Claims
- 1. A process for manufacturing galvannealed steel sheets by plating a steel strip in a zinc plating bath containing aluminum, the balance of its composition being zinc and impurities, adjusting its coating weight, and subjecting said strip to alloying treatment in a heating furnace so that its coating has an iron content of 8 to 12%, characterized in that said bath has an aluminum content of at least 0.05%, but less than 0.13%, and a temperature not exceeding 470.degree. C., said strip having a temperature not exceeding 495.degree. C. when entering said bath, said aluminum content of said bath and said temperature of said strip satisfying the following relationship:
- 437.5.times.Al%+448.gtoreq.T.gtoreq.437.5.times.Al%+428
- where
- Al%: the percent aluminum content of said bath;
- T: the temperature, in degrees Celsius, of said strip entering said bath, so that an alloying reaction forming a .zeta. phase in said bath is sufficiently promoted so that Z/D is in excess of 20, wherein:
- Z/D=(I.zeta..sub.(421) -I.sub.BG)/(I.delta..sub.1(249) -I.sub.BG).times.100
- wherein I.zeta..sub.(421) is the peak intensity of the .zeta. phase at d=1.900;I.sub.BG is the background intensity; and I.delta..sub.1(249) is the peak intensity of the .delta..sub.1 phase at d=1.990, and that said furnace is a high-frequency induction furnace in which said strip is heated so as to have a temperature not exceeding 495.degree. C. when leaving said furnace, said strip being held at that temperature, and cooled, thereby to form a plated film having a surface layer consisting essentially of a .zeta. phase and a layer under said surface layer consisting essentially of .delta..sub.1 phase.
- 2. A process for manufacturing galvannealed steel sheets by plating a steel strip in a zinc plating bath containing aluminum, the balance of its composition being zinc and impurities, adjusting its coating weight, and subjecting said strip to alloying treatment in a heating furnace so that its coating has an iron content of 8 to 12%, characterized in that said bath has an aluminum content of at least 0.05%, but less than 0.13%, and a temperature not exceeding 470.degree. C., said strip having a temperature not exceeding 495.degree. C. when entering said bath, said aluminum content of said bath and said temperature of said strip satisfying the following relationship:
- 437.5.times.Al%+448.gtoreq.T.gtoreq.437.5.times.Al% +428
- where
- Al%: the percent aluminum content of said bath;
- T: the temperature, in degrees Celsius, of said strip entering said bath, so that an alloying reaction forming a .zeta. phase in said bath is sufficiently promoted so that Z/D is in excess of 20, wherein:
- Z/D=(I.zeta..sub.(421) -I.sub.BG)/(I.delta..sub.1(249) -I.sub.BG).times.100
- wherein I.zeta..sub.(421) peak intensity of the .zeta. phase at d=1.900; I.sub.BG is the background intensity; and I.delta..sub.1(249) is the peak intensity of the .delta..sub.1 phase at d=1.990, and that said furnace is a high-frequency induction furnace in which said strip is heated so as to have a temperature not exceeding 495.degree. C. when leaving said furnace, said strip being held at that temperature, and cooled, thereby to form a plated film having a surface layer consisting essentially of a .zeta. phase and a layer under said surface layer consisting essentially of .delta..sub.1 at phase, and that said strip is plated with an iron or iron-alloy top coating having an iron content of at least 50% and a coating weight of at least 1 g/m.sup.2.
Priority Claims (1)
Number |
Date |
Country |
Kind |
2-415498 |
Dec 1990 |
JPX |
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Parent Case Info
This application is a continuation of application Ser. No. 07/920,595 filed as PCT/JP91/01801, on Dec. 27, 1991 now abandoned.
US Referenced Citations (3)
Number |
Name |
Date |
Kind |
5049453 |
Suemitsu et al. |
Sep 1991 |
|
5105454 |
Saito et al. |
Apr 1992 |
|
5141781 |
Suzuki et al. |
Aug 1992 |
|
Foreign Referenced Citations (1)
Number |
Date |
Country |
62-256959 |
Nov 1987 |
JPX |
Continuations (1)
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Number |
Date |
Country |
Parent |
920595 |
Aug 1992 |
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