CA1331128C

CA1331128C - Continuous treatment of cold-rolled carbon high manganese steel

Info

Publication number: CA1331128C
Application number: CA000563296A
Authority: CA
Inventors: George Krauss; Philip M. Roberts
Original assignee: Signode Corp
Current assignee: Signode Corp
Priority date: 1987-04-10
Filing date: 1988-04-05
Publication date: 1994-08-02
Anticipated expiration: 2011-08-02
Also published as: DE3811270A1; AU625223B2; KR880012777A; JPS64221A; KR950008532B1; AU607480B2; GB8808405D0; GB2203169A; JP2677326B2; GB2203169B; DE3811270C2; AU6796990A; AU1436288A; MX165036B; CA1333990C

Abstract

CONTINUOUS TREATMENT OF COLD-ROLLED CARBON
MANGANESE STEEL
Abstract Of The Disclosure Cold-rolled, non-microalloyed carbon manganese steel (0.11% to 0.18% C, 1.20% to 1.40% Mn) is preheated at 700° to 1000°F, heated to 1500° to 1725°F, and quenched to 800° to 950°F in a continuous process to develop minimum yield strength of 480 MPa, minimum tensile strength of 585 MPa, and 14% minimum elongation.

Description

CONTINUOUS TREATMENT OF COLD-ROLLED
CARBON MANGANESE STEEL
Background Of The Invention :
There exists today a group of steels whic~
are characterized by among other things enhanced mechanical properties including higher yield strengths and tensile strengths than plain carbon structural -~
steels. These are known as high-strength, low-alloy (HSLA) steels. Different types of HSLA steels are available, some of which are carbon-manganese steels and others of which are microalloyed by additions of such elements as niobium, vanadium, and titanium to achieve enhanced mechanical properties. The original `~
demand for HSLA steels arose from the need to obtain improved strength-to-weight ratios to reduce dead weight in transportation equipment. In addition to the original uses, HSLA steels are used today in a ~- -wide range of applications including vehicles, con-struction machinery, materials-handling equipment, bridges and buildings.
Commercial HSLA steels typically have minimum yield strengths of 275 to 345 MPa and minimum - 1 - : : :
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1331128 :

tensile strengths of 410 to 480 MPa. The mechanical properties and other characteristics of HSLA steels are set forth in standard specifications such as Society of Automotive Engineers (SAE) J410c. Micro-alloyed HSLA steels have even higher strengths on theorder of minimum yield strengths of 345 to 550 MPa and minimum tensile strengths of 450 to 655 MPa. These steels use additions of alloying elements such as niobium, vanadium, titanium, zirconium and rare earth elements in concentrations generally below 0.10 to 0.15% to achieve higher strength levels. Heat treat-ment is not involved because the properties of micro-alloyed HSLA steels result from controlled rolling on continuous hot strip mills.
One grade of high-strength low-alloy steel under SAE J410c is grade 950 A,B,C,D, which is charac-terized by a minimum yield strength (0.2% offset) of 345 MPa, minimum tensile strength of 4~0 MPa, and minimum elongation (5 cm specimen) of 22%. This material exhibits its mechanical properties as hot rolled, and when later cold reduced to sheet thick-ness, is subjected to a low temperature recovery anneal for an extended period of time to maintain the as-rolled mechanical properties. -Another grade of microalloyed, high-strength, low-alloy steel under SAE J410c is grade 970X, which is characterized by a minimum yield strength (0.2% ~ r~

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offset) of 480 MPa, minimum tensile strength of 585 MPa, and minimum elongation (5 cm specimen) of 14%.
As stated, this material exhibits its mechanical properties as hot rolled. When later cold reduced to sheet thickness, these steels are also subjected to a low temperature recovery anneal for an extended period of time to maintain the controlled rolled mechanical properties. In addition to the increased cost because of the addition of microalloying elements, this recovery anneal is disadvantageous because of either the extended times required for box annealing or the enormous investment required for equipment ~or con-tinuous annealing. ;
There thus exists today a need for steels possessing the desired combination of strength and ductility required for HSLA steel applications but which can be produced economically from cold reduced ~
sheet stock without the need for extended recovery -;
annealing. Moreover, there exists a need for such steels wherein the higher mechanical properties, ;~
particularly yield strength and tensile strength, are : : ~
achieved without the intentional inclusion of micro-alloying agents such as niobium, titanium and vanadium, which otherwise would add significantly to the cost of the steel.

.

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Summary Of The Invention It is among the principal objectives of this invention to provide a method for treating cold reduced steel compositions characterized by a rela-tively low carbon content and the absence of expensive ~ -microalloying agents which nevertheless exhibit in the treated condition mechanical properties, i.e., yield strength, tensile strength, and elongation, meeting the specifications for microalloyed HSLA steels, for example, grades 950 A,B,C,D and 970X of SAE J410c.
Moreover, it is among the principal objectives of this invention to provide such a method for producing cold reduced steels having the uniformly higher mechanical properties of the microalloyed HSLA steels which can be produced in a continuous process at relatively high ;
speed and very economically.
To these ends, the present invention is directed to a non-microalloyed low carbon manganese steel compositions and to a heat treatment method therefor. One steel composition included within this invention has a carbon content ranging from .11 to .18% by weight carbon and 1.20 to 1.40% by weight -.I r~
manganese. Microalloying elements such as niobium, titanium and vanadium are not added to the steel composition to achieve enhanced mechanical properties.
The steel, which is cold reduced to a desired sheet thickness, e.g., in the range of 0.078 to 0.236 mm, is ~` ' .
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passed continuously through three heating stages. The first stage is a preheating stage wherein the tempera~
ture of the cold rolled sheet is raised to a tempera-ture in the range of about 700F to 1000F. The steel is then heated to a temperature in the range of 1500F
to 1575F, quenched at a temperature in the range 850F to 950F, and then cooled to room temperature.
The heat treatment is carried out continuously at a line speed in the range of 50 to ;~
300 feet/minute whereby a continuous length of steel -~
strip of desired gauge and width is passed continuously ~ ~-and sequentially through the three heating stages.

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' " , ' --6- 13311~8 A preferred steel composi-tion is a steel having about 0.11 to 0.18% by weight carbon and about 1.20 to 1.40~ by weight manganese, the balance being iron and the normal residuals from deoxidation. When treated in accordance with the second heat treatment schedule described above, the treated steel exceeds the minimum yield strength of ~-480 MPa, minimum tensile strength of 585 MPa, and minimum elongation of 14~ specified for grade 970X SAE
J410c specifications.
The method of this invention for t.reating steels having the relatively low carbon and the manganese content recited and the absence of micro-. '. :~ .

133112~

alloying agents results in a cold reduced product having mechanical properties meeting or exceeding some existing HSLA steel specifications for microalloy steels. The present invention is thus characterized by the higher mechanical properties of some of the commercial microalloyed high-strength low-alloy steels but obtainable in a non-microalloyed, cold reduced low carbon steel and by the economies inherent in the absence of microalloying agents, and the continuous process for the treatment of a cold reduced product.
Brief Description Of The ~rawin~s Fig. 1 is a schematic illustration of the treatment process.
Detailed Description Of The Preferred Mode The carbon-manganese steel compositions treated by the method of this invention contain in one case from about 0.04 to 0.15~ by weight carbon and 0.25 to 0.70% by weight manganese and in another case from about 0.11 to 0.18% by weight carbon and 1.20 to 1.40~ by weight manganese. The steel is killed, pref-erably, aluminum killed and continuously cast, to achieve uniformity of mechanical properties. As a result, the composition can contain residual silicon and aluminum from the deoxidation process. The steel may also be a silicon killed or semi-killed steel.
Referring to Fig. 1, hot rolled coils of steel, which may be pickled and oiled, are cold -8- 1 3 3 ~ 1 2 8 reduced through a series of cold rolling passes to a sheet 10 having a desired reduced thickness, for example, on the order of 0.078 to 0.236 mm. The cold rolled and reduced sheet 10 is then passed over roller 11 and down into a preheating bath 12 which may be a bath of molten lead maintained at a temperature in the range of 700 to 1000F. The lead bath may be heated by any of a number of means, e.g., natural gas or electricity. Alternatively to a lead bath, other media capable of providing a liquid bath having a temperature in the range of 700 to 1000F may be used. ~
The material then passes upwardly out of the bath and ~ Y~-over an elevated roller 14. The material then passes down into a second molten lead bath 16 which is the ;
quench bath.
In the heating stage, the material is heated to a temperature in the range of 1625 to 1725F
depending on composition. In the quench stage, the material is quenched at a temperature in the range of 650 to 950F depending on composition. That is, the lower manganese composition is heated in the range of 1625C to 1725F and quenched in the range of 650 to 750F while the higher manganese composition is heated in the range of 1500 to 1575F and quenched in the range of 800 to 950F. Heating of the material in the heating stage is accomplished ~y resistance heating. That is, the preheat bath 12 and the quench ~-~
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,,~". ~,.~,;.~, g bath 16 are maintained at a potential of about 90 volts and current of 8000 amperes with the quench bath being grounded. As a consequence, the sheet material 10 passing between the preheat bath and the quench bath shunts the current and is thereby resistance heated. The length of material passing through the heating stage, current, and travel speed are con-trolled to subject the material in the heating stage to the desired treatment temperature. A protective atmosphere is maintained in the heating stage by enveloping the sheet material 10 in an atmosphere housing 18 which is flushed with a protecti~e exo- ;
thermic gas. The gas prevents the sheet material from oxidizing as it passes from the preheat bath 12 to the quench bath 16. Alternatively to resistance heating, the material 10 may be heated by other heating means such as induction, infrared, and gas heating.
The quench bath 16 is also a lead bath which can be heated by such means as electric immersion heaters or radiant gas tubes to the desired tempera-ture. After quenching, the material then passes out of the quench bath 16 and vertically upward over a roller 20 and through a charcoal chute 22 which contains ignited charcoal designed to prevent the lead from being dragged out of the quench bath on the sheet material. The sheet material which is now at a temperature of about 500F is then passed through a _g 1331~28 downstream water tank or water spray (not shown) to bring its temperature down to about 150F. However, all of the transformation of the steel is completed by -~
the time the material leaves the quench bath 16.
After cooling, the material may be coiled for shipment or subsequently processed by known techniques or combination of known techniques, e.g., acid and/or ,I'~ i.Y.
abrasive cleaning, painting, plating, flattening, tension leveling, and the like. ;
The sheet material continuously passes .: ~
through the preheat, heat and quench stages. Typical line speeds are on the order of 15 to 100 meters per minute. The preheat, heat, and quench stages are approximately 3 to 8 meters long. As a consequence, the material is heated or quenched very rapidly in each stage on the order of only 6-15 seconds, for example, at a line speed of 30 meters per minute.
Representative equipment for accomplishing such heating is disclosed in United States Patent Nos.
2,224,988 and 2,304,225 to Wood et al. Again, heating and quenching media other than molten lead can be used for both the preheat and quench baths.
It is believed that the relatively short -~
cycle times in the preheat, heat, and quench stages result in grain refinement and consequently increased strength. That is, in the preheat and heat stages, ;
the strain introduced into the material from cold ;~ ~ -~ .

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rolling causes recrystallization of the ferrite to a ;~
fine grain structure. The short cycle times limit grain growth keeping the grain size small, typically under 0.01 mm and frequently 0.003 to 0.004 mm and finer. In addition, small amounts of austenite form at the grain boundaries on heating and act to pin the grain boundaries against movement again serving to limit gain growth and resulting in higher strength levels. At the same time, the carbides in the pearlite are spheroidized and imperfections removed increasing the ductility of the steel. During the quench, the carbides precipitate introducing ductility and removing the potential for subsequent strain aging.
Example I
Using the equipment described in Fig. 1, 5 cm wide by 0.11 cm thick steel strip cold reduced from 0.2 cm material was heat treated. The steel was aluminum killed for uniformity of properties and the composition contained 0.10% carbon, 0.40% manganese, 0.012% silicon and 0.057% aluminum, the silicon and aluminum components being residuals from the deoxidation of the steel before casting. The strip material traveled at a rate of 33 meters per minute.
The length of the strip under the lead in the preheat bath was 3 meters, in the quench bath 6 meters, and in the heating stage 7.3 meters. Roller 14 was 2.4 meters above the lead baths. An optical pyrometer was ~ 3 ~ ~ 2 8 used to measure strip temperature. The treatment schedule and resulting mechanical properties are set ~ r *~
forth in Table I.

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As may be seen from Table I, the mechanical properties resulting from the treatment process exceeded the minimum mechanical properties specified .
for grade 950 A,B,C,D (345 MPa yield strength, 480 MPa tensile strength, 22% elongation).
A second, similar steel composition was run using the same process conditions. This composition comprised 0.04/0. n 6~ carbon and 0.25/0.35% manganese.
The treatment schedule and resulting mechanical properties are set forth in Table II. ;~

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133~28 This material although lower in tensile strength than the previous example was characterized by excellent degree of elongation and thus would be expected to have a high degree of formability.
Example II
Using the equipment described in Fig. l, 5 cm wide by 0.11 cm thick steel strip cold reduced from 0.2 cm material was heat treated. The steel was -aluminum killed for uniformity of properties and the composition contained 0.14~ carbon, 1.33% manganese, 0.22~ silicon and 0.019% aluminum, the silicon and~
aluminum components being residuals from the deoxidation of the steel before casting. The strip material traveled at a rate of 33 meters per minute.
The length of the strip under the lead in the preheat ;~
bath was 3 meters, in the quench bath 6 meters, and in the heating stage 7.3 meters. Roller 14 was 2.4 meters above the lead baths. An optical pyrometer was used to measure strip temperature. The treatment schedule and resulting mechanical properties are set forth in Table III. ~ ;~

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As may be seen from Table III, the mechanical properties resulting from the treatment process exceeded the minimum mechanical properties specified -;
for grade 970X (480 MPa yield strength, 585 MPa tensile strength, 14% elongation). Both samples exhibited excellent ductility in combination with the .- - ..: :..:
higher strength levels.
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The method of the present invention is ;
applicable to a range of steel compositions within the compositional limits set forth above. As the preceding ;
specific example shows, the treatment method provides low carbon high manganese cold reduced steels with the ~-~
desired combination of strength and ductility charac~
terizing commercial microalloyed and hot rolled high-strength low-alloy steels.
Thus having described the invention, what is claimed is~

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Claims

1. A method of treating steel in a continuous process wherein the steel is cold reduced and has a composition of from about 0.11% to 0.18% by weight carbon and 1.20% to 1.40% by weight manganese, without the addition of microalloying agents for the purpose of achieving enhanced mechanical properties, com-prising the steps of:
(1) preheating the steel to a temperature in the range of 700° to 1000°F;
(2) heating the steel to a temperature in the range of 1500° to 1575°F; and (3) quenching the steel at a temperature in the range of 800° to 950°F;
the treated steel having a minimum of 480 MPa yield strength; 585 MPa tensile strength; and 14%
elongation.

2. A method for treating steel in a continuous process wherein the steel is cold reduced and has a composition of about 0.14% by weight carbon and 1.33%
by weight manganese, without the addition of microalloying agents for the purpose of achieving enhanced mechanical properties, comprising the steps of:

(1) preheating the steel to a temperature in the range of 700° to 1000°F;
(2) heating the steel to a temperature in the range of 1500° to 1575°F; and (3) quenching the steel at a temperature in the range of 800° to 950°F;
the treated steel having a minimum of 480 MPa yield strength; 585 MPa tensile strength; and 14%
elongation.

3. The method of any one of claims 1 and 2 wherein the material is preheated by passing through a molten lead baths and is heated by a resistance heating stage each in less than about 15 seconds.

4. The method of any one of claims 1 and 2 wherein the material is aluminum killed steel sheet or strip.