CA1063389A - High strength low carbon steel and method - Google Patents
High strength low carbon steel and methodInfo
- Publication number
- CA1063389A CA1063389A CA208,805A CA208805A CA1063389A CA 1063389 A CA1063389 A CA 1063389A CA 208805 A CA208805 A CA 208805A CA 1063389 A CA1063389 A CA 1063389A
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- titanium
- maximum
- total
- columbium
- steel
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Abstract
ABSTRACT OF THE DISCLOSURE
Deep drawing steel is strengthened by alloy-nitrogen precipitation strengthening to a minimum average yield strength of 50 ksi. A deoxidized, low carbon steel sheet stock or article formed therefrom, containing from about 0.02% to 0.2% titanium in solution, from about 0.025% to 0.3% columbium in solution, from about 0.025% to about 0.3% zirconium in solution, alone or in admixture, is heat treated at 1100° - 1350°F in an atmosphere contain-ing ammonia in an amount insufficient, at the temperature and time involved, to permit formation of iron nitride.
Deep drawing steel is strengthened by alloy-nitrogen precipitation strengthening to a minimum average yield strength of 50 ksi. A deoxidized, low carbon steel sheet stock or article formed therefrom, containing from about 0.02% to 0.2% titanium in solution, from about 0.025% to 0.3% columbium in solution, from about 0.025% to about 0.3% zirconium in solution, alone or in admixture, is heat treated at 1100° - 1350°F in an atmosphere contain-ing ammonia in an amount insufficient, at the temperature and time involved, to permit formation of iron nitride.
Description
6~38~
This invention relates to a cold reduced steel sheet stock and deep drawn articles formed therefrom of high yield strength, and to a method of strengthening stamped or deep drawn articles after forming. Currently available high strength sheet stock cannot be extensively formed directly by stamping or deep drawing because of its limited ductility and drawability. The method of this invention involves the novel concept of producing stamped or deep drawn parts from a low strength, deep drawing quality steel, and subsequently strengthening the parts by alloy-nitrogen precipitation -strengthening. Cold rolled and annealed sheet stock can also be strengthened in the same manner before forming to attain higher yield strength than has hitherto been possible.
The hardening of steel surfaces by heat treating in an ammonia-containing atmosphere to form an iron-nitrogen ~, austenitic structure which is transEormed by quenching to a martensitic structure having high surface hardness, has been practiced for many years. Prior art nitriding practices are described in ASM Metals Handbook, 1948 Edition, pages 697-702, and the reference cited therein. Under present practice, nitriding is performed on particular types of steels (such as Nitralloy type, austenitic stainless steels, SAE and similar steels) in the machined and heat treated condition to provide a great wear resistance, retention o~ surEace hard-ne~s at elevated temperature and resistance to certain types ofcorrosion. Reference may also be made to United States Patent 3,399,~85 issued August 27, 1968 to H.E. Knechtel and H. H.
Podgurski, relating to nitriding of a "Nitralloy" type steel.
This invention relates to a cold reduced steel sheet stock and deep drawn articles formed therefrom of high yield strength, and to a method of strengthening stamped or deep drawn articles after forming. Currently available high strength sheet stock cannot be extensively formed directly by stamping or deep drawing because of its limited ductility and drawability. The method of this invention involves the novel concept of producing stamped or deep drawn parts from a low strength, deep drawing quality steel, and subsequently strengthening the parts by alloy-nitrogen precipitation -strengthening. Cold rolled and annealed sheet stock can also be strengthened in the same manner before forming to attain higher yield strength than has hitherto been possible.
The hardening of steel surfaces by heat treating in an ammonia-containing atmosphere to form an iron-nitrogen ~, austenitic structure which is transEormed by quenching to a martensitic structure having high surface hardness, has been practiced for many years. Prior art nitriding practices are described in ASM Metals Handbook, 1948 Edition, pages 697-702, and the reference cited therein. Under present practice, nitriding is performed on particular types of steels (such as Nitralloy type, austenitic stainless steels, SAE and similar steels) in the machined and heat treated condition to provide a great wear resistance, retention o~ surEace hard-ne~s at elevated temperature and resistance to certain types ofcorrosion. Reference may also be made to United States Patent 3,399,~85 issued August 27, 1968 to H.E. Knechtel and H. H.
Podgurski, relating to nitriding of a "Nitralloy" type steel.
-2-: .
,..:
1~6338~
The nitriding of s~eels containing nitride-forming alloying elements is discussed in Transactions AIME, volume 150 (1942), pages 157 - 171, by L. S. Darken.
! The nitrlding of iron-aluminum alloys in an ammonia-hydrogen atmosphere is described in Transactions Met. Soc. AIr~~
volume 245 (1969), pages 1595 - 1602 and in Transactions Met. Soc. AIME, volume 245 (1969), pages 1603 - 1608, . .
~y H. H. Podgurski et al.
. A comparison of nitrided iron-aluminum alloys and iron-titanium alloys is given in Transactions Met.
Soc. AIME, volume 242 (1968), pages 2415 - 2422, by V. A. Phillips and A. V. Seybolt. It was concluded in this article that an alloy contalning 1% titanium developed substantially higher surface hardenin~ than a 1% aluminum alloy due to the very small particle size of the titanium nitride which was ~ormed, less than about 15 Angstroms.
It was suggested that the nitride particles must be withln a range of about 10 to 40 Angstroms or smaller in diameter, in order to produce maximum hardening. The particle si~e Or aluminum nitrides in the aluminum-bearing alloy was sub-stantially coarser.
aoron~ lclum Columbium and ~lrconlum in Iron .?...~
and ~teel, by R. A. Grange et al, John '~lley ancl Sons, Inc., publishers, pages 173 - 179, discusses columbium as an alloy-ing element in nitriding steels. It was concluded thereinthat columblum readil~ combines with nitrogen at temperatures above 750F i~ present in excess Or the amount required to comblne wlth all the carbon to increase the surface hard-ness of the steel.
-1~3;3~39 United States Patent 3,671,334, isslled June 20, 1972 to J. H. Bucher et al, discloses a medium-carbon columbium-modifled renitrogenized steel containing less than about 0.02% total of aluminum, zlrconium, vanadlum and titanium. Sufficient ~ree nitrogen is added to the molten steel before teeming to impart strain aging proper-tles thereto. In the hot rolled or cold rolled condition the steels have a yield strength of 50 to 70 ksi, which is increased to a range of 70 to 90 ksi after straining and aglng.
Unlted States Patent 3,673,008, lssued June 27, 1972, to M. E. Wood, discloses carbonitriding of a columbium-containing steel by heating an article formed from the cold worked steel to a temperature above the strain recrystalllza-tion temperature but below the A3 critical temperature of the steel in a carbonitriding atmosphere contalning h~drocarbons and ammonia.
The purpose of the Wood patent is to prevent strain lnduced grain coarsening by addition of ~rom 0.006% to 0.018%
columbium to a carbon steel containing from 0.05% to 0.15%
carbon. The carbonitriding is stated to produce a hard, wear-resistant case on the rormed article. A person skilled ln the art would conclude that the undesirable rerritlc ~raln coarsenlng is inhibited b~ columblum-carbide precipitates.
Such carbide precipitates must exist in a fine dispersion in order to provlde resistance to graln coarsenlng. A fine dlsperslon is obtained in steels o~ the type disclosed in Wood because of the high carbon content which significantly lowers the Al critical temperature (to about 1333F). The behaviG~ ~;
:..
, ` ~6338~
of low carbon steels to which th~ present invent~on relates '` would be considered non-analogous to that Or medium or high carbon steels slnce low carbon steels have a substantially hlgher Al critical temperature, and columbium-carbide particles formed by following the process of the Wood patent would be , expected by a person skilled in the art to be too coarse to , . ~
i inhlbit ferritic grain growth.
The case hardening of relatively massive parts by nitridlng, as practiced conventionally, is distinguishable from the concept of st~engthening hot rolled or cold rolled , low carbon sheet, stock. The prior art suggestions of addi- , ,, tion Or alloylng elements such as columbium for the purpose of case hardening or prevention Or grain coarsening, would ,, not provide a person skilled in the art with a teaching which would lead to the solution of the problem of irlcreasing the strength o~ stamped or deep drawn parts formed ~rom deep drawing quality steel sheet stock.
~..
, Despite the above background, no successrul approach ' has as yet been made to the problem o~ increasing the strength of deep drawn parts or stampings formed ~,rom sheet stock without loss of the necessary ductility and drawability Or the steel required to make the part.' Present practice is still governed by the rundamental precept that enhancement o~
strength is accomplished only by a sacrifice in ductility, drawability, and!or stretchabilit,y. rrO the best of appli-cant's knowledge the prior art has never previously suggested the application of alloy-nitrogen precipitation strengthening ~-to a deep drawing quality, low carbon steel. As is well ~nown/ when such steel in sheet I'orm is subjected to drawing : ' . . . . : , . :
.
~L~63389 or stamping, ~he finlshed artlcle will have areas o~ low yield strength where the part has not been work hardened by ~training or de~ormation, and will have other areas Or high yield strength hardened by straining or deformation in forming the article. Typically the yield strength of unstrained areas is the same as or slightly higher than the yield strength of the steel sheet from which the part was formed, i.e. about 20 - 30 ksi. The areas which have been work hardened mayhave yield strengths ranging upw'ardly from about 30 ksi to about 80 or lO0 ksi, depending upon the severity of straining or deformation. ~hen such article is sub~ected to heat treatment, the strained areas exhibit recrystallization and excessive graln growthl wlth consequent undeslrable softening.
The prlor art approach, illustrated by the above-mentloned B'ucher patent', which utilizes strain-aging by carbon or nltrogen to strengthen a formed article, cannot b'e applied where deep drawing properties are required.
Steels which can be strengthened more than a negligible amount by strain-age hardenlng inherently possess relatively high strengt'h and low ductillty in the hot rolled or cold rolled condltion and hence cannot be sub~ected t,o deep drawing.
Moreover, the gain in strength resultin~ ~rom strain-age harden-ing is relatively small, on the order of aboùt lO ksi, and virtùally no strengthening in unstrained areas o~ parts rormed rrom auch steels can be achieved.
~j33~
It is a principal object of the present invention to provide a process for producing articles by drawing or stamping from a deep drawing quality steel of a specific : composition and subsequently to kreat the articles after forming by a nitriding treatment which enhances the strength thereof.
It is a further object of the invention to provide a cold rolled sheet stock, and a method for production thereof, in the thickness range of 0.02 to 0.09 inch having a yield strength of at least about 70 ksi.
; According to the invention there is provided cold reduced and annealed steel sheet stock having a thickness : -between 0,5 and 2.29 mm, an average yield strength of at least 70 ksi, and sufficient formability to permit fabrication into articles other than deep drawn, consisting essentially of rom 1 0.002% to less than 0.010% carbon, from 0.05~ to 0.6% manga-; nese, rom 0.02% to 0,04% total aluminium, about 0.035~ maximum sulfur, about 0.01% maximum oxygen, residual silicon and phos-phorus, at least one nitride-forming element chosen from the group consisting of titanium, titanium and columbium, titanium and zirconium, and titanium and mixtures of columbium and zirconium, with total titanium ranging between 0.08~ and 0.10%, total columbium ranging between 0.03~ and 0~06~,total zirconium ranging between 0,03% and 0~06~r su~ficient nitrogen to com-25 bine substantially completely with said aluminium and said -I nitride-forming element, and remainder iron except for inci-dental impurities r all percentages being by weight.
A method of increasing the yield strength of a low carbon steel sheet stock according to the invention comprises the steps of providing a deep drawing quality steel containing from 0.002% to 0.015% carbon, 0.012% maximum nitrogen, 0 to
,..:
1~6338~
The nitriding of s~eels containing nitride-forming alloying elements is discussed in Transactions AIME, volume 150 (1942), pages 157 - 171, by L. S. Darken.
! The nitrlding of iron-aluminum alloys in an ammonia-hydrogen atmosphere is described in Transactions Met. Soc. AIr~~
volume 245 (1969), pages 1595 - 1602 and in Transactions Met. Soc. AIME, volume 245 (1969), pages 1603 - 1608, . .
~y H. H. Podgurski et al.
. A comparison of nitrided iron-aluminum alloys and iron-titanium alloys is given in Transactions Met.
Soc. AIME, volume 242 (1968), pages 2415 - 2422, by V. A. Phillips and A. V. Seybolt. It was concluded in this article that an alloy contalning 1% titanium developed substantially higher surface hardenin~ than a 1% aluminum alloy due to the very small particle size of the titanium nitride which was ~ormed, less than about 15 Angstroms.
It was suggested that the nitride particles must be withln a range of about 10 to 40 Angstroms or smaller in diameter, in order to produce maximum hardening. The particle si~e Or aluminum nitrides in the aluminum-bearing alloy was sub-stantially coarser.
aoron~ lclum Columbium and ~lrconlum in Iron .?...~
and ~teel, by R. A. Grange et al, John '~lley ancl Sons, Inc., publishers, pages 173 - 179, discusses columbium as an alloy-ing element in nitriding steels. It was concluded thereinthat columblum readil~ combines with nitrogen at temperatures above 750F i~ present in excess Or the amount required to comblne wlth all the carbon to increase the surface hard-ness of the steel.
-1~3;3~39 United States Patent 3,671,334, isslled June 20, 1972 to J. H. Bucher et al, discloses a medium-carbon columbium-modifled renitrogenized steel containing less than about 0.02% total of aluminum, zlrconium, vanadlum and titanium. Sufficient ~ree nitrogen is added to the molten steel before teeming to impart strain aging proper-tles thereto. In the hot rolled or cold rolled condition the steels have a yield strength of 50 to 70 ksi, which is increased to a range of 70 to 90 ksi after straining and aglng.
Unlted States Patent 3,673,008, lssued June 27, 1972, to M. E. Wood, discloses carbonitriding of a columbium-containing steel by heating an article formed from the cold worked steel to a temperature above the strain recrystalllza-tion temperature but below the A3 critical temperature of the steel in a carbonitriding atmosphere contalning h~drocarbons and ammonia.
The purpose of the Wood patent is to prevent strain lnduced grain coarsening by addition of ~rom 0.006% to 0.018%
columbium to a carbon steel containing from 0.05% to 0.15%
carbon. The carbonitriding is stated to produce a hard, wear-resistant case on the rormed article. A person skilled ln the art would conclude that the undesirable rerritlc ~raln coarsenlng is inhibited b~ columblum-carbide precipitates.
Such carbide precipitates must exist in a fine dispersion in order to provlde resistance to graln coarsenlng. A fine dlsperslon is obtained in steels o~ the type disclosed in Wood because of the high carbon content which significantly lowers the Al critical temperature (to about 1333F). The behaviG~ ~;
:..
, ` ~6338~
of low carbon steels to which th~ present invent~on relates '` would be considered non-analogous to that Or medium or high carbon steels slnce low carbon steels have a substantially hlgher Al critical temperature, and columbium-carbide particles formed by following the process of the Wood patent would be , expected by a person skilled in the art to be too coarse to , . ~
i inhlbit ferritic grain growth.
The case hardening of relatively massive parts by nitridlng, as practiced conventionally, is distinguishable from the concept of st~engthening hot rolled or cold rolled , low carbon sheet, stock. The prior art suggestions of addi- , ,, tion Or alloylng elements such as columbium for the purpose of case hardening or prevention Or grain coarsening, would ,, not provide a person skilled in the art with a teaching which would lead to the solution of the problem of irlcreasing the strength o~ stamped or deep drawn parts formed ~rom deep drawing quality steel sheet stock.
~..
, Despite the above background, no successrul approach ' has as yet been made to the problem o~ increasing the strength of deep drawn parts or stampings formed ~,rom sheet stock without loss of the necessary ductility and drawability Or the steel required to make the part.' Present practice is still governed by the rundamental precept that enhancement o~
strength is accomplished only by a sacrifice in ductility, drawability, and!or stretchabilit,y. rrO the best of appli-cant's knowledge the prior art has never previously suggested the application of alloy-nitrogen precipitation strengthening ~-to a deep drawing quality, low carbon steel. As is well ~nown/ when such steel in sheet I'orm is subjected to drawing : ' . . . . : , . :
.
~L~63389 or stamping, ~he finlshed artlcle will have areas o~ low yield strength where the part has not been work hardened by ~training or de~ormation, and will have other areas Or high yield strength hardened by straining or deformation in forming the article. Typically the yield strength of unstrained areas is the same as or slightly higher than the yield strength of the steel sheet from which the part was formed, i.e. about 20 - 30 ksi. The areas which have been work hardened mayhave yield strengths ranging upw'ardly from about 30 ksi to about 80 or lO0 ksi, depending upon the severity of straining or deformation. ~hen such article is sub~ected to heat treatment, the strained areas exhibit recrystallization and excessive graln growthl wlth consequent undeslrable softening.
The prlor art approach, illustrated by the above-mentloned B'ucher patent', which utilizes strain-aging by carbon or nltrogen to strengthen a formed article, cannot b'e applied where deep drawing properties are required.
Steels which can be strengthened more than a negligible amount by strain-age hardenlng inherently possess relatively high strengt'h and low ductillty in the hot rolled or cold rolled condltion and hence cannot be sub~ected t,o deep drawing.
Moreover, the gain in strength resultin~ ~rom strain-age harden-ing is relatively small, on the order of aboùt lO ksi, and virtùally no strengthening in unstrained areas o~ parts rormed rrom auch steels can be achieved.
~j33~
It is a principal object of the present invention to provide a process for producing articles by drawing or stamping from a deep drawing quality steel of a specific : composition and subsequently to kreat the articles after forming by a nitriding treatment which enhances the strength thereof.
It is a further object of the invention to provide a cold rolled sheet stock, and a method for production thereof, in the thickness range of 0.02 to 0.09 inch having a yield strength of at least about 70 ksi.
; According to the invention there is provided cold reduced and annealed steel sheet stock having a thickness : -between 0,5 and 2.29 mm, an average yield strength of at least 70 ksi, and sufficient formability to permit fabrication into articles other than deep drawn, consisting essentially of rom 1 0.002% to less than 0.010% carbon, from 0.05~ to 0.6% manga-; nese, rom 0.02% to 0,04% total aluminium, about 0.035~ maximum sulfur, about 0.01% maximum oxygen, residual silicon and phos-phorus, at least one nitride-forming element chosen from the group consisting of titanium, titanium and columbium, titanium and zirconium, and titanium and mixtures of columbium and zirconium, with total titanium ranging between 0.08~ and 0.10%, total columbium ranging between 0.03~ and 0~06~,total zirconium ranging between 0,03% and 0~06~r su~ficient nitrogen to com-25 bine substantially completely with said aluminium and said -I nitride-forming element, and remainder iron except for inci-dental impurities r all percentages being by weight.
A method of increasing the yield strength of a low carbon steel sheet stock according to the invention comprises the steps of providing a deep drawing quality steel containing from 0.002% to 0.015% carbon, 0.012% maximum nitrogen, 0 to
3~
0.08% aluminum, 0.05~ to 0.6% manganese, 0.035% maximum sulfur, 0.01~ maximum oxygen, 0.01~ maximum phosphorus, 0.015% maximum silicon, a nitride-forming element chosen from the ~roup con-sisting of titanium, columbium, zirconium, and mixtures thereof, in amounts such that titanium in solution is from 0.02% to 0.2%, columbium in solution is from 0.025% to 0.3%, and zirconium in solution is from 0.025% to 0.3~, the sum total of titanium, columbium and zirconium not exceeding 0.3%, and remainder iron except for incidental impurities, all percentages being by weighti reducing said steel to final thickness; and heating the resulting sheet stock in an atmosphere comprising ammonia and hydrogen at a temperature between 1100 and 1350 (593 and 732C) for a period of time sufficient to cause reaction of said nitride-forming element with the nitrogen of said ammonia to form small, uniformly dispersed nitrides, in order to increase the average yield strength of said sheet stock to a minimum of 60 ksi (414 MN/m2), the concentration of ammonia in said atmosphere ranging between 2~ and 10~ by volume and being insufficient, at the temperature and time involved, to permit formation of iron nitride.
Unlike prior art nitriding practice, the alloy-nitrogen precipitation strengthening process of the present invention avoids the formation of an iron-nitrogen austenitic ~tructure by heating at a higher temperatuxe, ~or a 9horter time, and with a lowèr ammonia concentration in the atmosphere than a typical nitriding operation.
Moreover, no quench is applied after the heat treat-ment, contrary to conventional practice in nitriding. The presentlnre~bi~ndoes not obtain or seek the properties desired in nitriding other types of steels, viz. high surface hardness ' ., ~ .
~L~633~
at elevated temperature, great wear resistance, increased endurance limit, and resistance to certain types o~ corrosion.
The process of the present invention involves relatively low and hence inexpensive alloying additions to low .
carbon steel, and relatively low heat treatment temperature for a relatively short period of time, thereby providing a commercially economical process which does not require special-ized facilities or equipment.
A composition suitable for the practice of the invention comprises, in bxoad ranges:
` Carbon - 0~002 to 0.015%
: Nitrogen - 0~012% maximum Aluminum - 0 to 0.08%
Manganese - 0.05 to 0.6%
Sulfur - 0.035% maximum Oxygen - 0.01% maximum : Phosphorus - 0.01% maximum : Silicon - 0.015% maximum Titanium - 0.02 to 0:~2% in solution Columbium - 0.025 to 0.3~ in solution Zirconium - 0.025 to 0.3~ in solution Iron - Remainder, except for incidental impurities In the above composition all percentages are by weight, and the titanium, columbium and zirconium may be present singly or in admixture, the sum total not exceeding about 0.3~.
A preferred composition is as follows:
Carbon - Less than 0.010 Nitrogen - 0.00~
~luminum - 0.02 to 0.04~ (total) Manganese - 0.05 to 0.6~
Sulfur - 0.035% maximum Oxygen - 0.01% maximum Phosphorus - Residual Silicon - Residual ~0 Titanium - 0.08 to 0.10% (total) Columbium - 0.03 to 0.06% (total) Zirconium - 0.03 to 0.06% (total) Iron - Remainder, except for incidental impurities _g_ ... . . . -. , . . ~
. . ' . ' . : . , .
. . . : .: . . , ,.::. ~ . . .
38~
As indicated previously, the ammonia concentration in the annealing atmosphere is maintained at a concen~ration sufficiently low, at the temperature and time involved, to avoid the formation of iron nitride or an austenitic structure, thereby avoiding high surface hardness, low toughness and embrittlement. Preferably, the atmosphere in which the heat treatment is conducted contains from 3~ to 6% ammonia by volume, with the balance,hydrogen., An inert gas, such as nitrogen or argon may be substituted for part of the hydrogen, provided proper adjustments are made in the ratio of ammonia to hydrogen contents so that the formation of iron nitride or an iron-nitrogen austenite does not occur.
It has been found that a heat treatment conducted in this atmosphere within the temperature range of 1100 to 1350F, preferably 1100to 1300F, results in relatively rapid dif~usion of nitrogen into the steel and reaction of the nitrogen with the nitride-~orming alloying element to form small, uni~ormly dispersed nitride particles, probably ranging in size between about 20 and about 30 Angstroms.
A time of one to three hours at temperature is ordinarily sufficient.
When heat treating a drawn or stamped article having strained and work hardened areas, it i~ pre~erred to , ' provide both titanium and columbium, or both titanium and `'; , ztrconium, as the nitride-~orming element~. The presence of at leas,t about 0.0~5% columbium or zirconium (as determined ,~' by analysis at room temperature) prevents the recrystallization and cQnse~uent so~tening of the strained areas of the formed a~ticle when subjected to heat. Thus, in the '~
preferred practice of the invention as applied to deep : . .
,:
j33~39 drawn or stamped articles, the yield strength in the un-strained areas is increased to a minimum of 50 ksi and ,, the yleld stren~th of the strai,ned areas is maintained or even increased.
Where the method of the invention is applied to the strengthening of a cold rolled sheet stock in the unformed condition having a thickness ranging between about 0.02 inch and about 0.09 inch, preferably 0.02 to 0.06 inch, the yield strength will be increased to at least about 70 ksi, a value never previously attainable in a low carbon steel. Such a product has sufficient formability to permit rabrication into articles of various types (other than deep drawn) wherein bends are mainly involved.
According to studies reported by L. S. Darken and ~. W. Gurry in Physical Chemistry of Metals, Mc~lraw-Hill Book Company, Inc. (1953), pages 372 - 395, the maxlmum ammonia ', concentrations which can be used within the temperature ranges of 1100 to 1350F and still avoid the formation of iron nitride, are as ~ollows:
1100F - about 10% ammonia 1200F - about 6% ammonia, 1300F - about 3% ammonia 1350F - about 2% ammonla The above values represent equilibrium between ammonia-hydrogen mixtures and solid phases of the iron nitride syskem at one atmosphere pressure.
It will be apparent from the above information that the temperature and ammonia concentration are interdependent and should be varied inversely with respect to one another in the practice of the present process. Similarly, time is ' , . .
a further interdependent variable also inversely proportional to the temperature and ammonla concentration. It has been found that the rate of diffusion of the nitrogen into the steel is the controlling factor since the reaction rate of nitrogen with the alloying elements ls relatively rapid.
Below a temperature of 1100F the rate of diffusion is so slow that the time required at temperature is commercially uneconomical. Above 1350F~ the ammonia concentratlon must be kePt so low that the driving force for diffusion becomes lnsufflcient. In addition, the nitrides formed at 1350F and above are coarser in size and hence contribute less strengthening effect. Flnally, when heat treating deep drawn or stamped articles having cold worked areas, a temperature above 1350F
should be avoided because of excessive grain growth and con- ' sequent softening. ~, Within a preferred temperature range of 1100 to 1300F and a preferred ammonia concentration of 3% to 6%
by volume, the heat treatment time can range between about 1 hour and about 2 hours. Under such conditions nitrogen dlffuse,s to a depth sufficient to increase the average yield strength of' material having an as-received yleld strength of 30 ksi to a minimum Or 50 ksi.
The thickness o~ steel sheet treated ln accordance with the process of the invention does not constitute a llmitation, althou,~h its greatest utilit,y resides in the ', treatment of hot rolled thin bar ranging in thickness from about 0.06 inch to 0.25 inch and cold rolled strip material ranging in thickness from about 0.02 inch to about 0.09 inch.
' Thin cold rolled material (i.e. up to about 0. o6 inch) heat trea~ed at about 1300F for 1 to 2 hours will be stren~thened by alloy-nltrogen precipitation ln rinel,y dispersed rorm .
-~2-~0 6 33 8~
substantlally all the way through the thlckness and will achieve a yield strength o~ about 70 ksi. Thicker hot rolled material can be heat treated at somewhat lower temperatures, ln which case it will be nitrided only part way through, but to a depth sufriclent to obtain an average yield strength in excess o~ 50 ksi and up to about 85 ksi.
- Experimental data are presented in the tables below ~or a series o~ heats of steels containing titanium, colu~bium, zirconium or mixtures thereof. For purposes of comparison a typl`cal drawing quality aluminum-killed steel ~heet containing no nitride-~orming alloy other than aluminum has been included.
TABLE I
,~
Compositon - Percent by Wei~ht .
; 15 Example HeatC N O S Mn Al Ti Cb Zr 1 BlOgO073 .040 .015 .0029 - 30 0o65(itntslol) ~ ~
2 V845-2 .0042 .0036 .0041 .019 .31 .004 .109 3 V845-3 .0043 .0045 .002~ .019 .31 .031 .110 . 4 800162-V .0044 .0057 .0012 .011 .4 .030 - .12 5 2250350-V .002 .0036 - .019 .32 .047 .095 .066 -s 6 2260778-V .0042 ~ 0031 - .011 .33 .029 .049 .039 7 V796-3 .0055 .0050 .0018 .017 .30 .12 8 2260566-V .001l .0030 ~ 015 .33 .01lO .084 .063 -9 22609~11-V .003 .0045 - .01l~ .32 ~ 044 .078 .058 -. Example 1 was a mill-produced, aluminu~-killed drawlng-quality heat which was not sub~ected to vacuum de-gasslng, but was mlll hot-rolled and laboratory cold-rolled.
Exampleæ 29 3 and 7 were laboratory-produced vacuum melted heats, subJectedlto laboratory hot-rolling and cold rolling.
' --, . . ~
, ~3 3 89 Examples 4, 5, 6, 8 and 9 were mill-produced, vacuum degassed heats, aluminum-killed, mill hot-rolled and laboratory cold-rolled.
In Table II below, properties and nitrogen contents at various stages of proc~ssing are reported for representative heats. In the "As-Received" condition all samples were cold-rolled to 0.040 inch thlckness and fully annealed. Samples were also subJected to 20% cold re~uction to 0.03~ inch thickness after anneal~ng in order to simulate strained and/or deformed areas of drawn articles.
~ TABLE II
- Heat Treated in 3% NH3 - 97% H2 By Volume Example 1 Y.S. T.S.
Condition ksi ksi %Elon~. Y.P.E. %N
15As-Received 26.6 43.9 46. o 0.0 .015 1100F-1 hr. 35.9 50.5 37.0 3.7 .015 ~, -2 hrs. 38.8 53.5 30.5 3 3 -___ i 1200F-1 hr. 47.9 62.4 26.5 3.0 .056 -2 hrs. 52.0 64.9 23~0 3.o .o78 201300F 1 hr. 51.4 70.5 24. 5 2.2 .13 . .
-2 hrs. 51.4 71.2 21.5 1.6 ____ Cold Rolled 20% 59 ~ 6 59.6 10.5 0.0 ----1100F-1 hr. 51.7 G3. o 19.0 2.6 -___ 25-2 hrs. 49.9 62.0 25.5 ?.5 ~~~~
1200F-l hr. 56.1 69. o 19.0 2.6 -----2 hrs. 57.1 69.9 19.0 2.6 --~-1300F-l hr. I~.g 70.9 20.0 0.0 ____ -2 hrs. 48.9 73.7 17.0 o.o ____ - :l 4 -: . ; .: . . . .. . . . .. . .
10~3~89 TABLE II ( Continued ) Heat Treated in 3% NH3 - 97% H, B!l Volume Example 2 Y,~. T.S.
Condltion ksi ksi ~Elon~. ~Y.P.E. %N
As-Received 18.3 42.1 42.0 0.0 .0036 1100F-1 hr.37.6 54.2 23.5 0.7 .012 -2 hrs.71.0 81.5 17.0 1.4 ___ `~ 1200~-1 hr.76.4 92.6 14.0 1.4 .047 -2 hrs .102.6 114.5 12.5 1.5 . 79 101300F-1 hr.88.3 98.3 14.0 1.5 .094 -2 hrs .85.2 97.6 13.0 0.0 _~_ ;
:' :
Cold-Rolled 20~60.0 63.1 7.0 0.0 .0036 1100F-1 hr.57.9 64.3 16.0 0.0 ___ - 2 hrs.85.0 91.2 12.D o.8 ---151200F-1 hr.96.3 102.5 10.0 1.1 ---; -2 hrs .107.0 113.5 12.0 1.8 ___ ~, 1300F-1 hr.90.6 98.3 11.0 1.8 ---', -2 hrs .85.8 100.2 7.0* 0.0 ---,, .
-I Example 3 Y.S. T.S.
Condi~ion ksi ksi %Elon~. %Y.P.E. %N
As-Received 17.8 42.3 42.0 0.0 .0045 1100F-1 hr.33.5 51.2 30.0 0.7 .011 -2 hrs.68.4 79.6 18.0 1.~
1200F-1 hr.79 9 89.5 11. O 0. O 47 25 -2 hrs .104.51:16.7 9. O 0.0 .092 1300~;-1 hr.88.1 98.6 12.5 0. O .11 -2 hrs.90.7 102.7 12.0 o.o ---' Cold-Rol.led 20% 58.3 65.6 ll .5 0.0 .0045 1100F-1 hr.58.9 64.8 15.0 o . o --_ 30 -2 hrs .85.7 92.1 10.5 - ~~~
1200F-1 hr.89.9 96.4 10.5 0.0 ----2 hrs .112. g119.4 10.0 0. O ---1300F-1 hr.g5.2 104.4 10.5 0.0 ----2 hrs.93-9 105.1~ ll.0 0.0 --- -!'` ' ~63;38~
TABLE II (Continued~
Heat_Treated in 3% NH3 - 97% H~ By Volume Exam~le 4 Y.S. '''-~.
Condltion ksi ksi ~Elon~. %Y.P.E. ~N
5As-Recelved 23.8 48.6 37.0 0.0 .0057 1100F-1 hr. 39.8 50.8 35.5 5.o .012 -2 hrs. 55.9 65.1 15.0* 4.3 ___ 1200F-1 hr. 51.2 59.3 18.0 2.7 .029 -2 hrs. 71.2 79.9 12.0 3.3 .069 101300F-1 hr. 56.6 70.9 20.0 3.2 .o74 -~
-2 hrs. 62.5 77.1 22.0 2.9 ___ ~' Cold-Rolled 20% 67.9 73.2 4.0 0.0 .0057 ;
1100F-1 hr. 60.8 66.8 14.0 1.8 ----2 hrs. 74.6 78.1 9. 0.8 ---151200F-1 hr. 66.7 71.0 13.0 4.3 ___ -2 hrs. 82.3 86.9 11.0 3.9 ---1300F-1 hr. 80.8 87.3 13.0 3.5 --~
-2 hrs. 78.9 85.6 5.0* 1.8 -__ Example 5 ~.S. T~.
20 Condltionksl ksi %Elong. %Y.P.E. %N
As-Recelved 22.1 45.5 4- .0036 1100F-1 hr. 37.4 55.6 28 ! -3 010 -2 hrs. 66.3 76.7 19.0 1.1 --1200F-1 hr. 70.0 81.1 17.0 0.9 029 -2 hrs. 103.6 111.6 11.0 2.0 079 1300~-1 hr. 87.0 9~.5, 15.5 1 9 .098 -2 hrs. 89.ll 100.4 13.0 1 5 ---Cold-Rolled 20% 69.0 73.4 3.5 0.0 .0036 1100~-1 hr. 65.6 70.3 11.0 0.0 --~
-2 hrs. 90.5 92.9 9.0 0.0 --_ 1200F-1 hr. 90.5 94.5 11.5 1 7 ___ -2 hrs. 114.3 118.4 10.0 2 6 ---1300F-1 hr. 94.8 100.1 12.5 3.5 ----2 hrs. 97.2 103.5 12.0 2.5 ~~~
.:
.
, :., . ., . ., ., . :
...:,~ . . . -,. . '' ' ' : ' 1~63389 TABLE II (Continued) , Heat Treated in 3% NH3 - 97% H~ Bv Volume Example 6 Y.S. T.S.
Condltion ksi ksi %Elong. %Y.P.E. %N
5As-Recelved 20.7 44.840.5 0 .0031 ; 1100F-1 hr. 43.8 55.729.5 2.5 .0093 -2 hrs. 68.o 74.119.0 3.6 ___ 1200F-1 hr. 74.8 82.115.0 2.7 .032 -2 hrs. 89.6 98.713.0 2.0 .067 101300F-1 hr. 73.8 81.917.0 2.5 .o80 -2 hrs. 76.5 86.816.0 2.1 ---: . . ..
Cold-Rolled 20% 63.4 68.25.0 0.0 .0031 1100F-1 hr. 64.2 69.314.0 3.7 -2 hrs. 84.o 88.311.0 3-7 151200~F-1 hr. 82.8 86.611.0 3.0 -2 hrs. 94.3 98.714.0 2.8 1300F-1 hr. 82.4 88.616.0 2.8 -2 hrs. 81.4 88.616.0 2.8 Example 7 Y.S. T.S.
20Conditlon ksi ksi ~Elon,~. %Y.P.
As-Received 23.1 48.537.5 0.0 1100F-1 hr. 32.7 49.837.5 1.5 -2 hrs. 41.5 54.232.0 4.1 1200F-1 hr. 54.1 62.724.0 5.1 25-2 hrs. 66.6 71.519.0 3.8 1300F-1 hr. 62.9 73.617.5 3.3 -2 hrs. 68.3 79.818.0 3.7 ,.
Cold-Rolled 20% 68.7 73.23.5 0.0 1100F-1 hr. 57.7 63.313.0 0.0 30-2 hrs. 65.2 69.814.5 2.2 1200F-1 hr. 64.6 70.512.0 2.0 -2 hrs. 86.~ 90.29.5 2.5 1300F-1 hr. 80.5 87.012.0 3.9 -2 hrs. 83.9 92.813.0 2.8 *Broke near or outside ~a~e mark.
~17-.. : .
1a~633~
Table II indicates that the alumin~lm-killed drawin~ -' quality steel of sample 1 showed very little stren~thening when nitrided under the same conditions as the remaining steels Or examples 2 - 7. The moderate increase in yield '' , 5 strength is due primarily to the return of the yield point elongation. In addition, some strengthenin~ occurs as a result Or nitrogen in solid solution in the steel.
~' A more direct comparison of the strengthening e~fe,ct of titanium to that of aluminum is obtained from , 10 examples 2 and 3~ example 2 containing onl,Y titanium as a nitride ~ormer, with example 3 containing the same amount of titanium plus 0.031% aluminum. It is apparent that no beneficLal effect with respect to strengthening is obtained by addition of aluminum. The only difference is that the steel of example 2 containing only titanium developed yield point elongation while that of example 3 did not do so at the same total nitrogen concentrations. This is of course "
, due to'the fact that aluminum was available to scavenge ' ~, nitrogen, thus resulting in less nitrogen in solid solution. ,, ,, 20 It is further apparent that the,increase in yield strength produced in the columbium-bearing and zirconium-bearing steels of examples 4 and 7, respectivQl,y, is not as ~reat as that ~or the t:itanlum-bearln~ steel.s. I[owever, ,yleld strengt~æ Ln excess Or 50 ksi were obtained in both cases by heat treatment at 1200 - 1300~ for one hour.
Example 5 was an embodiment o~ a relativel~y highly alloyed titanium and columbium-bearing steel which exhibited an increase in yield strength substantially the same as that ', , of examples 2 and 3.
.
Example 6, illustrative o~ lower alloying additions of titanium and columbium than example 5, exhibited signifi-cant increase in yleld strength, althou~h not as high as that of the more highl,y allo,ved example 5.
The yield strengthsreported for samples subJected to , 20% cold reduction (simulating-the strain or deformation resulting from deep drawing) show that the strengthening whlch accompanies the precipitation of alloy nitrides is additive ~`
to the cold work strengthehing so that a net gain in strength ls obtained even though there is a small loss in strength due to partial recovery. The strength advantage in nitrided cold : . .
worked material over nitrided as annealed material is attrlbut-able to alloy nitride nucleation and precipitation on dis-', locations and to enhanced solubility of nitrogen in a strained ; , ~' 15 lattice.
~-~ The elongation values after nitriding are relatively high in view of the yield strength levels attalned. These elongation values indicate that some limited formin~ could be ~ perrormed after nitriding strip material such as bending or a , 20 restrike operation. The samples sub~ected to 2Q% cold re-duction increased in elongation values alon~ with an increase in yield stren~th, because of recover,y.
As indlcated previously the heat treatment step o~ "
the present process results in an increase in nitro~en in solid solution in the steel as well as nitrogen combined as nitrides with titanium, columbium, and/or zirconium. It has been found tha~ the total amount of nltrogen taken up by the steel can exceed that required to satisfy normal e~uilibriwn , ~L~6338~ -solutlon requirements ~lus that needed to convert the alloys to nitrides, This excess nitrogen can be attributed.to nitrogen trapped on dislocations, adsorbed at the nitride- ~ :
ferrite interface, and as enhanced lattice solubility in strained ferrlte.
~ Of rurther significance in Table II are the values - reported for nitriding at 1300F in 3% ammonia for 2 hours.
In most instances a decrease in yiel-d strength from the - maximum values obtained at 1200F for two hours occurred., due to the formation of coarser alloy nitride particles. A thin . austenite rim formed on the surfaces of samples nitrided at 1300F; therefore, to avoid the formation of an iron nitrogen : austenite rim, the ammonia concentration should be slightl,y less than 3% at 1~00F for a two-hour heat treatment.
A comparison of the strengthening achieved by heat treating in a 3% ammonia - 97% hydrogen mixture with that achieved in a 6% ammonia - 94% hydrogen mixture is given in Table III below " "' '', ~63~9 v, e to ~ ~o ~ i o oa~ o o ~ o o ¢ ~:: Z: ~Z~ Z ~; ~ Z
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~3~389 It is apparent from Table III that any given steel achieves a higher yield strength when nitrided in 6% ammonia under the same time and temperature conditions than is attained by nitriding in 3% ammonia. Higher strength is ob-tained by nltriding at 1200F for one hour in 6~ ammonia thanby nltriding at 1200F for two hours in 3% ammonia. However, due to diffuslon phenomena, the surface to mid-thickness strength gradient would be greater in a 6% ammonia atmosphere than in a 3% ammonia atmosphere. For some applications it may be desirable to obtain a lower average strength with a lesser gradient to mid-thickness. The present invention makes it possible to select readily temperature, time and ammonia concentrations which will result in a wide range o~
average yield strengths and surface to mid-thickness strength gradients~
Table III again indicates that nitridlng ln 6%
ammonia at 1300F results in formation of an iron-nitrogen austenite rim which will transform either to martensite or -an eutectoid structure, depending upon the cooling rate.
In example 5, an austenite rim about 1 mil thick resulted rrom annealing in 6% ammonia at 1300F for one hour.
As the thlckness Or the steel stock subJected to the heat treatment o~ the present inventlon increas0s, the time requlred to reach saturation at the equilibrlum nitrogen content in solution (for a given temperature and ammonia con-centration in the atmosphere) increases as the square of the thlckness. For example,for nitrogen diffusion in pure iron at 1200F, to reach an average fractional saturation (i.e.
N Avg/N Equll.) of 0.7 it has been ~ound that one hour is .
i33~3~
.:
requlred for sheet stock of 0.040 inch thickness, while 5.6 hours ls re~uired for sheet .stock of 0.090 inch thlckness.
: However, an important feature of the present lnvention is the dlscovery that marked increases in average yield strength can be realized within relatively short times, (i.e. not , ..... .
~ more than two hours), by partial alloy-nitrogen preci~ltation - strengthening. Table IV below indicates the substantial : incr~ase in yield strength achieved by nitriding the titanium- ~ .. .
... . . .
columbium bearing steel o~ example 8 ln a 3% ammonia - 97%
.~ 10 hydrogen atmosphere within the temperature range o~ 1100 -1300F for 1 - 2 hours. It will be noted that nltriding at 1200F for only one hour resulted in an average yleld strength o~ 66.5 ksi. Even greater strengthenln~ could i, . .
be attained by heat treating f`or lon~er periods Or time or by increasing the ammonia concentration to 6%. In Table IV
1300F again proved to be an unacceptable temperature when .
using a 3% ammonia atmos~here because Or formation of an austenlte rlm.
".
;. '.
--21J-- , , , ` 3L~6338~
TABLE IV
--Properties of 0.090 Inch Hot Rolled Steel of Example 8 Heat Treated in 3% ~H3 - 97% H2 Y.S. T.S.
Conditlon ksi ksi %Elong. %Y.P.E~ ~N
As Hot Rolled 27.3 49.1 40.5 0.0 .0027 1100F-1 hr. 46.8 63.3 30.0 0.0 .010 ,.: . .
-2 hrs. 56.8 70.3 22.0 . .013 101200F-1 hr. 66.5 81.1 21.0 0.0 .027 -2 hrs. 83.9 94.9 16.0 1.0 .043 1300F-1 hr. 77.7 91.1 20.0 1.3 .066 -2 hrs. 83.9 94.2 13.0 1.8 .o80 The criticality Or providing at least about 0.02%
titanium in solutlon is illustrated in Table V below.
TABLE V
Erfect Or Amount Or Available Nitride~Formin~ Element o.oo44%C 0.010%C C Saturated(0.024%C) Available titanium ~
- in Solut~on .47% 0.025% %
Y.S.-ksi(nitrlded in 6% NH3, 1200QF-2 hrs.) 10-2.5 85~3 64.o Y.S.-ksi(nitrided and then denitrided) 81.8 65.9 44.7 .S.-ksi(Y.S.nitr:l-ded-Y.~.denltrlded) 20.7 19.ll 19.3 In Table V a steel of the invention containing 0.077% ;
total titanium, ~.037% total columbium, 0.031% aluminum, 0.0035%
nitrogen, and remainder substantially lron, was carburized rrom an original carbon level Or 0.0044% to a carbon level Or 0.010% and to saturation with carbon in order to var~ the amount Or titanium in solutlon available to react with ammonia in the nitriding operation. It is apparent ~rom Table V that .
~e63313~
a substantial decrease in yield strength occurs progressively with decrease of available titanium in solution from 0.047%
to 0.025% and to 0% successively.
' In order to ascertain the degree of strengthening contributed by nitrogen taken into solid solution in the steel, the samples of Table V were denitrided by heating in a hydrogen atmosphere at about 1200F for 2 hours. Table V
reports the, yield strength in the denitrided condition and furt,her sets forth the differential at each Or the di~ferent carbon contents, from which it is apparent that nitrogen taken into solid solution contributes about 20 ksi to the yield ,-strength. It is further apparent that denitrided material ~' at the 0.010% carbon level (resulting in 0~025~ available titanium in solution) retains a substantially increased yield strength of 65.9 ksl ln the denitrLded condtion.
It has been discovered that the excess nitrogen content of the steels following alloy-nitrogen precipitation `' strengthening at higher allowable ammonia contents can present weldability problems and result in high ductile to brittle ' notched sheet Charpy impact transition temperature. The welding problems involve porosity resulting from the liberation ' Or this excess nitro~en as nitrog~n gas. Th~e problems can be overcome by special welding techniques. The use of high ammonia concentrations, ~ust less than that which results in the formatlon of iron nitrides or an iron-nitrogen austenite, i9 desirable from the standpoint of producing maxlmum strengthen-ing in the shortest possible time. However, it has been dls- ~ ' covered that if nitriding conducted for the purpose of strength-ening (and which results in an undesirably high excess nitrogen 1~633~39 content in solid solution) is followed by a denitriding step, such as annealing in hydrogen gas, the excess nitrogen is removed with only a 10 to 20 percent reduction in yield strength. Removal of the excess nitrogen eliminates welding porosity and significantly reduces the ductile to brittle ; transition temperature while improving the impact energy values. The present invention thus provides low carbon, high strength steel stock suitable for welding applications. ~-Table VI demonstrates the above observations regarding the erfect of denitriding. A steel of the invention lnitially containing 0. oo6~ carbon, 0.077% titanium, 0.037% columbium, 0.031% aluminum, 0.0035% nitrogen, and balance substantially lron, cold rolled to o.o58 inch thickness and annealed, was nltrided as lndicated ln Table VI. Sample A
was not denitrided, Sample B was partially denitrided, and Sample C was stlll further denitrided. Both the yield strength and the ductile to brittle transition temperature decreased gradually with ehe lecrease in nltrogen in solutlon.
:
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-~7-338~ :
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Steel having the composition speciried herein can be melted by an,yconventional operatlon such as open hearth, basic oxygen furnace or electric furnace. The rnolten steel is then vacuum degassed in order to achieve the desired carbon and nitrogen levels, killed preferably wlth A],and the nitride-forming alloying element or elements are added to the ladle after degassing with suitable mixing. The melt is then teemed into ingots, or cast into slabs. The solldified ingots or slabs are then subJected to conventional not rolling and to conventional subsequent processlng steps to obtain sheet stock of the desired final thickness. The steel is then subJected to the process Or the present invention either in the rorm of sheet or strip, or after forming into articles by drawl g or Atamp:ns.
~.
' . " ' ' . .
' .
.
0.08% aluminum, 0.05~ to 0.6% manganese, 0.035% maximum sulfur, 0.01~ maximum oxygen, 0.01~ maximum phosphorus, 0.015% maximum silicon, a nitride-forming element chosen from the ~roup con-sisting of titanium, columbium, zirconium, and mixtures thereof, in amounts such that titanium in solution is from 0.02% to 0.2%, columbium in solution is from 0.025% to 0.3%, and zirconium in solution is from 0.025% to 0.3~, the sum total of titanium, columbium and zirconium not exceeding 0.3%, and remainder iron except for incidental impurities, all percentages being by weighti reducing said steel to final thickness; and heating the resulting sheet stock in an atmosphere comprising ammonia and hydrogen at a temperature between 1100 and 1350 (593 and 732C) for a period of time sufficient to cause reaction of said nitride-forming element with the nitrogen of said ammonia to form small, uniformly dispersed nitrides, in order to increase the average yield strength of said sheet stock to a minimum of 60 ksi (414 MN/m2), the concentration of ammonia in said atmosphere ranging between 2~ and 10~ by volume and being insufficient, at the temperature and time involved, to permit formation of iron nitride.
Unlike prior art nitriding practice, the alloy-nitrogen precipitation strengthening process of the present invention avoids the formation of an iron-nitrogen austenitic ~tructure by heating at a higher temperatuxe, ~or a 9horter time, and with a lowèr ammonia concentration in the atmosphere than a typical nitriding operation.
Moreover, no quench is applied after the heat treat-ment, contrary to conventional practice in nitriding. The presentlnre~bi~ndoes not obtain or seek the properties desired in nitriding other types of steels, viz. high surface hardness ' ., ~ .
~L~633~
at elevated temperature, great wear resistance, increased endurance limit, and resistance to certain types o~ corrosion.
The process of the present invention involves relatively low and hence inexpensive alloying additions to low .
carbon steel, and relatively low heat treatment temperature for a relatively short period of time, thereby providing a commercially economical process which does not require special-ized facilities or equipment.
A composition suitable for the practice of the invention comprises, in bxoad ranges:
` Carbon - 0~002 to 0.015%
: Nitrogen - 0~012% maximum Aluminum - 0 to 0.08%
Manganese - 0.05 to 0.6%
Sulfur - 0.035% maximum Oxygen - 0.01% maximum : Phosphorus - 0.01% maximum : Silicon - 0.015% maximum Titanium - 0.02 to 0:~2% in solution Columbium - 0.025 to 0.3~ in solution Zirconium - 0.025 to 0.3~ in solution Iron - Remainder, except for incidental impurities In the above composition all percentages are by weight, and the titanium, columbium and zirconium may be present singly or in admixture, the sum total not exceeding about 0.3~.
A preferred composition is as follows:
Carbon - Less than 0.010 Nitrogen - 0.00~
~luminum - 0.02 to 0.04~ (total) Manganese - 0.05 to 0.6~
Sulfur - 0.035% maximum Oxygen - 0.01% maximum Phosphorus - Residual Silicon - Residual ~0 Titanium - 0.08 to 0.10% (total) Columbium - 0.03 to 0.06% (total) Zirconium - 0.03 to 0.06% (total) Iron - Remainder, except for incidental impurities _g_ ... . . . -. , . . ~
. . ' . ' . : . , .
. . . : .: . . , ,.::. ~ . . .
38~
As indicated previously, the ammonia concentration in the annealing atmosphere is maintained at a concen~ration sufficiently low, at the temperature and time involved, to avoid the formation of iron nitride or an austenitic structure, thereby avoiding high surface hardness, low toughness and embrittlement. Preferably, the atmosphere in which the heat treatment is conducted contains from 3~ to 6% ammonia by volume, with the balance,hydrogen., An inert gas, such as nitrogen or argon may be substituted for part of the hydrogen, provided proper adjustments are made in the ratio of ammonia to hydrogen contents so that the formation of iron nitride or an iron-nitrogen austenite does not occur.
It has been found that a heat treatment conducted in this atmosphere within the temperature range of 1100 to 1350F, preferably 1100to 1300F, results in relatively rapid dif~usion of nitrogen into the steel and reaction of the nitrogen with the nitride-~orming alloying element to form small, uni~ormly dispersed nitride particles, probably ranging in size between about 20 and about 30 Angstroms.
A time of one to three hours at temperature is ordinarily sufficient.
When heat treating a drawn or stamped article having strained and work hardened areas, it i~ pre~erred to , ' provide both titanium and columbium, or both titanium and `'; , ztrconium, as the nitride-~orming element~. The presence of at leas,t about 0.0~5% columbium or zirconium (as determined ,~' by analysis at room temperature) prevents the recrystallization and cQnse~uent so~tening of the strained areas of the formed a~ticle when subjected to heat. Thus, in the '~
preferred practice of the invention as applied to deep : . .
,:
j33~39 drawn or stamped articles, the yield strength in the un-strained areas is increased to a minimum of 50 ksi and ,, the yleld stren~th of the strai,ned areas is maintained or even increased.
Where the method of the invention is applied to the strengthening of a cold rolled sheet stock in the unformed condition having a thickness ranging between about 0.02 inch and about 0.09 inch, preferably 0.02 to 0.06 inch, the yield strength will be increased to at least about 70 ksi, a value never previously attainable in a low carbon steel. Such a product has sufficient formability to permit rabrication into articles of various types (other than deep drawn) wherein bends are mainly involved.
According to studies reported by L. S. Darken and ~. W. Gurry in Physical Chemistry of Metals, Mc~lraw-Hill Book Company, Inc. (1953), pages 372 - 395, the maxlmum ammonia ', concentrations which can be used within the temperature ranges of 1100 to 1350F and still avoid the formation of iron nitride, are as ~ollows:
1100F - about 10% ammonia 1200F - about 6% ammonia, 1300F - about 3% ammonia 1350F - about 2% ammonla The above values represent equilibrium between ammonia-hydrogen mixtures and solid phases of the iron nitride syskem at one atmosphere pressure.
It will be apparent from the above information that the temperature and ammonia concentration are interdependent and should be varied inversely with respect to one another in the practice of the present process. Similarly, time is ' , . .
a further interdependent variable also inversely proportional to the temperature and ammonla concentration. It has been found that the rate of diffusion of the nitrogen into the steel is the controlling factor since the reaction rate of nitrogen with the alloying elements ls relatively rapid.
Below a temperature of 1100F the rate of diffusion is so slow that the time required at temperature is commercially uneconomical. Above 1350F~ the ammonia concentratlon must be kePt so low that the driving force for diffusion becomes lnsufflcient. In addition, the nitrides formed at 1350F and above are coarser in size and hence contribute less strengthening effect. Flnally, when heat treating deep drawn or stamped articles having cold worked areas, a temperature above 1350F
should be avoided because of excessive grain growth and con- ' sequent softening. ~, Within a preferred temperature range of 1100 to 1300F and a preferred ammonia concentration of 3% to 6%
by volume, the heat treatment time can range between about 1 hour and about 2 hours. Under such conditions nitrogen dlffuse,s to a depth sufficient to increase the average yield strength of' material having an as-received yleld strength of 30 ksi to a minimum Or 50 ksi.
The thickness o~ steel sheet treated ln accordance with the process of the invention does not constitute a llmitation, althou,~h its greatest utilit,y resides in the ', treatment of hot rolled thin bar ranging in thickness from about 0.06 inch to 0.25 inch and cold rolled strip material ranging in thickness from about 0.02 inch to about 0.09 inch.
' Thin cold rolled material (i.e. up to about 0. o6 inch) heat trea~ed at about 1300F for 1 to 2 hours will be stren~thened by alloy-nltrogen precipitation ln rinel,y dispersed rorm .
-~2-~0 6 33 8~
substantlally all the way through the thlckness and will achieve a yield strength o~ about 70 ksi. Thicker hot rolled material can be heat treated at somewhat lower temperatures, ln which case it will be nitrided only part way through, but to a depth sufriclent to obtain an average yield strength in excess o~ 50 ksi and up to about 85 ksi.
- Experimental data are presented in the tables below ~or a series o~ heats of steels containing titanium, colu~bium, zirconium or mixtures thereof. For purposes of comparison a typl`cal drawing quality aluminum-killed steel ~heet containing no nitride-~orming alloy other than aluminum has been included.
TABLE I
,~
Compositon - Percent by Wei~ht .
; 15 Example HeatC N O S Mn Al Ti Cb Zr 1 BlOgO073 .040 .015 .0029 - 30 0o65(itntslol) ~ ~
2 V845-2 .0042 .0036 .0041 .019 .31 .004 .109 3 V845-3 .0043 .0045 .002~ .019 .31 .031 .110 . 4 800162-V .0044 .0057 .0012 .011 .4 .030 - .12 5 2250350-V .002 .0036 - .019 .32 .047 .095 .066 -s 6 2260778-V .0042 ~ 0031 - .011 .33 .029 .049 .039 7 V796-3 .0055 .0050 .0018 .017 .30 .12 8 2260566-V .001l .0030 ~ 015 .33 .01lO .084 .063 -9 22609~11-V .003 .0045 - .01l~ .32 ~ 044 .078 .058 -. Example 1 was a mill-produced, aluminu~-killed drawlng-quality heat which was not sub~ected to vacuum de-gasslng, but was mlll hot-rolled and laboratory cold-rolled.
Exampleæ 29 3 and 7 were laboratory-produced vacuum melted heats, subJectedlto laboratory hot-rolling and cold rolling.
' --, . . ~
, ~3 3 89 Examples 4, 5, 6, 8 and 9 were mill-produced, vacuum degassed heats, aluminum-killed, mill hot-rolled and laboratory cold-rolled.
In Table II below, properties and nitrogen contents at various stages of proc~ssing are reported for representative heats. In the "As-Received" condition all samples were cold-rolled to 0.040 inch thlckness and fully annealed. Samples were also subJected to 20% cold re~uction to 0.03~ inch thickness after anneal~ng in order to simulate strained and/or deformed areas of drawn articles.
~ TABLE II
- Heat Treated in 3% NH3 - 97% H2 By Volume Example 1 Y.S. T.S.
Condition ksi ksi %Elon~. Y.P.E. %N
15As-Received 26.6 43.9 46. o 0.0 .015 1100F-1 hr. 35.9 50.5 37.0 3.7 .015 ~, -2 hrs. 38.8 53.5 30.5 3 3 -___ i 1200F-1 hr. 47.9 62.4 26.5 3.0 .056 -2 hrs. 52.0 64.9 23~0 3.o .o78 201300F 1 hr. 51.4 70.5 24. 5 2.2 .13 . .
-2 hrs. 51.4 71.2 21.5 1.6 ____ Cold Rolled 20% 59 ~ 6 59.6 10.5 0.0 ----1100F-1 hr. 51.7 G3. o 19.0 2.6 -___ 25-2 hrs. 49.9 62.0 25.5 ?.5 ~~~~
1200F-l hr. 56.1 69. o 19.0 2.6 -----2 hrs. 57.1 69.9 19.0 2.6 --~-1300F-l hr. I~.g 70.9 20.0 0.0 ____ -2 hrs. 48.9 73.7 17.0 o.o ____ - :l 4 -: . ; .: . . . .. . . . .. . .
10~3~89 TABLE II ( Continued ) Heat Treated in 3% NH3 - 97% H, B!l Volume Example 2 Y,~. T.S.
Condltion ksi ksi ~Elon~. ~Y.P.E. %N
As-Received 18.3 42.1 42.0 0.0 .0036 1100F-1 hr.37.6 54.2 23.5 0.7 .012 -2 hrs.71.0 81.5 17.0 1.4 ___ `~ 1200~-1 hr.76.4 92.6 14.0 1.4 .047 -2 hrs .102.6 114.5 12.5 1.5 . 79 101300F-1 hr.88.3 98.3 14.0 1.5 .094 -2 hrs .85.2 97.6 13.0 0.0 _~_ ;
:' :
Cold-Rolled 20~60.0 63.1 7.0 0.0 .0036 1100F-1 hr.57.9 64.3 16.0 0.0 ___ - 2 hrs.85.0 91.2 12.D o.8 ---151200F-1 hr.96.3 102.5 10.0 1.1 ---; -2 hrs .107.0 113.5 12.0 1.8 ___ ~, 1300F-1 hr.90.6 98.3 11.0 1.8 ---', -2 hrs .85.8 100.2 7.0* 0.0 ---,, .
-I Example 3 Y.S. T.S.
Condi~ion ksi ksi %Elon~. %Y.P.E. %N
As-Received 17.8 42.3 42.0 0.0 .0045 1100F-1 hr.33.5 51.2 30.0 0.7 .011 -2 hrs.68.4 79.6 18.0 1.~
1200F-1 hr.79 9 89.5 11. O 0. O 47 25 -2 hrs .104.51:16.7 9. O 0.0 .092 1300~;-1 hr.88.1 98.6 12.5 0. O .11 -2 hrs.90.7 102.7 12.0 o.o ---' Cold-Rol.led 20% 58.3 65.6 ll .5 0.0 .0045 1100F-1 hr.58.9 64.8 15.0 o . o --_ 30 -2 hrs .85.7 92.1 10.5 - ~~~
1200F-1 hr.89.9 96.4 10.5 0.0 ----2 hrs .112. g119.4 10.0 0. O ---1300F-1 hr.g5.2 104.4 10.5 0.0 ----2 hrs.93-9 105.1~ ll.0 0.0 --- -!'` ' ~63;38~
TABLE II (Continued~
Heat_Treated in 3% NH3 - 97% H~ By Volume Exam~le 4 Y.S. '''-~.
Condltion ksi ksi ~Elon~. %Y.P.E. ~N
5As-Recelved 23.8 48.6 37.0 0.0 .0057 1100F-1 hr. 39.8 50.8 35.5 5.o .012 -2 hrs. 55.9 65.1 15.0* 4.3 ___ 1200F-1 hr. 51.2 59.3 18.0 2.7 .029 -2 hrs. 71.2 79.9 12.0 3.3 .069 101300F-1 hr. 56.6 70.9 20.0 3.2 .o74 -~
-2 hrs. 62.5 77.1 22.0 2.9 ___ ~' Cold-Rolled 20% 67.9 73.2 4.0 0.0 .0057 ;
1100F-1 hr. 60.8 66.8 14.0 1.8 ----2 hrs. 74.6 78.1 9. 0.8 ---151200F-1 hr. 66.7 71.0 13.0 4.3 ___ -2 hrs. 82.3 86.9 11.0 3.9 ---1300F-1 hr. 80.8 87.3 13.0 3.5 --~
-2 hrs. 78.9 85.6 5.0* 1.8 -__ Example 5 ~.S. T~.
20 Condltionksl ksi %Elong. %Y.P.E. %N
As-Recelved 22.1 45.5 4- .0036 1100F-1 hr. 37.4 55.6 28 ! -3 010 -2 hrs. 66.3 76.7 19.0 1.1 --1200F-1 hr. 70.0 81.1 17.0 0.9 029 -2 hrs. 103.6 111.6 11.0 2.0 079 1300~-1 hr. 87.0 9~.5, 15.5 1 9 .098 -2 hrs. 89.ll 100.4 13.0 1 5 ---Cold-Rolled 20% 69.0 73.4 3.5 0.0 .0036 1100~-1 hr. 65.6 70.3 11.0 0.0 --~
-2 hrs. 90.5 92.9 9.0 0.0 --_ 1200F-1 hr. 90.5 94.5 11.5 1 7 ___ -2 hrs. 114.3 118.4 10.0 2 6 ---1300F-1 hr. 94.8 100.1 12.5 3.5 ----2 hrs. 97.2 103.5 12.0 2.5 ~~~
.:
.
, :., . ., . ., ., . :
...:,~ . . . -,. . '' ' ' : ' 1~63389 TABLE II (Continued) , Heat Treated in 3% NH3 - 97% H~ Bv Volume Example 6 Y.S. T.S.
Condltion ksi ksi %Elong. %Y.P.E. %N
5As-Recelved 20.7 44.840.5 0 .0031 ; 1100F-1 hr. 43.8 55.729.5 2.5 .0093 -2 hrs. 68.o 74.119.0 3.6 ___ 1200F-1 hr. 74.8 82.115.0 2.7 .032 -2 hrs. 89.6 98.713.0 2.0 .067 101300F-1 hr. 73.8 81.917.0 2.5 .o80 -2 hrs. 76.5 86.816.0 2.1 ---: . . ..
Cold-Rolled 20% 63.4 68.25.0 0.0 .0031 1100F-1 hr. 64.2 69.314.0 3.7 -2 hrs. 84.o 88.311.0 3-7 151200~F-1 hr. 82.8 86.611.0 3.0 -2 hrs. 94.3 98.714.0 2.8 1300F-1 hr. 82.4 88.616.0 2.8 -2 hrs. 81.4 88.616.0 2.8 Example 7 Y.S. T.S.
20Conditlon ksi ksi ~Elon,~. %Y.P.
As-Received 23.1 48.537.5 0.0 1100F-1 hr. 32.7 49.837.5 1.5 -2 hrs. 41.5 54.232.0 4.1 1200F-1 hr. 54.1 62.724.0 5.1 25-2 hrs. 66.6 71.519.0 3.8 1300F-1 hr. 62.9 73.617.5 3.3 -2 hrs. 68.3 79.818.0 3.7 ,.
Cold-Rolled 20% 68.7 73.23.5 0.0 1100F-1 hr. 57.7 63.313.0 0.0 30-2 hrs. 65.2 69.814.5 2.2 1200F-1 hr. 64.6 70.512.0 2.0 -2 hrs. 86.~ 90.29.5 2.5 1300F-1 hr. 80.5 87.012.0 3.9 -2 hrs. 83.9 92.813.0 2.8 *Broke near or outside ~a~e mark.
~17-.. : .
1a~633~
Table II indicates that the alumin~lm-killed drawin~ -' quality steel of sample 1 showed very little stren~thening when nitrided under the same conditions as the remaining steels Or examples 2 - 7. The moderate increase in yield '' , 5 strength is due primarily to the return of the yield point elongation. In addition, some strengthenin~ occurs as a result Or nitrogen in solid solution in the steel.
~' A more direct comparison of the strengthening e~fe,ct of titanium to that of aluminum is obtained from , 10 examples 2 and 3~ example 2 containing onl,Y titanium as a nitride ~ormer, with example 3 containing the same amount of titanium plus 0.031% aluminum. It is apparent that no beneficLal effect with respect to strengthening is obtained by addition of aluminum. The only difference is that the steel of example 2 containing only titanium developed yield point elongation while that of example 3 did not do so at the same total nitrogen concentrations. This is of course "
, due to'the fact that aluminum was available to scavenge ' ~, nitrogen, thus resulting in less nitrogen in solid solution. ,, ,, 20 It is further apparent that the,increase in yield strength produced in the columbium-bearing and zirconium-bearing steels of examples 4 and 7, respectivQl,y, is not as ~reat as that ~or the t:itanlum-bearln~ steel.s. I[owever, ,yleld strengt~æ Ln excess Or 50 ksi were obtained in both cases by heat treatment at 1200 - 1300~ for one hour.
Example 5 was an embodiment o~ a relativel~y highly alloyed titanium and columbium-bearing steel which exhibited an increase in yield strength substantially the same as that ', , of examples 2 and 3.
.
Example 6, illustrative o~ lower alloying additions of titanium and columbium than example 5, exhibited signifi-cant increase in yleld strength, althou~h not as high as that of the more highl,y allo,ved example 5.
The yield strengthsreported for samples subJected to , 20% cold reduction (simulating-the strain or deformation resulting from deep drawing) show that the strengthening whlch accompanies the precipitation of alloy nitrides is additive ~`
to the cold work strengthehing so that a net gain in strength ls obtained even though there is a small loss in strength due to partial recovery. The strength advantage in nitrided cold : . .
worked material over nitrided as annealed material is attrlbut-able to alloy nitride nucleation and precipitation on dis-', locations and to enhanced solubility of nitrogen in a strained ; , ~' 15 lattice.
~-~ The elongation values after nitriding are relatively high in view of the yield strength levels attalned. These elongation values indicate that some limited formin~ could be ~ perrormed after nitriding strip material such as bending or a , 20 restrike operation. The samples sub~ected to 2Q% cold re-duction increased in elongation values alon~ with an increase in yield stren~th, because of recover,y.
As indlcated previously the heat treatment step o~ "
the present process results in an increase in nitro~en in solid solution in the steel as well as nitrogen combined as nitrides with titanium, columbium, and/or zirconium. It has been found tha~ the total amount of nltrogen taken up by the steel can exceed that required to satisfy normal e~uilibriwn , ~L~6338~ -solutlon requirements ~lus that needed to convert the alloys to nitrides, This excess nitrogen can be attributed.to nitrogen trapped on dislocations, adsorbed at the nitride- ~ :
ferrite interface, and as enhanced lattice solubility in strained ferrlte.
~ Of rurther significance in Table II are the values - reported for nitriding at 1300F in 3% ammonia for 2 hours.
In most instances a decrease in yiel-d strength from the - maximum values obtained at 1200F for two hours occurred., due to the formation of coarser alloy nitride particles. A thin . austenite rim formed on the surfaces of samples nitrided at 1300F; therefore, to avoid the formation of an iron nitrogen : austenite rim, the ammonia concentration should be slightl,y less than 3% at 1~00F for a two-hour heat treatment.
A comparison of the strengthening achieved by heat treating in a 3% ammonia - 97% hydrogen mixture with that achieved in a 6% ammonia - 94% hydrogen mixture is given in Table III below " "' '', ~63~9 v, e to ~ ~o ~ i o oa~ o o ~ o o ¢ ~:: Z: ~Z~ Z ~; ~ Z
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~3~389 It is apparent from Table III that any given steel achieves a higher yield strength when nitrided in 6% ammonia under the same time and temperature conditions than is attained by nitriding in 3% ammonia. Higher strength is ob-tained by nltriding at 1200F for one hour in 6~ ammonia thanby nltriding at 1200F for two hours in 3% ammonia. However, due to diffuslon phenomena, the surface to mid-thickness strength gradient would be greater in a 6% ammonia atmosphere than in a 3% ammonia atmosphere. For some applications it may be desirable to obtain a lower average strength with a lesser gradient to mid-thickness. The present invention makes it possible to select readily temperature, time and ammonia concentrations which will result in a wide range o~
average yield strengths and surface to mid-thickness strength gradients~
Table III again indicates that nitridlng ln 6%
ammonia at 1300F results in formation of an iron-nitrogen austenite rim which will transform either to martensite or -an eutectoid structure, depending upon the cooling rate.
In example 5, an austenite rim about 1 mil thick resulted rrom annealing in 6% ammonia at 1300F for one hour.
As the thlckness Or the steel stock subJected to the heat treatment o~ the present inventlon increas0s, the time requlred to reach saturation at the equilibrlum nitrogen content in solution (for a given temperature and ammonia con-centration in the atmosphere) increases as the square of the thlckness. For example,for nitrogen diffusion in pure iron at 1200F, to reach an average fractional saturation (i.e.
N Avg/N Equll.) of 0.7 it has been ~ound that one hour is .
i33~3~
.:
requlred for sheet stock of 0.040 inch thickness, while 5.6 hours ls re~uired for sheet .stock of 0.090 inch thlckness.
: However, an important feature of the present lnvention is the dlscovery that marked increases in average yield strength can be realized within relatively short times, (i.e. not , ..... .
~ more than two hours), by partial alloy-nitrogen preci~ltation - strengthening. Table IV below indicates the substantial : incr~ase in yield strength achieved by nitriding the titanium- ~ .. .
... . . .
columbium bearing steel o~ example 8 ln a 3% ammonia - 97%
.~ 10 hydrogen atmosphere within the temperature range o~ 1100 -1300F for 1 - 2 hours. It will be noted that nltriding at 1200F for only one hour resulted in an average yleld strength o~ 66.5 ksi. Even greater strengthenln~ could i, . .
be attained by heat treating f`or lon~er periods Or time or by increasing the ammonia concentration to 6%. In Table IV
1300F again proved to be an unacceptable temperature when .
using a 3% ammonia atmos~here because Or formation of an austenlte rlm.
".
;. '.
--21J-- , , , ` 3L~6338~
TABLE IV
--Properties of 0.090 Inch Hot Rolled Steel of Example 8 Heat Treated in 3% ~H3 - 97% H2 Y.S. T.S.
Conditlon ksi ksi %Elong. %Y.P.E~ ~N
As Hot Rolled 27.3 49.1 40.5 0.0 .0027 1100F-1 hr. 46.8 63.3 30.0 0.0 .010 ,.: . .
-2 hrs. 56.8 70.3 22.0 . .013 101200F-1 hr. 66.5 81.1 21.0 0.0 .027 -2 hrs. 83.9 94.9 16.0 1.0 .043 1300F-1 hr. 77.7 91.1 20.0 1.3 .066 -2 hrs. 83.9 94.2 13.0 1.8 .o80 The criticality Or providing at least about 0.02%
titanium in solutlon is illustrated in Table V below.
TABLE V
Erfect Or Amount Or Available Nitride~Formin~ Element o.oo44%C 0.010%C C Saturated(0.024%C) Available titanium ~
- in Solut~on .47% 0.025% %
Y.S.-ksi(nitrlded in 6% NH3, 1200QF-2 hrs.) 10-2.5 85~3 64.o Y.S.-ksi(nitrided and then denitrided) 81.8 65.9 44.7 .S.-ksi(Y.S.nitr:l-ded-Y.~.denltrlded) 20.7 19.ll 19.3 In Table V a steel of the invention containing 0.077% ;
total titanium, ~.037% total columbium, 0.031% aluminum, 0.0035%
nitrogen, and remainder substantially lron, was carburized rrom an original carbon level Or 0.0044% to a carbon level Or 0.010% and to saturation with carbon in order to var~ the amount Or titanium in solutlon available to react with ammonia in the nitriding operation. It is apparent ~rom Table V that .
~e63313~
a substantial decrease in yield strength occurs progressively with decrease of available titanium in solution from 0.047%
to 0.025% and to 0% successively.
' In order to ascertain the degree of strengthening contributed by nitrogen taken into solid solution in the steel, the samples of Table V were denitrided by heating in a hydrogen atmosphere at about 1200F for 2 hours. Table V
reports the, yield strength in the denitrided condition and furt,her sets forth the differential at each Or the di~ferent carbon contents, from which it is apparent that nitrogen taken into solid solution contributes about 20 ksi to the yield ,-strength. It is further apparent that denitrided material ~' at the 0.010% carbon level (resulting in 0~025~ available titanium in solution) retains a substantially increased yield strength of 65.9 ksl ln the denitrLded condtion.
It has been discovered that the excess nitrogen content of the steels following alloy-nitrogen precipitation `' strengthening at higher allowable ammonia contents can present weldability problems and result in high ductile to brittle ' notched sheet Charpy impact transition temperature. The welding problems involve porosity resulting from the liberation ' Or this excess nitro~en as nitrog~n gas. Th~e problems can be overcome by special welding techniques. The use of high ammonia concentrations, ~ust less than that which results in the formatlon of iron nitrides or an iron-nitrogen austenite, i9 desirable from the standpoint of producing maxlmum strengthen-ing in the shortest possible time. However, it has been dls- ~ ' covered that if nitriding conducted for the purpose of strength-ening (and which results in an undesirably high excess nitrogen 1~633~39 content in solid solution) is followed by a denitriding step, such as annealing in hydrogen gas, the excess nitrogen is removed with only a 10 to 20 percent reduction in yield strength. Removal of the excess nitrogen eliminates welding porosity and significantly reduces the ductile to brittle ; transition temperature while improving the impact energy values. The present invention thus provides low carbon, high strength steel stock suitable for welding applications. ~-Table VI demonstrates the above observations regarding the erfect of denitriding. A steel of the invention lnitially containing 0. oo6~ carbon, 0.077% titanium, 0.037% columbium, 0.031% aluminum, 0.0035% nitrogen, and balance substantially lron, cold rolled to o.o58 inch thickness and annealed, was nltrided as lndicated ln Table VI. Sample A
was not denitrided, Sample B was partially denitrided, and Sample C was stlll further denitrided. Both the yield strength and the ductile to brittle transition temperature decreased gradually with ehe lecrease in nltrogen in solutlon.
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Steel having the composition speciried herein can be melted by an,yconventional operatlon such as open hearth, basic oxygen furnace or electric furnace. The rnolten steel is then vacuum degassed in order to achieve the desired carbon and nitrogen levels, killed preferably wlth A],and the nitride-forming alloying element or elements are added to the ladle after degassing with suitable mixing. The melt is then teemed into ingots, or cast into slabs. The solldified ingots or slabs are then subJected to conventional not rolling and to conventional subsequent processlng steps to obtain sheet stock of the desired final thickness. The steel is then subJected to the process Or the present invention either in the rorm of sheet or strip, or after forming into articles by drawl g or Atamp:ns.
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Claims (15)
- The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
l. Cold reduced and annealed steel sheet stock having a thickness between 0.5 and 2.29 mm, an average yield strength of at least 70 ksi, and sufficient formability to permit fabrication into articles other than deep drawn, consisting essentially of from 0.002% to less than 0.010% carbon, from 0.05% to 0.6% manganese, from 0.02% to 0.04% total aluminum, about 0.035% maximum sulfur, about 0.01% maximum oxygen, residual silicon and phosphorus, at least one nitride-forming element chosen from the group consisting of titanium, titanium and columbium, titanium and zirconium, and titanium and mixtures of columbium and zirconium, with total titanium ranging between 0.08% and 0.10%, total columbium ranging between 0.03% and 0.06%, total zirconium ranging between 0.03% and 0.06%, sufficient nitrogen to combine substantially completely with said alumi-num and said nitride forming element, and remainder iron except for incidental impurities, all percentages being by weight. - 2. A deep drawn article formed from a cold-reduced and annealed steel sheet stock having a thickness between 0.5 and 2.29 mm consisting essentially of less than 0.010% carbon, from 0.05% to 0.6% manganese, from 0.02% to 0.04% total alumi-num, about 0.035% maximum sulfur, about 0.01% maximum oxygen, residual silicon and phosphorus, at least one nitride forming element chosen from the group consisting of titanium, titanium and columbium, titanium and zirconium, and titanium and mixtures of columbium and zirconium, with total titanium ranging between 0.08% and 0.10%, total columbium ranging between 0.03% and 0.06%, total zirconium ranging between 0.03% and 0.06%, sufficient nitrogen to combine substantially completely with said aluminum and said nitride forming elements, and remainder iron except for incidental impurities, all percentages being by weight, said article having an average yield strength of at least about 50 ksi.
- 3. A method of increasing the yield strength of a low carbon steel sheet stock, comprising the steps of pro-viding a deep drawing quality steel containing from 0.002%
to 0.015% carbon, 0.012% maximum nitrogen, 0 to 0.08% aluminum, 0.05% to 0.6% manganese, 0.035% maximum sulfur, 0.01% maximum oxygen, 0.01% maximum phosphorus, 0.015% maximum silicon, a nitride-forming element chosen from the group consisting of titanium, columbium, zirconium, and mixtures thereof, in amounts such that titanium in solution is from 0.02% to 0.2%, columbium in solution is from 0.025% to 0.3%, and zirconium in solution is from 0.025% to 0.3%, the sum total of titanium, columbium and zirconium not exceeding 0.3%, and remainder iron except for incidental impurities, all percentages being by weight; reducing said steel to final thickness; and heating the resulting sheet stock in an atmosphere comprising ammonia and hydrogen at a temperature between 1100° and 1350°F (593°
and 732°C) for a period of time sufficient to cause reaction of said nitride-forming element with the nitrogen of said ammonia to form small, uniformly dispersed nitrides, in order to increase the average yield strength of said sheet stock to a minimum of 60 ksi (414 MM/m2), the concentration of ammonia in said atmosphere ranging between 2% and 10% by volume and being insufficient, at the temperature and time involved, to permit formation of iron nitride. - 4. The method according to claim 3, wherein said steel initially consists essentially of from 0.002% to 0.015%
carbon, 0.012% maximum nitrogen, 0 to 0.08% aluminum, from 0.05% to 0.6% manganese, from 0.02% to 0.2% titanium in solu-tion, 0.035% maximum sulfur, 0.01% maximum oxygen, residual silicon and phosphorus, and remainder iron except for inci-dental impurities, all percentages being by weight. - 5. The method according to claim 3, wherein said steel initially consists essentially of from 0.002% to 0.015%
carbon, 0.012% maximum nitrogen, 0 to 0.08% aluminum, from 0.05% to 0.6% manganese, from 0.025% to 0.3% columbium in solution, 0.035% maximum sulfur, 0.01% maximum oxygen, resi-dual silicon and phosphorus, and remainder iron except for incidental impurities, all percentages being by weight. - 6. The method according to claim 3, wherein said steel initially consists essentially of from 0.00% to 0.015%
carbon, 0.012% maximum nitrogen, 0 to 0.08% aluminum, from 0.05% to 0.6% manganese, from 0.025% to 0.3% zirconium in solution, 0.035% maximum sulfur, 0.01% maximum oxygen, resi-dual silicon and phosphorus, and remainder iron except for incidental impurities, all percentages being by weight. - 7. The method according to claim 3, wherein said steel initially consists essentially of less than 0.010 carbon, from 0.05% to 0.6% manganese, from 0.02% to 0.04%
total aluminum, 0.004% maximum nitrogen, 0.035% maximum sulfur, 0.01% maximum oxygen, residual silicon and phosphorus, from 0.08% to 0.10% total titanium, from 0.03% to 0.06% total columbium, and remainder iron except for incidental impurities, all percentages being by weight. - 8. The method according to claim 3, wherein said steel initially consists essentially of less than 0.010% carbon, from 0.05% to 0.6% manganese, from 0.02% to 0.04% total aluminum, 0.004% maximum nitrogen, 0.035% maximum sulfur, 0.01% maximum oxygen, residual silicon and phosphorus, from 0.08% to 0.10%
total titanium, from 0.03% to 0.06 total zirconium, and re-mainder iron except for incidental impurities, all percentages being by weight. - 9. The method according to claim 3, wherein said sheet stock is cold rolled to a thickness of from 0.02 to 0.09 inch (0.5 to 2.29mm).
- 10. The method according to claim 3, wherein said atmosphere contains from 3% to 6% ammonia by volume, and re-mainder hydrogen.
- 11. The method according to claim 3, wherein said atmosphere contains ammonia and hydrogen, and the balance an inert gas, with the ammonia to hydrogen ratio adjusted in such manner that the formation of iron nitride or iron-nitrogen austenite is avoided.
- 12. The method according to claim 9, wherein said heating is conducged at 1100° to 1300°F (593° to 705°C) for a period of time inversely proportional to the temperature and directly proportional to the square of the thickness.
- 13. A method according to claim 3, 4 or 5, including the steps of annealing said steel to soften and impart excellent drawing quality properties after reducing to final thickness, forming an article from said reduced and annealed steel, and strengthening said article by subjecting it to said heating in an atmosphere comprising ammonia and hydrogen.
- 14. The method according to claim 3, 4 or 5, including the steps of annealing said steel after reducing to final thickness, forming an article by deep drawing said reduced and annealed steel, and strengthening said article by subjecting it to said heating in said atmosphere comprising ammonia and hydrogen.
- 15. The method according to claim 3, 4 or 5, in-cluding as a final step denitriding said sheet stock in a hydrogen atmosphere at a temperature between 1100° and 1350°F
1593° and 732°C) for a period of time sufficient to decrease the ductile to brittle transition temperature to at least about - 60°F (-51°C).
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA208,805A CA1063389A (en) | 1974-09-09 | 1974-09-09 | High strength low carbon steel and method |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA208,805A CA1063389A (en) | 1974-09-09 | 1974-09-09 | High strength low carbon steel and method |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1063389A true CA1063389A (en) | 1979-10-02 |
Family
ID=4101085
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA208,805A Expired CA1063389A (en) | 1974-09-09 | 1974-09-09 | High strength low carbon steel and method |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1063389A (en) |
-
1974
- 1974-09-09 CA CA208,805A patent/CA1063389A/en not_active Expired
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