US 4,046,601 AGranted

Method of nitride-strengthening low carbon steel articles

AssigneeArmco Steel Corporation

InventorRollin E. Hook

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Abstract

A method for internal strengthening of articles formed from deep drawing quality low carbon steel strip or sheet containing about 0.02 to about 0.3% titanium in solution, up to about 0.3% columbium in solution, columbium when present being in an amount of at least 0.025% in solution, with the sum total of titanium and columbium in solution not exceeding about 0.3%, comprising the steps of heating the articles at a temperature of about 570.degree. to about 580.degree. C in a nitriding fluid for a time sufficient to form a total nitrogen concentration adjacent the surfaces of the articles at least substantially equal to the amount theoretically required to combine completely with the nitride-forming elements in the steel; and diffusion annealing the articles in a non-nitriding fluid at a temperature of about 595.degree. to about 815.degree. C for a time sufficient to complete through-thickness strengthening by reaction of the nitrogen with the nitride-forming elements. A preferred nitriding fluid is a fused cyanate salt bath.

Claims14

Dependent claims

A method for internal strengthening of articles formed from a deep drawing quality steel sheet of strip stock, which comprises:

providing a vacuum decarburized and deoxidized steel melt containing, in weight percent, from about 0.002 to about 0.015% carbon, up to about 0.012% nitrogen, up to about 0.08% aluminum, about 0.05 to about 0.6% manganese, up to about 0.035% sulfur, up to about 0.01% oxygen, up to about 0.01% phosphorus, up to about 0.015% silicon, about 0.02 to about 0.3% titanium in solution, up to about 0.3% columbium in solution, and remainder iron except for incidental impurities, columbium when present being added in an amount of at least 0.025% in solution, with the sum total of titanium and columbium in solution not exceeding about 0.3%;
reducing said steel to final thickness by steps involving casting, hot rolling to strip thickness, removing the hot mill oxide scale, and cold rolling to strip or sheet thickness;
annealing to obtain a fully recrystallized, ductile, deep drawing steel strip or sheet;
forming said article from said annealed strip or sheet;
heating said article at a temperature of about 570.degree. to about 580.degree. C. in a nitriding fluid chosen from the group consisting of a fused salt nitriding bath, and a gaseous nitriding atmosphere, whereby to form on the surfaces of said article an epsilon iron nitride layer, said heating being conducted in said fluid for a period of time, proportional to the thickness of said steel strip or sheet, sufficient to provide a total nitrogen concentration at least substantially equal to the amount theoretically required to combine completely with said titanium, aluminum and columbium in said steel;
and
diffusion annealing said article in a non-nitriding fluid at a temperature of about 595.degree. to about 815.degree. C. for a period of time, dependent on said strip or sheet thickness, sufficient to complete through-thickness strengthening by reaction of said nitrogen with said titanium, aluminum and columbium.

4 more elements

A continuous process for internal strengthening of articles formed from a deep drawing quality cold rolled and fully recrystallized steel sheet or strip stock containing, in weight percent, from about 0.02 to about 0.015% carbon, up to about 0.012% nitrogen, up to about 0.08% aluminum, about 0.05 to about 0.6% manganese, up to about 0.035% sulfur, up to about 0.01% oxygen, up to about 0.01% phosphorus, up to about 0.015% silicon, about 0.02 to about 0.3% titanium in solution, up to about 0.3% columbium in solution, and remainder iron except for incidental impurities, columbium when present being added in an amount of at least 0.025% in solution, with the sum total of titanium and columbium in solution not exceeding about 0.3%, comprising the steps of passing said articles through a fused nitriding salt bath at a temperature of about 570.degree. to about 580.degree. C. for a period of time sufficient to provide a total nitrogen concentration at least substantially equal to the amount theoretically required to combine completely with said titanium, aluminum and columbium in said steel;

and subjecting said articles to a continuous diffusion anneal in a non-nitriding fluid at a metal temperature of about 595.degree. to about 815.degree. C. for a period of time sufficient to complete through-thickness strengthening by reaction of said nitrogen with said titanium, aluminum and columbium.

Dependencies are inferred from the claim text.