US3783040A - Low carbon high strength steel - Google Patents

Abstract

LOW CARBON STEELS HAVING HIGH STRENGTH, E.G. FOR STRUCTURAL PLATE, COMPRISE MOLYBDENUM AND/OR BORON AS AN INGREDIENT COATING TO AVOID FERRITE-PEARLITE TRANFORMATION AND A SECOND INGREDIENT, IN ECONOMICAL AMOUNT, OF ELEMENTS FROM THE CLASS OF MANGANESE, NICKEL, CHROMIUM ABND COPPER COATING TO RAISE THE LEVELS OF BOTH STRENGTH AND TOUGHNESS SILULTANEOUSLY WHILE AFFORDING TRANSFORMATION IN THE BAINITE REGION, SUCH TRANSFORMATION BEING THEREBY PREFERABLY INITIATED AT A LOW TEMPERATURE, SO THAT UPON COOLING THE PRODUCT DIRECTLY FROM A HOT ROLLING OPERATION, E.G. BY AIR COOLING, THE STEEL TRANSFORMS TO A MICROSTRUCTURE EXHIVITING A PLATE-LIKE MORPHOLOGY AND HAVING YIELD STRENGTH OF AT LEAST ABOUT 95 K.S.I. AND GOOD IMPACT TOUGHNESS, NEED FOR HEAT TREATMENT, SUCH AS IN TEMPERING AFTER QUENCHING, IS OBVIATED. PROPERTIES CAN BE FURTHER ENHANCED BY A MINOR AMOUNT OF A THIRD INGREDIENT, FOR EXAMPLE COLUMBIUM, WHICH COOPERATES IN ATAINING DESIRED RESULTS.

Description

Jan. 1, 1974 J, B BALLANCE ET AL 3,783,040

LOW CARBON HIGH STRENGTH STEEL Original Filed May 26, 1970 6 Sheets-Sheet 2 IZO Z I 00* r ya 80 1 I I I I I 1 I I I200 #00 won 900 500 700 600 600 E TEMPE/077M915 {mm/M21 0 "F' F ici. E

v INVENTORS JOHN B. BALLAIUCE STEPHEN J. MATAS BY KONRAD JAKUNDIG M1 J. (QM/M,

ATTORNEY Jan. 1, 1974 J, a BALLANCE ET AL 3,783,040

LOW CARBON HlGH STRENGTH STEEL Original Filed May 26, 1970 6 Sheets-Sheet I5 Q 5k o o 0 E 40* o E Q i 0 O O E k 0 0 E o 0 U A -Zd0 i 1 I I l l 1 5 EMPERzifl/Rf imam/2750),

I :IET 3 INVENTORS JOHN B. BALLAMCE STEPHEN J. ATAS BY KONR'AD LAJ JUNDlG ATTORNEY Jan. 1, 1974 J. B. BALLANCE ET AL LOW CARBON HIGH STRENGTH STEEL Original Filed May 26, 1970 6 Sheets-Sheet 4 I4 COPPER VAR/4770M O lV/CKEL VAR/A 770N A MANGANESE VAR/A770 U=CHROMUM VAR/A r/o/v 1 M1, 0, A/L' I/A/I /A T/O/I/ I I I I I I I 320 300 250 260 240 220 200 m /60 M0 lzo mg IIIII IIIIII INVENTORS JOHN B. BALLAN E STEPHEN J, MATAS KONRADIAKUNDIG IM J. (9W2...-

ATTORNEY N Jan. 1-, 1974 J. B. BALLANCE ETAL 3,733,040

LOW CARBON HIGH STRENGTH STEEL Original Filed May 26, 1970 6 Sheets-Sheet 5 Jan. 1., 1974 J. B. BALLANCE ETAL 3,783,040

LOW CARBON HIGH STRENGTH STEEL nal Filed May 26 1970 6 Sheets-Sheet 6 igi United States Patent Oflice 3,783,040 Patented Jan. 1, 1974 Int. Cl. C22c 39/20, 39/54 US. Cl. 148-36 20 Claims ABSTRACT OF THE DISCLOSURE Low carbon steels having high strength, eg for structural plate, comprise molybdenum and/or boron as an ingredient coacting to avoid ferrite-pearlite transformation and a second ingredient, in economical amount, of elements from the class of manganese, nickel, chromium and copper coacting to raise the levels of both strength and toughness simultaneously while affording transformation in the bainite region, such transformation being thereby preferably initiated at a low temperature, so that upon cooling the product directly from a hot rolling operation, e.g. by air cooling, the steel transforms to a microstructure exhibiting a plate-like morphology and having yield strength of at least about 95 k.s.i. and good impact toughness. Need for heat treatment, such as in tempering after quenching, is obviated. Properties can be further enhanced by a minor amount of a third ingredient, for example columbium, which cooperates in attaining desired results.

BACKGROUND OF THE INVENTION This is a continuation of Ser. No. 40,483, filed May 26, 1970, and now abandoned.

This invention relates to high strength, low carbon steel notably of a low-alloy category, at least in the sense of avoiding relatively large proportions of expensive alloying elements. The invention is more particularly concerned with steels of the stated class as designed for structural uses, or in a specific sense, with steel objects, such as plate or other rolled shapes or articles which are appropriate for uses requiring yield strength of the order of 100,000 pounds per square inch and upward, i.e. 100 k.s.i. (the abbreviation k.s.i. being used herein, conventionally, to mean 1000 pounds per square inch) and having satisfactory impact properties. A particular object is to provide for the production of such steel objects with unusual economy of cost, e.g. as regards factors such as alloying elements and methods of fabrication and treatment.

At the present time steel plate and the like with yield strengths of 100 k.s.i. appropriate for use in construction is conventionally available in the heat treated form, achieving the required mechanical properties, including yield strength and impact or toughness values, by the usual quenching, and tempering operations. An important specific feature of the present invention is to provide steels which are able to attain, in the specific microstructure of bainite, a suitably high order of strength and toughness, very advantageously without requiring heat treatment, such as quenching and tempering, of the hot rolled objects.

Whereas the common structure of quenched and tempered steels has classically been tempered martensite and it has been recognized that the bainitic structure can represent another alternative for avoidance of ferrite-pearlite structures to which austenite usually transforms if not quenched to martensite and which are undesirable for high strength structural applications, effective and economically practical attainment of bainite, particularly bainite obtained from transformation of austenite at lower temperatures, does not appear to have been achieved under circumstances affording high strength results in hot rolled structural steel products. Special procedures for production of bainite, involving partial quenching and then isothermal transformation by holding at a selected temperature, are manifestly costly, and have not generally been regarded as offering much practical advantage for low-cost structural steels.

Some efforts have been made to produce steels which, because of their composition, will transform to bainite upon air cooling from a hot-formed or austenitized state, it being recognzied that certain elements such as molybdenum or alternatively boron affect conventional transformation curves in such manner, e.g. being understood to shift the ferrite-pearlite transformation so as to require longer times before the beginning of transformation, and thus permit cooling to a predominantly bainitic microstructure while bypassing, so to speak, the ferrite and pearlite regions. Some such proposals have been limited in their indicated potentiality, e.g. as for massive forgings, and in all cases, so far as is known, there has been failure to recognize or discover an alloy composition or any underlying alloying principles which would permit the attainment of truly high mechanical properties after direct bainitic transformation, by essentially simple cooling, from an as-rolled state.

Thus with a limited content of molybdenum and manganese, with boron addition, steels readily transforming to bainite have been produced, for which the strength and toughness affords relatively little improvement over conventional pearlitic-ferritic structures, for structural applications. In another disclosure, concerned with large forgings or castings, high and correspondingly expensive additions of nickel, as of the order of 5% or more, have been indicated, while a different proposal has been concerned with additions of nickel and copper to a molybdenum-containing low carbon composition (which may also include boron), but with yield strengths, in the asrolled and cooled (and presumably bainite-transformed) condition, that fall short of the high values mentioned above.

Still another effort in the art, toward the provision of high strength steels without requiring heat treatment, has asserted merit for special alloy combinations, such as a combined addition of 3% Mo and 3% Ni (which are costly elements) or an alternative of 3% Cr and 2.5% Mn, but the products are described as formed by a complex program of hot rolling and successive reheating.

Such products, however, have not been disclosed as attaining a bainitic microstructure. There are further dis closures asserted to afford useful results for hot rolled articles with avoidance of quenching and tempering, but these proposals have involved no consideration of or reference to a bainitic structure and have not suggested the complete combination of alloying elements (e.g. including one or another or more of such elements as nickel, boron and columbium along with others, as defined below) that has been discovered to be of unusual practical advantage in accordance with the principles of the present invention of high strength bainitic steel.

As will be understood, bainite is a microstructure of steel that can be defined as consisting of an aggregate of ferrite and carbide which in general is produced 'by transformation from austenite at temperatures lower than those where ferrite and pearlite form and higher than that of the beginning of martensite formation on cooling. Present views as to the nature of the bainitic structure are further explained below, it being particularly noted that in the bainite attained in the steels of this invention there appears to be a correlation between the presence of a plate-like structure and the achievement of superior levels of strength and toughness.

SUMMARY OF THE INVENTION For the attainment of novel and unusually effective high strength bainitic steels, and especially hot-formed, low carbon steel products that are characterized by a sub stantially bainitic structure, it has been discovered that unusually effective results are obtained by a novel combination of metallurgical ingredients, coacting to produce superior mechanical properties, including a yield strength of at least about 95 k.s.i., as well as satisfactory impact properties, for example an impact strength measured by energy absorption of at least about 12 foot-pounds, or very preferably higher, at 20 F. in the standard Charpy V-notch test. More specifically, the composition of steel according to the invention, being basically a low carbon steel, comprises a first metallurgical ingredient which consists advantageously of molybdenum and boron, or at least one of them, and which is effective in function and amount to establish a time-temperature-transformation relationship such that the conventional ferrite-pearlite structure Will be formed only at a very slow cooling rate, and a second metallurgical ingredient which consists of at least two elements selected from the group consisting of manganese, chromium, nickel and copper, and which at least includes chromium and nickel for special advantage in many cases, this second metallurgical ingredient being effective in function and amount to establish the starting temperature of bainite transformation at a relatively low point, for example below about 950 F.

The first ingredient is preferably present in total amount up to about 1%. The second ingredient is present in total amount sufficient for achievement of the desired results in the complete composition; such total amount is at least 4% in many instances, notably for slower rates of cooling the product, e.g. below 180 F. per minute as presently contemplated for air cooling of /2-inch plate, but the amount may be less (preferably upwards of 3%) where cooling is faster.

The composition also includes, for steels of unusually effective character, a third metallurgical ingredient that consists of at least one element selected from the group consisting of columbium, tantalum, tungsten, titanium, vanadium and zirconium, and preferably comprises at least 0.03% to 0.2% columbium for exceptionally useful results, this third ingredient being effective in function and amount to magnify at least one mechanical property, e.g. strength or toughness or both, of the bainitic microstructure.

As stated, the first ingredient comprises molybdenum or boron or both, and in many examples below, specifically comprises 0.3% to 1% molybdenum, advantageously with an effective amount of boron, e.g. 0.0005% to 0.01%. Present evidence indicates that satisfactory results are obtainable With the inclusion of only one of these two elements when the total amount of the second ingredient is at least about 4%, but that both are in most cases essenial, for example in amounts of the order stated herein, when the total for the second ingredient is below 4%. Additional compositional characteristics include a presently preferred range of carbon up to 0.25 and the following ranges for the elements of the second ingredient: to 3% Mn, 0.5 -to 4% Cr, 0.8 to 3.5% Ni and 0 to 2% Cu. All percentages herein are stated by weight, and the balance of the composition is, of course, iron and incidental constiuents.

It has been found that the stated ingredients, in a low carbon steel composition having not more than a reasonable content of silicon (e.g. from 0 to 0.8%) coact in an unusual manner to provide a steel which when directly cooled from the completion of hot rolling (say, at temperatures of 1450 F. or above) in a simple, continuous manner that can ordinarily be achieved by air cooling, transforms to a high strength, satisfactorily tough bainitic microstructure, yielding a product well suited for structural uses. According to present understanding, the first ingredient functions to shift the ferrite-pearlite region or nose of the transformation curve to the right, i.e. in such direction on the time scale that the indicated continuous cooling operation essentially completely bypasses such region, i.e. permits the metal to cool below the pearlite zone before expiration of the relatively longer time that would be required to enter such zone; in consequence the cooling operation is directed to the bainite transformation zone.

The second ingredient, in the defined compositional range, has the property of depressing the starting temperature of bainite transformation (B and by doing so, enhancing both the yield strength and the impact strength of the resulting product. That is to say, it has been found that as the content of this second ingredient is increased, there is progressive and essentially simultaneous improvement in both of the stated mechanical properties, being thus in contrast to the conventional experience of alloying practice that yield or tensile strength is improved only at the expense of toughness and toughness or impact strength is usually improved with sacrifice of tensile values. The increase in the second ingredient is simultaneously effective in a generally proportional manner for lowering the B temperature in such a manner that reasonable variation of cooling rate can be permitted without allowing the metal to bypass the bainite zone, and indeed correspondingly assuring the formation of the desired bainitic microstructure in all cases.

Finally, the inclusion of the third ingredient further coacts with the otherwise defined composition to magnify or enlarge one or more of the desired mechanical properties, as for example in that with the inclusion of a preferred content of columbium, a suitably high level of yield strength and effective impact strength may be achieved with less amount of other alloying elements (specifically in the second ingredient) than otherwise possible.

Referring further to proportions of alloying elements that appear at present to be desirable for particularly effective results, the nickel content is advantageously at least 0.95%, the second of the above-named ingredients preferably containing manganese in amounts of 0.5% and upwards, i.e. along with nickel and chromium, it being generally preferred that the total elements of this ingredient be above 4% substantially unless faster cooling rates are used as indicated above, for example as in air cooling inch or thinner products. Some specific advantage, notably in reaching an economical composition as well as an unusually effective one, is the provision of at least 1% each of the elements chromium and nickel, the amount of nickel, which may be expensive, being conveniently not more than 2.5%. Limitation of nickel is best realized in compositions including copper, e.g. up- 'wards of 0.3% or preferably at least 05%. Copper not only contributes cooperatively to the desired function of the second alloying ingredient, especially in matter of toughness, and indeed specifically constitutes a replacement for at least some portion of a total nickel amount that might otherwise be preferred, but also aids corrosion resistance and imparts or enhances age hardening properties, e.g. such that the steel may on aging exhibit substantially higher tensile values.

Further modifications of proportions of alloying ingredients are noted below as applicable to particular situations or requirements of practice or use as for example in that in some cases, e.g. with higher total amounts of the second alloying ingredient, the proportions or composition of the first ingredient may conceivably be reduced, or the situation may in a number of cases be such that desired results are obtained by assuring suflicient presence of only one element of the group consisting of molybdenum, boron and columbium and its alternatives. In all cases, the desired and attained object is to afford a steel composition which is readily transformable to a bainitic structure having usefully high mechanical properties, notably tensile properties.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graphical illustration plotting various heats of steel to show the relation between changes in yield strength and toughness for /a-inch plate which has been air cooled after hot rolling, as occurring with various compositions, including compositions embodying the present invention.

FIG. 2 is a graph showing the relation between tensile values and bainite transformation starting temperature for various tested heats of steel, including compositions of the invention.

FIG. 3 is a graph showing changes in toughness relative to bainite transformation starting temperature for the heats of FIG. 2.

FIG. 4 is a graph illustrating the relation between yield strength and the difference between bainite and martensite transformation starting temperatures, as affected by variation in alloying elements, involved in the invention.

FIG. 5 is a graph showing the effect of alloying element variations, similar to FIG. 4, in reference to the relation between toughness and the above-stated temperature difference.

FIG. 6 is a photomicrograph produced by transmission electron microscopy, original magnification 10,000 times, of one example of an improved bainitic steel product of the invention.

FIG. 7 is a like photomicrograph of another example of an improved product.

DETAILED DESCRIPTION The invention and its products and procedures are best explained in relation to an extended series of test heats of steel, including numerous compositions presently understood to embody the principles of the invention, as well as other compositions for comparison purposes, each of the beats and products of the first-mentioned category thus representing a specific, working example.

In the tabulated data set forth below, wherein the heats are identified by numbers prefixed with the letters B or V, each heat represented a separate melt of steel, prepared in an induction-heated furnace in accordance with standard practice and incorporating the various alloying elements in conventional form and manner with exception that elements such as boron, columbium, vanadium, zirconium, titanium, tantalum and tungsten, when used, were only added to the melt after completion of the heat, and specifically after deoxidation.

It will be appreciated that in making up melts, for example as representative of the invention, alloying ingredients can be added in conventional forms.

In general, these were heats of a quantity of 70 pounds each in at least most cases, or quantities of like order appropriate for the described treatment, testing and examination. The series identified by the letter B were so-called air induction heats, i.e. in the sense that the induction furnace was in communication with the atmosphere, whereas heats denoted by the letter V were vacuum induction heats, wherein the melt was maintained under high vacuum, for observation of possible effect thereof, as for degassing. It did not appear, however, that any notable advantage was related to the vacuum heats as compared with the air heats. In all cases the steel was killed prior to or upon casting in ingot form, conveniently with addition of aluminum metal in suitable amount appropriate for some degree of killing (meaning an extent of killing that would be recognized as having at least some useful significance), e.g. a normal proportion of 2 pounds or more per ton of steel, for instance 2 /2 pounds per ton. Although other conventional deoxidizing operations are applicable, such as by use of silicon or other metals, or even by the so-called vacuum carbon deoxidation technique alone, aluminum killing is found highly effective and economical, it being understood that the steel should be produced in some degree of the killed state when additions of elements prone to oxidation, e.g. boron, titanium and the like, are to be made for their desired effects on properties.

The several test heats were cast as suitable ingots, e.g. 4-inch square ingots, from which shape appropriate for hot rolling were prepared; in general this shape conversion involved hot forging to 1% inch sheet bars, being slabs having a cross-section of about 1.5 inches by 6 inches, although it will be understood that in production heats, conventional practice of rolling to slabs or billets is employed.

The prepared sheet bar or like shape, which was generally allowed to cool, was conditioned by soaking at a suitable elevated temperature, e.g. in the austenitizing range, such as 2l50 to 2350" F. In general, these test heats were thus soaked for one hour at about 2350 F., although it was noted that higher temperatures can be advantageously employed, and indeed would appear preferable in commercial operation, e.g. up to 2400 F. or above if desired. As will be understood, higher soaking or austenitizing temperatures promote the evening out of any inhomogeneities in the ingot, and of course, the procedure can embody successive soaking stages as may be conventional, e.g. prior'to working the ingot in the blooming mill and again after hot rolling to slab, i.e. just prior to ultimate fabrication of desired product by hot rolling.

In the test practice for the several enumerated heats, the sheet bars at the soaking temperature were hot rolled to approximately /2 inch plate in straightaway manner, i.e. with no cross rolling employed; specifically the practice involved rolling to 0.550 inch plate in five passes. The final rolling temperature was around 1500 F.; in some cases it was specially designed to reach about 1900 F. and in other cases 1100 F., present indication being that hot rolling should preferably be concluded in the range upwards of 1400 F. After hot rolling the plates were allowed to air cool, e.g. standing on edge so as to avoid uneven transformation rates.

Following air cooling, tests were made in a standard manner to determine mechanical properties, including ultimate tensile strength, yield strength (0.2% offset), and in a number of cases elongation, the values of the latter being found satisfactory for all products deemed to come within the invention, e.g. of the order of 20% or above, or even substantially higher. Toughness was examined by the standard Charpy V-notch test, determining energy absorption in foot-pounds (required for breakage on impact) at various temperatures. In all cases effort was made to perform the Charpy tests on specimens taken across the rolling direction of the plate, it being considered, and indeed noted, that higher impact properties are observed lengthwise of the rolling direction and it being deemed important to appraise the toughness in the more diflicult crosswise situation. In all instances more than one V-notch test was performed, the tabulated results being the less favorable of the set in each instance. Special attention was paid to low temperature impact properties, readings being taken at sub-zero temperatures, e.g. to find the value for a designated temperature or to find by conversion or direct readings, the lowest temperature at which a given impact value, for example 15 foot-pounds, can be obtained.

Microscopic examination, especially by electron transmission microscopy, was also performed on samples from numerous heats, the result being an observational determination that in general, all such instances involving compositions of the invention, and by extrapolation, other test heats within the presently understood limits of the invention, can be correlated with the presence of microstructural features of bainitic character as explained here inbelow.

7 The following Table I sets forth the compositions of a considerable number of heats of the described series of tests, these being specifically the air induction heats and the composition in each case consisting of the elements noted, balance iron and incidental impurities.

TABLE I.AIR HEATS Composition (weight percent) Heat Si Cr Mo Ni Cu Cb B 07 Zr; B756, 0.05 v; B757, 0.08 v; B758,

1 Also: B658, 0.02 Ti; B702, 0. 0.16 V.

As explained above, all of these heats were duly processed, including hot rolling to plate, and immediately thereafter air cooling to final product state. The determinations of the mechanical properties of the plate products, for the heats of Table I, are as follows:

TABLE II Ultimate Yield tensile Charpy V-N, foot-lbs. at strength, strengt Heat number k.s.i. k .s.i +70 F. -20 F.

B122 105. 4 153.0 35 22 B123 111. 0 162. 7 40 27 B435 107. 9 161. 6 24 15 B436 120. 7 172. 1 39 26 B437 100. 6 150. 8 11 6 B438 111.8 171. 3 36 21 B439 105. 0 161. 4 32 12 B440 105.0 162. 1 36 18 B441 95. 3 141. 7 6 102. 7 158. 9 21 9 109. 0 162. 7 18 8 102. 7 174. 6 28 17 106. 3 158. 9 12 6 107. 5 178. 9 24 14 103. 4 168. 3 13 111. 3 175. 5 26 13 107. 4 167. 1 26 14 119. 9 180. 0 30 20 110. 2 170. 5 20 6 118. 8 174. 9 23 18 141. 3 208. 4 17 14 102. 8 158. 5 20 8 117. 7 171. 7 26 20 96. 1 145. 9 12 5 102. 0 157. 5 15 7 104. 0 160. 0 18 6 108. 3 167. 2 24 12 103. 1 158. 1 24 16 114. 8 159. 7 26 18 147. 2 245. 0 15 14 148. 2 243. 7 18 17 109. 3 172. 8 36 24 132. 5 187. 5 38 34 135. 5 203. 6 35 28 144. 0 211. 0 28 24 145. 0 221. 4 30 28 122. 2 175. 3 26 18 119. 0 170. 2 22 16 119. 8 173. 3 21 14 118. 6 169. 5 19 12 117. 8 179. 4 20 12 116. 7 178. 7 20 14 115. 9 178. 3 24 18 118. 7 174. 9 22 16 119. 0 174. 9 24 16 122. 2 189. 2 24 18 124. 6 174. 9 24 18 107. 1 165. 4 20 10 109. 8 167. 9 23 13 117. 5 174. 3 23 16 121. 8 179. 1 23 16 121. 0 182. 9 24 19 120. 6 182. 5 22 15 B679 123. 8 181.0 22 18 B680 120.6 186. 1 25 18 B681 132. 2 182.8 22 16 B682 144.2 204. 8 19 16 B683- 126. 6 184. 2 22 18 B698 124.5 182.2 25 18 B699 125. 5 185. 0 21 17 B700 126. 8 186.3 19 17 B701 120. 2 178.3 24 17 B702 136. 4 192. 0 17 13 B703 120.0 186.0 24 19 B704 127. 7 190. 8 19 13 B746 98. 0 142. 8 24 14 B747 106.0 160. 0 25 19 B748 107'. 2 164. 5 22 10 B749 105. 0 152. 2 32 18 B750 99.6 145.0 30 21 B751 106.0 153.0 26 20 B752 103. 2 148. 0 23 20 B753 103. 2 149. 4 20 12 B754 100. 0 143. 4 20 14 B765 92. 0 132.2 23 16 B756 92.6 135. 8 24 18 B757 107. 0 152. 4 11 10 B758 94. 1 136. 4 23 18 B759 105.1 150. 0 24 20 In Table III there are described a series of vacuum heats that resulted in products understood to fall Within at least the broader aspects of the invention, together With the mechanical properties of the plate, after hot rolling and air cooling. A chief function of these heats Was to investigate various additions to the alloy, with a minimum of variation of major ingredients except for one or two instances. In all cases the contents of carbon, manganese, silicon, chromium, molybdenum and copper were essentially the same, i.e. to the extent that no significant effect Was noted in the minor incidental variation of such proportions. Specifically these heats contained 0.10- 0.15% C, 1.21--1.32% Mn, 0.27-0.36% Si, 1.47-1.55 Cr, 0.530.62% Mo, 0.71-0.80% Cu, 0.0050.0055% B, and 0.270.36% Si except that heat V916 contained no silicon, in contrast to 0.29% Si in heat V917. The remaining compositional characteristics (balance iron and incidentals, as before) and the mechanical properties were as follows:

TAB LE 111.-VAC C UM HEATS in comparison with other heats show that although these heats were of some utility, colunibiurn is an element of special advantage as constituting the third ingredient. Like- Oomposition (weight percent) as to elements I Ultimate varied Yield tensile Charpy V-N, foot-lbs. at Heat strength, strength, number Ni Cb K.s.i. K.s.i. +70 F. 20 F.

V916 l. 62 0. 06 131.9 189. 6 28 16 7 59 0. 06 122. 3 195. 6 30 22 119.0 182.1 24 10 117. 0 185. 7 17 12 118. 0 178. 6 13 8 117. 0 177. 4 29 22 117. 0 174. 6 34 21 118. 0 173. 8 30 14 110.1 162. 7 22 12 117. 0 183.3 21 16 113. 1 174. 6 20 16 111. 1 175. 8 36 24 117.1 183. 7 35 22 113.1 178. 2 38 24 117. 1 173. 8 36 24 115. 1 170. 6 32 18 114.1 175. 4 2B 14 103. 2 160. 7 18 8 119. 0 190. 1 32 12 In general, a basic criterion of an improved steel product according to the invention is a yield stress of at least about 95 k.s.i., it being noted that indeed a large proportion of the successful heats itemized in the above tables achieved values of 100 k.s.i. and in many cases well above, for best practical use. Toughness being a mechanical property of some essentiality in structural steel, a Charpy impact value of at least about 12 foot-pounds at -20 F. is presently regarded as a minimum for most cases, and indeed an aim for commercial practicality is a minimum of at least foot-pounds at F. Again it will be observed that a large number of the successful heats, hereinabove, satisfied these criteria, with many of them reaching or exceeding the upper minimum. It will be understood, of course, that Charpy or other impact tests are somewhat sensitive to minor uncontrollable variation of result (despite averaging or other accommodation in making such readings), for example in the sense that presumably identical heats of steel, identically treated, may show a variation of impact values of the order of a few footpounds or so.

Based on these criteria, with particular emphasis on yield strength requirements, it will be seen that a large number of heats in the tabulation represented distinctly useful results, and in many cases alforded specially advantageous products both as to yield strength and impact toughness. As explained below, the tabulation also includes (for comparison) a number of heats, particularly in the series up to heat B510 in Table I, that fall definitely short of satisfaction, in a manner readily correlated with their failure to embody compositional requirements of the invention.

In one specific sense, a particular limit is that the elements of the second metallurgical ingredient (i.e. nickel, chromium, with or without one or both of manganese and copper) should total at least 4% unless the cooling rate of the product, after completion of hot rolling, can be significantly faster than is usually attainable by air cooling of fairly thick plate or the like, e.g. of the order of a half inch or more, which now represents a considerable part of the demand for these structural steels. Products wherein the composition significantly exceeded this value were in general eminently successful; special desirability is presently believed to be indicated for steels wherein the second ingredient totals at least 4.2%. On the other hand, it is noted that even under these cooling rate conditions, steels in the lowest part of the range of 4% upwards nevertheless exhibited satisfactory values for at least one of the properties of yield strength and impact toughness, for example despite small departures in some cases below one or another of the indicated minima. Among other specific observations from the extended test series, columbiurn-free heats B757 and B758 taken wise heats V979, V980 and V981, compared mutually and also with other heats wherein the content of nickel was in the range upwards of 1%, indicate definite advantage in a nickel proportion of at least 0.95%, and indeed most usefully 1% or more.

The compositional relationship of steels of the invention to yield strength and impact toughness is graphically shown in FIG. 1, Where a considerable number of heats of Table I, in the series from heat B435 to heat B514, are plotted against a horizontal coordinate of yield strength and a vertical coordinate of toughness values, the determinations for the latter measurement being specially taken for or translated to the lowest temperature at which the given steel product exhibited a Charpy energy absorption value of 15 foot-pounds. In respect of the air-cooled plate products of all of the heats B437, B441,

B445, B447, B471, B474, B475 and B504, wherein the second metallurgical ingredient was less than 4%, the toughness values were low, indeed undesirably for the purposes of the invention. Heats B442, B443 and B451, although satisfactory in tensile strength, and indeed tending to be higher in such property than the heats first enuerated, failed to achieve the particularly advantageous minimum reading of 15 foot-pounds at least as low as 20 F. In none of these products: did the proportion of the second ingredient greatly exceed 4%, e.g. not above 4.2%. At the same time, practically all of the rest of the plotted products, showing Charpy 15 foot-pound readings at temperatures of -20 F. or below, were well above 4% in the second ingredient, and except for B446 and B505, had such totals of 4.2% or higher.

The major significance, however, of FIG. 1 resides in the unexpected principle demonstrated therein, namely that as the alloying content of the specific qualitative composition represented by the second ingredient increased, the mechanical properties of both impact toughness and yield strength rose to or remained in a definable region of marked superiority. This trend is illustrated by the broken line curve A which down to a minimum at C corresponds to progressive increase of the significant elements with continuous increase in both impact and tensile properties, the versatility and range of the invention being further demonstrated by the upturn of the curve at the right, where very high yield strength is obtainable but nevertheless with good impact toughness, as the proportion of the second metallurgical ingredient rises from values substantially above 5%, e.g. being 6.65% in heat B509, 5.6% in heat B508, 5.45% in heat B470 and 6.15% in heat B413, with correspondingly high values in other heats.

For simplicity the plot of FIG. 1 was developed from the selected heats, but further test heats, e.g. other examples in Tables I to III above, were found to agree very 1 1 well with the regional distribution in FIG. 1, fully confirming the results.

Although the invention is not limited to any particular scientific hypothesis, the further graphical presentations in FIGS. 2 to 5 are believed to afford an unusually plausible configuration, in a theoretical sense, of the nature of the present alloys in their adaptability to transformation to a high strength bainitic structure. To prepare these graphs, as representing a model of the development of the present alloys through compositional ranges from unsatisfactory to effective properties, calculations were made of the bainite transformation starting temperatures and martensite starting temperatures for selected sets of test heats, i.e. showing variation in content of nickel, copper, chromium and manganese, drawn from Tables I and II, including heats both unsuccessful and successful as to desired minimum properties. These heats included: Ni variation, B441, B437, B435; Cu variation, B504, B505, B507, B506, B510; Cr variation, B474, B447; Mn variation, B437, B452; Ni+Mn+Cr variation, B471, B472, B470.

These bainite and martensite starting temperatures (B and M respectively) in degrees F. were calculated in accordance with mathematical procedure reported by Steven and Haynes, Journal of the Iron and Steel Institute, vol. 183 (1956), page 349. Appropriate modification of the formulations involved conversion to degrees F. and enlargement to take account of copper as being effective in similar fashion to nickel in accordance with evidence from experimental results in the development of the present invention. These calculations produced B and M values for isothermal transformation. Correlation from data directly determined from experimental heats shows that in alloys of the series reported in the above tabulations, B values for continuous cooling transformations (that would be more rigorously appropriate to the present analysis) would be about 150 F. lower. Such factor being believed to be approximately constant, the significance of the graphs is thus valid on the isothermal transformation values.

An incidental but significant conclusion developed in this manner and indeed confirmed by other determinations, is that the desired results of the invention can be correlated to a composition where upon cooling the product directly and continuously from its final hot rolled temperature, the bainite starting transformation temperature is below 950 F., indeed preferably substantially below, the present theory being that depression of such temperature is a substantial function of the second alloying ingredient. As will be apparent, the concomitant of depression of the bainite starting temperature (B speci cally in the production of a fully developed, low-trans formation-temperature bainitic microstructure, is the achievement of the desired high level of mechanical properties.

FIG. 2 shows a plot of points representative of the entire total of the selected sets of test heats from which calculations were made, the small circles representing ultimate tensile strength and the triangles representing yield strength, and the graph being related to a vertical coordinate of increasing tensile values in k.s.i. and the horizontal coordinate in decreasing B in degrees F. As will be seen, each of the two sets of clusters follows an up ward curve demonstrating increase of mechanical property and decrease of B temperature, it being further the fact that the plotted points generally corresponded to increase of alloying elements of the second ingredient, ranging from a low total at the lower lefthand end of each curve, to a high total at the upper righthand end.

In FIG. 3 a similar plot of the same total set of selected tests is made against decreasing B values on the horizontal coordinate and vertically decreasing toughness values on the vertical coordinate, i.e. in the latter case the minimum temperatures for attainment of a footpound reading, from extremely low sub-zero temperature upwards. Again considering that the alloying content (second ingredient) increases as the plotted points progress from an upper lefthand position to a lower righthand position, it is seen that the alloying ingredients coact to achieve progressively lower B. values while greatly increasing the toughness.

FIGS. 4 and 5 show specifically the effect of variation of independent alloying elements, viz. Cu, Ni, Mn and Cr, and of the group Mn, Cr, Ni, as to yield strength (FIG. 4) and impact toughness (FIG. 5), measured along the respective vertical coordinates and as to the difference of calculated bainite and martensite transformation starting temperatures, i.e. the value B. M measured horizontally. In FIG. 4, clarity is promoted by aligning the values for the several heats in accordance with the specific carbon levels for which the sets of tests were respectively selected. The range of values B -M indicates, in another way, the corresponding change of the bainite transformation zone, i.e. a lowering of the B value in the sense of coming progressively closer to the M value. Again it is apparent that the individual and collective effect is improvement in the mechanical properties with decrease of bainite transformation temperatures, effecting marked progressive increase in mechanical values as the specially significant elements are individually and collectively increased so as to approach, reach or traverse compositions representative of the inventionthe latter being: heats B435 (nickel variation); B505 (in a broad aspect) and B506, B507 and B510 (Cu); B452 (Mn); and B472 and B470 (group, without Cu). The set of graphs of FIGS. 2 to 5 thus demonstrates, in the form of a model measured against experimentally determined facts (being the mechanical properties of the test heats), the underlying principles of the invention respecting the correlation of alloy composition with attainment and strength of bainitic structure.

In addition to the basic qualitative and quantitative characteristics of the composition as explained above, i.e. relative to the named ingredients and a suitable proportion of the total second ingredient, certain subsidiary characteristics have been found definitive of special advantage, e.g. as to economy of alloying elements, especially those of more expensive nature, and also as to assured attainment of desired and preferably superior results. Thus certain preferred minimum values for nickel and chromium have been noted and likewise for one or both of the elements molybdenum an boron. Moreover it appears that the total of chromium and manganese (whether one or both are present) need not ordinarily be, and indeed very appropriately should not be, more than about 4.5%, a particularly advantageous maximum being 4%. At the same time it has been observed that the total of nickel, copper and manganese, again whether one or more of them may be present, is preferably at least 2%, and sometimes advantageously at least about 3%. While upper limits of elements such as nickel, chromium and manganese have been indicated above, and in one instance or the other excellent products have been attained at or close to the maximum, practical advantage is realized with lesser amounts, e.g. up to 2.5% Ni, up to 2% Cr and up to about 1.5% Mn. Finally, it appears presently unnecessary, for most purposes, to exceed a total of about 7.5% for the group of the second ingredient, excellent results having in fact been achieved with totals in the range of 6% and below.

While major improvement is presently conceived to depend on inclusion of all three stated ingredients (or at least the first and second) and while in a number of cases it is notably desirable to include both molybdenum and boron in the first ingredient at or above the stated minimum proportions, the extensive testing of a program of heats including those set forth above, has indicated that very useful products of some utility may be achieved despite departure in one respect or another from various preferred requirements given above. Thus in some cases 13 one or another, or conceivably more than one, of the elements molybdenum and boron, and columbium (or us alternative), may be omitted, especially in a higher alloy steel, i.e. providing the total of the second ingredient 1s above 4%, and advantageously well above, indeed preferably of the order of 4.5% or higher.

When molybdenum is included, it is preferably in amount of 0.3% and above, and when boron is included, it is preferably in the range of 0.0025 to 0.01% (or perhaps to 0.02%), but smaller quantities of each are effective in some cases, e.g. molybdenum down to 0.2% (or conceivably 0.1%), and boron down to 0.002%, or even to 0.0005 As indicated above, where the content of the second alloying ingredient is 4% or preferably somewhat higher, good results are attainable in a number of cases even though at least one of these elements is omitted; such compositions are represented, in effect, by heats B654, B655, B667 and B698. In the situation of leaner alloys, i.e. which have the second ingredlent below 4% (as in the range down to 3%), and which are possible when faster cooling rates are feasible, it appears to be desirable to include both molybdenum and boron for best assurance of attaining a bainitic microstructure.

As indicated in heats V968 to V977 inclusive (see also B756 to Z758), the elements vanadium, ziconium, titamum, tantalum and tungsten, i.e. when added as alloying elements, are capable of functioning as alternatives for columbium, at least to a useful degree, so that when the third ingredient is employed, it may be broadly defined as consisting of an element or elements of the group that consists of columbium and the others named. Combinations of these metals appear useful, particularly combinations with columbium which, as noted above, is itself of special or unusual value in cooperation with the other ingredients, notably for enhancing one or both of the mechanical properties imparted by the second ingredient. In general, amounts of these additives range from 0.01 (preferably 0.03) to 0.6% each, or a total at least in that range. For columbium, additions of 0.03 to 01% appear most suitable or adequate, and for the other metals like or larger proportions seem to be preferable in most cases, eg at or above 0.05%, ranging up to 0.3%.

In a general sense, the compositions of the invention require a low carbon content (e.g. above a practicable minimum of 0.01%) and thus may be defined as low carbon steels, without criticality, in the normal understanding of the term. It appears that as a rule the carbon content should not be greater than 0.30%, or preferably not more than 0.25%, and that specific advantage is realized with a. carbon content in the range of 0.02 to 0.2%; the lower proportions of carbon are deemed appropriate to achieve better weldability and apparently also for impact toughness, e.g. for good balance of the latter with tensile values, that may tend to rise as carbon increases. Some study indicates, for example heats B746, B747 and B748, that a preferred, superior carbon range is 0.1 to 0.15%, indeed as distinguished from extremely low values, e.g. below 0.05%.

i In ordinary steel making practice, silicon is almost invariably present in the melt, and it is found that modest proportions of this element, usually less than 1%, are well tolerated in the present compositions, and may indeed be included byintention, as in amounts upwards of 0.1%, if specially desired for some expected contribution, e.-g. to a small extent in the area of strength or toughness (noting the comparison of heat V917 with heat V916), or in enhancing hardenability or other properties. In general, silicon may range from to 0.8%, preferably not higher than 0.6%, indication also being that a silicon content of the order of 0.9% and above has no special virtue and is preferably avoided Incidental constituents, conveniently classed as impurities and including the usual unavoidable residuals, are also well tolerated, and have mostly not been shown in the above tables. For instance, sulphur may range up to 0.03%, preferably 0.15% or below, and likewise phosphorus in an amount usually acceptable in low carbon steel, and preferably, for example, not more than 0.01%. The above preferred maximum for sulphur is given in the light of evidence that as the sulphur level rises from a low value, there is some tendency for the impact toughness to decrese, notably as measured at room temperature. Small or minor additions of other elements, which have no appreciably deleterious effect and which may therefore be classed as incidentals, are correspondingly tolerated. Thus low amounts of aluminum, e.g. as remaining from the killing step, were noted in a number of the tabulated heats, but even up to about 0.1% or that order were apparently of no consequence. In a general sense any minor addition that does not interfere with the functions of the desired ingredients of the present steels, as for example even cobalt where shown above, can be considered incidental. As will be understood, small amounts of impurities are difficult to avoid in steelmaking, particularly when economical practice of adding alloying elements may involve such addition in term or other available high alloy form. In passing it may be noted that one convenient mode of incorporating nickel, and copper with it, is by utilization of a well-known, special purpose metal that consists chiefly of about two-thirds nickel and one-third copper.

While it is conceivable that the alloy compositions which have been described can be subjected to heat treatment of one sort or another or more particularly that supplemental treatments may be employed either before or after conversion of the formed or rolled product to bainite (as for example, a normalizing treatment immediately after hot rolling, or age-hardening or other special processing of the bainitic product), a particular virtue of the invention is that in general the hot rolled object needs simply to be cooled, as by air cooling, to effectuate the desired transformation resulting in. a strong, tough steel article. In this particular sense, the contemplated procedure involves cooling at a rate such that bainite is not bypassed in favor of martensite, nor should there even be any curtailed quench which would require a constanttemperature holding, in order to reach the bainite trans formation region. At the opposite extreme, the cooling rate should be fast enough to avoid entrance into the ferrite-pearlite zone; under extremely slow cooling, e.g. slower than 2 F. per minute, it may not be possible, because of the composition selected, to bypass the latter zone and to achieve the desired bainitic transformation. The chemistry of the alloy thus in some measure governs the cooling rate to be adopted, as can readily be determined, by test if necessary, for a given composition. Indeed as also indicated, the availability of particular cooling rates or procedures may affect the choice of composition; for example, when cooling rates substantially above 180 F. per minute are feasible (due to the thin character of the rolled product, such as A inch, or otherwise), a leaner chemistry as to the second ingredient may be adopted if desired, although in such case it seems to be at least preferable that the alloy include both molybdenum and boron, to serve the function of the first ingredient.

The actual rate attainable in air or equivalent cooling depends, of course, on the dimensions and nature of the plate or other objects, the equipment or positioning whereby objects or groups of them are exposed to the cooling action of air (the latter term including equivalent gases), and the like, and the rate changes somewhat during progress of cooling, so that numerical values are gener ally to be regarded as approximation or averages. By way of example, and without excluding the use of higher or lower rates, air cooling is presently considered to be quite readily attainable at rates of the order of to 300 F. per minute for sections such as one-half inch plate, and the examples of compositions of the invention, as given above, are in general well suited to those rates, for reaching the bainitic area. Substantially faster cooling,

conceivably even approaching 1200" F. per minute, is contemplated to be compatible with the attainment of the described results and may be used when practicable, and indeed under such circumstances some rearrangement of alloy chemistry as explained above (including a progressively leaner composition in certain respects, for instance as the rate rises above about 180 F. per minute) may then be employed if desired.

The stated values of cooling rates, including the broader range down to about 2 F. per minute mentioned above, are presently deemed to be limits of practicability; indeed it will be understood that any given alloy composition may not be susceptible of accomplishing the bainitic transformation at every cooling rate within a given range or even in more than a fraction of the range. It is presently believed, however, that these ranges for products of various dimensional characteristics are definitive in the sense that a convenient cooling operation, using air or equivalent means and requiring no costly equipment, is conceivable at any rate within the appropriate range and that each of the present alloy compositions is susceptible of cooling directly to bainite at some rate or range of rates within the broader limits.

In the foregoing sense, it will be seen that adaptability to direct bainite transformation is of very notable advantage and the process of producing the desired steel articles, e.g. hot rolled plate or other shapes, is correspondingly simplified. Thus the desired melt of steel is made and the heat completed in a perfectly conventional operation, as in a basic oxygen furnace, or open hearth (as the nature of alloying elements may permit), and austem'tizing is preferably achieved at appropriate time, e.g. at least for the slab that is to be rolled. The resulting hot slab or other shape is then hot formed, very preferably by continuous hot rolling through the necessary series of passes (e.g. without intermediate cooling) in an ordinary manner, yielding the hot rolled object, such as plate, at a high final temperature, presently conceived to be in the range of 1400 F. upwards, usually 1450 to 1800 -F. Thereupon the object is air cooled to low or room temperature with the result of a completed article having the desired bainitic microstructure and high mechanical properties.

The austenitizing step, at temperatures mentioned earlier, serves its usual functions, and indeed is observed to promote the efficacy of the columbium ingredient; maximum solution of Cb is apparently attained, in the metal of ingot or slab, at temperatures of about 2250 F. and above, 2300" F. being notably effective. In some cases, supplemental treatments may be applied to the completed, cooled, bainite-transformed plate or the like, as for example a stress-relieving treatment, specially desirable to provide a better product in the case of the cooper-containing compositions. While stress relief procedures may generally involve heating to temperatures and times upwards of 950 F. and 1 hour, special advantage is noted, for the last-mentioned alloys, in treatments at 1150 to 1250 F. for 5 to 8 hours or more.

In the hot-rolled bainitic steels of this invention, the nature of the microstructure, as studied by transmission electron microscopy, depends on the temperature at which transformation from austenite to bainite occurs. As the transformation temperature is lowered, particularly as affected by increase'in the second metallurgical ingredient, the bainitic structure becomes increasingly plate-like. Indeed it presently appears that the achievement of the highest level of properties herein described can be correlated, at least in many instances, with the provision of such balance of metallurgical ingredients as suffices to obtain a well-defined plate-like structure.

In general, insufliciently improved mechanical properties were characteristic of products where the microstructure (as in the product of B441) was predominantly acicular ferrite, which under the electron microscope appears equiaxed, this structure being recognized by its 16 low dislocation density. Broadly, the invention contemplates a microstructure, of high dislocation density, which lacks a predominance of acicular ferrite and which has at least an appreciable content or distribution of platelike structure (herein classed as predominantly platelike), believed indicative of bainite. Useful new products, having good strength and toughness, included those where the microstructure appeared to be a mixture of acicular ferrite and upper bainite,-and Where there was at least some characterizing morphology of appreciably plate-like nature, considered predominant even though relatively less than well-defined; examples were heats B448 and B506. Further instances of plate-like morphology, more completely plate-like but still less than best-defined throughout, included products of B449 and B435. Very well-defined plate-like microstructures, indicative of correspondingly very low transformation temperatures (whether considered, as structures, to be upper and/or lower bainite, or other bainitic mixture) and characterized by a highly superior combination of strentgh and toughness, were numerous among the tabulated heats, some examples being heats B444, B452, B470, B507 and B510.

As indicated, these studies of morphology were made with transmission electron microscopy, at magnifications of the order of 10,000 times and above, as distinguished from optical examinations at necessarily much lower magnification, that seemed to be inconclusive. The results of the studies are the present understanding or belief as to the significance of a plate-like morphology which is recognizable, and herein considered bainitic, whether it is Well-defined or substantially less than Well-defined, the studies being further interpreted to mean that as the selection or balance of metallurgical ingredients is varied to provide better definition of plate-like structure, higher levels of properties are achieved.

By way of example, FIG. 6 is a photomicrograph produced by transmission electron microscopy with original magnification at 10,000 times, of an appropriately prepared section of the plate product of heat B452.

By Way of further example, FIG. 7 is a photomicrograph, similarly produced with like magnification, from the plate product of heat B506.

Presently preferred examples of the compositions for the invention include heats B436, B438, B444, B446, B450, B452, B470, B472, B505, B506 and B507, from Table I. According to the sum of investigations to date, it is considered that superior results, considering practicalities of steel-making and the attainment of plate or other objects having yield strength of k.s.i. or above and preferred impact toughness as has been stated, are attained with a basic composition within the following specific ranges: 0.06 to 0.15% C, 1.0 to 1.5% Mn, 0.5 to 1.5% Cu, 1.0 to 2.5% Ni, 1.0 to 2.5% Cr, and 0.35 to 0.55%. M0, with low levels of phosphorus and sulphur. This composition is prepared to have a total of elements of the second ingredient greater than 4%, and the ingot material is conveniently processed in the manner described, including austenitizing of slab at temperatures upwards of 2250 F., hot rolling in straightaway manner, beginning at 1600 F. or above and ending at not less than about 1450 F., with subsequent air cooling of the hot rolled product.

A further, specific example of nominal or target composition for a melt to be produced by B.O.F. practice, plus ladle additions of elements functioning in minor amounts if desired, is: 0.06 to 0.25% C, 1.2 to 1.5% Mn, 0.6 to 1.2% Cu, 1.0 to 1.5% Ni, 0.8 to 1.5% Cr, 0 to 0.5% Mo, with incidental amounts of phosphorus and sulphur. All of these preferred examples, when prepared within the compositional requirements of the invention. satisfy the desired criteria of mechanical properties, including yield strength upwards of 100 k.s.i. and Charpy impact strength of at least about 15 foot-pounds at -20 F.

It will be seen that the invention affords new steels, particularly new steel products of good utility for high strength applications and having special advantages as to composition, simplicity of production and cost. A further feature of the invention, especially in its broader aspects, is its considerable flexibility in composition, according to the principles and limits set forth, and notably so in permitting considerable variation of any given element, especially within the group constituting the second ingredient. This makes it convenient to suit various requirements of melting practice, or changes in availability of alloying constituents, or variations in cost, from time to time, of one element or another. In other words, se-

lection can be predicted on these factors, especially in-.

cluding economy, as occasion requires, but with the result in all instances of a high strength steel appropriate for structural applications.

The application of the principles described herein to steels that have a leaner content of alloying elements, and that are presently indicated to be useful where the product of hot forming can be cooled rapidly, is illustrated by compositions wherein: the first ingredient comprises, in the steel, 0.3 to 0.5% Mo and 0.0005 to 0.01% B; the second ingredient (for controlling the transformation temperature) comprises, in the steel, 0.8 to 1.5% Ni, 0.5 to 1.5% Cr, 0.25 to 1% Mn, and to 1.5% Cu, the total of the second ingredient being 3 to 4%; and the steel further comprises 0.01 to 0.25% C, 0 to 1% Si, plus 0 to 1% of the third ingredient (if desired), balance ironand incidentals. As explained, such compositions are significant when cooling can be effected rather rapidly, for example at a rate above 180 F. per minute and not more than 1200 F. per minute; specific availability for such cooling, e.g. by air at least in the lower part of this range, occurs where the hot rolled product is relatively thin, usually in the range up to inch, for instance 0.08 to inch plate.

Thus a product of this character is made by appropriate adaptation of procedure described above, namely establishing a steel of the selected basic composition, with minor additions if wanted, and then after the usual preliminary treatment, producing the desired thin plate or other object by hot forming, e.g. rolling, beginning at a temperature of at least about 2000 F. and finishing at a temperature of at least about 1400" F. The object is then subjected to substantially continuous cooling, i.e. to or approaching room temperature, at a rate selected in the above range upwards of about 180 F. per minute, so as to elfect transformation to the desired plate-like microstructure (with its indicated high dislocation density and a lack of any predominance of acicular ferrite), that is understood to be representative of desired results, e.g. a yield strength of at least about 95 k.s.i. and Charpy impact toughness of at least about 12 foot-pounds at -20 F.

It can be observed from Tables I and II that vary high strength products, e.g. having yield strength of the order of 140 k.s.i. or more, were obtained in a number of instances where the second ingredient represented over preferably at least about 5.5% and indeed advantageously at least 6% of the steel. Indeed it appears that special utility may reside in a steel product made in the manner last described above wherein the hot rolled object is cooled in the stated rapid range, notably for relatively thin plate or the like, and wherein a relatively rich content of alloying elements is employed. The composition being otherwise the same, a particularly suitable chemistry for the second ingredient may then comprise 2 to 3.5% Ni, 3 to 4% Cr, 0.5 to 3% Mn and 0 to 2% Cu, for a total of 6% or more, and the ultimately attained yield strength being at least 140 k.s.i., with good impact toughness.

It is to be understood that the invention is not limited to the specific compositions and operations hereinabove set forth but may be carried out in other ways without departure from its spirit.

What is claimed is:

1. A high strength, low carbon steel object having a predominantly plate-like microstructure and yield strength of at least about k.s.i., and produced by hot rolling and by substantially continuous cooling in air from hot rolled condition, the steel of said product having a composition such that during the aforesaid cooling directly after hot rolling it transforms to said microstructure, said low carbon steel consisting essentially of 0.01 to 0.3% C, not more than 1% Si, and the following materials, balance iron and incidental impurities; a first material consisting of 0.1 to 1% M0 or 0.0005 to 0.01% B or both Mo and B in said amounts; 0.8 to 3.5% Ni, 0.5 to 4% Cr, 0 to 3% Mn, 0.5 to 2% Cu, said elements Ni, Cr and Cu, and Mn when present, constituting a second material and being present in total amount of at least about 4%; and a total of 0 to 0.6% of a third material consisting of one or more elements selected from the group consisting of columbium, tantalum, tungsten, vanadium and zircomum.

2. A steel object as defined in claim 1, in which said first material comprises 0.2 to 0.62% Mo.

3. A steel object as defined in'claim 1, in which said third material is present in amount of 0.03 to 0.6% of said steel.

4. A steel object as defined in claim 1, in which said third material comprises 0.03 to 0.6% Cb.

5. A steel object as defined in claim 4, which contains at least 0.5% Mn, at least 0.95% Ni and not more than 0.2% C, there being at least 0.3% Mo when present and at least 0.0025 B when present.

6. A steel object as defined in claim 1, in which the content of nickel is not more than 2.5%.

7. A steel object as defined in claim 1, in which the following elements are present within the following ranges: 0.06 to 0.15% C, 1.0 to 1.5% Mn, 0.5 to 1.5% Cu, 1.0 to 2.5% Ni, 1.0 to 2.5% Cr, and 0.35 to 0.55% Mo.

8. A steel product as defined in claim 1, in which the following elements are present within the following ranges: 0.06 to 0.25% C, 1.2 to 1.5% M11, 0.6 to 1.2% Cu, 1.0 to 1.5% Ni, 0.8 to 1.5% Cr, and 0 to 0.5% Mo.

9. A high strength, low carbon steel product having a predominantly plate-like microstructure and yield strength of at least about 95 k.s.i., the steel of said product having a composition such that upon air cooling directly after hot rolling it transforms to said microstructure during such cooling, said low carbon steel consisting essentially of 0.01 to 0.3% C, 0 to 0.8% Si, 0.2 to 1% Mo, 0 to 0.01% B, 0.8 to 2.5% Ni, 0.5 to 4% Cr, 0 to 3% Mn, 0.3 to 2% Cu, and 0 to 0.6% of material selected from the group consisting of columbium, tantalum, tungsten, vanadium and zirconium, balance iron and incidental impurities, the total of Ni, Cr, Cu and Mn being at least 4%, the total of Cr and Mn being not more than 4.5%, and-the total of Ni, Cu and Mn being at least 2%.

10. A steel product as defined in claim 9, which contains at least 0.01% of said material of said group of Cb, Ta, W, V and Zr.

11. A steel product as defined in claim 9, which contains at least 0.5% Mn and not more than 0.62% Mo.

12. A steel product as defined in claim 11, which contains at least 0.01% of said material of said group of Cb, Ta, W, V and Zr.

13. A steel product as defined in claim 12, which contains at least 0.03% of said material.

14. A steel product as defined in claim 12, which contains at least 0.95 Ni.

15. A steel product as defined in claim 11, which contains said material comprising columbium in amount of at least 0.03%.

16. A high strength, low carbon steel product having a predominantly plate-like microstructure and yield strength of at least about 95 k.s.i., the steel of said product being killed at least to an appreciable extent and having a composition such that upon cooling at a predetermined rate directly after hot rolling it transforms to said plate-like microstructure during said cooling at said rate instead of transforming to ferrite-pearlite and instead of transforming to martensite, said low carbon steel consisting essentially of 0.02 to 0.25% C, to 0.8% Si, 0.2 to 1% Mo, with or without 0.001 to 0.01% B, 0.95 to 2.5% Ni, 0.5 to 4% Cr, 0.5 to 3% Mn, 0.3 to 2% Cu, and 0 to 0.6% of material selected from the group consisting of columbium, tantalum, tungsten, vanadium and zirconium, balance iron and incidental impurities, the total of Ni, Cr, Cu and Mn being at least 4%, the total of Cr and Mn being not more than 4.5%, and the total of Ni, Cu and Mn being at least 2%.

17. A steel product as defined in claim 16, in whic the content of molybdenum is not more than 0.62%.

18. A steel product as defined in claim 16 which contains at least 0.01% of said material selected from said group.

19. A high strength, low carbon steel object having a predominantly plate-like microstructure and yield strength of at least about 95 k.s.i., and produced by hot rolling and by substantially continuous cooling in air from hot rolled condition, the steel of said product having a composition such that during the aforesaid cooling directly after hot rolling it transforms to said microstructure, said low carbon steel consisting essentially of 0.01 to 0.3% C, not more than 1% Si, 0.2 to 0.62% Mo, 0 to 0.01% B, 0.95 to 3.5% Ni, 0.5 to 4% Cr, 0 to 3% Mn, 0 to 2% 20 Cu, and,0.01 to 0.6% of material selected from-the group consisting of columbium, tantalum, tungsten, vanadium and zirconium, balance iron and incidental impurities, the' total of Ni, Cr, Mn and Cu being at least 4%, and the total of Cr and Mn being not more than 4.5%.

20. A steel object as defined in claim 19, which contains at least 0.5% Mn and at least 0.03% of the aforesaid material selected from said group.

References Cited UNITED STATES PATENTS OTHER REFERENCES Przegalinski, 8.: Properties and the Application Range of Bainiti-c Steels, Prace Inst. Hutniczych, 1965, 17 (2-4) 179-187.

WAYLAND W. STALLARD, Primary Examiner US. Cl. X.R.

a UNITED STATES PATENT oFFIcE CERTIFICATE OF CORRECTION Patent No. 3,783, 0 10 Dated January 1 19'! Inventor(s) John B. Ballance, Stephen J, Mates, and

Konrad J. .Ao Kundig It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 7, Table I heat Non B 4 18, "percentage of Si, -255" should read 0,225

same table, heat Non B lil, percentage of Cr, "5.1" should read --l. l--

Col. 8, Table II, heat B755, ultimate tensile strength, "132.2" should read --l3l.2--

Col. 10, line 6 4, "continuous" should read --continu1ng-- Col. 13, line 25, "Z758" should read --B758-- Col. 17, line 56, "vary" should read ---very--,-

Signed and sealed this 1st day of October"l974,'

(SEAL) At test:

MCCOY M. GIBSON JR, C. MARSHALL DANN Attesting Officer Commissioner of Patents USCOMM-DC 60376-3 69 US. GOVERNMENT PRINTING OFFICE: I! 0-36l-3l4 FORM PO-105O (10-69)

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