DESC:Field of the invention:

The present invention relates to steel industry and more particularly relates to a method of preparing Extra Fine structure (EFS) H13 grade die steel for better fatigue strength and die life.

Background of the invention:

Normally the steel used for manufacturing the tools working at higher temperature need to have well defined characteristics to fit them for the purpose to which they are intended. Hence the steel used for these purposes should have some qualities like good machinability, uniform and extra fine structure (EFS) and free from micro segregation and banding, hot toughness etc. Also the steel used for hot die environments, the materials are subjected to repeated heating and cooling, requires high-temperature strength, oxidation resistance, thermal fatigue resistance and like.

Commonly used hot die steel like 3Cr2W8V 4Cr5MoSiV (H13 Steel) have very low temperature strength, poor resistance to oxidation and thermal fatigue resistance. While using these materials for die casting and hot extrusion die they subject to deformation and reduces the life of mold. This also causes serious issues like reduced surface quality of the molded parts due to the inner cavity fatigue cracks.

Accordingly, there exists need to Extra Fine structure (EFS) H13 grade die steel for better fatigue strength and die life to overcome the drawbacks in the prior art.

Objects of the invention:

An object of the present invention is to provide an EFS H13 grade Die steel that gives a fine grain structure and improved surface quality best suited to make moulds.

Another object of the present invention is to provide a manufacturing method for EFS H13 grade die steel that gives improved die life and material properties in accordance with the invention.

Summary of the invention:

Accordingly, in one aspect, the present invention provides a formulation for die steel. The formulation for a manufacturing the die steel comprises components with mass percent as follows: 0.35 percent to 0.40 Carbon, 0.90 percent to 1.10 Silicon, 0.30 percent to 0.40 Manganese, 0.20 percent (maximum) Nickel, 5 percent to 5.50 Chromium, 1.2 percent to 1.40 Molybdenum, 0.85 percent to 0.95 Vanadium, 0.030 percent (maximum) Aluminum and 0.025 percent to 0.035 Niobium..

In another aspect, the present invention provides a method for preparation of a die steel. At first step, the method involves heating/melting the die steel compositions in the furnace at temperature. Thereinafter, the method involves air quenching process, wherein air at a particular velocity (please provide range) is impinged on compositions of the die steel held at a particular temperature (please provide range), thereby carrying out homogenization of the structure without changing the microstructure of the die steel to provide high strength, increased fatigue life and finer grain size of the die steel. The next step is air hardening cycle, wherein the die steel composition is soaked at 980°C temperature for 3 hours and cooled to room temperature by exposing at high speed air. At further step, spherodise annealing cycle is carried out, wherein the die steel composition is soaked at 850°C temperature for 8 hours and cooled to 650°C temperature. At last step, the method involves switching off the furnace and removing the die steel at room temperature. The addition of the air hardening cycle before the spherodise annealing cycle resulted in fine grain structure leading to better toughness of the die steel and as fine grain structure absorbs more energy before rupturing, thereby reducing cycle time from 72 hours to 38 hours.

Brief description of the drawings:

The objectives and advantages of the present invention will become apparent from the following description read in accordance with the accompanying drawings wherein,

Figure 1 shows a schematic of micro structure of a steel in accordance with the prior art;

Figure 2 shows a schematic of micro structure of an EFS H13 die steel, in accordance with the present invention;

Figure 3 shows a flow diagram of process of manufacturing the EFS H13 die steel, in accordance with the present invention; and

Figure 4 shows a curve showing the fatigue strength of the EFS H13, in accordance with the present invention.

Detailed description of the invention:

The foregoing objects of the present invention are accomplished and the problems and shortcomings associated with the prior art, techniques and approaches are overcome by the present invention as described below in the preferred embodiments. The present invention is illustrated with reference to the accompanying drawings and tables.

The present invention provides a method of developing a superior quality EFS (Extra Fine structure) H13 grade die steel (herein after referred to as “the die steel”). The die steel is prepared by a formulation of the minerals and a heat treatment process.

The formulation for a manufacturing the die steel comprises components with mass percent as follows: 0.35 percent to 0.40 Carbon, 0.90 percent to 1.10 Silicon, 0.30 percent to 0.40 Manganese, 0.20 percent (maximum) Nickel, 5 percent to 5.50 Chromium, 1.2 percent to 1.40 Molybdenum, 0.85 percent to 0.95 Vanadium, 0.030 percent (maximum) Aluminum and 0.025 percent to 0.035 Niobium.

Referring to the figure 2, the microstructure of the die steel, finely distributed carbides in ferrite matrix is shown. The addition of Niobium into the steel imparts a very fine grain size ASTM 9 (American Society for Testing and Materials) standard Niobium Carbides. The fine grain structure of the die steel is obtained by treating the above mentioned formulation of steel (Niobium composition) under air quenching process followed by alloy specific Spherodised annealing (herein after referred to as “Spherodised annealing”) process

Referring to the figure 3, a method (100) involved in manufacturing of the EFS H13 grade die steel is described. The method (100) provides designing of alloy, designing of air quenching process, designing of alloy specific Spherodise annealing process and finally trials with various processes like temperature, holding time, velocity of air, holding of material, temperature of material, and like.

At first step, the method (100) involves heating/melting the die steel compositions in the furnace at temperature. The combination of air quenching process and Spherodise annealing are designed by considering the composition of the die steel. In the Air quenching process, air at a particular velocity is impinged on the steel material held at a particular temperature. This leads to homogenization of the structure without changing the microstructure. The Air quenching process improves the grain size and thus prepares the base material for further Spherodise annealing cycle. The Air quenching process leads to high strength, increased fatigue life and finer grain size of the final die steel.

The alloy specific Spherodise annealing cycle comprises Air hardening cycle where the die steel composition is soaked at 980°C for 3 hrs, and cooled to room temperature by exposing at high speed air. This is followed by a normal Spherodise annealing cycle of soaking the die steel composition at 850°C for 8 hours, and cooling it to 650°C, switch off the furnace and remove at room temperature.

Before conducting the Spherodise annealing cycle, the die steel is subjected to air hardening cycle. This resulted in fine grain structure and better toughness in the final die steel product. As the fine grain structure absorbs more energy before rupturing the cycle, time required for whole process has reduced from 72 hours to 38 hours.

Referring to the figure 4, a curve of rotating bending fatigue testing is shown. In the experiment, 20 samples were tested of each type. From the observations the Fatigue limit is 1 million cycles. The Fatigue Stress (Mpa) of EFS - H13 grade die steel obtained for 1 million cycles is 32.3Mpa and the Fatigue strength of EFS H13 steel is 370MPa for 106 cycles.

Mechanical Properties of the EFS H13 grade die steel is shown in a table below.

Table:
UTS 1609/1810 Mpa
YS 1406/1602 Mpa
YS/UTS Ratio 0.87/0.88
Percentage Elongation 14.3/15.9%

Advantages of the invention:
1. Improved Die life.
2. Improved material Properties.
3. EFS H13 die steel is useful for critical application.
4. Finer grain size ASTM 8-9.
5. Very uniform microstructure and uniform distribution of properties.
6. High Fatigue life, high strength and improved wear resistance in comparison with other die steel grades.
7. Enhanced die life: improved by 30-40 % by using this material.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omission and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the application or implementation without departing from the scope of the present invention.
,CLAIMS:We claim:

1. A die steel, wherein chemical composition and weight percentage of the die steel comprising:
Carbon at a concentration ranging from 0.35 to 0.40 percent by weight;
Silicon at a concentration ranging from 0.90 to 1.10 percent by weight;
Manganese at a concentration ranging from 0.30 to 0.40 percent by weight;
Nickel at a maximum concentration of 0.20 percent by weight;
Chromium at a concentration ranging from 5 to 5.50 percent by weight;
Molybdenum at a concentration ranging from 1.2 to 1.40 percent by weight;
Vanadium at a concentration ranging from 0.85 to 0.95 percent by weight;
Aluminum at a maximum concentration 0.030 percent by weight; and
Niobium at a concentration ranging from 0.025 to 0.035 percent by weight.

2. A method of preparation of a die steel comprising the steps of:
heating/melting the die steel compositions as claimed in claim 1 in the furnace at temperature;
air quenching process, wherein air at a particular velocity is impinged on compositions of the die steel held at a particular temperature, thereby carrying out homogenization of the structure without changing the microstructure of the die steel to provide high strength, increased fatigue life and finer grain size of the die steel;
air hardening cycle, wherein the die steel composition is soaked at 980°C temperature for 3 hours and cooled to room temperature by exposing at high speed air;
spherodise annealing cycle, wherein the die steel composition is soaked at 850°C temperature for 8 hours and cooled to 650°C temperature;
switching off the furnace and removing the die steel at room temperature;
wherein, addition of the air hardening cycle before the spherodise annealing cycle resulted in fine grain structure leading to better toughness of the die steel and as fine grain structure absorbs more energy before rupturing, thereby reducing cycle time from 72 hours to 38 hours.

Dated this 05th day of March 2019

Madhavi Vajirakar
Agent for the Applicant
(IN/PA-2337)