Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2001-10-19
2003-07-01
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C148S333000, C148S334000, C148S337000, C148S518000, C148S522000, C148S533000, C148S540000, C148S546000, C148S601000, C148S602000, C148S653000, C148S654000, C148S661000, C148S662000, C420S104000, C420S105000, C420S123000
Reexamination Certificate
active
06586117
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high strength cold rolled steel sheet and a high strength zinc-coated steel sheet suitable for use in parts such as automotive panels which require a good external appearance, good workability, and good shape accuracy, i.e., shape retention. The present invention also relates to a steel for preparing such a steel sheet and to a method for manufacturing the steel sheet.
2. Description of the Related Art
Automotive panels and other exterior members of automobiles are required to have an excellent appearance and a good strength exemplified by dent resistance. A primary cause of flaws in the external appearance of such panels is surface strains caused by elastic restoration after press forming. Therefore, a material having a low yield strength is suitable for such panels. However, if the yield strength of a panel after forming is too low, the panel has poor dent resistance, and indentations remain when the panel is pressed with a finger.
Japanese Published Unexamined Patent Application Hei 2-111841(1990) discloses a steel sheet which is soft at the time of forming and which has a yield stress which increases at the time of bake finishing after forming. However, due to a deterioration of strain aging properties of the steel sheet, there is a practical limit to the extent to which the yield stress of that steel sheet can be increased.
A multi-phase structure steel sheet is known to have good strain aging properties and a good bake hardenability. Japanese Published Unexamined Patent Application Hei 4-173945(1992) describes a method for the manufacture of such a steel sheet. However, in order to manufacture a steel sheet with a multi-phase structure, it is necessary to add large amounts of C or Mn, so the yield strength of the steel sheet becomes too high, and it is difficult to use the steel sheet in automotive panels.
Japanese Published Unexamined Patent Application No. 2000-109965 discloses a method of manufacturing a steel sheet having a multi-phase structure and a low yield strength. However, the steel sheet has a low r-value, so it is not completely satisfactory with respect to formability.
SUMMARY OF THE INVENTION
The present invention provides a steel suitable for forming cold rolled steel sheet and zinc-coated steel sheet having the ability to undergo aging at room temperature (strain aging), good shape accuracy, good dent resistance, and good press-formability and which can be utilized for exterior members of automobiles. The present invention also provides a method for the manufacture of this steel sheet.
A method of improving the formability of a steel sheet with a multi-phase structure by retaining austenite has already been disclosed in Japanese Published Unexamined Patent Application Hei 11-131145(1999), for example. However, according to that disclosure, in order to obtain retained austenite, it is necessary to add large amounts of Si or Al. In a method in which the amount of bainite is made extremely large, the yield strength becomes too high, and it becomes easy for stretcher strains to occur, so the resulting sheet is not appropriate for application to automotive panels. Furthermore, if the amount of Si is made too high, in hot dip galvanizing, there are problems with respect to the wettability at the time of manufacture and with respect to the ability to perform galvannealing (alloying treatment).
The present inventors found that by adding a suitable amount of Mo to a steel with a reduced level of C, during tension of the steel sheet in the direction perpendicular to rolling, a low yield point of at most 300 MPa, which is a suitable for application to automotive panels, is realized. Furthermore, they found that by maintaining this steel in a prescribed temperature range after annealing, a suitable amount of austenite is retained. By forming a metal structure substantially of ferrite and a bainite/martensite hard phase and retained austenite, adequate workability can be guaranteed without a deterioration in strain aging properties.
According to one form of the present invention, a steel for use in forming high strength steel sheet comprises, in mass %, C: at most 0.04%, Si: at most 0.4%, Mn: 0.5-3.0%, P: at most 0.15%, S: at most 0.03%, Al: at most 0.50%, N: at most 0.01%, and Mo: 0.01-1.0%.
The steel may further include at least one of Cr: less than 1.5%, Ti: at most 0.15%, Nb: at most 0.15%, and B: at most 0.01%.
In preferred embodiments, the steel has a metal structure containing retained austenite with a volume ratio of at least 0.5% and less than 10%, and a remainder which is a multi-phase structure of ferrite and a hard phase of at least one of bainite and martensite.
The steel may be formed into a high strength cold rolled steel sheet suitable for use as an automotive panel. In preferred embodiments, in a tensile test in a direction perpendicular to the rolling direction of the cold rolled steel sheet, the yield point is at most 300 MPa, the amount of work hardening with a 2% prestrain and the amount of BH are both at least 30 MPa, and the yield ratio is at most 75%.
The cold rolled steel sheet may be subjected to zinc coating by a variety of plating methods to form a zinc-coated steel sheet.
According to another form of the present invention, a method of manufacturing a high strength galvanized steel sheet includes casting a slab of the above-described steel, performing hot rough rolling either directly or after heating to a temperature of at most 1300° C., commencing hot finish rolling either directly or after reheating or holding, completing finish rolling at a temperature of at least 780° C., performing coiling after cooling to a temperature of 750° C. or below at an average cooling rate of at least 3° C./second, optionally performing cold rolling either directly or after scale removal, heating to an annealing temperature of at least 700° C. and then cooling to a temperature of 600° C. or below at an average cooling rate of at least 3° C./second, holding in a temperature range of 450-600° C. for at least 10 seconds, performing hot dip galvanizing after cooling, and then optionally carrying out alloying.
According to another form of the present invention, a method of manufacturing a high strength steel sheet includes casting a slab of the above-described steel, performing hot rough rolling either directly or after heating to a temperature of at most 1300° C., commencing hot finish rolling either directly or after reheating or holding, completing finish rolling at a temperature of at least 780° C., performing coiling after cooling to a temperature of 750° C. or below at an average cooling rate of at least 3° C./second, optionally performing cold rolling either directly or after scale removal, heating to an annealing temperature of at least 700° C. and then cooling to a temperature of 600° C. or below at an average cooling rate of at least 3° C./second, holding in a temperature range of 250-600° C. for at least 10 seconds, and then cooling. If desired, the resulting steel sheet may be electroplated with a metal or an alloy having zinc as a primary component to obtain a high strength zinc-coated steel sheet.
DESCRIPTION OF PREFERRED EMBODIMENTS
A steel according to the present invention can be used to form a cold rolled steel sheet, or a zinc-coated steel sheet formed from either a cold rolled steel sheet or a hot-rolled steel sheet. In the present invention, any type of Zn-based plating can be used. Zinc-coated steel sheet according to the present invention can be produced by various types of manufacturing methods such as hot dip plating, electroplating, vapor deposition plating, and flame spraying. The plating composition can be, for example, pure Zn, a composition having Zn as a primary component such as Zn—Fe, Zn—Ni, Zn—Al, Zn—Mn, Zn—Cr, Zn—Ti, or Zn—Mg, or it may be a composition including one or more other alloying elements and impurity elements for improving corrosion resistance or other property, such as Fe, Ni, Co, Al, Pb, Sn, Sb, Cu, Ti, Si, B, P, N, S, or O. I
Nakagawa Hiroyuki
Nakazawa Yoshiaki
Nomura Shigeki
Burns Doane , Swecker, Mathis LLP
Koehler Robert R.
Sumitomo Metal Industries Ltd.
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