RU2243286C1 - High-strength stainless steel - Google Patents

Abstract

FIELD: iron metallurgy, in particular high-strength stainless steel for high-loaded pats, operated in aggressive media.

SUBSTANCE: Claimed steel contains (mass %): carbon 0.005-0.05; chromium 12.0-15.0; nickel 3.9-10.4; molybdenum 1.25-4.0; aluminium 0.1-1.4; titanium 0.1-1.2; copper 0.1-2.5; niobium 0.05-0.60; and balance: iron, provided that Ni=(24.0+26.5)-1.338Cr and (4Al+2Ti+Cu) = 0.7-7.0.

EFFECT: high-strength steel with improved impact resistance and ductility.

2 tbl, 1 ex

Description

The invention relates to ferrous metallurgy, and in particular to stainless steels intended for the manufacture of highly loaded parts operating in aggressive environments.

Known foundry martensitic steel of the following composition, wt.%: Carbon 0.05-0.10; silicon 0.80-1.50; manganese 0.30-0.80; chrome 15.0-18.0; nickel 2.80-3.80; aluminum 0.005-0.05; titanium 0.005-0.01; calcium 0.005-0.05; REM 005-0.06; niobium 0.05-0.15; iron - the rest (ed. certificate. No. 1285054, M.cl. C 22 C 38/50, 03.23.87).

Steel has an insufficient level of strength, since the mechanism of dispersed hardening of steel is achieved due to the insignificant introduction of niobium, aluminum and titanium.

The closest in technical essence and the achieved result is stainless steel containing components in the following ratio in wt.%: Carbon up to 0.05; chrome 9.0-13.0; nickel 8.0-12.0; molybdenum 1.5-3.0; titanium 0.9-1.3; copper 1.5-4.0; aluminum 0.8-1.6; iron - the rest (ed. certificate. No. 370266, M. Cl. C 22 C 38/50, 11/15/1973, prototype).

Steel refers to high-strength corrosion-resistant martensitic steels. The increased content of aluminum, titanium and copper leads to the release of a significant amount of intermetallic phases in steel, which affects the toughness and reduces the ductility of steel. The homogeneous martensitic steel structure, providing high strength, favors brittle fracture. The disadvantage of steel is that brittle fracture occurs at high dynamic loads due to low toughness and insufficient ductility.

The problem solved by the invention is the production of steel with high strength and high resistance to dynamic loads.

This problem is solved in that the stainless steel, including carbon, chromium, nickel, molybdenum, aluminum, titanium, copper, iron, additionally contains niobium in the following ratio of components, wt.%:

Carbon 0.005-0.05

Chrome 12.0-15.0

Nickel 3.9-10.4

Molybdenum 1.25-4.0

Aluminum 0.1-1.4

Titanium 0.1-1.2

Copper 0.1-2.5

Niobium 0.05-0.60

Iron Else

When the relations Ni = (24.0-26.5) -1.338 Cr and (4Al + 2Ti + Cu) = 0.7-7.0.

Steel may contain impurities, wt.%: Manganese up to 0.7; silicon up to 0.7; sulfur up to 0.025; phosphorus up to 0.025.

Compared with the prototype, this steel is distinguished by the introduction of niobium with a new quantitative and qualitative ratio of components and the condition for the ratio of nickel content to chromium, as well as a given total ratio of aluminum, titanium and copper.

The technical result is to increase the toughness and ductility of steel while maintaining high strength, which reduces the risk of brittle fracture at high dynamic loads.

The introduction of niobium into steel is necessary for the binding of carbon to niobium carbides, which are fine particles uniformly distributed over the grain volume. The presence of niobium carbides ensures the production of fine-grained steel, which contributes to an increase in the toughness and ductility of steel. A niobium content of less than 0.05% is insufficient for carbon bonding in steel, which can lead to the formation of chromium carbides, which are released during cooling along grain boundaries, which leads to a sharp decrease in toughness and ductility. At the same time, the introduction of niobium in excess of 0.60% is excessive, since it does not lead to further improvement of the service properties of steel, and due to the high cost of niobium it will lead to excessive material costs.

A carbon content of more than 0.05% promotes the formation of chromium carbides in steel, worsens the plastic properties and especially reduces the toughness. The production of carbon in steel of less than 0.005% is impractical, since it requires the use of highly pure materials, special methods of steel smelting and leads to a higher cost of the process. At the same time, an additional positive effect with a decrease in carbon is not achieved.

A content of chromium steel of less than 12 wt.% Leads to a sharp decrease in its corrosion resistance. An increase in the chromium content increases the corrosion resistance of the steel. When the chromium content is more than 15 wt.% In the steel, an additional excess phase is formed - ferrite. As a result, strength properties decrease, ductility worsens.

The nickel content in steel depends on the chromium content and is determined by the ratio Ni = (24.0-26.5) -1.338 Cr.

This ratio of nickel and chromium provides steel with a martensitic structure and an insignificant austenite content in the range from 3 to 15%. The martensitic structure of steel with austenite veins, which has a higher ductility than martensite, provides an increase in impact strength. When the nickel content is less than 3.9 wt.% And (or) less than the calculated value with a coefficient (K) of less than 24.0, determined by the ratio, the steel acquires a homogeneous martensite structure with an insufficient level of plasticity and impact strength.

At a maximum chromium content of 15 wt.%, The nickel content in the ratio should be at least 3.9 wt.%.

When the nickel content is more than 10.4 wt.% And (or) more than the calculated value with a coefficient (K) of more than 26.5, determined by the ratio, an excess content of residual austenite of more than 15% occurs in steel. Strength properties are reduced.

With a minimum chromium content of 12 wt.%, The nickel content in the ratio should be no more than 10.4 wt.%.

When the molybdenum content is less than 1.25 wt.%, The corrosion resistance is reduced. The increase in molybdenum content is more than 4.0 wt.%. will not lead to further increase in corrosion resistance of steel.

Aluminum, titanium and copper provide significant hardening of steel during heat treatment due to the precipitation of dispersed intermetallic compounds Ni ₃ (Al, Ti, Cu).

The degree of dispersion hardening of steel depends on the sum of the contents of aluminum, titanium and copper in the following proportion: (4Al + 2Ti + Cu) = 0.7-7.0. If the sum of the element content is less than 0.7 and (or) the quantitative content of titanium and aluminum and copper is less than 0.1 wt.%, The hardening effect will be insignificant due to the small volume of the formed intermetallic particles.

With an increase in the sum of the elements, the strength characteristics of steel will increase, but at the same time, the toughness decreases. When the sum of the elements is more than 7.0 and (or) the quantitative content of titanium is more than 1.2 wt.%, Aluminum is more than 1.4 wt.% And copper is more than 2.5 wt.%, Significant hardening of steel is achieved, but plastic characteristics and toughness.

Example.

Steel was smelted in an electric arc furnace. Steel was cast into 1.15 tons ingots, and the ingots were rolled into billets of 100 mm square. The billets were rolled on a fine mill 250 onto bars with a diameter of 22 mm. The rods were subjected to heat treatment according to the regime: heating to 620 ° C, holding for 4 hours, followed by cooling in air.

On finished rods, mechanical properties and toughness were determined.

Testing of mechanical properties was carried out according to GOST 1497-84; impact strength according to GOST 9454-78.

The chemical composition of the steel smelted with different contents of the components and the steel of the prototype, the test results of the mechanical properties and impact strength are given below in table. 12.

The optimal steel composition options are options No. 1, 2, 3, 6, 7. In option No. 4, an excessively high nickel content with respect to chromium is introduced (K = 26.8). As a result, the steel acquired an austenitic-martensitic structure with reduced mechanical properties.

In option No. 5, an insufficient amount of nickel with respect to chromium was introduced (K = 23.8), the steel acquired a martensitic structure with high strength, but with reduced ductility and toughness.

When steelmaking according to option No. 8, the total ratio of titanium, aluminum and copper is below the lower limit (0.7). Steel has acquired insufficient strength. In option No. 9, the introduction of an excess content (more than 7.0) of the sum of titanium, aluminum and copper led to a further increase in the yield strength and tensile strength, but at the same time, the plastic characteristics significantly decreased.

In option No. 10, an insufficient amount of niobium equal to 0.04 wt.% Was introduced; steel has a reduced ductility and impact strength.

In option No. 11, an excess niobium content of 0.65 wt.% Was introduced, the properties correspond to the optimal option No. 7, that is, the excess niobium content did not lead to an additional improvement in the properties of steel.

In option No. 12, the content of elements in the steel is introduced below the lower limit. Steel has a martensitic structure with insufficient strength.

In option No. 13, the content of elements in the steel is introduced above the upper limit, the steel has an austenitic structure with intermetallic hardening. Steel has low strength properties and high plastic characteristics and impact strength.

Thus, the proposed composition of the steel, subject to the ratios, allows to increase the toughness and ductility in comparison with the prototype while maintaining a high level of strength properties of steel.

Claims (1)

High-strength stainless steel, including carbon, chromium, nickel, molybdenum, aluminum, titanium, copper and iron, characterized in that it additionally contains niobium in the following ratio, wt.%: carbon 0.005-0.05 chrome 12.0-15.0 nickel 3.9-10.4 molybdenum 1.25-4.0 aluminum 0.1-1.4 titanium 0.1-1.2 copper 0.1-2.5 niobium 0.05-0.60 iron rest when the relations Ni = (24,0

26.5) -1.338 Cr and (4Al + 2Tl + Cu) = 0.7

7.0.