STEEL PRODUCT HAVING IMPROVED MECHANICAL PROPERTIES

• US 3,857,741 A
• Filed: 03/19/1973
• Issued: 12/31/1974
• Est. Priority Date: 02/17/1972
• Status: Expired due to Term

• Current Assignee: LTV Steel Company, Inc. (LTV Corp.)
• Original Assignee: Frank A. Hultgren, Richard A. Kot
• Inventors: Hultgren, Frank A., Kot, Richard A.
• Primary Examiner(s): Stallard, W.
• Application Number:
  US05/342,700
• Time in Patent Office: 652 Days
• Field of Search: 148/36,37,12
• US Class Current: 148/320
• CPC Class Codes: C21D 7/13 by hot working

Abstract

Steel products, which have been subjected to a desired deformation, are characterized by superior strength, including markedly improved toughness or tensile properties or both, as a result of procedure whereby the steel is made temporarily superplastic at an elevated temperature and is deformed while in such state. This procedure for converting steel, notably ordinary and alloy grades of low carbon, ferritic character, to a superplastic state, e.g. affording very high ductility, and for deforming such superplastic steel in a desired manner, embraces: rapidly heating a body of steel to a temperature, advantageously in the alpha-plus-gamma phase field, where the steel is then found, over a brief interval, to experience a transitional state of severe microstructural instability and to be characterized by superplasticity; and applying stress to the body in such interval to effect the desired deformation. The new products, which are compositionally of the character required for the process, are found to have much higher strength than the original steel.

Claims

1. A STEEL PRODUCT HAVING A SELECTED SHAPE PRODUCED BY DEFORMATION OF A BODY OF STEEL WHICH HAS A COMPOSITIONAL CHARACTER THAT IS NORMALLY FERRITIC AND IS SUCH THAT AS EXHIBITED BY THE EQUILIBRIUM PHASE DIAGRAM FOR SAID COMPOSITIONAL CHARACTER THERE IS AN ALPHA-GRAMMA-TRANSITION TEMPERATURE VALUE CAPABLE OF PROVIDING TEMPORARY MICROSTRUCTURAL INSTABILITY, SAID STEEL PRODUCT HAVING SUBSTANTIALLY GREATER IMPACT STRENGTH THAN SAID BODY OF STEEL, SAID STEEL PRODUCT BEING FERRITIC AND HAVING A MICROSTRUCTURE COMPOSED ESSENTIALLY OF FERRITE GRAINS CHARACTERIZED BY A COARSE CELL SUBSTRUCTURE WITHIN EACH OF THE GRAINS, AND SAID STEEL PRODUCT BEING PRODUCED BY:

SUBJECTING SAID BODY OF STEEL TO RAPID HEATING, AT A RATE OF AT LEAST ABOUT 10*F. PER SECOND, TO THE AFORSAID TEMPERATURE VALVE, HOLDING THE BODY AT SAID VALUE FOR PROVIDING HIGH DUCTILITY IN SAID BODY DURING AN INTERVAL OF MICROSTRUCTURAL INSTABILITY WHILE TRANSFOR-

2. A steel product as defined in claim 1, in which the steel thereof is essentially a carbon standard steel having a carbon content in the range up to about 1%.

3. A steel product as defined in claim 1, in which the steel thereof has a carbon content in the range up to about 0.5%.

4. A steel product having a selected shape produced by deformation of a body of steel which has a compositional character containing less than 0.8% carbon, that is normally ferritic and is such that it exhibits a phase field, in a range of elevated temperature values, which at equilibrium contains both ferrite and austenite, said steel product having substantially greater impact strength than said body of steel, said steel product being ferritic and having a microstructure composed essentially of ferrite grains characterized by a coarse cell substructure within each of the grains, and said steel product being produced by:

subjecting said body of steel to rapid heating, at a rate of at least about 10* F. per second, to a temperature in the aforesaid range, such that upon reaching such temperature the steel experiences temporary microstructural instability, while phase transformation is occuring, and subjecting the body to rapid deformation to said selected shape by applying stress thereto while the body is undergoing said instability at said temperature.

5. A steel product as defined in claim 4, in which the steel thereof is essentially a carbon standard steel of hypoeutectoid character having a carbon contEnt in the range up to about 0.5%.

6. A steel product as defined in claim 5, in which the temperature to which the body of steel is heated is about 1,450* F.

7. A steel product having a selected shape produced by deformation of a body of steel which has a compositional character that is normally ferritic and is such that as exhibited by the equilibrium phase diagram for said compositional character there is an alpha-gamma-transition temperature value capable of providing temporary microstructural instability, said steel product having substantially greater strength in at least one of the properties of impact strength and tensile strength than, and at least as great strength in each of said properties as, said body of steel, said steel product being ferritic and having a microstructure composed essentially of ferrite grains characterized by a coarse cell substructure within each of the grains, and said steel product being produced by:

subjecting said body of steel to rapid heating, at a rate of at least about 10*F. per second, to the aforesaid temperature value, holding the body at said value for providing high ductility in said body during an interval of microstructural instability while transformation occurs in the steel from alpha iron at least partially to gamma iron, and subjecting the body to rapid deformation to said selected shape by applying stress thereto while the body is held at the said temperature value in said interval.

8. A steel product as defined in claim 7, in which the steel thereof is essentially a non-alloy carbon steel having a carbon content in the range under 0.8%.

9. A steel product as defined in claim 7, in which the steel thereof has a carbon content in the range up to about 0.5%.

10. A steel product as defined in claim 7, in which the steel thereof is essentially a non-alloy carbon steel of hypoeutectoid character having a carbon content in the range up to about 0.5%.