. above the critical point, and quenched in the same equipment used in Example I. Samples of the resultant fully martensitlc product were given a laboratory tempering treatment involving heating at 400°F. for 50 minutes, which treatment was Intended to simulate the lacquer or enamel baking to which tin plate is frequently subjected. No effect on mechanical properties was noted. Other samples were heated for 0 minutes at higher temperatures, the resultant properties being shown in Table IV, as follows: Table IV Temp.. °F. Tensile Strength, psl % Elongation in 2 inches As quenched 198,300 3.5 600 138,600 1.8 700 138,300 2.1 8OO 118,000 3.6 900 104, 800 4.5 As will be appare t, for a substantially equivalent strength level, the product of Run 2 in Example I had as good or better ductility (2.5 elongation) than the product of Example II tempered at 600-700°F.

Additional samples were obtained from fully martensitlc steel strips having carbon contents of .09% and .11% and were given tempering treatments designed to simulate the heating cycle experienced in conventional hot dip galvanizing.

Table V Tensile % Carbon, Strength Elongation wt . Tempering Treatment pal in 2 Inches .09 2 mln. at 600°F., 5 sec. at 860°F. 1^2,500 2.0 .09 " " " 800°F., « " " " 133,800 .09 " " « 900°F. « » " » 119,400 .09 M » " 1000°F. » w " « 106,500 3.5 .09 1 mln. at 1100°F. " » " » 104,000 2.5 .11 2 mln. at 600°F., 5 sec. at 860°F. 133, 800 1-5 .11 » » « 800°F., « « « « 131,600 I. .11 H » » 900°F., " " « » 119,300 1.5 .11 " « " 1000°F., « » » " 107,600 3. .11 1 « « 1100°F., « « " « 102,000 4 A comparison of the foregoing results with the results of Runs 1, 3, 4 and 5 shown In Table m of Example I again shows that, for substantially the same carbon content and strength level, the product of the present Invention has equal or, in many cases, better ductility than can be ob-tained by quenching fully to martensite and tempering. Moreover, these results are obtained without the necessity for a separate tempering treatment.

EXAMPLE III A series of continuous runs were made on commercial scale equipment of the type illustrated in Figures 1-3 using lake water at ambient temperature as the quench medium. The starting material In eaoh instance was cold rolled full hard strip of 32-3/4 inch width having a tensile strength of about 100,000 pel and an elongation in two inches of less than 1%. The chemical analyses and gauges of the test colls are shown in the following table; Table VI Run Gauge Chemical Analysis (wt . ) No. (Inch T § 31 Cu 1 .0057 .09 .45 .011 .022 .001 .03 .018 2 .0084 .09 . .012 .022 .010 .02 .016 3 .0080 .11 • 34 .012 .021 .004 .03 .013 4 .0083 .19 .42 .016 .022 .011 .03 .014 The processing data for the four runs are shown in Table VII, as follows'.

Table VII Line Water Temp Water Flow Run Speed Steel Quench (°F.) (gal. /.mm.) No. (ft ./Mli Temp.«(0F. Entry Inlet 35 Sprays 40 625-750 1630-1720 60-65 66-74 725 700 450- 75 1730-1750 60-65 66-74 725 700 475-510 1670-1760 6Ο-65 66-74 725 700 490-520 17OO 60-65 66-74 725 700 ^Temperature of heated strip just before contact with the quench water .

Photomicrographs of test specimens from the quenched strips showed that the miorostructure in each case was entirely tempered martensite.

The coils from each run were dry rolled for flatness on a twin stand four high temper mill. The work rolls had .010 inch crowns and were blasted with No. l4 grit to provide a rough surface. Because of the high hardness of the quenched material adequate flattening of the strip was obtained on a single pass with less than 1/2 reduction and without undue roughening of the strip surface. The colls from these runs were then tin plated in a conventional acid electrolytic tinning line. The average properties of the final tin plate product are hown n able be e e "being repeated for convenience.

Table VIII Run No. 1 2 Carbon, wt . % .09 -09 .11 .19 Gauge, inch .0057 ·Οθ84 .Οθ8θ 008j Tensile strength, 174,000 172,000 189,000 222,000 psi Elongation, % 1.5-2.0 2.5-5.Ο 2.0-2.5 2.0-2.5 in 5 inches Olsen ductility, .15 .158 .151 .151 inch Rockwell hardness, 55 55 58 65 50-N Pickle lag* 6 4 5 ATC* .02 .04 .05 .05 *Pickle lag test and ATC (alloy tin couple) test are described in "Tlnplate Testing" (May i960) by Tin Research Institute, Appendix XII and Appendix XIV, respectively.

From the foregoing data it will be seen that the product had exceptionally high strength even in very thin gauge 8. Moreover, the ductility of the product was uniformly good even at the highest carbon content in Run No. 4. Corrosion resistance, as measured by the piokle lag and ATC tests, was excellent. Although not shown In Table VIII, the differ-ence between longitudinal and transverse measurements of tensile strength was slight so that the product was essentially isotropic .

In addition, Rockwell (30-N) hardness traverses were made at one inch intervals across the width of each strip with the following results; Median Value Run No. 4- Deviation 1 53 t 3 2 5 .5 - 2.5 3 58 ! 1 4 65 ί 1 Thus, it will be evident from the uniform hardness of the strips that uniform quenching was carried out and that a very uniform martensitic mlcrostructure was obtained.

The test results show that by the heating and quenching process of the present invention a full hard cold rolled strip having a tensile strength of about 100,000 psi and very poor ductility (less than 1% elongation in two Inches) is transformed into a material having equal or better tensile strength and vastly improved ductility, this result being obtained by a simple heating and quenching sequence without any subsequent tempering step. In conventional tin plate processing sequences, which rely upon cold reduction to provide strength, it is impossible to obtain this desirable combination of properties. For example, when annealing is employed after cold reduction, ductility is Improved at the expense of tensile strength. When double cold reduction is employed, the ductility is poor and even then the strength levels achieved are much less than can be obtained by the present Invention. Moreover, the martensitic and the partially martensitic tin plate of the present invention are substantially isotropic. For example, the difference between longitudinal and transverse measurements of tensile strength will generally be on the order of 2-^ or less, whereas in a typical double reduced tin plate product such difference is 10$ or more.

A further important advantage of steel strip made resistance to breakage as compared with steel strip materials heretofore used in continuous processes. For example, double reduced tin plate has a relatively high transition temperature and is highly susceptible to fracture or tearing if edge cracks develop in the strip. This undesirable property is especially noticeable in nitrogenlzed double reduced tin plate. The steel atrip herein described is much less sensitive in this respect since edge cracks do not tend to propagate as readily and less breakage of the strip is encountered. As will be appreciated, the foregoing advantage becomes increasingly important as the thickness of the strip decreases.

DATED the day of 1965 S. HOROWITZ & CO.

Agents for Applicants

Claims (1)

1. Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is : 1. A thin gauge steel product in sheet or strip form comprising a plain carbon steel having a carbon content of from about .03 wt.$ to about .25 wt .% and a microstructure consisting at least partially of martensite, said product having a tensile strength of at least about 90, 000 psi and an elongation in two inches of at least about 1.5 · 2. The product of claim 1 further characterized in that it is substantially isotropic. 3. The product of claim 1 further characterized in that said steel has a thickness of from about .0002 to about .100 inch. 4. The product of claim 1 further characterized in that said steel has a thickness of from about .002 to about .050 inch. 5. The product of claim 1 further characterized in that said microstructure is a duplex microstructure consisting essentially of ferrite and martensite, said tensile strength is from about 90, 000 to about 130, 000 psi, and said elongation is at least about 2 .5 . 6. The product of olalm further characterized in that said elongation is from about 2. $ to about 13 . 7. The product of claim 5 further characterized in that said steel has a thickness of from about .0002 to about .100 inch and comprises a plain carbon steel having a carbon content of from about .03 wt. to about .25 wt.$ and a manganese content of from about .20 wt.$ to about .60 wt.$. 8. The product of claim 1 further characterized in that said microstructure consists essentially of tempered martensite and said tensile strength is at least about 130, 000 psi. that aaia tensile strength Is from about 150,000 to about 250,000 pel and said elongation is from about 1. $ to about 10#. 10. The product of claim 8 further characterized in that said steel has a thickness of from about .0002 to about .100 inch and comprises a plain carbon steel having a carbon content of from about .05 wt . to about .25 wt.% and a manganese content of from about .20 wt.$ to about *60 wt. . 11. The product of claim 1 further characterized in that said steel is coated with another metal. 12. The product of claim 1 further characterized in that said steel has a coating selected from the group consisting of tin, zinc, and aluminum. 15. The product of claim 5 further characterized in that said steel has a tin coating and a thickness of from about .002 to about .015 inch. 14. The product of claim 8 further characterized in that said steel has a tin coating and a thickness of from about .002 to about «,015 inch. 15. A process for making thin gauge high tensile strength steel sheet or strip which comprises heating plain carbon thin gauge steel sheet or strip having a carbon content of from about .05 wt.$ to about .25 wt.$ to a temperature at least above the critical point so as to at least partially austenltize the steel, and uniformly quenching the sheet or strip from said temperature to obtain a mierostructure oonslst-jling at least, partially of martenslte, l6» The process of oliaim 15 further characterized in that said sheet or strip is heated to an intermediate temperature above the critical point but below the. critical In that said temperature is from about IJJQ0?. to about l670°F., dependent upon the carbon content. 18. The process of claim 17 further characterized in that said temperature is from about l400°F. to about l600°F. 19. The process of claim 15 further characterized in that said sheet or strip is heated to a temperature above the critical point whereby said steel is fully austenit ized. 20. The process of claim 19 further characterized in that said temperature is from about 1525°F. to about 2100°F., dependent upon the carbon content. 2l„ The process of claim 15 further characterized in that said sheet or strip has a thickness of from about .0002 to about .100 inch. 22. The process of claim 15 further characterized in that said sheet or strip has a thickness of from about .002 to about .050 inch. 25. The process of claim 15 further characterized in that an intermediate gauge plain carbon steel sheet or strip is cold reduced by at least about θ% to obtain a work hardened sheet or strip of desired gauge between about .0002 and .100 inch which is then heated and quenched, said work hardened sheet or strip having a tensile strength on the order of about 100,000 psi and an elongation in two Inches of less than about 1%. 24. The process of claim 15 further characterized in that the quenched sheet or strip is thereafter rolled to flatten the same. 25. The process of claim 15 further characterized in that the quenched sheet or strip is thereafter tin plated. 26. A thin gauge steel product in sheet or strip accompan ing drawings . 27. A process for making thin gauge high tensile strength steel sheet or strip substantially as here in described with reference t o the accompanying drawings . DATED the day of 1965 S. HOROWITZ & CO. Agents for Applicants