FIELD OF INVENTION
The current invention relates to the process leading to improvement of surface hardness and tensile strength of precipitation hardened steel without affecting the size of precipitates. Laser beam has been used to selectively harden the surface of the steel through phase transformation, without changing the size, of the precipitates so that overall properties of the steel can be improved.
BACKGROUND OF INVENTION
Surface hardening by different methods (for example, flame, induction heating etc.) is well established but has several limitations . Surface hardening of steel using laser has attracted much attention during the past two decades. High power laser beam of specific spot size and power density can be used as a potential case hardening tool due to various advantages like high degree of controllability, high reproducibility, treatment of complex areas with precision, case depth controllability, excellent amenability to automation, high processing speed, to name a few. Furthermore, the typical shallow laser hardened zone facilitates in minimizing distortion and significant reduction or total elimination of post-treatment processing requirements as compared to other case hardening techniques.
Typically, when a laser beam of specific power density and spot size is scanned on a steel surface with a specific pre-determined speed, its surface temperature reaches the extent of austenetization temperature and upon self-quenching results in martensitic transformation in the steel surface to a certain depth. The extent of martensite formation in the microstructure and its case depth is governed by the hardenability (chemical composition) of the steel and processing

parameters adopted. Although many works are reported in utilizing this technique commercially for medium and high carbon steels [1,2], the approach for utilizing this method for nano precipitate hardened steels such as microalloyed steel is a new concept, that too for the purpose of increase in strength without changing the original size, morphology of the precipitates.
Prior art search has also been done on laser processing of materials, especially on steels. For instance, patent No: CN1121115 states that long cylinder of medium carbon steel, medium carbon alloy steel etc, were surface hardened by involving carbon-nitrogen co-cementing treatment. Similarly, Patent Nos: JP59179776 and JP59185723 used laser carburization method for surface hardening of pure iron and low carbon steel, whereas Patent Nos: US4533400, US4539461, US5073212 developed laser surface hardening methods and apparatus for surface hardening of gear and turbine blade steel, respectively. A new method was introduced namely laser quenching in Patent No: US5182433 and it was effectively used in Patent Nos: US5313042 and US6379479. Laser phase transformation and ion implantation process were used for ferrous and non-ferrous metals to improve the hardness and corrosion resistance as patented in Patent No: US6454877.
The US patent US6218642, discloses a method of surface hardening of steel work piece using laser beam to obtain equivalent or superior ductility with enhanced wear resistance. The selected surface areas of steel work pieces are heat treated using the laser beam to increase the hardness in the required surfaces. Laser beam of less intensity is subsequently applied, for relieving stress. Application of laser beam, reduced processing time without weakening metal section and its durability. The method can be used for cutting rules, knife blades etc.

The European patent EP2161095, discloses method of surface treatment of
turbine component using laser or electron radiation. In this method the surface
of the steam turbine is remelted by laser radiation or electron radiation and then
surface-alloying is done to increase mechanical stability and corrosion
resistance of the surface of the steam turbine. The method gives steam turbine part with good smoothness, high strength and high corrosion resistance and thereby improves the efficiency of the turbine blade. This method can be used for treating surface of a steam turbine made of austenitic or ferritic-martensitic steel.
The European patent EP0893192, discloses the method of imparting residual compressive stresses on steel (machine) components by inducing martensite formation in surface/subsurface microstructure. In this invention, the steel component, such as a bearing race, is locally melted using laser beam along its surface of the component. The remelted steel layer gets rapidly solidified to transform some of the austenite into martensite. Subsequently after tempering, most of the laser-treated case becomes martensitic and the solidified steel acquires a residual compressive stress due to volume expansion associated with martensite transformation. This process improves fatigue performance and crack resistance of the component and can be used to improve the life of bearing assembly

The Chinese patent CN101225464, discloses an invention that relates to a method to improve the anti-oxidation performance in high temperature steam atmosphere of ferrite/martensite refractory steel. The properties of rapidly heated and rapidly cooled layer results in phase transformation with grain refinement on the steel surface. This improves chromium element diffusion from base body to oxygenation level, thereby improving high temperature and steam oxidation resisting properties of ferrite/ferrite refractory steel.
The European patent EP0585843A2 discloses the alloying elements and microstructures suited for realizing a marked increase in strength of low-carbon or ultra-low carbon steel plate using a high-density energy source such as a laser. More particularly, the invention relates to a highly formable steel plate which can be enhanced in strength in necessary areas by laser treatment after forming or the laser treatment according to the invention can be performed prior to the forming as well. In contrast, the present invention is directed to laser surface hardening and although the similar term used laser treatment is used in many places of the patent text, the proposed invention relates quite different approaches from the above invention.
Further, the present invention relates to the formation of hard phases up to few microns along the thickness and leaving the core microstructure of the microalloyed as it is, using laser surface treatment technique using microstructural changes by phase transformation without altering the morphology of the precipitates and also the chemistry of the base steel or melting the surface.

Still further, the present invention is differentiated, as the application of the above invention targeted mainly automotive body members, in contrast, the application of the present invention not only relates to the increased strength but can also provide improved dent/wear resistance property and overall endurance limit for fatigue, which are however very much significant for the automotive members. More particularly, the present invention can be used in a variety of steel sheet products of a thickness of 1mm or below.
The main drawback of the prior art is that laser beam hardening process has only been used for medium and high carbon steels, which have limited use in automotive industry as these steels show poor formability. In addition, it emphasizes the application of surface hardening only to improve the surface related properties (for example, wear resistance, oxidation resistance, corrosion resistance etc), whereas the current invention is related to the improvement of overall mechanical strength of the microalloyed steel, adaptable for use in automotive industry.
OBJECTIVES OF INVENTION
The main goal of the work is to improve overall strength of precipitate hardened steel sheet using laser surface hardening method by developing hardened layer upto certain depth on one side of steel sheet across sheet thickness.

This work is also aimed at designing a scheme of different combination of laser process methodologies for microalloyed steel grades. The optimized processing conditions like laser power, scanning speed, steel chemistry, thickness and pattern will be identified in terms of improved mechanical properties of the steel sheet.
It is therefore an object of the invention to propose a process to improve overall mechanical strength of microalloyed steel by laser surface hardening applicable to produce components for automotive industry with better surface property.
Another object of the invention is to propose a process to create a composite structure by developing hardened layer of the steel blank by employing laser surface hardening without changing basic morphology of the precipitates in microalloyed steel.
A still another object of the invention is to propose a process to generate dual phase structure up to a certain depth from the surface by employing laser surface hardening of microalloyed steel.
A still further embodiment of the invention is to compare the mechanical strength of laser surface processed steels with untreated bare steels of same grade.
A still further embodiment of the invention is to improve mechanical strength of laser surface processed steels with varying treatment depth as compared to untreated bare steels of same grade

SUMMARY OF INVENTION
Optimum laser processing parameters for surface hardening are identified for hot rolled microalloyed steels. One surface (sides) of steel blanks are surface hardened by a laser beam with the optimized process variables, (mainly laser power and laser scanning speed) to improve the surface hardness and overall mechanical strength which is adaptable for automotive component where surface related properties are important. The effects of laser beam processing (LP) on the microstructure and micro-hardness of the working steel sheets are recorded along with evaluation of tensile properties. Laser beam processing of the surface causes harder dual phase structure with grain refinement up to a certain depth in the treated surface. This composite structure of the blank across the cross section consists of a harder layer and the softer core, which accomplishes an increase in overall surface properties.
Heat Treatment Processing Variables
According to the invention, conventionally un hardenable steel with better formability is identified for the surface treatment, namely micro alloyed steel sheet (%C). Table 1 shows typical chemical composition of the steel grades considered for experimentation. The initial microstructure of the steel is primarily ferritic with fine precipitates (Fig 4a and b). The setup utilized for laser hardening shown as schematic in Fig 1 constitutes a diode laser beam delivered by a 1500-micron optical fiber (1) and focusing optics/optical head (2) to produce laser beam of circular spot with of 3 mm diameter (3) onto the surface of steel blank (4). The steel blank is fixed to the table (6) with the help of clamps (5) and the laser beam is moved at a predetermined scanning speed

to result in the hardened layer at the interaction region (7). The diode laser beam was applied using several combinations of laser process variables to achieve a definite surface temperature for phase transformation. The process variables for laser surface hardening have been identified as 176-450 W of laser power and a scan speed of 150-650 mm/min and surface hardness was measured for each combination of laser power and scan speed. These variables can be selected with respect to the desired depth of hardened layer and hardness level. The beam is moved on the clamped steel sheet using a 6-axis Robot.
The higher hardened case achieved at optimum processing condition of a laser power of 452 W and scan speed: 650 mm/min of microalloyed steel blanks were measured up to depth of 200 μm from the surface.
Microstructure contains a combination of prior-ferrite and martensitic/bainitic dual phase structure and few pockets of residual austenite (Fig. 4). This fraction of martensite was enough to achieve 373Hv hardness level as compared to its base hardness of 270Hv for laser treated steel. These results are very much unique for surface hardening of such low carbon (0.04%) and microalloyed steel grades.
Surface hardening of each type of steel sheet was done on one surface (Fig. 2). The main application of these types of sheets will be to manufacture auto-components which require tailored mechanical properties viz.,strength, crash resistance in different locations. Additionally, it will also give better wear resistance for skin panel components as the hardness level is improving by more than 100%.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Schematic of processing setup utilized for laser hardening of steel sheet (1: 1500-µm fiber carrying diode laser beam, 2: optical head for focusing laser beam, 3: 3-mm diameter circular diode laser beam spot, 4: steel blank, 5: Clamps used for fixing steel sheet, 6: working table and 7: laser interaction region (hardened layer).
Figure 2 - Schematic representation of laser surface treatment of the steel sheet.
Figure 3 - Hardness profile on the laser treated surface across the laser tracks. (a) highest hardness peak (b) lowest hardness peak.
Figure 4 - Optical and SEM micrograph of base and laser treated surface. (a) and (b) base, (c) and (d) higher hardness sample, (e) and (f) lower hardness sample.
DETAIL DESCRIPTION OF THE INVENTION
Table 1 shows the chemical composition of the exemplary steel grades selected for laser surface treatment according to the invention.


The base material with stated composition according to Table 1 was produced thorough conventional LD process and rolled through Hot strip mill. The hot strip mill reheating temperature is kept ≥1200° C, finish rolling temperature was kept above 850oC, cooling rate at runout table was kept >30oCs1 and final coiling was done at 630-660oC. The microstructure consists of predominantly ferrite with trace amount of second phases (Bainite/martensite). Nano interphase precipitates are dispersed within the ferrite phase.
The surface microstructure of the laser treated area is shown in Fig. 4. At the same time, hardness profile taken across multi-tracks of laser treated area on the surface are presented in Fig. 3. The hardness level increased to 373-380Hv as compared to its base hardness of 280Hv due to the formation harder phase. The SEM micrograph shown in Fig. 4 indicates the formation of dual phases (prior-ferrite and martensite/bainite) which are responsible for the increased hardness values.


References:
1. W.M. Steen, Laser Material Processing, Springer, London 1991.
2. B. Ehlers, et al., Proceedings of the ICALEO, Section G, 1998, pp. 75–84.
3. M.F. Ashby et al., Acta Metall. Vol. 32, No. 11.(1984), 1935-1948.
4. Patent No: CN1121115, Surface hardening treatment method for inner wall of long cylinder, 1996-04-24.
5. Patent No: JP59179776, Surface hardening method by carburization hardening of pure Iron and low carbon steel by laser, 1984-10-12.
6. Patent No: JP59185723, Low strain surface hardening method of cold worked parts, 1984-10-22.

7. Patent No: US4533400, Method and apparatus for laser hardening of steel, 1985-08-06.
8. Patent No: US4539461, Method and apparatus for laser gear hardening, 1985-09-03.
9. Patent No: US5073212, Method of surface hardening of turbine blades and the like with high energy thermal pulses, and resulting product, 1991-12-17.
10. Patent No: US5182433, Method of laser quenching, 1993-01-26.
11. Patent No: US5313042, Laser hardening device, 1994-05-17.
12. Patent No: US6379479, Steel member surface treatment method, 2002-04-30.
13. Patent No: US6454877, Laser phase transformation and ion implantation in metals, 2002-09-24.
14. Patent No: US6218642, A method of surface hardening of steel work piece using laser beam to obtain equivalent or superior ductility with enhanced wear resistance.
15. Patent No: EP2161095, A method of surface treatment of turbine component using laser or electron radiation.

16. Patent No: EP0893192, The method of imparting residual compressive stresses on steel (machine) components by inducing martensite formation in surface/subsurface microstructure.
17. Patent No: CN101225464, A method to improve the anti-oxidation performance in high temperature steam atmosphere of ferrite/martensite refractory steel.

WE CLAIM :
1. A laser surface hardening process for hardening a micro alloyed steel sheet, the process comprising:
- providing hot rolled (HR) micro alloyed steel sheet grades (low carbon micro alloyed) with a chemical composition in weight percentage, C: 0.04-0.07, Mn: 1.4-1.7, S: <0.01, P: <0.025, Si: 0.05-0.2, Al: <0.08, Mo: 0.18-0.22, and Ti:0.08-0.12;
- selecting a laser track scanning pattern for surface hardening of the steel sheet;
- optimizing and applying laser processing variables to reach austenitizing temperature capable for phase transformation of the initial microstructure to harder dual phase microstructure of the steel sheet;
- selecting and adapting associated laser optics to operate the laser beam such that appropriate laser spot size of circular shape impinges on the sheet and scanned in the direction guided by the 6-axis robot movement;
- controlling the surface temperature of the steel to eliminate any possibility of melting based on optimizing the processing conditions and on-line surface temperature effect
- periodically reviewing the development of desired microstructure of the sample, including measuring hardness level and fraction of different phases.

2. The process as claimed in claim 1, wherein the hardened surface of the
steel sheet comprisesharder dual phase structure.
3. The process as claimed in claim 1, wherein the dual phase structure
' comprises prior-ferrite; martensitic/bainitic dual phase structure and few
pockets of residual austenite.
4. The process as claimed in claim 1, wherein the processing parameters
comprises 176-450 W of laser power and a scan speed of 150-650
mm/min. 5. The process as claimed in claim 1, wherein the hardness of steel sheet
increases in the range of 13% to 40%. . 6. The process as claimed in claim 1, wherein said micro alloyed steel sheet is nano precipitate strengthened steel.