Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2001-03-05
2002-07-23
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C427S433000, C427S436000, C148S320000, C148S331000, C148S333000, C148S334000, C148S337000, C148S533000, C148S534000, C148S648000, C148S653000, C148S654000, C148S661000, C148S662000, C148S663000, C148S664000
Reexamination Certificate
active
06423426
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a high-strength galvanized steel sheet and in particular to an improvement in ductility of a high-strength galvanized steel sheet produced in a continuous galvanizing line.
BACKGROUND ART
Recent years, an improvement in fuel efficiency of cars has been demanded from the viewpoint of preservation of global environment. Further, in order to protect passenger in cars upon collision, an improvement in safety of car bodies also has been demanded. For such reasons, car bodies are being positively made lighter and stronger. It is said that in order to satisfy the lightening and strengthening of car bodies simultaneously, high strengthening of raw materials of their parts is effective. Recently, high-strength steel sheets have been positively used for car parts.
A great number of car parts that are made of steel sheets are formed by press forming. Therefore, excellent press formability is required to steel sheets for car parts. In order to attain excellent press formability, it is firstly important to ensure high ductility. For this reason, it is strongly demanded that high-strength steel sheets for car parts have high ductility.
As a high-strength steel sheet excellent in ductility, there is suggested a structure strengthening type steel sheet with a composite structure of ferrite and a low-temperature transformation phase. A dual phase type steel sheet having a composite structure of ferrite and martensite is typical of this structure strengthening type steel sheet. Recently, a high-ductility steel sheet using transformation-inducing plasticity resulting from retained austenite has become practicable.
Incidentally, for car parts, high corrosion resistance is also demanded in the case of certain regions to which they are applied. As raw materials of parts which are applied to such regions, are preferable galvanized steel sheets which are mainly alloyed galvanized steel sheet.
Therefore, in order to promote further lightening and strengthening of car bodies, a high-strength galvanized steel sheet excellent in both corrosion resistance and ductility has come to be an indispensable material.
At present, however, a great number of galvanized steel sheets are produced in continuous galvanizing lines. In many cases, annealing equipment and galvanizing equipment are continuously installed in these continuous galvanizing lines. Thus, cooling after annealing is interrupted at galvanizing temperature by galvanizing treatment after the annealing. Therefore, it is difficult to make an average cooling rate in the whole of the steps large.
Accordingly, concerning high-strength galvanized steel sheets produced in a continues galvanizing line, it is difficult that martensite and retained austenite, which are generally produced under a cooling condition whose cooling rate is large, are caused to be contained in the steel sheets that have been subjected to galvanizing treatment.
An example of a method for producing a structure strengthening type high-strength galvanized steel sheet in a continuous galvanizing line is a method that makes it easy to produce a low-temperature transformation phase, such as martensite by adding an alloying element causing a rise in hardenability, such as Cr or Mo, in a great amount to the steel. However, there arises a problem that the addition of a large amount of the alloying element causes a rise in production costs.
For example, Japanese Patent Application Publication No. 62-40405 suggests a manufacturing method of a structure strengthening type alloyed galvanized steel sheet having high-strength, using a continuous galvanizing line for heating sheet steel comprising C: 0.005-0.15%, Mn: 0.3-2.0%, and Cr: 0.03-0.8% to a temperature between Ac
1
transformation temperature and Ac
3
transformation temperature, subjecting the steel sheet to galvanizing treatment in the middle of cooling the steel sheet, subjecting the steel sheet to alloying treatment wherein the steel sheet is heated to a temperature between 500° C. and Ac
1
transformation temperature, and subsequently cooling the steel sheet to 300° C. This manufacturing method of alloyed galvanized steel sheet having high-strength is characterized in that the cooling after the heating to a temperature between Ac
1
transformation temperature and Ac
3
transformation temperature, and the cooling to 300° C. after the alloying treatment are performed at cooling rates that are not less than a critical cooling-rate defined by an equation related to the Cr and Mn content. A dual phase type steel sheet comprising, in a ferrite base, a low-temperature transformation structure made mainly of martensite is thereby prepared and a steel sheet having an alloyed galvanized layer thereon is produced.
In the art described in the Japanese Patent Application Publication No. 62-40405, however, it is necessary to adjust cooling conditions after the annealing and the galvanizing treatment in the continuous galvanizing line so as to match with the compositions of respective steel sheets. In such adjustment of the cooling conditions, problems arise because of restrictions about equipment of the continuous galvanizing line. It cannot be said that the ductility of the steel sheet produced by the technique described in the Japanese Patent Application Publication No. 62-40405 is sufficient.
Incidentally, there are proposed manufacturing methods for obtaining a high-strength galvanized steel sheet utilizing tempered martensite for excellent formability, using a continuous galvanizing line, which is different from the structure strengthening type galvanized steel sheet having high-strength described in the Japanese Patent Application Publication No. 62-40405.
For example, Japanese Patent Application Laid-Open No. 6-93340 suggests a manufacturing method of a high-strength, alloyed, galvanized steel sheet comprising heating and holding at not less than recrystallization temperature and not less than Ac
1
transformation temperature, subsequent rapid cooling to not more than Ms point temperature, heating at a temperature that is not less than the Ms point temperature and that is not less than galvanizing-bath temperature and alloying-furnace temperature, and subsequent immersion into a galvanizing-tank.
Japanese Patent Application Laid-Open No. 6-108152 suggests a manufacturing method of a high-strength, alloyed, galvanized steel sheet excellent in bendability, comprising a recrystallization annealing step involving holding at a temperature from (Ac
3
transformation temperature−50° C. ) to 900° C. for at least 1 second or more, a step of performing galvanization, and a subsequent step of conducting reheating treatment at a temperature from Ac
1
transformation temperature to 250° C. (both inclusive), in which cooling is performed from a temperature higher than Ms point temperature to the Ms point temperature or lower at a cooling rate that is not less than a critical cooling rate, which depends on the amounts of alloying elements, after the recrystallization annealing step and before the reheating treatment step.
Both of the techniques described in the Japanese Patent Application Laid-Open No. 6-93340 and the Japanese Patent Application Laid-Open No. 6-108152 are manufacturing methods of a high-strength, alloyed, galvanized steel sheet in which the steel sheet is quenched from an austenite temperature range to a temperature not more than Ms point temperature before galvanizing or alloying treatment to produce a steel sheet having martensite structure and then this is reheated to produce tempered martensite.
However, both of the steel sheets produced by the techniques described in the Japanese Patent Application Laid-Open No. 6-93340 and the Japanese Patent Application Laid-Open No. 6-108152 insufficiently satisfy ductility that is presently required as raw materials of car parts or the like. Thus, a further improvement in ductility is desired.
SUMMARY OF THE INVENTION
The present invention provides a high-strength galvanized steel sheet which has sufficient ductility for a raw material of car
Kobayashi Takashi
Sakata Kei
Shinohara Akio
Kawasaki Steel Corporation
Koehler Robert R.
Young & Thompson
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