FIELD OF THE INVENTION

[0001] The present invention relates to a steel composition. Particularly, the present invention relates to a steel composition or steel for high temperature carburizing and its method of preparation.

BACKGROUND AND INTRODUCTION

[0002] Carburizing is a case hardening heat treatment process in which metal (Steel/Iron) is heated in the presence of a carbon-bearing material, such as charcoal or carbon monoxide, leading to diffusion of the element“Carbon” through the surface of the metal. The Steel/Iron has higher solubility of Carbon in the austenite phase. Hence, the carburizing treatment is carried out in the temperature range where the metal is in austenite phase which increases the diffusion of Carbon. Due to this diffusion of Carbon, the surface and sub-surface layers of the metal become rich in Carbon. After Carburizing, the metal is cooled rapidly in a quenching medium, which results in hardening of the outer surfaces with high carbon content. Higher carbon content in the surface layer of metal leads to transformation from austenite to martensite, while the core of metal (where the carbon content remains same as earlier) remains soft and tough due to ferritic and/or pearlitic micro structure. Carburizing process is controlled by different parameters such as carbon potential, carburizing time and temperature. Depending on the amount of time and temperature, the affected area can vary in carbon content. Longer carburizing times and higher temperatures typically increase the depth of carbon diffusion.

[0003] The case formed on the metal surface during carburizing increases the hardness of the surface. This hard surface with a tough core helps in increasing the wear resistance of the metal. To increase wear life, engineers have been trying to increase the case depth of carburizing process by increasing the diffusion of Carbon during the process. This can be achieved either through increasing the carburizing temperature or the carburizing time. Increase in the carburizing time leads to lower productivity and hence, is avoided. Use of higher temperature may accelerate the diffusion process and hence, can reduce the cycle time. But, as the carburizing process is done in the temperature range where the metal is in an austenite phase, the metal has the tendency of grain coarsening during the process. One way of retarding the grain coarsening is to modify the chemistry of material which may resist the grain coarsening process.

[0004] Various steel compositions made for high temperature carburizing are disclosed in the prior art. The prior art disclosures have attempted to use different alloying elements in the steel composition to make the steel suitable for high temperature carburizing. In high temperature carburizing, the diffusion of carbon is increased which results in lower cycle time. However, this causes formation of abnormal grains which deteriorates mechanical properties of a carburized part.

[0005] Accordingly, there is a need to develop a steel composition which can overcome the drawbacks of the prior art and provide a steel material which is devoid of abnormal grains and has improved mechanical properties after high temperature carburizing.

OBJECTS OF THE INVENTION

[0006] It is an object of the present invention to provide a steel composition or steel which has superior grain coarsening resistance and fatigue characteristics after carburizing heat treatment. [0007] It is another object of the present invention to provide a method of preparation of the steel composition.

[0008] It is yet another object of the present invention to reduce the carburizing cycle time.

[0009] It is still another object of the present invention to perform carburizing treatment at a higher temperature in a shorter period of time.

SUMMARY OF THE INVENTION

[00010] The present invention provides a steel composition for high temperature carburizing comprising: a) 0.11 to 0.3 wt. % of Carbon, b) 1.1 to 1.4 wt. % of Manganese, c) 0.15 to 0.35 wt. % of Silicon, d) 1 to 1.3 wt. % of Chromium, e) < 0.0006 wt. % of Boron, f) 0.04 to 0.05 wt. % of Titanium, g) 0.035 to 0.056 wt. % of Niobium, h) <0.2 wt. % of Nickel, i) <0.06 wt. % of Molybdenum, j) <0.025 wt. % of Sulphur, k) <0.025 wt. % of Phosphorous, 1) 0.02 to 0.03 wt. % of Aluminium, m) <190 ppm of Nitrogen, and n) the rest is Iron (Fe).

[00011] Typically, the weight ratio of Titanium to Niobium is in the range of 1: 0.7 to 1: 1.4. [00012] In one embodiment of the present invention, the weight ratio of Titanium to Niobium is 1:0.92.

[00013] In one embodiment of the present invention, the weight ratio of Titanium to Niobium is 1:1.11.

[00014] In one embodiment of the present invention, the composition comprises: 0.205 wt.% of Carbon, 1.251wt. % of Manganese, 0.269 wt. % of Silicon, 1.195 wt. % of Chromium, 0.0001 wt. % of Boron, 0.0435 wt. % of Titanium, 0.0465 wt. % of Niobium, 0.039 wt. % of Nickel, 0.018 wt. % of Molybdenum, 0.019 wt.% of Sulphur, 0.011 wt.% of Phosphorous, 0.02 wt. % of Aluminium, 77 ppm of Nitrogen and the rest is Iron (Fe).

[00015] In one embodiment of the present invention, the composition comprises: 0.2 wt. % of Carbon, 1.296 wt. % of Manganese, 0.249 wt. % of Silicon, 1.252 wt. % of Chromium, 0.0006 wt. % of Boron, 0.0404 wt. % of Titanium, 0.0373 wt. % of Niobium, 0.039 wt. % of Nickel, 0.029 wt. % of Molybdenum, 0.019 wt.% of Sulphur, 0.021 wt.% of Phosphorous, 0.015 wt. % of Aluminium, 159 ppm of Nitrogen and the rest is Iron (Fe). [00016] In one embodiment of the present invention, the composition comprises 0.197 wt.% of Carbon, 1.129 wt. % of Manganese, 0.17wt. % of Silicon, 1.231 wt. % of Chromium, 0.0004 wt. % of Boron, 0.0503 wt. % of Titanium, 0.0556 wt. % of Niobium, 0.039 wt. % of Nickel, 0.031 wt. % of Molybdenum, 0.017 wt.% of Sulphur, 0.021 wt.% of Phosphorous, 0.027 wt. % of Aluminium, 183 ppm of Nitrogen and the rest is Iron (Fe).

[00017] In accordance with another aspect there is provided a steel article made from the steel composition of the present invention.

[00018] In one embodiment of the present invention, the article is selected from shafts and gears.

[00019] In one embodiment of the present invention, the steel article made from the steel composition is characterized by one or more following features:- a fine grain structure with ASTM grain size ranging from 7.5 to 9; - effective case depth: 0.8-1 mm; - retained austenite: 12.5 to 13.1%;- tempered Martensite at surface and

Bainitic structure at core. [00020] In accordance with still another aspect there is provided a process for manufacturing the steel or steel part; the process comprising the following steps: a. providing a steel composition comprising a) 0.11 to 0.3 wt.% of Carbon, b) 1.1 to 1.4 wt. % of Manganese, c) 0.15 to 0.35 wt. % of Silicon, d) 1 to 1.3 wt. % of Chromium, e) <0.0006 wt. % of Boron, f) 0.04 to 0.05 wt. % of Titanium, g) 0.035 to 0.056 wt. % of Niobium, h) <0.2 wt. % of Nickel, i) <0.06 wt. % of Molybdenum, j) <0.025 wt.% of Sulphur, k) <0.025 wt.% of Phosphorous, 1) 0.02 to 0.03 wt. % of Aluminium, m) <190 ppm of Nitrogen, and n) the rest is Iron (Fe); b. subjecting said composition to forging and carburizing to obtain the steel or steel part.

DESCRIPTION OF THE INVENTION

[00021] The present invention is focused on reducing the carburizing cycle time for steel or a steel part. This is achieved through accelerating carbon diffusion by providing additional thermal energy i.e. increasing carburizing temperature. In this high temperature carburizing, the diffusion of carbon is increased but, this causes formation of abnormal/coarse grains which deteriorates mechanical properties of the carburized part. The present invention attempts to avoid abnormal grain growth which typically occurs due to fast grain boundary movement at a high temperature. Thus, the present invention provides a solution to eliminate abnormal grain growth and thereby prevents deterioration of mechanical properties of the carburized part.

[00022] Accordingly, the present invention provides a steel composition containing a specific combination of alloying elements. The alloying elements and their weight percentage are chosen such that, precipitates of these elements have the potential of pinning the austenite grain boundary as these precipitates do not dissolve at the carburizing temperature.

[00023] In one embodiment of the present invention, the steel composition for high temperature carburizing comprises: 0.11 to 0.3 wt.% of Carbon, 1.1 to 1.4 wt. % of Manganese, 0.15 to 0.35wt. % of Silicon, 1 to 1.3 wt. % of Chromium, <0.0006 wt. % of Boron, 0.04 to 0.05 wt. % of Titanium, 0.035 to 0.056 wt. % of Niobium, <0.2 wt. % of Nickel, <0.06 wt. % of Molybdenum, <0.025 wt.% of Sulphur, <0.025 wt.% of Phosphorous, 0.02 to 0.03 wt. % of Aluminium, <190 ppm of Nitrogen and the rest is Iron (Fe).

[00024] In one embodiment of the present invention, Titanium to Niobium ratio is in the range ofl: 0.7 to 1: 1.4.

[00025] In one embodiment of the present invention, the ratio of Titanium to Niobium is 1:0.92.

[00026] In one embodiment of the present invention, the ratio of Titanium to Niobium is 1: 1.11.

[00027] In one embodiment of the present invention, the steel composition for high temperature carburizing comprises: 0.11 to 0.3 wt.% of Carbon, 1.1 to 1.4 wt. % of Manganese, 0.15 to 0.35wt. % of Silicon, 1 to 1.3 wt. % of Chromium, <0.0006 wt. % of Boron, 0.04 to 0.05 wt. % of Titanium, 0.035 to 0.056 wt. % of Niobium, <0.2 wt. % of Nickel, <0.06 wt. % of Molybdenum, <0.025 wt.% of Sulphur, <0.025 wt.% of Phosphorous, 0.02 to 0.03 wt. % of Aluminium, <190 ppm of Nitrogen and the rest is Iron (Fe), wherein the ratio of Titanium to Niobium is in the range of 1: 0.7 to 1: 1.4. [00028] In one embodiment of the present invention, the steel composition for high temperature carburizing comprises: 0.11 to 0.3 wt.% of Carbon, 1.1 to 1.4 wt. % of Manganese, 0.15 to 0.35wt. % of Silicon, 1 to 1.3 wt. % of Chromium, <0.0006 wt. % of Boron, 0.04 to 0.05 wt. % of Titanium, 0.035 to 0.056 wt. % of Niobium, <0.2 wt. % of Nickel, <0.06 wt. % of Molybdenum, <0.025 wt.% of Sulphur, <0.025 wt.% of Phosphorous, 0.02 to 0.03 wt. % of Aluminium, <190 ppm of Nitrogen and the rest is Iron (Fe), wherein the ratio of Titanium to Niobium is in the range of 1 :0.92 and 1: 1.11.

[00029] In one exemplary embodiment of the present invention, the steel composition for high temperature carburizing comprises: 0.205 wt.% of Carbon, 1.251wt. % of Manganese, 0.269wt. % of Silicon, 1.195 wt. % of Chromium, 0.0001 wt. % of Boron, 0.0435 wt. % of Titanium, 0.0465 wt. % of Niobium, 0.039 wt. % of Nickel, 0.018 wt. % of Molybdenum, 0.019 wt.% of Sulphur, 0.011 wt.% of Phosphorous, 0.02 wt. % of Aluminium, 77 ppm of Nitrogen and the rest is Iron (Fe).

[00030] In another exemplary embodiment of the present invention, the steel composition for high temperature carburizing comprises: 0.2 wt.% of Carbon, 1.296 wt. % of Manganese, 0.249wt. % of Silicon, 1.252 wt. % of Chromium, 0.0006 wt. % of Boron, 0.0404 wt. % of Titanium, 0.0373 wt. % of Niobium, 0.039 wt. % of Nickel, 0.029 wt. % of Molybdenum, 0.019 wt.% of Sulphur, 0.021 wt.% of Phosphorous, 0.015 wt. % of Aluminium, 159 ppm of Nitrogen and the rest is Iron (Fe).

[00031] In still another exemplary embodiment of the present invention, the steel composition for high temperature carburizing comprises: 0.197 wt.% of Carbon, 1.129 wt. % of Manganese, 0.17wt. % of Silicon, 1.231 wt. % of Chromium, 0.0004 wt. % of Boron, 0.0503 wt. % of Titanium, 0.0556 wt. % of Niobium, 0.039 wt. % of Nickel, 0.031 wt. % of Molybdenum, 0.017 wt.% of Sulphur, 0.021 wt.% of Phosphorous, 0.027 wt. % of Aluminium, 183 ppm of Nitrogen and the rest is Iron (Fe).

[00032] The present invention also provides steel articles made from the steel composition of the present invention. The articles include but are not limited to shafts, gears and the like. In one aspect the present invention also provides a process for manufacturing the steel or steel part by forging and carburizing the steel composition described herein above.

[00033] In one embodiment of the present invention, carburizing is carried out in the range from900 to 1050°C. In one embodiment, carburizing is carried out at about 1020 °C.

[00034] In one embodiment of the present invention, there is provided a process for manufacturing the steel or steel part; the process comprising the following steps: a. providing a steel composition comprising a) 0.11 to 0.3 wt.% of Carbon, b) 1.1 to 1.4 wt. % of Manganese, c) 0.15 to 0.35 wt. % of Silicon, d) 1 to 1.3 wt. % of Chromium, e) <0.0006 wt. % of Boron, f) 0.04 to 0.05 wt. % of Titanium, g) 0.035 to 0.056 wt. % of Niobium, h) <0.2 wt. % of Nickel, i) <0.06 wt. % of Molybdenum, j) <0.025 wt.% of Sulphur, k) <0.025 wt.% of Phosphorous, 1) 0.02 to 0.03 wt. % of Aluminium, m) <190 ppm of Nitrogen, and n) the rest is Iron (Fe), wherein the ratio of Titanium to Niobium is in the range of 1: 0.7 to 1: 1.4; b. subjecting said composition to forging and carburizing to obtain the steel or steel part, wherein the carburizing is carried out at 900 to 1050°C preferably at 1020 °C. [00035] In accordance with the present invention, the forging comprises hot forging or cold forging.

[00036] The steel article/part made using the steel composition of present invention and process of the present invention exhibits improved mechanical strength with no abnormal grain growth.

[00037] It is observed that due to the invented chemistry, the abnormal grain growth is restricted at higher carburizing temperature which leads to superior mechanical properties and about 40% reduction in cycle time of case hardening process (carburizing).

[00038] The invention is now illustrated with the help of non limiting examples. The examples provided are merely for illustration purpose and should not be construed as limitation to scope of invention.

[00039] Various steel composition were prepared by varying the alloying elements (Ti, Nb and Mo) and their weight percentages as shown in Table 2. The basic steel composition as shown in Table 1 was kept same for all the compositions. [00040] Table 1: Basic metal composition

[00041] Table 2: Alloying Element Variation

[00042] One article was hot forged from each of the above steel composition and then carburized at a temperature of about 1020°C. These articles were checked for abnormal grain growth and the findings were enlisted in Table 3. [00043] Table 3: Alloying Element Variation

[00044] Findings:

[00045] The detailed analysis of different articles made from above compositions provided in tables 2 and 3, shows that:

- Only addition of Titanium (0.03-0.04) did not give desired results.

- Addition of only Niobium with varying percentage (low percentage 0.015 -0.025, medium percentage 0.06-0.07, high percentage 0.09-0.11) also showed abnormal grain growth (AGG). Combination of Titanium with Niobium (0.03-0.05Ti + 0.02-

0.04 Nb and 0.03-0.05 Ti + 0.05-0.065 Nb) showed no abnormal grain growth and comparatively better fatigue strength.

- Abnormal grain growth was observed in the composition with higher percentage of Niobium with Titanium (0.09-0.10 Nb + 0.003 - 0.005TΪ).

- Combination of Titanium, Niobium and Molybdenum (0.03- 0.04 Ti + 0.09-0.1 lNb + 0.20-0.25 Mo) showed abnormal grain growth.

[00046] Thus, the results clearly indicate the significance of a combination of Titanium and Niobium at a specific proportion.

[00047] Based on these findings, optimized chemistry/composition was designed. Two articles (GEAR & SHAFT) manufactured from the optimized composition were further analyzed in detail.

[00048] Analysis of first article (GEAR):

- There was no abnormal grain growth at the surface and core. Further, a fine grain structure was observed with ASTM grain size ranging from 7.5 to 8.5.

- Effective case depth: 0.8-0.9 mm

- Retained Austenite: 13.07%

- At surface, tempered Martensite is observed while at core Bainitic structure is observed.

[00049] Analysis of the second article (SHAFT):

There was no abnormal grain growth at the surface and core.

Further, a fine grain structure was observed with ASTM grain size ranging from8 to 9.

Effective case depth: 1.0 mm

Retained Austenite: 12.56%

At surface, tempered Martensite is observed while at core

Bainitic structure is observed.

[00050] The use of the expression“at least” or“at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results. [00051] Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.

[00052] The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.

[00053] While considerable emphasis has been placed herein on the specific features of the preferred embodiment, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other changes in the preferred embodiment of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.