Steel product with heat-resistant, corrosion-resistant plating layers

DETAILED DESCRIPTION OF THE INVENTION The steel product pertaining to the present invention includes sheets, pipes, couplings, clamps, bolts, nuts, and the like.
The steel product also includes a comparatively thin lap-welded steel tube (10 mm or below in outside diameter) with copper plating for welding.
(Such a steel tube may be used for automotive hydraulic and fuel piping.
) It has three plating layers.
The lower layer is a 0.
2-10 .
mu.
m thick nickel plating layer.
With a thickness less than 0.
2 .
mu.
m, this layer does not cover the ground of a steel product completely, nor does it improve heat resistance and corrosion resistance so much.
With a thickness in excess of 10 .
mu.
m, this layer is liable to peel and crack during bending and hence does not improve corrosion resistance for its increased thickness.
This nickel plating layer may be formed by electroplating using a Watts bath, which provides a plating layer having less stress.
The intermediate layer is a Zn-Ni alloy plating layer, which may be formed by electroplating using a chloride bath or sulfate bath.
The content of nickel varies depending on the bath composition and current density; but it should be 2 to 20%, preferably 12 to 15%, for improved corrosion resistance, bendability, and ease with which the chromate film is formed thereon afterwards.
The top layer is a chromate film, which may be formed from a chromate solution or dichromate-sulfuric acid solution or a commercial chromate treating solution (e.
g.
, ZN-80 YMU, a product of Ebara-Udylite Co.
, Ltd.
).
The thus obtained steel product with plating layers exhibits good corrosion resistance even in high-temperature environments and on its bent parts, as demonstrated in Examples which follow.
EXAMPLES The invention will be described in more detail with reference to the following examples.
Example 1 A lap-welded steel tube, measuring 8 mm in diameter, 0.
7 mm in wall thickness, and 380 mm long, was made of cold rolled carbon steel sheet designated as SPCC according to JIS G-3141, with the surface thereof coated with an about 3-