FIELD OF INVENTION
The present invention relates to an apparatus and process to increase the
surface hardness of low carbon steel sheets by application of laser beam with the
treatable surface being covered by a layer of carbon soot.
BACKGROUND OF THE INVENTION
Wear-resistant coatings containing hard-phase particles embedded in a more
ductile matrix can be produced by several methods. In flame or plasma spray
type processes, hard particles such as carbides, borides, or silicides, coated or
mixed with more ductile materials such as cobalt, nickel or iron alloys, are heated
and propelled toward a substrate by a flame or plasma. The laser spray process
employs the laser beam to deliberately heat the coating material to temperatures
so high that volatization of the powder will propel it toward the substrate and
cause it to be deposited on the substrate. This process is only suitable for the
deposition of coating materials with a suitably high vapor pressure at the
temperature they achieve in the beam. This high vapor pressure is generally
achieved at temperatures above the melting point of the coating material.
It is known in the art to use laser beams in surface alloying for achieving case
hardening. For example, U.S. Pat. No. 4,157,923 discloses several embodiments
for processing selected surface areas such as for surface wear characteristics or
extended fatigue life. It is further known as disclosed in U.S. Pat. No. 4,125,926

to build up an aluminum or aluminum alloy surface, such as a piston ring groove,
by applying a powder of metal harder than the aluminum and melting it by
electromagnetic energy so as to provide a diffusion zone of an alloy of the metal
powder and aluminum or aluminum alloy at the interface for establishing a
harder surface.
US 4299 860 describes a process to impregnate the surface (including a thin sub-
surface region) of a metal substrate with particles of a different and harder
wear-resistant material to impart wear resisting characteristics to the surface. A
metal which is to have its surface made more wear resistant first has its surface
subjected to relative movement of a laser beam which melts a localized area
progressively by a plurality of passes of controlled depth and width across the
surface. Small particles of hard wear-resistant material, such as carbides, nitrides
and borides are forcibly injected by being propelled at high velocity, preferably
by an inert gas, into the melt pool under or behind the laser beam. Any particle
which passes through the laser beam do so with sufficient velocity so as not to
be melted thereby. The particles are interspersed throughout the width and
depth dimensions of the melt's cross-sectional extent and are fixedly embedded
therein by metallurgical bond upon rapid solidification of the melt as the laser
beam passes on. Since the injected particles are well dispersed throughout the
melt, there is presented a wear resistant surface formed by the particles
throughout the pass wherever it may be desired to provide a wear resistant
surface.
The cited patent further suggests that instead of blowing wear resisting particles

into the melt, a wear resistant surface could be provided by sprinkling or
otherwisely depositing the particles onto the metal surface in front of the laser
beam. However, this process is ineffective since a surface prepared by such an
arrangement, only a few titanium carbide (TiC) particles can be incorporated
deep into the melt, because the TiC has a lower density than the steel and would
tend to float on the melt.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a process to increase surface
hardness of low-carbon steel sheets, which eliminates the disadvantages of prior
art.
Another object of the invention is to propose a process to increase surface
hardness of low-carbon steel sheets, which allows the surface of the low-carbon
steel sheets to be subjected to a plurality of laser-beam movement enabling
progressive melting of localized area.
A still another object of the invention is to propose a process to increase surface
hardness of low-carbon steel sheets, in which carbon soot is adapted to cover
the surface to be treated.
A further object of the invention is to propose an apparatus to increase the
surface hardness of low-carbon steel sheets through laser beam application
combined with coverage of the treatable surface by a layer of carbon soot.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows a schematic view of the apparatus configured in
accordance with the invention,
Figure 2(a) (d) shows different configurations of laser-processed zones on
the surface of the low-carbon steels sheet,
Figure 3 shows a graphical representation exhibiting a comparison of
the surface hardness characteristics of a low-carbon steel
sheet achieved according to the invention vis-a-vis achieved
by the prior art processes,
Figure 4 shows a profile of a typical corrugated sheet
DETAILED DESCRIPTION OF THE INVENTION
Fig. 1 shows a low carbon steel sheet (0.8-1.2mm thick) mounted on a platform.
The steel surface is covered with a thin layer of carbon soot. A laser beam (LB) is
swept across the sheet at a velocity of 0.7mm/sec. Other parameters e.g.
current, residence period, spot size, frequency are shown in TABLE 1. The power
of the beam (LB) is optimized such that only a small depth (130 - 200 micron) of
the steel surface (1) melts. The laser (LB) used is Nd-Y AG

pulsed laser. The line speed is controlled such that the generated laser spots
overlap with each other. In this manner, a track consisting of overlapping spots
is formed. In order to surface harden a bigger area a multiple tracks with a
degree of overlapping is employed. Different configurations are adapted to make
the laser process amenable for imparting sufficient hardness, which are
schematically shown in figures 2(a) to 2(d).

Example
Once the laser processing is done, the samples are cut across the cross section
and the micro hardness of each sample is measured along a track in depth wise
direction. The hardness increase for a low carbon steel sheet of thickness 1.2mm
with a carbon level of 0.07wt% and other parameters are shown in Figure 3,
after carrying-out the following treatments.
(a) Laser treatment on steel sheet without carbon soot but N2 use

(b) Same as (a) with carbon soot deposited on steel sheet surface
(c) Laser treatment with carbon soot deposited on the steel sheet surface
without N2 use
Each track formed has the following geometric features:
width-1.6-2.7 mm and depth 130-200um.
Figure 3 - illustrates a comparison on the surface hardness achieved through the
inventive process. It is observed that the carbon soot and the laser beam
penetration process increases the surface hardness by 90 to 130% up to a depth
of 200pm.
According to the inventive process the surface hardness of low carbon steel
sheets is increased by using carbon soot and laser beam in particular for roofing
applications. The following steps are implemented:
(a) providing a low-carbon steel sheet, the steel strip having a composition,
C: 0.09 (max), Mn: 0.6 (max) Si: 0.006 (max), S: 0.009 (max), P: .016
(max) Al:0.06(max), N: 35 ppm(max), with thickness in the range up to
1.2 mm including a microstructure of ferrite with hardness 120 VHN;
(b) covering the surface of the strip at least with one layer of carbon soot
and its derivatives;

(c) causing a laser beam to sweep across the steel strip in different
configurations at an optimized intensity such that a width and depth of
the processed zone being restricted up to 3 mm and 200 micron
respectively; and
(d) allowing the molten surface of the steel sheet comprising a plurality of
tracks each consisting of several spots with different degree of
overlapping to solidify which increases the surface hardness and refine
the microstructure, wherein the velocity of the laser beam is maintained
in a range of 0.6 to 0.8 mm/sec, and wherein the frequency, current and
residence period of the laser beam are respectively maintained between
2.5 to 3.0 Hz, 130-200 amp, and 6 to 8 milliseconds.

WE CLAIM
1. A process to increase surface hardness of low-carbon steel sheets, the
process comprising the steps of:
(a) providing a low-carbon steel sheet, the steel strip having a
composition, C: 0.09 (max), Mn: 0.6 (max) Si: 0.006 (max), S:
0.009 (max), P: .016 (max) Al:0.06(max), N: 35 ppm(max), with
thickness in the range up to 1.2 mm including a microstructure of
ferrite with hardness of 120 VHN;
(b) covering the surface of the strip at least with one layer of carbon
soot and its derivatives;
(c) causing a laser beam to sweep across the steel strip in different
configurations at an optimized intensity such that a width and
depth of the processed zone being restricted up to 3 mm and 200
micron respectively; and
(d) allowing the molten surface of the steel sheet comprising a plurality
of tracks each consisting of several spots with different degree of
overlapping to solidify which increases the surface hardness and
refine the microstructure, wherein the velocity of the laser beam is
maintained in a range of 0.6 to 0.8 mm/sec, and wherein the
frequency, current and residence period of the laser beam are
respectively maintained between 2.5 to 3.0 Hz, 130-200 amp, and
6 to 8 milliseconds.

2. The process as claimed in claim 1, wherein the laser beam is enabled to
generate a plurality of tracks on the surface of the steel sheet each
constituting several overlapping spots, wherein each spot formed with a
size between 1.6 to 2.7mm.
3. The process as claimed in claim 1, wherein a Nd-YAG pulsed laser is
adapted for application on the surface of the low-carbon steel sheet.
4. The process as claimed in any of the preceding claims wherein the surface
hardness of the low-carbon steel sheet is increased by 90 to 130% upto a
depth of 200pm.
5. A process to increase surface hardness of low-carbon steel sheets, as
substantially described and illustrated herein with reference to the
accompanying drawings.

The invention relates to a process to increase surface hardness of low-carbon
steel sheets, the process comprising the steps of providing a low-carbon steel
sheet, the steel strip having a composition, C: 0.09 (max), Mn: 0.6 (max) Si:
0.006 (max), S: 0.009 (max), P: .016 (max) A1:0.06(max), N: 35 ppm(max),
with thickness in the range up to 1.2 mm including a microstructure of ferrite
with hardness of 120 VHN; covering the surface of the strip at least with one
layer of carbon soot and its derivatives; causing a laser beam to sweep across
the steel strip in different configurations at an optimized intensity such that a
width and depth of the processed zone being restricted up to 3 mm and 200
micron respectively; and allowing the molten surface of the steel sheet
comprising a plurality of tracks each consisting of several spots with different
degree of overlapping to solidify which increases the surface hardness and refine
the microstructure, wherein the velocity of the laser beam is maintained in a
range of 0.6 to 0.8 mm/sec, and wherein the frequency, current and residence
period of the laser beam are respectively maintained between 2.5 to 3.0 Hz, 130-
200 amp, and 6 to 8 milliseconds.