CA1176009A - High strength, low alloy steel with improved surface and mechanical properties - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
High strength, low alloy steel is produced, preferably as a hot rolled article (e.g. strip), to have a rimmed skin of essentially ferrite while having a main body or core which is aluminum-killed and comprises, for superior mechanical properties including yield strength, a suitable quantity of columbium and/or vanadium, The carbon, manganese and sulfur contents of the base metal, which provides the skin and plus the addition, the core, are preferably limited to provide special results as low alloy steel and also, by the same limitations, to provide auto sulfide shape control and thus to avoid unwanted directionality regarding toughness and bendability. The steel is made by pouring a mold 80-95% full of the base composition, then allowing the steel to rim for several minutes, and after a shell has solidified, continuing to pour while adding Al and Cb or V to the teemed stream, thereby providing an ingot with the above killed core, which can be hot reduced as desired.

Description

BACKGROUND OF THE INVE~TION
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This invention relates to high strenyth, low alloy steel products and methods of producing such pro-ducts, the invention being more particularly relat?d to hot rolled steel products of the stated character with improved mechanical properties, particularly in form-ability as by shaping, bending and like operations involving drawing strain in the metal ox portions of it.
Certain conventional HSLA steels, e.g. produced as hot rolled strip, have successfully included one or morP
micro-alloying ingredients such as columbium~ vanadium and titanium, such steel being fully killed, i.e. usually aluminum-killed, and the strip or like article produced for use in as-hot-rolled condition having a yield strength in the range of about 50 ksi and above, indeed often as high as 80 or 90 ksi. In order to avoid directionality of results in forming processes such as bending and in impact strensth, e.g. relatively poor transverse proper-ties, so-called sulfide shape control agents have often been added, usually rare earth elements.

Although hot rolled, aluminum-killed products of the foregoing sort, sometimes with very low carbon (e.g. 0.06~ or less) and including columbium or ~oth columbium and vanadium for tensile properties and toughness, have been successful, it has been found that some difficul-ties neveltheless remain. For example, these hot rolled products have exhibited some limitations upon forming, notably bending, in that even though rare earths are added to minimize directionality or special chemistry is adopted for like purpose, cracking sometimes occurs, e.g. edge or 1 !L7~i0~9 1 other surface cracks, with small radius bends that are hoped to be possible at low carbon levels of steel.
In the case of some rimming grades of steel, particularly as desi~ned for deep drawing operations in cold rolled state, i.e. after extensive cold rolling with appropriate annealing, problems of an apparently different sort have been noted and sought to be overcome. This rimming steel, which customarily is made with 0.07 to 0.11%
carbon for convenience and economy in production, has superior drawing properties with a clean surface essential for cold rolled strip to be deep drawn or similarly formed, but manipulating or bending the strip or otherwise subject-ing it to drawing or deformin~ operations has a tendency to fluting, and the steel has also exhibited the surface defects or ~arkings known as stretcher strains. Other problems have been that the rimmed steel has some undesirable internal porosity and is subject to aging, i,e. so-called stra;n aging, which can also result in the stretcher strain markings men-tioned above.
With the view of avoiding various difficulties in cold rolled drawing grades derived from rimmed steel, it has been proposed to prepare ingots using a rimming steel composition as described above, by first pouring each ingot mold to about 80% to 90% full of the molten steel, then interrupting such pour and allowing the mold to rest while rimming a~tion occurs, e.g. ~or several minutes, and a skin or shell of rimmed steel solidifies adjacent to the mold wall. Thereupon pouring is continued, i.e. of the same steel from the same ladle, to fill the mold, while aluminum is added in sufficient amount to kill the en-tire, still-~L~7600~

1 molten steel core. When the ingot solidifies, it consists of a rimmed steel skin or layer around and integral with ~; a killed steel core, and can be processed by the usual steps of hot rolling, cold xolling, annea~ing and temper rolling as a~propriate to achieve a cold rolled product of strip or the like suitable for drawing applications and retaining the rimmed surface over aluminum-killed steel. It i5 said that such products avoid the aging and porosity problems of previous ri~med steel, and may avoid the difficulties of the latter as to fluting and stretcher strains. In some such proposals for cold rolled, deep drawing products, addition of special elements such as columbium (for hardening or strengthening) and rare earths (for sulîide shape control) to the molten core have been described. In another case, it was proposed to fill the mold to about 65% with rimming (effervescing) steel of 0.08~ C and 0.38% Mn, and after solidification of a shell, to complete filling with a killed steel, high carbon melt of 0.78~ C, 0.80~ Mn and 0.26-Q Si, said to yield a deep - 20 drawing product of high tensile strength, free of metalloid segregation.
None of this prior art, however, has indicated any significance for the class of steel now contemplated as high strength low alloy (HSLA) material with relatively quite low carbon content ana designed for employment as a hot rolled product, e.g. hot rolled strip having sufficient levels of yield and impact strenyth, and bending properties, to be attractive for automotive uses where high strength with less weight has become important. Indeed, these HSLA
3~ steels inherently lack the problems, such as aging, porosity, 760~9 1 or stretcher strains that have been noted with cold rolled, drawing grades of rimmed steel, but the above developments in the art have been noted because of their possible super-ficial resemblance, in some procedural steps, to sorne operations that are involved in the present inven-tion and that have been discovered to have unexpected benefit for the special class of as-hot-rolled HSLA steel products.
SUMMARY OF THE INVENTION
The invention is a high streng-th steel product consisting of hot rolled strip having a thickness of 0~05 to 0.5 inch produced by ho-t rolling to a deformation of at least 50% and as so produced having a yield s-trength of at least 50 ksi and high bendability, without cracking The high strength steel product consists of a skin of rimmed steel which is essentially ferrite and has a thickness of about 0.001 to 0.01 inch, over the principal surface area of the product and beneath -the skin a core of aluminum-killed steel. The steel of the skin and of the core is free of sulfide shape control elements and consists 20 essentially of 0.03 to 0.06~ carbon, 0.2 -to 0.6% manganese, 0.045 maximum % phosphorous, less than 0.025% sulphur, balance iron and incidental elements. The core also contains the following elements: 0 to 0.20% titaniurn, 0.02 to 0.2%
aluminum, 0.01 to 0.15% columbium and 0 to 0.20% vanadium.
The invention is also a me-thod of making a hot rolled, high strength, low alloy steel product consis-ting of hot rolled strip having a thickness of 0.05 to 0.5 inch, and comprises the following steps:
a) pouring an ingo-t mold 80o to 95O full of molten steel which is free of sulfide shape control elements and consists essentially of 60~9~

1 0.03 to 0.06~ carbon, 0.2 to 0.~% manganese, 0.0~5 maximum percent phosphorous, less than 0.025% sulphur, balance iron and incidental elements;
b) allowing said filling to undergo rimming action while a shell of rimmed steel about one inch to four inches thick solidifies next to the mold surrounding a still-molten core, and then comple-tinc3 pouring of said molten steel in-to said ingot mold while adding to the molcen steel in the rn~ld, aluminum to provide 0.02 to 0.20%
aluminum in the finished core and thereby to kill the core, and 0.01 to 0.15% columbium and 0 to 0.20~ of vanadium in the core; and c) after solidification of the ingot, converting the same by hot rolling, -to a hot reduced strip product having a skin of rimmed steel which is essentiall.y ferrite and has a thickness of abou-t 0.001 to 0.01 inch, over the principal surface area of the produc-t, and beneath the skin a core of aluminum-killed steel, t~le hot rolled strip product having bendability, without cracking.
The invention is also a high strength steel produc-t, and a method of making such a product, as described above, wherein the product has a bendability, without crackinc~, of essentially zero inside radius of bend over essentially 180.
For further de-tai.ls of the invention, as claimed, please refer -to claims 1-36 which follow.
Kindly note tha-t all composition percenta~es expressed in the disclosure and in the claims are in weic3ht percent.

~76(~9 ~I:n~fots pl o(l~lced in this way clrc hallcl:Led in conve;lt:ioncll manller, being subject -to usual operations before and afLcr removal from the rnold, including hot mill opera-tions that employ ho-t rolling conditions known to be appropriate Eor steel having the composition finally reached : in the ingot core. Thus the steel can be subjected to - ~

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~ - 5a -~'76~09 1 conventional ho-t deforma-tion to produce plate ! bar~ sheet or stri`p, most con~only hot rolled strip as in -thickness from abou~ 0.05 to 0~5 inch. As stated, the conditions of hot rolling and o~ Cooling and coiling the product are as can be found satisfactory for hot rolled steel having the composition of the in~ot core in the present invention.
Examples of such hot rolling practice are to be found, for instance, in Abraham et al. patent No. 4,142,922, granted March 6, 1979.
It is found that the present invention is greatly superior in avoiding surface cracks or nucleation of cracks during processing operations. In the first place, the so-called panel cracks or snakes, which have been found to be created in the surface of previous hot rolled HSLA steel by the presence of both columbiu~ and alu;~inum, do not appear in the essentially pure ferrite surface of the pre-sent products. Hence, there is no need to undertake costly operations of grinding or the like, for significant metal removal to obliterate panel cracks and to achieve strip having a suitable surface.

It is further found that the hot rolled products of the invention not only have tensile properties in the range of 45 ksi yield strength and above, preferably 50 ksi or higher (the minimum value obtained being dependent on the content of the alloying elements such as columbium and vanadium, as detailed below), but have unusal properties for formability, especially in bending. Not only may the ho~ rolled strip, if desired, be made to lack the undesirable directionality that sometimes characterizes toughness and bendability in ~SLA steels lacking sulfide shape control 1~'7~0~

1 elements, but the actual suitability for bending or like shaping without cracking is materially higher than in prior HSLA products of otherwise similar chemistry.
As will now be understood, when the ingot of this inven-tion or a bloom or slab from it is reduced by hot rolllng, the article in effect retains the same ri~ed shell or skin that characterized the ingot on soli~ification.
In other words, the ultimate rolled product, such as strip 0.05 to 0.3 inch thick, has a thin skin at its principal surface (i.e. consisting of both sides) which is in effect rimmed steel, essentially lacking aluminum and oxidized aluminum compounds as well as other alloying ingredients;
it is essentially ferrite, and is believed to contribute unusal results in the use of the product. In particular, this surface or surface layer of the present steel product is found to have greatly reduced tendency to surface notches, cracks or the like and tends to a reduced occur-rence, indeed absence, of such cracks that might be expected to arise on sharp bending or similarly severe forming operations. This is in contrast to the situation when such bending or forming is attempted with many steel products, where there are often some fine cracks or surface inclusions that result in nucleation of more significant cracks or lines of fracture, which cannot be tolerated in the finished part that requires the bent or similarly formed shape.
Thus in a special sense, the invention provides unusual avoidance of the development of cracks on bending.

Not only axe there no oxidation products (e.g. resulting from killing) at the surface such as ordinarily afford ~76V~

1 some potential for crack nucleation, but the skin, being a low strength ferrite composition, has a substantially higher fracture strain characteristic than the underlying, high stre~gth core. Hence, when the strip is bent, stretching or drawing an ou~ermost layer of the steel, plastic deformation can be more rea~ily achieved in such layer, avoiding development of cracks or the like. Under these circumstances, severe stresses (which involve localized drawing strains) can be applied to the product while avoiding deve~opment of cracks, which ordinarily tend to nucleate in the surface, and yet the major part of the product, being the high strength core, is properly capable of bending while sheathed, so to speak, from any surface tendency to develop fractures or cracks.
At the same time, these steel products are characterized by the high strength levels which are now known for HSLA steels of columbium- and/or vanadium-containing composition, and can also be characterized by what might be called auto-sulfide-shape control in the sense of being capable of having a special co~position that precludes any need for rare earth additions or the like in order to avoid poor properties of bendability or toughness in the transverse direction as compared with the longitudinal direction, these being the directions related to rolling. In other words, with C and Mn at preferred low levels and S
below 0.025~, the steel may be characterized by the advan-tages in the foregoing respect which are noted in the above-cited Abraham et al. paten~ No. 4,142,922.

Further details and examples of the invention are set forth hereinbelow.

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D T I ED D SCRIPTION
The manner of proc"lcing the hot rolled steel articles of this invention, and indeed specifically advan-tageous compositions for such puLpose, have been described above, at least to relatively close ranges for elements such as carbon and manganese. The base composition of the steel is essentially a low carbon chemistry suitable for HSLA steel, notably as distinguished from conventional rimmed steel compositions, which ordinarily contain carbo~-10 in the range of 0.07 to 0.11%, a carbon content which is particularly suita~le for steel to be cast with rimming action, especially to take advantage of the desirably inexpensive practice of rimmed steel production where it is normally unnecessary and would indeed be deemed undesir-able to adopt more costly treatments for achieving lower carbon values.
In contrast, the present in~ention is directed to a steel with carbon in the range of 0.03 to 0.06%, the last being an upper limit which also appears crucial for attainment of so-called auto-sulfide-shape control and thus avoidance of the use of rare earths or the like with their consequent expense and tendency to produce unwanted non-metallic surface inclusions.

The base metal may thus consist of the defined composition, with manganese in the range of 0.2 to 0. 6o 1 very preferably not more than 0.45%, while the phosphorus concentration is at 0~045% maximum, and sulfur at 0.04%
maximum, most particularly less than 0.025%, the balance being iron and incidental elements. This melt of steel, which is obtainable by appropriate procedure in a suitable 7~0~9 1 furnace such as a so~called basic oxyyen type furnace or one employing a similar process, is used in the first stage of the pourir.g of each ingot. That is to say, such metal held in tl~e ladle is ~eemed into the ingot ; 5 mold until the latter is, for instance~ 35% full, the flow of molten steel being then interrupted. While similar partial filling is carried on with one or more further ingot molds, the first poured steel, which has been undergoing rimminy action, freezes against the walls of the mold, thus yielding a solidified shell or skin, - such action requiring from two to five minutes or more, ; possibly up to 15 minutes depending on thickness desired for the solid rimmed zone. This step is then immediately followed by final filling (back-filling) with further steel from the ladle (which is brought back for the purpose), while at the same time adding other desired elements, it being understood that the ladle can then be moved along for similar back-filling of further partly filled mold or molds that may be waiting.
Thus, solid aluminum is added (if desired, as ferro-aluminum) in an amount to kill the ingot core.
Such amount is in the range of 0.02 to 0.20% final content of alu~inum in the core, pr~ferably 0.02 to 0.10%, a specific example being 0.05% aluminum. Likewise, along with the solid aluminum, other solid elements are added, e.g. at least one of the elements columbium and vanadium sufficient to provide amounts of O.Ol to 0.15~ Cb and/or 0.03 to 0.20% V in the core. Thus, for exam~le, if columbium is used alone as the additional alloying ele-ment, yield strength of 50 to 60 ksi requires O.Ol to 1~7600~

1 0.03~i Cb; for 60 to 70 ksi, 0.04 to 0.07% Cb; for 70 to 80 ~si, 0.10 to 0.13~ Cb; above 80 ksi, 0.14 to 0.15P~
Cb. If hoth columhium and vanadium are added, suitable amounts (likewise in the core) are: for 60 to 70 ksi yield strength, 0.02 to 0.0~~ C~ and 0.03 to 0.05% V, with a total Cb plus V of 0.05 to 0.08~; for 70 to 80 ksi, 0.02 to 0.04% Cb and 0.10 to 0.12% V, with a total Cb plus V of 0.12 to 0.1~%; for greater than 80 ksi, 0.04 to 0.15%
Cb and 0.10 to 0.20% V, total Cb plus V being at least 10 0.15%.
A convenient practice for addition of aluminum and other elements during the final filling of the mold in this invention is to make the addition as a special alloy of such elements with iron, prepared as fine particles, granules or other suitable solid pieces. In such alloy, the iron content may be as necessary (e.g. up to 70%), and the proportions of other elements, such as Al and Cb, are dependent on the amounts desired to be added, i.e. by using a predeter~
mined quantity of the alloy. The alloy may also con-tain V
and/or Mg, and other incidental elements. By way of example in such an alloy, suitable for injection, having iron up to 30%, the weight ratio of Al to Cb may advantageously be as follows:

Desired Product (Wt. of Al)/(Wt. of ~b) Ratio Strength Preferred Range 50 - 60 ksi 2.6 1-12 60 - 70 ksi 1.2 0.3-3.2 70 - 80 ksi 0.54 0.15-1.3 For experimental test purposes, a commercial-size, 27-ton ingot was cast in accordance with the foregoing ~6V~9 1 two-stage procedure, with a desired content of columbium, and was thereafter subjected to solidification and hot de-formation in conventiona] manner, including hot rolling to a final gauge of the hvt band, of 0.105 inch. The finish temperature of the ho~ band was 1600F and it was coiled at 1175F. This strip product exhibited yield strengths, for various part of the coil, in the range of 52.6 to 55.6 ksi (longitudinal; 56 to 58 transverse) for a columbium content of 0.022gO. The base composition of the steel was 0.05~ C, 0.32% Mn , O.007% P and 0.027~ S. In addition to the above-noted Cb content, the aluminum-killed core of the ingot and of the hot rolled strip contained 0.045~ Al. In all cases, superior formability was achieved, as evidenced by good bending with very minor or no edge cracks. Elonga-tion (in 2 inches~ was about 33~ in each direction.
Although other evidence has indicated that withcarbon not over 0.06%, manganese 0.2 to 0.6% and preferably not over 0.45% (as in the above steel) and sulfur below 0.025~, e.g. at ~.020% or less, effective auto-sulfide-shape control is attainable, and although the above test example of steel of the invention, with sulfur at 0.027~, did not exhibit complete sulfide shape control, this test steel showed good bendability. Thus even at a bend radius as small as l/2T (0.05 inch), bending as much as 180 was achieved with only very minor edge cracks, being results superior to the bending characteristics of much currently available hot rolled, high strength low alloy steel without special sulfide shape control. Stated in another way, the best bendability of previous HSL~ steel (without shape control) is often 2-1/2 to 3T at T of 0.3 inch, whereas ~76~19 1 the new steel even with S a~ 0.025 to 0.027% can show bendability of about 2T at T oE 0.3 inch.
The invention nevertheless is unusually effective as embodied with a composition to ~a7.e advantage of its potential for auto-sulfide-shape control; in such case, all of the advantages of the present killed, HSLA steel with superior surface characteristics are fully realized.
Such steel generally has a hending radiusin each direction without cracks at least as low as lT where T can be 0.3 tO inch.
As will be understood, an essential feature of the present invention, providing the pure ferrite skin or surface layer in the described products, is the two-stage pouring operation of the original ingot. As stated, 1~ the first filling of the ingot moldr to the extent of 80~
to 95% (e.g. 85% to 90~ or so), is allowed to incur rimming action, involving the usual boiling or similar effect, until a shell or skin is solidified. While it is difficult to measure the actual thickness of this shell, it appears that for optimum results for the ultimate hot rolled strip in the thickness ranges mentioned above, rimming action should proceed for a delay of 2 to 15 minutes, for example, preferably about 6 minutes, before the ingot mold is back-filled with additional ladle metal and with simultaneous injection of solid elements as mentioned above, particularly aluminum, and elements such as columbium, vanadium andtitanium. Under these circumstances, the hot rolled strip ~e.g. 0.1 to 0.5 inch thick) is found to have an essentially pure ferrite skinofO.001 to 0.010 inch thickness over both faces, preferably at least about 0.003 inch.

1 As will be understood, techniques and devices are available for feeding particles, granules or other pieces of the added elements, e g. aluminum or ferro-aluminum together with columbium o~ vanadium, or composite ferro-alloys, into the falling stream of molten steel or dlrectly into the mold while the further steel of the melt is being delivered from the ladle. As will also be appre-ciated, the chief desirability is to get these elements incorporated not later than completion of back-filling.

Although alùminum is greatly preferred and has very special advantages as the deoxidant for providing the killed composition of the core steel, it is conceived that for some purposes the steel could be killed otherwise, as by silicon. To such extent in appropriate situations, silicon may thus be considered as an equivalent killing agent, and hence as an added ingredient of the ultimate steel core, in lieu of aluminum. It will, of course, be understood that silicon is not an element of normally desired inclusion in any steel to ~mdergo rimming action,

2~ and would in no case be added except during the back-fill-ing operation that creates the killed state of the core.
The steel products of this invention, in addition to properties of high strength and superior formability, have good weldability. The final rolled strip has an excellent surface, free of cracks, snakes and the like, and is very suitable for such finishes as plating, paint and enamel. The distribution of aluminum and particularly that of columbium have been found very good throughout the core metal of the ingot and rolled products, for the purposes desired. Variations in composition have been 60~9 1 indicatecl, and variations are conceivable in rnethod oE
production, as in the mode of adding further elemen-ts.
Thus ins-tead of a single ferro-alloy, mixtures ` of ferro-alloys each of less than all elements can be used. Any such pieces, e.g. ferro-columbium granules and aluminum pellets, ~hould be well mixed before being supplied to the equipment that injects the material into the back-filling stream of steel. Especially for the back-filling, the teeming nozzle should be relatively large. The steel in the ladle should be as hot as possible consistent with good rimming action. The delay time should be long enough to achieve a sufficient rimmed zone around the ingot for a significant ferrite skin at all critical surfaces of the products. The added elements or one or more of them could be formed as ferro-alloy wire or rod (e.g. by powder metallurgy), to be inserted in the teeming stream or in the molten metal in the mold. Thus in general the invention is capable of being carried out in various ways without departure from the present disclosure or its spirit.
The advantage of the invention, for example when made as hot rolled s-trip, as described above, with a base melt having the aforesaid preferred composition with no-t more than 0.06% carbon, 0.36% manganese, and less than 0.025% sulphur (e.g. 0.017% sulphur) and carrying the described ferrite skin over a core also con-taining columbium (0.028%) and aluminum (0.098~), has been demonstrated by tests to include bendability, withou-t cracking, of essentially zero inside radius of bend over essentially 180, for example, at strip thickness of 0.3, 0.4, and 0.5 inch, as contrasted with strip that was rolled to the same thickness from a billet of the same steel from which the :Eerrite skin had been 60(~

.
1 ground away, anci that shows cracking on such bending in all cases.
~ s noted above, in the summary of the invention, all composition percentages expressed in the disclosure and in the following 18 claims are in wei.ght percent.

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Claims (36)

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

1. A high strength steel product consisting of hot rolled strip having a thickness of 0.05 to 0.5 inch produced by hot rolling to a deformation of at least 50%
and as so produced having a yield strength of at least 50 ksi and bendability, without cracking, of essentially zero inside radius of bend over essentially 180°, said product consisting of a skin of rimmed steel which is essentially ferrite and has a thickness of about 0.001 to 0.01 inch, over the principal surface area of the product and beneath said skin a core of aluminum-killed steel, the steel of said skin and core being free of sulfide shape control elements and consisting essentially of 0.03 to 0.06% carbon, 0.2 to 0.6% manganese, 0.045 maximum percent phosphorous, less than 0.025% sulphur, balance iron and incidental elements, said core also containing the following elements: 0 to 0.20% titanium, 0.02 to 0.2%
aluminum, 0.01 to 0.15% columbium and 0 to 0.20% vanadium.

2. A steel product as defined in claim 1 in which said skin and core contains 0.015% maximum percent phosphorous and 0.015 maximum percent sulphur, and up to 0.45% manganese.

3. A steel product as defined in claim 2, having yield strength of at least 80 ksi, said core containing at least 0.04% columbium and at least 0.10% vanadium, the total of columbium and vanadium in said core being at least 0.15%.

4. A steel product as defined in claim 2 in which said core contains at least 0.04% columbium.

5. A steel product as defined in claim 1 in which said core contains at least 0.04% columbium.

6. A steel product as defined in claim 1 in which said core contains at least 0.10% columbium.

7. A steel product as defined in claim 1 in which said core contains at least 0.14% columbium.

8. A method of making a hot rolled, high strength, low alloy steel product consisting of hot rolled strip having a thickness of 0.05 to 0.5 inch, comprising pouring an ingot mold 80% to 95% full of molten steel which is free of sulfide shape control elements and consists essentially of 0.03 to 0.06% carbon, 0.2 to 0.6% manganese, 0.045 maximum percent phosphorous, less than 0.025% sulphur, balance iron and incidental elements, allowing said filling to undergo rimming action while a shell of rimmed steel about one inch to four inches thick solidifies next to the mold surrounding a still-molten core, and then completing pouring of said molten steel into said ingot mold while adding to the molten steel in the mold, aluminum to provide 0.02 to 0.20% aluminum in the finished core and thereby to kill said core, and 0.01 to 0.15% columbium and 0 to 0.20% .
vanadium in the core; and after solidification of the ingot, converting the same by hot rolling, to a hot reduced strip product having a skin of rimmed steel which is essentially ferrite and has a thickness of about 0.001 to 0.01 inch, over the principal surface area of the product, and beneath the skin a core of aluminum-killed steel, said hot rolled strip product having bendability, without cracking, of essentially zero inside radius of bend over essentially 180°.

9. A method as defined in claim 8 in which the first-described pouring of steel is to about 85 to 90% of filling of the mold.

10. A method as defined in claim 8 in which the said addition to the molten steel core comprises aluminum and columbium in aforesaid amounts and is effected by adding portions of solid alloy consisting essentially of iron with alulminum and columbium in proportions required to provide said amounts of aluminum and columbium in the core.

11. A method as defined in claim 10 in which the final product is made to have a yield strength of at least 50 ksi, and the weight ratio of aluminum to columbium in said last-mentioned alloy is in the range of 1 to 12.

12. A method as defined in claim 11 in which said weight ratio is about 2.6.

13. A method as defined in claim 10 in which the final product is made to have a yield strength of at least 60 ksi and said introduced alloy contains columbium to provide at least 0.04% columbium in the ingot core, the weight ratio of aluminum to columbium in said alloy being in the range of 0.3 to 3.2.

14. A method as defined in claim 13 in which said weight ratio is about 1.2.

15. A method as defined in claim 10 in which the final product is made to have a yield strength of at least 70 ksi and said introduced alloy contains columbium to provide at least 0.10% columbium in the ingot core, the weight ratio of aluminum to columbium in said alloy being in the range of 0.15 to 1.3.

16. A method as defined in claim 15 in which said weight ratio is about 0.54.

17. A method as defined in claim 9 in which the addition of columbium to the molten steel in the mold during completion of pouring is sufficient to provide at least 0.10%
columbium in the ingot core.

18. A method as defined in claim 9 in which the additions to the molten steel in the mold during completion of pouring are aluminum to provide 0.04 to 0.10% aluminum columbium to provide 0.04 to 0.13% columbium and vanadium to provide 0.05 to 0.20% vanadium in the ingot core.

19. A high strength steel product consisting of hot rolled strip having a thickness of 0.05 to 0.5 inch produced by hot rolling to a deformation of at least 50%
and as so produced having a yield strength of at least 50 ksi and high bendability, without cracking, said product consisting of a skin of rimmed steel which is essentially ferrite and has a thickness of about 0.001 to 0.01 inch, over the principal surface area of the product and beneath said skin a core of aluminum-killed steel, the steel of said skin and core being free of sulfide shape control elements and consisting essentially of 0.03 to 0.06% carbon, 0.2 to 0.6% manganese, 0.045 maximum percent phosphorous, less than 0.025% sulphur, balance iron and incidental elements, said core also containing the following elements: 0 to 0.20%
titanium, 0.02 to 0.2% aluminum, 0.01 to 0.15% columbium and 0 to 0.20% vanadium.

20. A steel product as defined in claim 19 in which said skin and core contains 0.015% maximum percent phosphorous and 0.015 maximum percent sulphur, and up to 0.45% manganese.

21. A steel product as defined in claim 20, having yield strength of at least 80 ksi, said core containing at least 0.04% columbium and at least 0.10% vanadium, the total of columbium and vanadium in said core being at least 0.15%.

22. A steel product as defined in claim 20 in which said core contains at least 0.04% columbium.

23. A steel product as defined in claim 19 in which said core contains at least 0.04% columbium.

24. A steel product as defined in claim 19 in which said core contains at least 0.10% columbium.

25. A steel product as defined in claim 19 in which said core contains s at least 0.14% columbium.

26. A method of making a hot rolled, high strength, low alloy steel product consisting of hot rolled strip having a thickness of 0.05 to 0.5 inch, comprising pouring an ingot mold 80% to 95% full of molten steel which is free of sulfide shape control elements and consists essentially of 0.03 to 0.06% carbon, 0.2 to 0.6% manganese, 0.045 maximum percent phosphorous, less than 0.025% sulphur, balance iron and incidental elements, allowing said filling to undergo rimming action while a shell of rimmed steel about one inch to four inches thick solidifies next to the mold surrounding a still-molten core, and then completing pouring of said molten steel into said ingot mold while adding to the molten steel in the mold, aluminum to provide 0.02 to 0.20% aluminum in the finished core and thereby to kill said core, and 0.01 to 0.15% columbium and 0 to 0.20% vanadium in the core; and after solidification of the ingot, converting the same by hot rolling, to a hot reduced strip product having a skin of rimmed steel which is essentially ferrite and has a thickness of about 0.001 to 0.01 inch, over the principal surface area of the product, and beneath the skin a core of aluminum-killed steel, said hot rolled strip product having high bendability, without cracking.

27. A method as defined in claim 26 in which the first-described pouring of steel is to about 85 to 90% of filling of the mold.

28. A method as defined in claim 26 in which the said addition to the molten steel core comprises aluminum and columbium in aforesaid amounts and is effected by adding portions of solid alloy consisting essentially of iron with aluminum and columbium in proportions required to provide said amounts of aluminum and columbium in the core.

29. A method as defined in claim 28 in which the final product is made to have a yield strength of at least 50 ksi, and the weight ratio of aluminum to columbium in said last-mentioned alloy is in the range of 1 to 12.

30. A method as defined in claim 29 in which said weight ratio is about 2.6.

31. A method as defined in claim 28 in which the final product is made to have a yield strength of at least 60 ksi and said introduced alloy contains columbium to provide at least 0.04% columbium in the ingot core, the weight ratio of aluminum to columbium in said alloy being in the range of 0.3 to 3.2.

32. A method as defined in claim 31 in which said weight ratio is about 1.2.

33. A method as defined in claim 28 in which the final product is made to have a yield strength of at least 70 ksi and said introduced alloy contains columbium to provide at least 0.10% columbium in the ingot core, the weight ratio of aluminum to columbium in said alloy being in the range of 0.15 to 1.3.

34. A method as defined in claim 33 in which said weight ratio is about 0.54.

35. A method as defined in claim 27 in which the addition of columbium to the molten steel in the mold during completion of pouring is sufficient to provide at least 0.10%
columbium in the ingot core.

36. A method as defined in claim 27 in which the additions to the molten steel in the mold during completion of pouring are aluminum to provide 0.04 to 0.10% aluminum columbium to provide 0.04 to 0.13% columbium and vanadium to provide 0.05 to 0.20% vanadium in the ingot core.