FIELD OF THE INVENTION
The present invention relates to a cooling system for post-heat treatment of cooling cold-worked steel to produce dual phase steels.
BACKGROUND OF THE INVENTION
Cooling is one of the key operating steps for the production of dual phase (DP) steels that are vividly used in automotive industries.
Conventional cooling methods such as forced air jet/sprays or single-fluid nozzles cause sample distortion and require post quenching machining. However, cooling processes applying air-assisted water sprays with dispersing fine droplets size is technically superior over the conventional cooling methods. Spray cooling consisting of water-air mixture provides more uniform cooling with negligible distortion than liquid quenching, thus reducing post quenching machining.
EP 2402670 discloses an atomization cooling equipment and atomizing method that provide a refreshing cooling sensation in the open space through which people pass. While, EP 235514E describes a cooling technique for dissipating heat generated by high-powered electronic components, which employs a water spray system for condensers, vapor conveying ducts and cooling liquid conveying ducts. Further, EP 226771 detail about water sprayers for cooling of electronic equipment, and more particularly, cooling of electronics components. CN 201395615 discloses a cooling device for a roller bottom type continuous annealing furnace. The cooling device for the roller bottom type continuous annealing furnace comprises of air-water heat exchanger, a circulating blower, a cooling chamber and a furnace roller. The circulating blower is sequentially connected with a protective air inlet, a distribution box and a spray box by

pipeline. An adjusting valve is arranged between the distribution box and the spray box, and a nozzle of the spray body is arranged below the furnace roller. The cooling device can adjust the air spraying volume and position of the spray box, according to the length of materials. The cooling with adjustable speed is uniform and quick and cooling the efficiency is high. This cooling device is mainly used in the roller bottom type continuous annealing furnace for long rod materials and can also be used for reforming the present roller bottom type continuous annealing furnace, and has obvious economic benefits. However, inventions related to air/gas assisted spray cooling for steel processing (ROT) cooling/Continuous Annealing cooling) are non-existent. These apart, controlled/regulated cooling rates can not be simulated using the conventional techniques.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a cooling system for post-heat treatment cooling of cold-worked steel to produce dual phase steels.
Another object of the invention is to propose a cooling system for post-heat treatment cooling of cold-worked steel to produce dual phase steels, which is enabled to cool at uniform and faster rate the cold-rolled low-carbon-low-alloyed advanced high strength steels.
A still another object of the invention is to propose a cooling system for post-heat treatment cooling of cold-worked steel to produce dual phase steels, which produces dual-phase steels with desired tensile properties and having desired amount of martensite with fine ferrite.

SUMMARY OF THE INVENTION
The factors that influence the spray cooling effectiveness are spray droplet size, water flow rate, air-to-liquid ratio and nozzle-to-surface distance.
The cooling system developed according to the present invention is capable of cooling cold worked steel samples at a desired cooling rate, immediately after their heat treatment in a salt bath furnace, for a few minutes. The cooling system includes at least two air/helium gas assisted nozzles (sprayers), attached to two water reservoirs facing perpendicular to the heat treated work piece (sample). The nozzles are disposed to direct the air/helium gas assisted water spay mists of atomized water particles towards the work. By orientating the spray mist pattern in a predetermined manner, adjusting the air/helium pressure including the distance between the nozzle and the work piece, the water mist is made to envelope a substantial portion of the work piece to produce a uniform rate of cooling. The thermal cycles are recorded on-line in a data acquisition system, and are displayed on a display device. After heat treatment, two standard tensile samples were taken out from each heat treated material and subsequently tensile tests were conducted for the required properties.
Accordingly the present invention provides a cooling system consisting of water-air or water-helium mists for a cooling rate range of ~ 80-200°C/s corresponding to the cooling rates of H2 - HGJS (High Speed Gas Jet cooling) and ACC (Accelerated Cooling) techniques that are normally utilized to develop low carbon low alloyed advanced high strength steels.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - a schematic diagram of a cooling system according to the invention.
Figure 2 - a sectional view of the inventive system showing the spray nozzle with water reservoir.
Figure 3 - a sample with marked uniform area for cooling (solid black), and a tensile specimen (dotted black).
Figure 4 - shows a sample holder with DAS connector forming a part of the inventive system.
Figure 5 (a) and 5(b) - shows cooling rates using a) water-air, b) water-helium media.
Figures 6(a) and (6b) - shows ferrite martensitic micrographs of the dual phase steel sample of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 provides a schematic diagram of the inventive cooling system. The system is capable of producing steels with thermal cycles that match a wide range of continuous annealing cycles. All important parameters of the cooling process can be validated by simulation on a salt bath furnace (salt composition: NaCl-BaCl2-10-50% & 50-90% respectively-operating temperature range: 700-1090°C) with the help of an in-built command based computer program.

The system consists of a salt bath furnace as mentioned in Figure 1. A-pair of nozzles/sprayers is used for water-air/He mist cooling system, attached to two water reservoirs of ~250 ml capacity (Figures 1, 2). Prior to starting the tests, the operating parameters were critically optimized to obtain the desired results. The optimized air/He pressure was found to be in the range of 2-5 kg/cm2, while the separating distance between the nozzle and the work piece was estimated to be ~ 40-80 mm. In addition, the nozzles were also adjusted such that the work piece was adequately covered (~70mm X 45 mm) to obtain at least two standard tensile samples (as per ASTM 08) with uniform heating zones (Figure 3).
At least three thermocouples, one at the centre and other two at 30-mm away from the centre, along with the data acquisition system (DAS) connectors, were attached to a cold worked steel sheet sample of composition: 0.08-0.12C, 1.3-1.6Mn, 0.3-0.4Si, 0.06-0.27, 0.2-0.4MO, 0.006-0.010 Ti, Nb, 0.008-0.014S with the dimension of 230 mm X50 mm (Figure 4). The sample assembly was hung into the salt bath furnace for heating the sample in the intercritical temperature range of 850-900°C for 1-3 mins. After heating, the assembly was quickly transferred to the sample stand and kept vertically there (Figure 1), to be cooled by the water-air or water-helium mists originating from the nozzles/sprayers. The spray cooling essentially involves boiling and the formation of a steam layer on the top of the steel surface and the compressed air/helium provides the necessary kinetic energy required for penetration of the water droplets through this steam layer. The installation of water-air/helium mist sprayers provided he required air/helium volume for an effective cooling, up to the maximum air/helium pressure (~ 5 kg/cm2 in this case) that led to a stable operation.

The attainment of such an optimized supply of water-air/helium ratio through this system for effective cooling rate is not possible using a conventional single fluid nozzle.
Two representative continuous annealing cycles used in the present invention are shown in Figure 5a and b for water-air and water-helium cooling media, respectively. The samples were heat treated in the range of ~ 850-900°C for 2-3 mins in the salt bath furnace and subsequently cooled in the respective media. The cooling rates recorded through the DAS were 85 and 105 °C/s, for water-air and water-helium mists, respectively. The cooling was fairly uniform as is manifested by the cooling cycles in Figure 5(a, b) and helium was found to be more effective in resulting higher cooling rates. After heat treatment, the samples were thoroughly dried and subsequently cleaned with acetone and were machined to obtain tensile samples. For reproducibility of results, 4-Nos. of tensile sample were tested for each condition. Yield strength, tensile strength and percent elongation registered were ~350-502 MPa, 875-950MPa and 12-16%, respectively. Figures 6 (a, b) represents the optical and scanning electron micrographs obtained for the heat treated samples. The microstructure consisted of ~40-49% martensite in association with fairly ferrites of ~ <1 to 3-µm grain size.

WE CLAIM :
1. A cooling system for post-heat treatment cooling of cold-worked samples
to produce dual phase steels, comprising :
at least two spraying nozzles adaptable for one of air and helium gas; placed perpendicular to the sample
at least two water reservoirs, each reservoir accommodating one spraying nozzle rotatably mounted thereon
a cold-worked steel sample heated under intercritical temperature for a prescribed period in a salt bath furnace maintained at specified temperature and subsequently the sample being taken out of the furnace and vertically held in a stand; wherein the system is enabled to cool the sample by one of water-air and water helium mists originating form the nozzles allowing formation of a steam layer on the steel surface, wherein the water-mist is made to envelope a substantial portion of the work-piece to produce a uniform cooling rate range of 80-200°C/s and wherein the intercritical temperature range and the predetermined period are 850°-900°C and 1 to 3 minutes, respectively.
2. The system as claimed in claim 1, wherein the work-piece was annealed ID a salt-bath furnace with salt composition Nacl-Bacl2,10-50%, and 50-90% respectively.
3. The system as claimed in claim 1, comprising of a data acquisition system (DAS) to record the on-line data relating to thermal cycles.

4. The system as claimed in claim 1, comprising a display device to display the data from the DAS.
5. The system as claimed in claim 1, wherein the air or helium pressure including the separating distance between the nozzles and the work-piece is optimized according to desired results.

ABSTRACT

The invention relates a cooling system for post-heat treatment cooling of cold-worked steels to produce dual phase steels, comprising of: at least two spraying nozzles adaptable to one of air and helium gas, placed perpendicular to the sample; at least two water reservoirs; each reservoir accommodating one spraying nozzle rotatably mounted thereon; a cold-worked Steel Sample heated under intercritical temperature for a prescribed period in a salt bath furnace maintained at specified temperature, the sample being vertically held in a stand after the heat treatment; wherein the system is enabled to cool the sample by one of water-air and water helium mists originating form the nozzles allowing formation of a steam layer on the steel surface, wherein the water-mist is made to envelope a substantial portion of the work-piece to produce a uniform cooling rate range of 80-200°C/s and wherein the intercritical temperature range and the predetermined period is 850°-900°C and 1 to 3 minutes respectively.