HIGH STRENGTH AND TOUGHNESS LOW CARBON NANOSTRUCTURED BAINITIC STEEL AND PREPARATION METHOD THEREOF

Abstract

A nanostructured bainitic steel of low carbon comprising of the following components in percentage by mass: Carbon—(0.24-0.28%), Manganese—(1.8-2%), Silicon—(2-2.5%), Nickel—(1.5-1.8%), Molybdenum—(0.2-0.25%), Chromium—(0.2-0.25%), Aluminium—(0.2-0.25%), and Cobalt—(0.45-0.5%) and the balance being Iron and unavoidable impurities and a method for preparation thereof.

Claims

1. A nanostructured bainitic steel of low carbon comprising of the following components in percentage by mass: Carbon—(0.24-0.28%), Manganese—(1.8-2%), Silicon—(2-2.5%), Nickel—(1.5-1.8%), Molybdenum—(0.2-0.25%), Chromium—(0.2-0.25%), Aluminium—(0.2-0.25%), and Cobalt—(0.45-0.5%) and the balance being Iron and unavoidable impurities.

2. The nanostructured bainitic steel of low carbon comprising of the following components in percentage by mass: Carbon—0.26% Manganese—1.9% Silicon—2.49% Nickel—1.6% Molybdenum—0.21% Chromium—0.21% Aluminium—0.2% Cobalt 0.49% and the balance being Iron and unavoidable impurities.

3. A method for the preparation of the nanostructured bainite steel of low carbon as claimed in preceeding claims comprises the following steps: austenitization of steel at 945-955° C. for 15-20 minutes; a first stage of isothermal transformation of austenitized steel comprising the elements as claimed in claim 1, at 350-360° C. for 20-25 minutes; and a second stage of isothermal transformation at temperature of 250-255° C. for at least 6 hours.

Description

DESCRIPTION OF THE INVENTION

[0009] The present invention is directed towards a low carbon nanostructured bainitic steel having exceptional combination of strength, ductility, impact toughness and fracture toughness and a microstructure showing absence of detrimental blocky type retained austenite. The nanostructured steel is comparatively cost-effective for large production.

[0010] In one aspect, the present invention provides a nanostructured bainitic steel of low carbon comprising of the following components in percentage by mass: Carbon—(0.24-0.28%), Manganese—(1.8-2%), Silicon—(2-2.5%), Nickel—(1.5-1.8%), Molybdenum—(0.2-0.25%), Chromium—(0.2-0.25%), Aluminium—(0.2-0.25%), and Cobalt—(0.45-0.5%) and the balance being Iron and unavoidable impurities.

[0011] In an embodiment of the invention, the components in percentage by mass in the nanostructured bainitic steel of low carbon is: 0.26% Carbon—1.9% Manganese—2.49% Silicon—1.6% Nickel—0.21% Molybdenum—0.21% Chromium—0.2% Aluminium—0.49% Cobalt and the balance being Iron and unavoidable impurities.

[0012] In another aspect, the invention relates to a method for the preparation of the nanostructured bainite steel of low carbon. In this the as-cast steel comprising the alloying elements Carbon—(0.24-0.28%), Manganese—(1.8-2%), Silicon—(2-2.5%), Nickel—(1.5-1.8%), Molybdenum—(0.2-0.25%), Chromium—(0.2-0.25%), Aluminium—(0.2-0.25%), and Cobalt—(0.45-0.5%) was subjected to the steps of

[0013] Austenitization of steel of above mentioned composition at 945-955° C. for 15-20 minutes followed by [0014] first stage of isothermal transformation at 350-360° C. for 20-25 minutes; and [0015] second stage of isothermal transformation at 250-255° C. for at least 6 hours.

[0016] The as cast steel was initially mild-homogenized at 1000° C. for 48 hours followed by hot rolling at 1000° C. to reduce the thickness from 25 mm to 13.5 mm. The martensite start temperature, Ms was determined as 280° C. by doing dilatometry experiment in a BAHR DIL 805 dilatometer. The steel was austenitized at low temperature (945-955° C. for 15-20 minutes) to get small prior austenite grain (PAG). Bainitic transformation involved two stages of isothermal transformation.

[0017] First stage of isothermal transformation was done at 350-360° C. for 20-25 minutes.

[0018] In this regard, the product obtained after the first stage had the following characteristics: [0019] Small fraction of bainitic ferrite (˜50% of total bainitic ferrite that could form at the first stage (for transformation time were 3 hours to complete the bainitic transformation). These transformed bainitic ferrite will act like nucleating sites for further transformation at the second stage. [0020] Large fraction of untransformed retained austenite, blocky austenite as well films. [0021] Complete partitioning of carbon from transformed bainitic ferrite to retained austenite during the first stage [0022] Retained austenite with high carbon and high silicon content has higher stability [0023] Martensite start temperature, Ms.sub.2 of retained austenite was determined as 248° C. is much below Ms.sub.1.

[0024] Similarly, the product obtained after the second stage of austempering had the following characteristics: — [0025] The next (second stage) isothermal transformation was done at a very low temperature of 250-255° C. for at least 6 hours. The large driving force for bainitic transformation, ΔG—will not be nucleation limited transformation because of existing bainitic ferrite formed during the first step. [0026] Small amount of Al and Co was part of the alloying elements during steel-making to reduce the transformation time at the second stage of transformation [0027] Second stage bainitic ferrite is very fine [0028] Large volume fraction of bainite in fine scale—high strength with high toughness.

[0029] Scanning electron microscopy (SEM) was utilized to obtain the micrographs for measurement of bainitic lath thickness. The true bainitic lath thickness was 107±44 nm, where almost 50% of laths were below 100 nm in thickness. Also, the X-ray diffraction study showed the formation of 0.87 volume fraction of bainitic ferrite.

[0030] Thus, in the present invention, we have produced a low carbon carbide-free nano-structured bainitic steel (yield strength >1.2 GPa) through two-stage isothermal transformation that formed 0.87 volume fraction of bainitic ferrite with almost 50% of bainitic laths below 100 nm. The transformation time (less than 8 hours) is very less which is necessary for transferring the process to industry. The problem of coarse microstructure (coalescence of bainitic plates that degrades the strength in low carbon steels transformed at low temperatures) is solved without compromising the strength of steel. The yield strength above 1.2 GPa and ultimate tensile strength above 1.5 GPa was achieved along with a ductility above 18%. The plain strain fracture toughness (82 MPam.sup.0.5) and impact energy (31 J) are higher than the prior work on development of low carbon bainitic steel. This low carbon carbide free nano-bainitic steel is expected to have better weldability over existing high carbon bainitic steels.

[0031] The low carbon nanostructured bainitic steel of the present invention is targeted at multiple applications in the field of pipe line alloys, railway lines, railway wheels, bearings, automobile bodies, wind turbine gear box etc.

Examples

[0032] The following experimental examples are illustrative of the invention but not limitative of the scope thereof: [0033] Table 1 shows the results of various mechanical tests:

TABLE-US-00001 TABLE 1 Results of various mechanical tests Ultimate Fracture Yeild tensile toughness Impact strength strength Elongation Energy Hardness (MPa) (MPa) (%) (J) 1225 ± 26 1536 ± 2 18.5 ± 0.15 82 ± 1.4 31 ± 1.4 463 ± 12 indicates data missing or illegible when filed

[0034] The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. Since the modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to the person skilled in the art, the invention should be construed to include everything within the scope of the disclosure.