GB2203169A

GB2203169A - Continuous treatment of cold-rolled carbon manganese steel

Info

Publication number: GB2203169A
Application number: GB08808405A
Authority: GB
Inventors: George Krauss; Philip M Roberts
Original assignee: Signode Corp
Current assignee: Signode Corp
Priority date: 1987-04-10
Filing date: 1988-04-11
Publication date: 1988-10-12
Anticipated expiration: 2008-04-11
Also published as: AU607480B2; GB2203169B; KR950008532B1; DE3811270A1; DE3811270C2; JP2677326B2; MX165036B; AU1436288A; KR880012777A; AU6796990A; AU625223B2; GB8808405D0; CA1333990C; JPS64221A; CA1331128C

Description

CONTINUOUS TREATMENT OF COLD-ROLLED CARBON MANGANESE STEEL This invention

relates to cold-rolled carbon steel. There exists today a group of steels which 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 is weight in transportation equipment. In addition to the original uses, HSLA steels are used today in a wide range of applications including vehicles, construction machinery, materials-handling equipment, bridges and buildings.

Commercial HSLA steels typically have minimum yield strengths of 275 to 345 MPa and minimum 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. Microalloyed HSLA steels have even higher strengths on the order 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 treatment is not involved because the properties of microalloyed 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 characterized by a minimum yield strength (0.2% offset) of 345 MPa, minimum tensile strength of 480 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 thickness, 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% 1 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 thicknessf 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 for continuous 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 microalloying agents such as niobium, titanium and vanadium, which otherwise would add significantly to the cost of the steel.

It is among the principal objectives of this invention to provide a method for treating cold reduced steel compositions characterized by a relatively low carbon content and the absence of expensive microalloying agents which nevertheless exhibit in the treated condition mechanical properties, i.e., yield is 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.

Such a method in accordance with this invention for treating steel in a continuous process wherein the steel is cold reduced and has a composition of from about 0.04% to 0.18% by weight carbon and 0.,25% to 1. 40% by weight manganese, without the addition of microalloying agents for the purpose of achieving enhanced mechanical properties, comprises the steps of:

(1) preheating the steel to a temperature in the range of 7000 to 1000'F; (2) heating the steel to a temperature in the range of 15000 to 1725'F; and (3) quenching the steel at a temperature in the range of 6500 to 950'F; the treated steel having a minimum of 275 MPa yield strength; 345 MPa tensile strength; and 14% elongation. One steel composition included within this invention has a carbon content ranging from.04 to.15% by weight carbon and 0.25 to 0.70% by weight 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 passed continuously through the three heating stages. In the second stage the steel is heated to a temperature in the range of 1625'F to 1725'F, quenched at a temperature in the range 650OF to 750'F, and then cooled to room temperature. Another 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 manganese. The steel is also cold reduced to a desired sheet thickness, e.g., in the range of 0.078 to 0.236 mm and is passed continuously through the two heating stages, and the steel being quenched at a temperature in the range 850F to 95CF.

The heat treatment in both cases 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.

One presently preferred steel composition is a steel having about 0.10 to 0.15% by weight carbon i and about 0.25 to 0.70% by weight manganese, the balance being iron and the normal residuals from deoxidation. When treated in accordance with the first heat treatment schedule described above, the treated steel exceeds the minimum yield strength of 345 MPa, minimum tensile strength of 480 MPa, and minimum elongation of 22% specified for grade 950 A,B,C,D of SAE J410c specifications. Another lower carbon composition-containing from about 0.04 to 0.07% by weight carbon and about 0.25 to 0.40% by weight manganese when treated by the method of this invention exhibits a minimum yield strength of 345 MPa, minimum tensile strength of 410 MPa, and a relatively high elongation of 28%.

Another presently preferred steel composition 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 treating steels having the relatively low carbon and the manganese content recited and the absence of microalloying 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 highstrength 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 procQss for-the treatirlent of a.cold.reduced product, The invention will be further described by way of example with reference to the accompanying drawing which Is a schematic illustration of a preferred treatment process.

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, preferably, 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 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 7000 to 10001F. 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 10001F may be used. The material then passes upwardly out of the bath and 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 1725'F depending on composition. In the quench stage, the material is quenched at a temperature in the range of 6500 to 950OF depending on composition. That is, the lower manganese composition is heated in the range of 1625' to 17251F and quenched in the range of 650' to 750OF while the higher manganese composition is heated in the range of 15000 to 1575OF and quenched in the range of 8000 to 9500F. Heating of the material in the heating stage is accomplished by resistance heating. That is, the preheat bath 12 and the quench -9bath 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 controlled 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 protective exothermic 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 temperature. 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 500OF is then passed through a -10downstream 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 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 is -11rolling causes recrystallization of the ferrite to a fine grain structure. The short cycle times limit grain growth keeping the grain size small, typically und er 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% manganesef 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 1 used to measure strip temperature. The treatment schedule and resulting mechanical properties are set forth in Table I.

TABLE I

Sample Strip Quench Tensile Yield % Elongation Code Preheat 'F Temp. 'F Temp. 'F Strength(MPa) Strength (bfPa) (5 cm_gauge) YSITS Hardness 1-0 860 1725 690 505.4 455.1 26.7.90 86 1 26.7.91 86 2-0 775 1670 720 503.3 456.5 W 1 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 0.04/0.06% carbon and 0.25/0.35% manganese.

The treatment schedule and resulting mechanical properties are set forth in Table II.

is 1 TABLE 11

Sample Strip Quench Tensile Yield % Elongation Code Preheat F Temp. 'F Temp. 'F Strength (MPa) Strength (MPa) (5 cm gauge) YSITS Hardness 1-0 860 1725 690 417.2 371.6 28.7.89 75 2-0 775 1670 720 413.7 264.7 28.0.88 73 1 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. 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 0.14% carbon, 1.33% manganesef 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.

11 TABLE III

Sample Strip Quench Tensile Yield % Elongation Code Preheat OF Temp. 'F Temp. OF Strength (MPa) Strength (11Pa) (5 cm gauge) YSITS Hardness 3-m 795 1535 855 639.1 559.9 18.7.88 95 4-M 820 1500 950 593.0 524.0 22.0.88 92 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.

The method of the present invention is applicable to a rpLnge 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 characterizing commercial microalloyed and hot rolled high-strength low- alloy steels.

i

Claims

19 CLAIMS:1. A method of treating steel in a continuous process wherein

the steel is cold reduced and has a composition of from about 0.04% to 0. 18% by weight carbon and 0.25% to 5 1.40% 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 7000 to 1000OF; (2) heating the steel to a temperature in the range of 15000 to 1725'F; and (3) quenching the steel at a temperature in the range of 6500 to 950OF; the treated steel having a minimum of 275 MPa yield strength; 345 MPa tensile strength; and 14% elongation.

2. A method of treating steel as claimed in Claim 1 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 the treated steel having a minimum of 480 MPa yield strength; 585 MPa tensile strength; and 14% elongation.

3. A method for treating steel as claimed in Claim 1 or 2 wherein the steel has a composition of about 0.14% by weight carbon and 1.33% by weight manganese, without the addition of microalloying agents, and wherein the steel is heated in the second step to a temperature in the range of 15000 to 15750F; and is quenched at a temperature in the range of 8000 to 950OF; the treated steel having a minimum of 480 MPa yield strength; 585 MPa tensile strength; and 14% elongation.

4. A method of treating steel in a continuous process wherein the steel is cold reduced and has a composition of from about 0.04% to 0. 15% by weight carbon and 0.25% to 0.70% 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 7000 to 1000OF; (2) heating the steel to a temperature in the range of 1625' to 1725'F; and (3) quenching the steel at a temperature in the range of 6500 to 750'F; the treated steel having a minimum of 275 MPa yield strength; 345 MPa tensile strength; and 22% elongation.

5. A method for treating steel in a continuous process wherein the steel is cold reduced and has a composition of from about'0.10% to 0.15% by weight carbon and 0.25% to 0.70% 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 1000OF; 25 (2) heating the steel to a temperature in the range of 1625' to 1725'F; and (3) quenching the steel at a temperature in the range of 6500 to 750OF; the treated steel having a minimum of 345 MPa yield c W 21 strength; 480 MPa tensile strength; and 22% elongation.

6. The method of Claims 1 to 5 wherein the material is preheated by passing through a molten lead bath and is heated by a resistance heating stage each in less than 5 about 15 seconds.

7. The method of Claims 1 to 5 wherein the material is aluminium killed sheet or strip.

8. A method of treating steel substantially as hereinbefore described with reference to the accompanying drawings.

9. Steel treated by a method as claimed in any of Claims 1 to 8.

Publihhed 1988 at The Patent Office, State House, 66171 Holborn, London WC1R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BRS 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent. Con. 1187.