Magnetic strips and methods for making the same

• US 5,527,399 A
• Filed: 02/27/1995
• Issued: 06/18/1996
• Est. Priority Date: 08/30/1993
• Status: Expired due to Term

• Current Assignee: The Arnold Engineering Co. (The Kattegat Trust Co. Ltd.)
• Original Assignee: The Arnold Engineering Co. (The Kattegat Trust Co. Ltd.)
• Inventors: Anderson, Richard L., Manning, Neil R.
• Primary Examiner(s): WYSZOMIERSKI, GEORGE P
• Application Number:
  US08/394,705
• Time in Patent Office: 477 Days
• Field of Search: 148/101, 148/120, 148/121
• US Class Current: 148/120
• CPC Class Codes: C21D 1/18 Hardening C21D1/02 takes precedence; Quenching with or without subsequent tempering quenching devices C21D1/62
• CPC Class Codes: C21D 2211/008 Martensite
• CPC Class Codes: C21D 6/002 containing Cr C21D6/004 takes precedence
• CPC Class Codes: C21D 8/12 during manufacturing of articles with special electromagnetic properties
• CPC Class Codes: C21D 8/1255 with diffusion of elements, e.g. decarburising, nitriding
• CPC Class Codes: C23C 8/22 of ferrous surfaces
• CPC Class Codes: G08B 13/2408 using ferromagnetic tags
• CPC Class Codes: G08B 13/244 Tag manufacturing, e.g. continuous manufacturing processes
• CPC Class Codes: G08B 13/2442 Tag materials and material properties thereof, e.g. magnetic material details
• CPC Class Codes: H01F 1/147 Alloys characterised by their composition treatment thereof for enhancing their electromagnetic properties C21D8/12
• CPC Class Codes: H01F 1/14775 in the form of sheets
• CPC Class Codes: H01F 1/16 in the form of sheets H01F1/147 takes precedence

Abstract

Methods for preparing magnetic strips are provided in which the strips are manufactured to a thickness of less than about 0.005 inches and are made of a ferrous alloy having a carbon content of from about 0.4 to about 1.2 weight percent. The strips can be prepared by first manufacturing an alloy having a carbon content below about 0.5 weight percent to the desired thickness and then subjecting the strip to a carburizing step to raise the carbon content in the strip. The strips can also be prepared by controlling the chemistry of the initial alloy and controlling the processing of that alloy until the desired thickness and requisite magnetic properties are obtained. The strips are particularly useful in EAS systems.

Claims

1. A method for producing a thin magnetic strip that is readily slit and that exhibits superior magnetic properties, comprising:

(a) providing a ferrous alloy comprising at least about 85 weight percent iron and from about 0.45 to about 0.7 weight percent carbon, wherein said carbon is present in the form of spheroidal carbides within an iron-based matrix;

(b) cold rolling said ferrous alloy to reduce the thickness of said ferrous alloy;

(c) annealing said rolled ferrous alloy below the austenitizing temperature of said ferrous alloy;

(d) repeating the cold rolling and annealing steps until the alloy is reduced to a thin strip having a thickness between about 0.0005 and 0.005 inches;

(e) heating said thin strip to above the austenitizing temperature and subsequently quenching said thin strip to develop a martensitic structure in said thin strip; and

(f) tempering said martensitic thin strip to form a final thin strip,

wherein said final thin strip has a coercive level of at least about 35 oersteds and a residual induction of at least about 8,000 gauss.

2. The method of claim 1 wherein the size of the spheroidal carbides present in the ferrous alloy prior to the cold rolling step are below CS3 as determined by the ASTM A892 test procedure.

3. The method of claim 2 wherein the thin strip, prior to the tempering step, has a coercive level of at least about 50 oersteds and a residual induction of at least about 8000 gauss.

4. The method of claim 2 wherein the ferrous alloy further comprises from about 3.5 to about 5 weight percent chromium.

5. The method of claim 2 wherein the ferrous alloy further comprises from about 0.3 to about 1.5 weight percent molybdenum.

6. The method of claim 2 wherein the ferrous alloy further comprises from about 3.5 to about 5 weight percent chromium, from about 0.3 to about 1.5 weight percent molybdenum, and is essentially free of cobalt.

7. The method of claim 1 wherein the alloy is rolled to a strip thickness of below about 0.003 inches.

8. The method of claim 7 wherein the carbon content of the ferrous alloy is between 0.45 and 0.65 weight percent.

9. The method of claim 8 wherein the ferrous alloy has a chromium content of between 3 and 6 weight percent.

10. The method of claim 8 wherein the ferrous alloy has a chromium content of between 3.5 and 5 weight percent.

11. The method of claim 8 wherein the ferrous alloy has a molybdenum content of between 0.1 and 2 weight percent.

12. The method of claim 8 wherein the ferrous alloy has a molybdenum content of between 0.3 and 1.5 weight percent.

13. The method of claim 8 wherein the ferrous alloy has a chromium content of between 3.5 and 5 weight percent and a molybdenum content of between 0.3 and 1.5 weight percent.

14. The method of claim 13 wherein the ferrous alloy is essentially free of cobalt.