High strength and toughness low carbon nanostructured bainitic steel and preparation method thereof

FIELD OF THE INVENTION

The invention relates to the field of low carbon steel alloys. The invention, more specifically, relates in particular to high strength and high toughness nanostructured bainitic steel and method of preparation thereof.

BACKGROUND OF THE INVENTION

US20110126946 discloses a carbide free nano-bainitic steel with bainite content above 50% which is termed as super bainite. Initial studies have stated that the “transformation to super bainite steel best takes place between 8 hours and 3 days, although most economically in about 8 hours”. Strength of bainitic steels mainly arises from fine bainitic plates and adequate ductility is gained due to the presence of ductile phase austenite. A lower isothermal transformation temperature consumes more amount of austenite in addition to forming finer bainitic plates. However, the lowering of kinetics at lower temperatures as well as the possibility of formation of martensite creates a lower limit for the austempering temperatures at which bainite can be formed. The problem of martensite formation can be solved by choosing a steel with a higher carbon concentration since that will lower the martensitic start temperature (Ms). These high carbon carbide free bainitic steels austempered at low temperatures are extremely strong due to the formation of nano-scaled bainitic plates that form with large fraction of film type retained austenite. However, the problem with these steels is the high transformation time (in days) and poor weldability (formation of cracks in conventionally welded joints) along with considerable loss in mechanical properties like ductility, impact and fracture toughness .The transformation time should be short enough to make it economical for the manufacturing industries to produce these steels. Transformation time can be reduced with the addition of alloying elements like A1 and Co ]. However, this significantly adds to the cost of the steel. Moreover, welding of these high carbon steels needs pre -heating and post welding heat-treatment to regenerate the original micro structure circumventing the possibility of formation of cracks [12-14]. Hence, despite having very high strength, high carbon nanostructured bainitic steels do not meet many commercial/anticipated applications .

The solution to these problems can be a low carbon bainitic steel. In a low carbon steel, a large fraction of bainitic ferrite can be obtained even at higher austempering temperature when compared to a high carbon steel transformed at very low austempering temperature . Bainitic transformations for a lower carbon steel can be achieved at a lower temperature to potentially achieve a finer micro structure by suppressing the bainitic start temperature through addition of a large amount of substitutional alloying elements like Mn, Si, Ni etc . However, the microscopic studies done on low carbon bainitic steels have reported that bainitic plates that form at low transformation temperatures get coalesced and form thicker laths, resulting in degradation of strength of bainitic steels. In order to avoid the formation of coalesced bainite, multi-step bainitic transformation in a steel with 0.26 wt% C has been attempted Bainitic lath thickness of 120+31 nm was achieved after two stages was achieved for the steel. The effect of multi-step bainitic transformation in a medium carbon steel with 0.30 wt.% C has also been studied However, even after the three stages of transformation (> 90 h), the average bainitic lath thickness was 110+50 nm. The volume fraction of bainitic ferrite was 80% with 5% blocky austenite. The ultimate tensile strength and percentage elongation of three-step transformed steel was 1532 MPa and 12.5% respectively. A yield strength of around 1060 MPa in a steel with 0.26 wt% C has been achieved through multi-step bainitic transformation. The three stages of austempering took more than 76 hours. However, the ductility of the heat-treated steel was only 11.9%. In summary, it is possible to form high strength (> 1 GPa of yield strength) nano structured bainite in low carbon steels. However, there is a need in the art for developing a low carbon nano structured bainite that can be produced in relatively small transformation times with strength similar to what has been achieved for high carbon nanostructured steel as well as possessing improved ductility and toughness.

SUMMARY OF THE INVENTION

The present invention relates to a low carbon nanostructured bainitic of the following composition 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%), Cobalt - (0.45 - 0.5%) and the balance being Iron and unavoidable impurities.

In another aspect, the invention provides a method for the preparation of low carbon nanostructured bainite steel comprising the 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%) and the balance being Iron and unavoidable impurities, wherein the method comprised the steps of austenitization of steel comprising the elements at 945-955°C for 15-20 minutes followed by

• first stage of isothermal transformation of, comprising the elements as claimed in claim 1, at 350-360°C for 20-25 minutes; and · second stage of isothermal transformation at temperature of 250-255°C for at least 6 hours.

DESCRIPTION OF THE INVENTION

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 micro structure showing absence of detrimental blocky type retained austenite. The nanostructured steel is comparatively cost-effective for large production.

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. 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. 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 Austenitization of steel of above mentioned composition at 945-955°C for 15-20 minutes followed by

• first stage of isothermal transformation at 350-360°C for 20-25 minutes; and

• second stage of isothermal transformation at 250-255°C for at least 6 hours. 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. First stage of isothermal transformation was done at 350-360°C for 20-25 minutes.

In this regard, the product obtained after the first stage had the following characteristics:

- 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.

- Large fraction of untransformed retained austenite, blocky austenite as well films.

- Complete partitioning of carbon from transformed bainitic ferrite to retained austenite during the first stage

- Retained austenite with high carbon and high silicon content has higher stability

- Martensite start temperature, Ms ₂ of retained austenite was determined as 248°C is much below Msi. Similarly, the product obtained after the second stage of austempering had the following characteristics: -

- 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, AG - will not be nucleation limited transformation because of existing bainitic ferrite formed during the first step. - Small amount of A1 and Co was part of the alloying elements during steel-making to reduce the transformation time at the second stage of transformation

- Second stage bainitic ferrite is very fine

- Large volume fraction of bainite in fine scale - high strength with high toughness.

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.

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 micro structure (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 ^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.

The low carbon nano structured 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

The following experimental examples are illustrative of the invention but not limitative of the scope thereof:

Table 1 shows the results of various mechanical tests:

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.