Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-06-27
2003-05-13
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S230000, C148S333000, C420S128000, C428S659000
Reexamination Certificate
active
06562152
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high strength steel plate useful as automobile, building, electric and other members, and a process for producing the same, and more particularly to a high strength steel plate which has improved (excellent) punch stretching at the time of press forming and plating adhesion, a high strength alloyed galvanized steel plate, and a process for producing the same.
2. Background Art
For automobiles, studies have been made on a reduction in weight from the viewpoint of coping with a trend in recent years toward a reduction in fuel consumption. In this connection, regarding materials, an attempt to increase the strength has been made so as to ensure satisfactory strength even after a reduction in thickness for weight reduction purposes. Since, however, the workability of materials generally deteriorates with increasing the strength, steel plates satisfying both workability and strength requirements have been desired. Not only elongation in a tensile test but also n value and r value are used as measures of the workability. In recent years where simplification of the step of pressing by one-piece molding is an issue, it is important for the n value corresponding to even elongation to be large.
To this end, hot rolled steel plates and cold rolled steel plates utilizing strain induced transformation, wherein the austenite phase in the metal structure is transformed by working to hard martensite, have been developed. In this case, the transformation to martensite causes the introduction of a large quantity of dislocation in the steel plate and thus results in significantly hardened steel plates. Therefore, a high level of work hardening is maintained, and necking is suppressed, and the evenness of elongation is improved.
This steel plate is such that the steel does not contain expensive alloying elements and contains, as basic alloying elements, about 0.07 to 0.4% by weight of carbon, about 0.3 to 2.0% by weight of silicon, and about 0.2 to 2.5% by weight of manganese and, after the formation of austenite in a high temperature two phase region, transformation to bainite has been carried out at about 400° C., whereby austenite is retained in the metal structure even at room temperature. This steel is generally called “retained austenite steel,” “TRIP steel” or the like, and techniques associated with this steel is disclosed, for example, in Japanese Patent Laid-Open Nos. 230715-1989 and 79345-1989.
In these steel plates, however, since the unique transformation to bainite is utilized to retain austenite, a contemplated metal structure cannot be formed without close control of cooling rate from a temperature region, where two phases coexist, and close control of holding conditions (temperature and time) around 400° C. and this is causative of an impediment to good strength and ensuring of elongation and to an improvement in yield at the time of production.
Further, in the application of the steel plate to galvanized steel plates, which, at the present time, are becoming the mainstream in steel plates for automobiles, due to heat history at the time of plating, a failure of the favorable metal structure takes place, and, in addition, the adhesion to zinc is poor due to the presence of 0.3 to 2.0% by weight of silicon. Therefore, good surface corrosion resistance cannot be imparted, and this has impeded extensive industrial utilization of the steel plates.
In order to solve the above problem, for example, Japanese Patent Laid-Open Nos. 333552/1992, 70886/1993, and 145788/1994 disclose, for example, a method for improving the wettability by plating by the addition of nickel, a method for reducing silicon by the addition of aluminum having the same effect as silicon, and a method for multilayer plating of zinc plating and nickel plating having good adhesion to zinc plating.
In these methods, however, for example, the addition of the alloy or the increase in the number of steps increases the production cost, and, in addition, the contemplated metal structure remains unstable. That is, the problem has not been thoroughly solved.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a high strength steel plate which can ensure a contemplated retained austenite structure through simpler temperature control, has good adhesion to zinc plating, can be applied to highly corrosion resistant surface treated steel plates, and has good workability.
With a view to providing high strength steel plates which can attain the above object, the present inventors have made extensive and intensive studies on the relationship between the suitability for plating and the constituents of the steel plate, which has led to the completion of the present invention. The subject matter of the present invention is as follows.
Nitrogen has hitherto been known as an element for stabilizing an austenite phase. In a conventional production process wherein a high concentration of nitrogen is incorporated at the stage of molten steel, however, refining is difficult. Further, gas is evolved in semi-finished steel products during casting, and gas bubbles remain unremoved after solidification. This makes it impossible to produce good semi-finished steel products. For this reason, the application of high-nitrogen steel to steel plates for working, which is contemplated in the steel according to the present invention, has not been studied, and, thus, the workability and the suitability for plating are unknown. Accordingly, the present inventors have made studies on a method for incorporating nitrogen immediately before the formation of a product after the casting and have found that the incorporation of a large amount of nitrogen is effective in improving the workability and the suitability for plating.
Based on this finding, the present inventors have made further studies, for example, on the influence of elements, such as silicon, manganese, and carbon, and minor elements, such as calcium, sodium, and magnesium, and nitriding conditions and heat history for performing the control of the contemplated metal structure, which has led to the completion of the present invention. The main point is as follows.
(1) Basically, nitrogen is incorporated in a high concentration.
(2) The contents of silicon, aluminum and the like, which form nitrides, are regulated so as to fall within a suitable content range.
(3) Calcium, sodium, magnesium and the like are optionally added to control the formation of iron nitride.
(4) The contents of strengthening elements, such as carbon, silicon, manganese, and phosphorus, are controlled to regulate the strength of each phase constituting the metal structure and to regulate the strength and elongation as the steel plate.
(5) Heat history is controlled so that austenite is further stabilized and is retained in a large amount at room temperature.
Thus, according to the present invention, the following high strength steel plates and production process thereof are provided.
(1) A high strength steel plate having improved workability and plating adhesion, comprising, by weight, nitrogen: 0.03 to 2.0% and having a volume fraction of retained austenite of 3 to 20%.
(2) The high strength steel plate according to the above item (1), which further comprises, by weight, silicon: not more than 0.5%.
(3) The high strength steel plate according to the above item (1) or (2), which further comprises, by weight, carbon: not more than 0.08%.
(4) The high strength steel plate according to any one of the above items (1) to (3), which further comprises, by weight, at least one member selected from the group consisting of manganese: 0.5 to 3.0%, phosphorus: not less than 0.01%, and aluminum: not more than 0.3%.
(5) The high strength steel plate according to any one of the above items (1) to (4), which further comprises, by weight, at least one member selected from the group consisting of not more than 2.0% of nickel, not more than 2.0% of chromium, not more than 2.0% of calcium, not more than 2.0% of sodium, not more than
Murakami Hidekuni
Suehiro Masayoshi
Takada Yoshihisa
Kenyon & Kenyon
Nippon Steel Corporation
Yee Deborah
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