US3885997A - Method of producing a hot rolled and age hardened columbium-bearing steel product - Google Patents

Abstract

A high strength low alloy steel product consisting essentially of 0.06 to 0.14 percent carbon, 0.6 to 1.5 percent manganese, 0.02 percent maximum silicon, 0.07 percent maximum phosphorous, 0.04 to 0.12 percent columbium, balance iron is produced in the hot rolled and aged condition by hot rolling at a finishing temperature of 1550* to 1650*F, collecting the steel at a temperature of 1000* to 1250*F and age hardening by continuous heat treatment at a temperature of 1050* to 1250*F for 0.5 to 5 minutes. The above treated product may be further subjected to galvanizing.

Description

United States Patent. [191 Bucher et al.

[451 May 27, 1975 [75] Inventors: John H. Bucher; Daniel F. Jancula,

both of Pittsburgh, Pa.

[73] Assignee: Jones & Laughlin Steel Corporation,

Pittsburgh, Pa.

[22] Filed: June 13, 1974 [21] Appl. No.: 479,007

[52] U.S. Cl. l48/l2.3 [51] Int. Cl C2ld 7/14 [58] Field of Search 148/12 F, 12.3

[56] References Cited UNITED STATES PATENTS 3,666,570 5/1972 Korchynsky et a1. 148/12.3 3,761,324 9/1973 Elias et al. 148/36 3,765,874 10/1973 Elias et a1. 148/36 3,849,209 11/1974 lshizaki et al. 148/12 F Primary Examiner-W. Stallard Attorney, Agent, or FirmGerald K. White; T. A. Zalenski [5 7] ABSTRACT A high strength low alloy steel product consisting essentially of 0.06 to 0.14 percent carbon, 0.6 to 1.5 percent manganese, 0.02 percent maximum silicon, 0.07 percent maximum phosphorous, 0.04 to 0.12 percent columbium, balance iron is produced in the hot rolled and aged condition by hot rolling at a finishing temperature of l550 to 1650F, collecting the steel at a temperature of 1000 to 1250F and age hardening by continuous heat treatment at a temperature of 1050 to 1250F for 0.5 to 5 minutes. The above treated product may be further subjected to galvanizmg.

6 Claims, No Drawings METHOD OF PRODUCING A HOT ROLLED AND AGE HARDENED COLUMBIUM-BEARING STEEL PRODUCT Our invention is generally related to the production of columbium-bearing high strength low alloy steel products in the hot rolled, age hardened metallurgical state and having a minimum yield strength of 80,000 p.s.i. The process generally comprises subjecting a columbium-containing, low carbon steel to a thermomechanical practice which produces a fine grained, hot-rolled steel product in which incomplete precipitation of columbium carbides has occurred. Yield strengths on the order of 70,000 to 80,000 p.s.i. or greater are obtained at this stage. The hot-rolled strip is thereafter continuously heat-treated in order to cause further precipitation of columbium carbides and to thus obtain a minimum yield strength of 80,000 p.s.i. Such precipitation of columbium carbides or age hardening serves to increase the yield strength by about 3,000 to 15,000 p.s.i. Following aging, the hot-rolled and age hardened product may be galvanized in a continuous manner.

Columbium-bearing steel products are the subject of much interest and intensive effort in the art due to their desirable combination of high strength and relatively low cost. Typical of such activity are US. Pat. Nos. 3,010,822, 3,102,831, 3,619,303, 3,666,570, and 3,671,334 as well as an article appearing in Metallurgical Society of Transactions of the AIME, Volume 239, April, 1967, pages 478-485.

United States Patent Numbers 3,010,822, 3,102,831 and 3,666,590 relate to the controlled thermomechanical processing of columbium-bearing steels but do not, however, relate to an aging or precipitation heat-treatment to enhance yield strength levels. Moreover, one following the general teachings of US. Pat. No. 3,619,303 would be led to believe that an age hardening heat-treatment would be ineffective upon the product produced in accordance with the above three patents because of the lack of molybdenum in the respective steel compositions. US. Pat. No. 3,619,303 discloses a ferrite-carbide, low alloy,-constructional steel which is age hardenable. This patent teaches that columbium-bearing steels of the composition described therein will not exhibit any significant change in mechanical properties upon aging unless molybdenum is present. United States Patent Number 3,671,334 is directed to strainaged articles manufactured from renitrogenized, columbium-bearing, high-strength steels. The hot-rolled product is strained and aged to develop yield strengths in the vicinity of 70,000 to 90,000 p.s.i.-

Age hardening is due to the formation of iron carbides and nitrides. Columbium functions to increase strength primarily due to its role as a grain refining agent rather than as a precipitation hardening agent. The above mentioned AIME articles discloses solution treating and aging columbium-bearing ferritic steel.

The invention is based upon the discovery that columbium-bearing high strength steels can be age hardened to yield strength levels in excess of 80,000 p.s.i. without resort to costly alloying additions such as molybdenum through use of balanced carbon and columbium contents in combination with a controlled thermo-mechanical procedure and continuous age hardening treatment. Such procedure is advantageous in that the incompletely hardened hot rolled product can be conveniently age hardened by passage through heat-treating equipment that is located in-line with galvanizing facilities. Hence. one may advantageously employ existing equipment to perform the dual functions or age hardening and zinc coating.

It is thus an object of our invention to provide a process that is capable of producing an age hardened columbium-bearing steel product having a yield strength in excess of 80,000 p.s.i. and characterized by the absence of costly alloying ingredients.

It is a further object of our invention to produce a columbium-bearing galvanized steel product in the age hardened condition and having a yield strength in excess of 80,000 p.s.i.

These and other objectives and advantages of our invention will be apparent to those skilled in the art from the following description of the invention.

In order to produce an incompletely hardened or aged columbium-bearing hot rolled steel product and then to subsequently age harden such product to yield strength levels greater than about 80,000 p.s.i., careful control of chemical composition, thermo-mechanical processing, and heat-treatment is required. All of the above mentioned processing limits must be controlled to fall within predetermined limits in order to achieve a product having the desired mechanical properties.

Steel suitable for use in the invention may be manufactured by any conventional steelmaking process. Typical processes include the open hearth, basic oxygen, and electric furnace processes. Following steelmaking, molten steel is cast into conventional intermediate products such as ingots, billets, or slabs and then ultimately subjected to hot rolling into a hot rolled product such as strip.

Steel having the chemical composition shown in Table I is suitable for use in the practice of the invention.

In addition zirconium, titanium, or rare earths may be optionally present in an amount sufficient to form globular sulfides which will not become elongated during hot-rolling. This procedure enhances bendability and formability of the final product. Sulfide shape control with zirconium or rare earth additions is taught in US. Pat. No. 3,666,570. Further discussion of the role of zirconium and rare earths as well as titanium may be found in the article, Control of Sulfide Shape in Low Carbon Al-Killed Steel, E. J. Licky et al, Journal of Metals, July, 1965, pages 769-775.

Aluminum is present in an amount sufficient to produce a killed steel product. Typical aluminum concentrations for such purpose range from a minimum of 0.02 percent.

Nitrogen is present in residual amounts and is not purposefully added to the steels of the invention. It will be understood by those skilled in the art that residual nitrogen content may vary with the type of steelmaking process. As an example, steel produced by the open hearth process typically contains residual nitrogen in amounts on the order of 0.002 to 0.006 percent.

Phosphorous may be present in amounts up to 0.07 percent. Higher values should be avoided due to em brittling effects Although some degree of minimal strengthening may occur due to the formation of iron nitrides or, in the event that zirconium is employed as a sulfide shape control agent, zirconium nitrides, the primary secondary phase strengthening mechanism involves the precipitation of columbium carbides. A major factor of the invention resides in the selection of the columbium and carbon contents of the steel. The choice of the respective contents, along with the later described thermo-mechanical processing sequence, results in the incomplete formation or precipitation of columbium carbides in the hot-rolled condition. This attainment of this metallurgical state is a prerequisite for the subsequent yield strength increase obtained through age hardening. In order to obtain incomplete columbium carbide precipitation in the hot-rolled state, a relatively high columbium content and relatively low carbon content is required. The terms relatively high" and relatively low" are used in the context of the general ranges taught in US. Pat. No. 3,666,570 which discloses a general columbium content range of0.01 to 0.08 percent and a general carbon content range of 0.06 to 0.20 percent. For purposes of our invention columbium and carbon should be generally restricted to 0.04 to 0.12 percent and 0.06 to 0.14 percent, respectively. As may be observed, columbium is maintained at a relatively high level and carbon at a relatively low level. If one were to employ relatively high columbium and carbon levels or relatively low columbium and relatively high carbon levels, complete precipitation of columbium carbides would occur and, hence, the hot-rolled product would not be age hardenable. On the other hand, relatively low columbium and carbon contents are not capable of producing a sufficient degree of age hardening to achieve yield strength levels on the order of 80,000 psi. By employing the relatively high columbium and relatively low carbon levels of the invention, an ageable hot-rolled product capable of attaining a yield strength in excess of 80,000 may be obtained. Columbium and carbon contents on the order of 0.05 to 0.10 percent and 0.08 to 0.12 percent, respectively are preferred as such relatively finer control of ingredients tends to permit a greater degree of control of columbium carbide precipitation. An additional advantage associated with lower carbon levels is that of enhanced weldability.

Manganese in amounts of from about 0.6 to 1.5 percent promotes the achievement of a basic strength level to which secondary strengthening due to columbium carbide precipitation is additive. The preferred manganese range is about 1.0 to 1.2 percent and is selected so as to be generally toward the high side of the general range in order to further ensure that yield strength values in excess of 80,000 psi. are attained.

Silicon is maintained at a level of 0.02 percent maximum for purposes of optimizing zinc adherence during galvanizing.

Despite the teachings of US. Pat. No. 3,619,303 that molybdenum in amounts of from 0.04 to 0.50 percent is an indispensable alloying element if one wishes to age harden columbium-bearing steels, we have found that if one utilizes the described chemical composition and thermomechanical practice, significant age hardening of the hot-rolled product may be obtained. Thus, one

of the major advantages of the invention is related to the ability to achieve a yield strength increase on the order of 3,000 to 15,000 p.s.i. through an age harden- 5 ing treatment. This unexpected ability obviously leads to cost reductions that are of significance in the processing of large volumes of steel. The steel of the invention contains molybdenum only in residual quantities, i.e., no intentional molybdenum additions are made to the steel. Typically such levels are no more than about 0.01 or 0.02 percent. Residual levels are a function of scrap mix and other raw material input factors and consequently may vary somewhat in given instances. Generally, however, residual molybdenum does not exceed about 0.02 percent.

Having selected a steel composition that is capable of being transformed into a hot-rolled product characterized by the incomplete formation of columbium carbides, the cast or intermediate product is then subjected to hot rolling according to prescribed finishing and collection temperature ranges. Such processing parameters are necessary in order to achieve the desired precipitation state of the invention as well as a fine grain size. Fine grain sizes, typically on the order of ASTM No. 13 to ASTM No. 15, also promote high yield strength levels.

Hot-rolling should be performed with finishing temperatures on the order of about l550 to 1650F. Collection of the hot-rolled steel product is accomplished by either coiling or piling. Collection temperatures on the order of from l000 to 1250F are suitable for purposes of the invention. At this stage of manufacture, the steel typically has a yield strength of about 70,000 to about 80,000 p.s.i. or slightly higher.

Following hot-rolling, collecting, and, optionally, pickling, the steel is subjected to a continuous age hardening treatment which serves to promote further columbium carbide formation and to increase the yield strength by about 3,000 to 15,000 p.s.i. Continuous heat treatment at a temperature of from about l050 to 1250F for a time of from about 0.5 to 5 minutes is sufficient to attain the desired strength increase and thereby consistently produce an 80,000 p.s.i. minimum yield strength product having acceptable ductility and toughness. The choice of times and temperatures within those described above is somewhat dependent upon line speed and product thickness.

Suitable apparatus for conducting the age hardening step is a continuous annealing furnace which commonly precedes the galvanizing step in most commercial galvanizing processes for flat rolled steel products. Normally such annealing furnaces are operated at a temperature range of l500 to 1600F for travel time of 0.5 to 5 minutes. Such procedure would not result in the desired age hardening for the steel of the invention. Hence, it is necessary to operate the furnace at lower than normal temperatures. It is especially advantageous to employ the annealing furnace unit of a continuous galvanizing line to the product of the invention whenever it is also desired to zinc coat the steel due to the fact that the respective treatments are in-line with each other and the aged and zinc coated product is characterized by a commercially attractive combination of strength and ductility.

An example which demonstrates a specific embodiment of the invention is as follows. A steel having a composition of0. 12 percent carbon, 0.91 percent manganese, 0.052 percent phosphorous, 0.025 percent sulb. collecting said hot rolled steel at a temperature of fur, 0.053 percent columbium, 0.10 percent zirconium, from l000 to l250F; and 0.03 percent molybdenum, and 0.06 percent aluminum 0. age hardening said hot rolled steel by continuously was cast and hot-rolled into a slab using conventional heat treating the steel at a temperature of from practices. The slab was then hot-rolled into a 0.075 5 1050 t 125()F for a time of from 0.5 to 5 minutes inch thick Coll at a finishing temperature of 15800}: and so as to increase the yield strength of said hot rolled a coiling temperature of 1220F. The as-rolled yield Steel by f about 3 000 to 15 000 i strength of the hot-rolled product was 84,000 p.s.i.

Following pickling to prepare the coil surface for gal- A process for producing 21 killed high strength low vanizing, the hot-rolled coil was then pr ce sed 10 alloy steel product having a yield strength in excess of through a conventional continuous annealing furnace about 80,000 psi. in the hot rolled and age hardened to effect aging and then through a galvanizing bath to condition according to claim 1, wherein:

produce a galvanized product. Furnace temperature the steel consists essentially of 0.08 to 0.12 percent was maintained at ll70F for the front portion of the carbon, 1.0 to 1.2 percent manganese, and 0.05 to coil, ll90F for the middle portion of the coil, and 0,10 percent columbium.

ll50F for the back portion of the coil. Furnace time was on the order of three minutes. A 1% 02. minimum 3- A process f0! producing 3 killed high strength low spangle zinc coating was applied to both sides of the alloy steel product having a yield strength in excess of coil. The galvanized coil was then split into two coils, about 80,000 psi. in the hot rolled and age hardened A and B, for purposes of evaluation. Coils A and B repcondition according to claim 1, wherein:

resented the front and back portions of the original the steel consists essentially of .02% maximum mocoil, respectively. Coil A was tested at its front portion lybdenum. so as to be representative of the ll70F temperature. I Coil B was tested at both ends so as to be representative A Process for prodflcmg kllled hlgh S g low of temperatures of 1190F and b ll50F. alloy steel product having a yield strength in excess of The mechanical properties shown in Table II were aboul 80,000 in the h rolled i age hardened obtained from the two test coil portions: Condmon according to Clalm 9 Wheremi TABLE II Furnace Yieldtimate Total Temp. Strength Strength Elongation Hardness Coil (F) Orientation (k.s.i.) (k.s.i.) (70) (R,,)

A ll70 Longitudinal 87.9 94.0 20 92 Transverse 86.8 94.7 20 B ll90 Longitudinal 91.8 97.4 18 95 Transverse 91.8 97.0 18 B l 150 Longitudinal 84.5 92.1 20 92 Transverse 88.4 95.4 20

A may b b d f the above table, h various said steel further consists essentially of a sulfide inage hardening temperatures were sufficient to produce Clusion Shape COIltfOl ag nt Selected from the an increased yield strength level for a steel having the gr p isting of Zirconium, titanium, and rare composition of and processed in accordance with the earths in an mount Sufficient to form globular sulinventi0n tides in the hot rolled and age hardened condition.

5. A process for producing a killed high strength low We claim: alloy steel product having a yield strength in excess of I about 80,000 psi. in the hot rolled and age hardened 1. A process for produclng a il g Strength 10W condition according to claim 1, which further includes:

alloy steel product having a strength in excess Of passing said hot rolled and age hardened steel prodabout 80,000 psi. in the hot rolled and age hardened ct through a molten Zinc bath whereby Said hot condition, comprising: rolled and age hardened steel product is coated with zinc.

6. A process for producing a killed high strength low alloy steel product having a yield strength in excess of about 80,000 psi. in the hot rolled and age hardened condition according to claim 5, wherein:

a. hot rolling a steel consisting essentially of 0.06 to 0.14 percent carbon, 0.6 to 1.5 percent manganese, 0.07 percent maximum, phosphorous, 0.04 to 0.12 percent columbium, molybdenum in residual quantities, nitrogen in residual quantities, balance the Steel Consists essentially of a maximum of O 02 iron, at a finishing temperature of from l550 to percent Silicon. 165w. r. r

Claims (6)

1. A PROCESS FOR PRODUCING A KILLED HIGH STRENGTH LOW ALLOY STEEL PRODUCT HAVING A YIELD STRENGTH IN EXCESS OF ABOUT 80,000 P.S.I. IN THE HOT ROLLED AND AGE HARDENED CONDITION, COMPRISING: A. HOT ROLLING A STEEL CONSISTING ESSENTIALLY OF 0.06 TO 0.14 PERCENT CARBON, 0.6 TO 1.5 PERCENT MANGANESE, 0.07 PERCENT MAXIMUM, PHOSPHOROUS, 0.04 TO 0.12 PERCENT COLUMBIUM, MOLYBDENUM IN RESIDUAL QUANTITIES, NITROGEN IN RESIDUAL QUANTITIES BALANCE IRON, AT A FINISHING TEMPERATURE OF FROM 1550* TO 1650*F, B. COLLECTING SAID HOT ROLLED STEEL AT A TEMPERATUE OF FROM 1000* TO 125*F, AND C. AGE HARDENING SAID HOT ROLLED STEEL BY CONTINOUSLY HEAT TREATING THE STEEL AT A TEMPERATURE OF FROM 1050* TO 1250*F FOR A TIME OF FROM 0.5 TO 5 MINUTES SO AS TO INCREASE THE YIELD STRENGTH OF SAID HOT ROLLED STEEL BY FROM ABOUT 3,000 P.S.I.

2. A process for producing a killed high strength low alloy steel product having a yield strength in excess of about 80,000 p.s.i. in the hot rolled and age hardened condition according to claim 1, wherein: the steel consists essentially of 0.08 to 0.12 percent carbon, 1.0 to 1.2 percent manganese, and 0.05 to 0.10 percent columbium.

3. A process for producing a killed high strength low alloy steel product having a yield strength in excess of about 80,000 p.s.i. in the hot rolled and age hardened condition according to claim 1, wherein: the steel consists essentially of .02% maximum molybdenum.

4. A process for producing a killed high strength low alloy steel product having a yield strength in excess of about 80,000 p.s.i. in the hot rolled and age hardened condition according to claim 1, wherein: said steel further consists essentially of a sulfide inclusion shape control agent selected from the group consisting of zirconium, titanium, and rare earths in an amount sufficient to form globular sulfides in the hot rolled and age hardened condition.

5. A process for producing a killed high strength low alloy steel product having a yield strength in excess of about 80,000 p.s.i. in the hot rolled and age hardened condition according to claim 1, which further includes: passing said hot rolled and age hardened steel product through a molten zinc bath whereby said hot rolled and age hardened steel product is coated with zinc.

6. A process for producing a killed high strength low alloy steel product having a yield strength in excess of about 80,000 p.s.i. in the hot rolled and age hardened condition according to claim 5, wherein: the steel consists essentially of a maximum of 0.02 percent silicon.