Ultra-rapid cooling technology and application of hot rolled strip

1 Introduction

The development of the steel industry is closely related to people's production and life. The rapid increase of steel output in China has led to serious shortage of resources. Due to the impact of the international financial crisis, the competition in the steel industry has become increasingly strong. “Reducing costs and increasing efficiency” has become a major issue in the development of the steel industry. In order to reduce the amount of steel used, users are increasingly demanding high-performance steel. Article 8 of the "Steel Industry Development Policy" promulgated by the state in 2005 also requires efforts to improve the efficiency of steel use. The use of high-strength, toughness, and forming properties steel has become a national goal.

2, the role of ultra-fast cooling

C-Mn steel (or adding a small amount of Nb, Ti), using direct hot rolling plus ultra-rapid cooling, using a combination of fine grain strengthening and phase transformation strengthening, to improve the strength, toughness, formability and other properties.

Compared with traditional precipitation-strengthened high-strength steel, Cr-Mo alloyed hot-rolled duplex steel, heat-treated dual-phase steel or TRIP steel, the steel produced by ultra-rapid cooling has the advantages of low cost, low environmental load and easy regeneration. The characteristics of environmental materials meet the requirements of the sustainable development strategy of the steel industry.

3. Principle and arrangement of ultra-fast cooling device

The ultra-rapid cooling (UFC) device encrypts the water outlet by increasing the water outlet aperture to increase the water pressure to ensure that the small flow of water also has sufficient energy and impact to break the water film. According to its characteristics, it can be considered that the ultra-rapid cooling system directly impacts the heat exchange surface by the fluid, so that the flow boundary and the thermal boundary layer are greatly thinned, thereby greatly improving the heat/mass transfer efficiency, and thus it has the characteristics of jet impact heat transfer. From the perspective of heat transfer mechanism, the heat transfer intensity of the entire cooling system is improved by expanding the area of ​​the single-phase forced convection zone and reducing the film boiling heat exchange zone, thereby achieving the ultra-rapid cooling of the tropical steel.

An ultra-rapid cooling device is added after the finishing train frame to achieve ultra-quick cooling of the steel after rolling through the finishing train to obtain the required control temperature. The steel after the finish rolling deformation accumulates a large amount of defects such as dislocations, and rapidly nucleates in these regions by ultra-rapid cooling, eventually forming fine grain structure.

Before the existing fine adjustment section (in the second half of the coarse adjustment section), an ultra-rapid cooling device is added, which is used together with the previous section of ultra-rapid cooling and conventional laminar cooling system to improve the function of the post-rolling control cooling system. To realize the production of steel such as duplex steel, TRIP steel and super steel.

3.1 Add a section of ultra-fast cooling device at the exit of the roughing mill

An ultra-rapid cooling device is added at the exit of the roughing mill: the spacing is 30-40mm, the diameter of the outlet is 3~5mm, and the cooling water has a pressure of 2~10 bar, which is sprayed to the steel plate like a shower head.
The role of this ultra-rapid cooling device is:
1) controlling the strip temperature of the finishing mill;
2) When the temperature of the strip is too high, the production efficiency is improved;
3) Improve the surface quality of steel;
4) Improve deep draw performance.

In order to ensure toughness and weldability to achieve high strength, ultra-fast cooling technology will play an extremely important role. The Fushan Plant Heavy Plate Workshop first applied rapid cooling technology to heavy plate production in the world in 1980. In 1998, it developed a “Super-OLAC” device that uses the new water flow control technology to achieve the maximum theoretical cooling rate. The high-speed cooling rate during rapid cooling not only enhances the strength by strengthening the phase transformation, but also contributes to the refinement of the structure and reduces the toughness while reducing the alloying elements. TMCP technology using these state-of-the-art rapid cooling units has enabled the production of a wide range of high strength pipeline steels.

Super QLAC-H developed by Fushan Hot Rolling Steel Plant, the cooling rate can reach 700 °C / s (thickness 3mm), installed at the exit of the rolling mill, the steel plate is fully and evenly cooled, the equipment can be large The amplitude increases the cooling temperature control accuracy, thereby improving the quality of the steel (through grain refinement and homogenization to achieve high strength and high toughness, stable weldability, and improved processing performance).

3.2 Arrangement of ultra-fast cooling device

The meaning of each stage is as follows:
a) Ultra-rapid cooling section L1 main cooling mode: Microalloyed steel mainly based on the precipitation of carbonitrides controlling microalloying elements.
b) Ultra-rapid cooling section L1+ ultra-rapid cooling section L2 combination mode: microstructure and ferrite and bainite-based dual-phase steel and TRIP steel, etc. c) ultra-rapid cooling section L1+ laminar cooling combination mode: low Cost of high-strength steel production.
d) Adding the main cooling mode of the ultra-rapid cooling section in front of the hot coil box: the microstructure is deep-drawn high-quality structural steel mainly composed of ferritic rolling.

After the project is implemented, the following control objectives are achieved:
1) The cooling rate of the ultra-rapid cooling system is ≥300 °C / s;
2) The ultra-rapid cooling system can achieve the cooling rate and cooling temperature required for the production of ultra-fine grain steel and advanced high-strength steel, and can achieve precise control of the temperature drop path;
3) The strip is cooled evenly, and the ultra-rapid cooling target coiling temperature is generally in the range of 550-680 ° C, and the surface of the strip has no visually visible red and black zebra strips;
4) When the control accuracy of the final rolling temperature within the range of ±20 °C within the range of ±20 °C is above 96%, and the control accuracy of the final rolling thickness within 1/4 of the standard tolerance is above 96%, the ultra-rapid cooling coiling temperature is guaranteed. The control accuracy of the same plate temperature difference within ±20 °C is greater than 96%;
5) If the super-rapid cooling section added in front of the hot-rolling box is designed for the maximum cooling capacity: for the intermediate billet with a thickness of 40 mm, the surface temperature of the billet entering the hot-rolling box can reach 750 ° C after passing the ultra-rapid cooling section.

4. Laboratory research

The purpose of ultra-rapid cooling: control the cooling rate for the purpose of improving the microstructure and properties of the steel.
Increasing the cooling rate lowers Ar3, increases the phase transition nucleus, and suppresses grain growth after phase transformation, so that F grains are refined.
The grain refining effect of the controlled cooling on the recrystallization A phase transformation was not remarkable for the grain refining effect of the non-recrystallized A.
The hot rolling mill production process was simulated by a laboratory hot rolling mill and an ultra-rapid cooling test device.
Granular bainite/martensitic duplex steel and ferritic/martensitic duplex steel with a thickness of 4 mm were obtained by ultra-rapid cooling (UFC) and accelerated cooling (ACC) processes.
The experiment proves that the dual-phase steel has high strength, toughness, forming and other properties. During the trial process, there is no cracking edge, small rebound, and the trial condition is good, which is superior to the traditional steel grade.

5. Economic benefits

1) The production of 345MPa, 400MPa and part 490MPa grade products with Q235 will no longer add alloying elements, and the production cost per ton of steel can be saved by 20-40 yuan. Because of the large output of this part of steel, the annual economic benefits are great. Generally it is more than 10 million yuan.
2) Flexible rolling of steel grades can be achieved. A slab rolling a variety of products. Reduced many processes, reduced a lot of costs, and improved the competitiveness of enterprises.
3) The total amount of microalloying elements added to the 600MPa grade product is not more than 0.03%. Compared with the existing varieties and ultra-high-strength steel, the production cost per ton of steel can be reduced to about 200 yuan. If the output is large, the year is large. The economic benefits are great. With an annual output of 100,000 tons, the efficiency is more than 20 million yuan.
4) A new generation of hot-rolled steel for automobiles, which will be widely used in the manufacture and manufacture of structural parts for automobiles, such as chassis longitudinals, beams, longitudinals, cross beams, wheel hubs and spokes, bumpers, etc., gradually replacing the original lower strength level. The steel not only promotes the light weight of the car, reduces fuel consumption, but also increases the safety of car collision. In addition to considerable economic benefits, it also brings huge social benefits.
The ultra-rapid cooling technology is applied to the internationally advanced steel grade 600-1000MPa high-strength automotive hot-rolled steel sheet, which is calculated according to the annual output of 50,000 tons of steel for various grades and low-cost high-strength automotive structural parts. The added value will increase by at least 200 yuan, and the economic benefit will increase by nearly 10 million yuan per year.

6 Conclusion

In today's resource-constrained world, direct hot rolling and ultra-rapid cooling are used, and the combination of fine-grain strengthening and phase-change strengthening is used to improve the strength, toughness, formability and other properties. The level of demand can also improve the mechanical properties of steel. It can be said that various characteristics of the high-strength and adaptable use environment are also the future development trend.

In the future, these high-strength, toughness, formability and other properties of steel will play an increasingly important role, contributing to the development of the world.

A Diesel Engine is an internal combustion engine using the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber. This is in contrast to spark-ignition engines such as a petrol engine (also called Gasoline Engine) or gas engine (using a gaseous fuel as opposed to gasoline), which uses a spark plug to ignite an air-fuel mixture. The engine was developed by Rudolf Diesel in 1893.

Our Diesel Engine Series are from single cylinder 170F to double cylinder 290F .We considers customers benefit and quality as our first responsibility. We continually try to develop, strengthen and innovate by scientific management, outstanding talents, advanced technology, high-quality products and first class service. We sincerely welcome all businessmen from all over the world to our company for cooperation. We also wish to march toward a bright future with everyone.

Diesel Engine

Diesel Engine Set, Diesel Engine, Mini Diesel Engine, Air Cooled Diesel Engine

Zhejiang Taizhou Wangye Power Co., Ltd , http://www.wyengine.com