Boron steel is a kind of steel use boron as the main alloying element, also known as the boron-treated steel. Just need to add a very small amount of boron in the steel can significantly improve the hardenability of steel, the effect was hundredfold of general metal powder such as manganese, chromium, nickel, so it can use trace boron instead of a large number of metal elements.
At the same time, boron can also improve the high temperature strength and creep strength of the heat resistant steels, improve the red hardness of high speed steel and the cutting ability of cutter. From this point of view, boron steel not only enhance the overall performance of the steel, but also reduce the production cost. For nearly 10 years, the international steel mills continue to accelerate the development and production of boron steel, boron steel’s smelting technology constantly mature, the variety of boron steel is also expanding.
The main types of boron steels have alloy structural boron steel, low alloy high strength boron steel and spring boron steel and so on. Alloy structural boron steel used more often, including quenched and tempered boron steel, surface treated boron steel and cold deformation boron steel. Quenched and tempered boron steel has high strength, high toughness, high abrasion resistance, can be used in automobiles, tractors, machine tools, mining machinery, power plant equipment. The carbon mass fraction of these steels are more than 0.25%, except added boron separately, also can be added to a variety of alloys such as manganese boron, chromium boron, manganese-titanium boron, manganese vanadium boron, chromium-manganese boron and many other alloys; surface treated boron steel is mainly the infiltration carbon boron steel, the carbon mass fraction is generally less than 0.25%, this type of steel with better performance of carburized, the carburized layer does not form a large amount of residual austenite, thus have high hardness, abrasion resistance, good fatigue resistance, and small notch sensitivity.
Hardenability effect of boron in steel is related to the amount of boron, carbon content and the austenitizing temperature of steel. Generally increases as the carbon content increases, the role of the boron is gradually reduced. In the best austenitizing temperature, the hardenability effect of boron is the best. The studies show that the mass fraction of the boron in steel should be about 0.0010%, too low or too high can not produce the best results.
Boron occurs eliguation while the boron steel solidified, lead to the slab have defects. Some research institutions found that the difficulty of producing boron steel in the continuous casting stage. They thought that, even if add a small amount of boron in steel, the slab will also produce surface cracks, internal cracks and center segregation, even appears bleed-out. After added boron, the solidification of the molten steel in the continuous casting will melt again, by in situ analysis showed that boron segregated at the grain boundaries and dendrite. Thus it can be seen the molten steel maintain the molten state for a long time and prolong clotting time is the main reason lead to slab defective.
Use boron-containing molten iron as raw material produce boron steel can reduce costs. Researchers use boron-containing molten iron as raw materials to produce boron steel, hot metal through desulphurization, desilication, pretreatment loaded into the converter, adding slagging agent, and then supply for oxygen blowing. According to the requirement of boron steel components, adding silicon, manganese and carbon, and the steel temperature was controlled at 1560 ~ 1610, then casting into ingots, and finally obtain boron steel. Some researchers believe that use boron-containing molten iron produce boron steel can reduce costs, but while oxygen blowing, the high concentration of oxygen in the converter will reduce the yield rate of boron, this is the difficulty of use boron-containing hot metal to produce boron steel.
In Boron steel production, if not control well the craft and boron content will bring many problems. Such as the bad hardenability, leading to the strength, hardness have fluctuations. Although it is possible to adjust the added amount of titanium in the steelmaking to improve these problems, but what was worth to noting is that the effect of general alloy elements improving the hardenability is increased with its content in the steel, while there is an optimum content of boron powder. The more boron powder content, boron phase along the grain boundary sheet consecutive precipitation, it will weaken the toughness of boron steel. Generally considered that 0.0005% to 0.0020% of the mass fraction of the acid soluble boron is the best, can increase the yield of boron in refining, and the added amount of boron is more stable.
Source:http://www.mhcmp.com
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