Claims
- 1. A process for continuously annealing a cold-rolled low carbon steel strip, comprising the continuous steps of:
- introducing a cold-rolled low carbon steel strip into a direct fired furnace, in which a fuel is burnt in a combustion air ratio of 0.8 or more, but less than 1.0 to produce a gaseous combustion product, and in which furnace said steel strip is heated in said gaseous combustion product atmosphere at an average heating rate of from 30.degree. to 100.degree. C./second in the temperature range of from 500.degree. C. to the Ac.sub.3 point of said steel strip, whereby a layer of oxides is formed on the surface of said steel strip, said oxides having a thickness limited to 1,000 angstroms or less;
- introducing said heated steel strip into a reducing atmosphere which consists essentially of a mixture of 4% or more of hydrogen gas and the balance consisting of nitrogen gas and which has a dew point of 10.degree. C. or less and in which atmosphere the temperature of said steel strip is maintained in the range of from 700.degree. C. to the Ac.sub.3 point of said steel strip for 10 seconds or more, whereby said layer of oxides is reduced;
- cooling said reduced steel strip to a desired temperature in such a manner that the cooling operation is started from a temperature of at least 600.degree. C. of said steel strip and carried out at an average cooling rate of from 10.degree. to 300.degree. C./second by bringing a cooling medium, consisting of a mixture of a gas and a liquid, into contact with said steel strip; and
- subjecting said cooled steel strip to a treatment for eliminating a layer of oxides which has been formed on the surface of said steel strip during said cooling operation;
- wherein said air combustion ratio refers to the ratio of the amount of air in volume supplied to combust a predetermined amount of fuel to the amount of air in volume stoichiometrically necessary for completely burning the predetermined amount of fuel.
- 2. A process as claimed in claim 1, wherein said cold-rolled steel strip is preheated to a temperature of 500.degree. C. or less before being placed in contact with said gaseous combustion product.
- 3. A process as claimed in claim 1, wherein, in said direct fired furnace, said steel strip reaches a temperature of from 500.degree. to 850.degree. C.
- 4. A process as claimed in claim 1, wherein, in said direct fired furnace, said gaseous combustion product is generated by the combustion of a fuel at a combustion air ratio (M), and said steel strip reaches a temperature (T), which ratio (M) and temperature (T) fall on or within an irregular pentagon, in a rectangular co-ordinate diagram, defined by the co-ordinates A, B, C, D and E,
- A (M:0.8, T:850)
- B (M:0.8, T:600)
- C (M:0.9, T:500)
- D (M:0.99, T:500) and
- E (M:0.99, T:850).
- 5. A process as claimed in claim 2, wherein said preheating operation is carried out by using exhaust gas discharged from said direct fired furnace.
- 6. A process as claimed in claim 1, wherein said gas in said cooling medium is selected from the group consisting of nitrogen, and mixtures of nitrogen and hydrogen.
- 7. A process as claimed in claim 1, wherein said liquid in said cooling medium is water.
- 8. A process as claimed in claim 1, wherein said cooling operation is terminated when the temperature of said steel strip reaches a level near an overaging temperature of said steel strip, said cooled steel strip is overaged and, then, said overaged steel strip is additionally cooled to a desired temperature.
- 9. A process as claimed in claim 8, wherein said overaging operation is carried out in a temperature range of from 300.degree. to 550.degree. C. for 3 minutes or less.
- 10. A process as claimed in claim 9, wherein said overaging operation is carried out in a temperature range of from 350.degree. to 450.degree. C.
- 11. A process as claimed in claim 1, wherein said oxide-eliminating treatment is carried out by using a aqueous solution containing at least one acid selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid, formic acid and oxalic acid.
Priority Claims (2)
Number |
Date |
Country |
Kind |
54-156844 |
Dec 1979 |
JPX |
|
54-156845 |
Dec 1979 |
JPX |
|
Parent Case Info
This application is a continuation-in-part of our co-pending application Ser. No. 139,275, filed on Apr. 10, 1980, and now abandoned.
US Referenced Citations (6)
Foreign Referenced Citations (2)
Number |
Date |
Country |
52-144331 |
Dec 1977 |
JPX |
53-17518 |
Feb 1978 |
JPX |
Continuation in Parts (1)
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Number |
Date |
Country |
Parent |
139275 |
Apr 1980 |
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