The performance shortcomings and improvement methods of ferritic stainless steel
Ferritic stainless steel has a body-centered cubic crystal structure, and because it does not contain precious metals and is low in price, it is also called economic stainless steel. Because of its good processing performance, it is widely used in kitchens, household appliances, decorations, and automobiles. The annual consumption of ferritic stainless steel in the world’s stainless steel production is 30-40%.
Its main performance disadvantages are:
The pitting resistance equivalent of ferritic stainless steel is generally 18-24. Compared with other stainless steel (for example, the pitting resistance equivalent of austenitic stainless steel is greater than 26, and the pitting resistance equivalent of duplex stainless steel is 30-40), the pitting resistance is The low corrosion equivalent indicates poor corrosion resistance. The main disadvantages of ferritic stainless steel are the room temperature brittleness caused by various reasons and the high sensitivity to intergranular corrosion.
Among them, [C] and [N] in ferritic stainless steel have low solid solubility ([N] solid melting degree=927℃, w[N]=230×10-6; [N] solid melting degree=593℃, w [N]=60×10-6) The formation and precipitation rate of carbonitrides is very fast, leading to the rapid formation of chromium-depleted areas on the grain boundaries. Therefore, the manufacture of ferritic stainless steel should mainly focus on how to reduce [N] content, improve steel purity and add trace elements niobium and titanium, so that [N] strongly forms carbonitrides in the steel to prevent the formation of grain boundary chromium-depleted areas. Refine the grains to prevent intergranular corrosion.
The main ways to improve:
Therefore, to improve the performance of ferritic stainless steel, the lower the content of [N], the better. At present, high-purity steel w [N] is already less than 230×10-6. Generally, the main technology for producing ultra-low [N] ferritic stainless steel under non-vacuum conditions is to weaken the dissociation of N2 during arc melting, reduce the violent stirring of refining, and reduce the contact time between molten steel and N2 in the air; produce ultra-low [N] ferritic stainless steel under vacuum conditions. The main technology of low [N] ferritic stainless steel is to control the increase of [N] during the heating of the alloy and to reduce the content of [N] when [C] is removed under a vacuum.