RU2532628C1 - Steel for manufacture of items with increased hardness penetration - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, and namely to steels of bainite class with increased hardness protection, and can be used at manufacture of large-sized items operating under conditions of considerable impact actions, high pressure vessels, cutting tools, and in special equipment. Steel contains the following, wt %: carbon 0.10-0.20; manganese 2.0-3.0; chrome 2.0-3.0; silicon 1.0-1.5; molybdenum 0.4-0.6; vanadium 0.08-0.12; and iron is the rest. After heating for hardening to the temperature of 930°C, exposure during 1 hour and cooling in the air, steel items have a structure of lower noncarbide bainite.

EFFECT: steel has increased stability of undercooled austenite in the area of bainite conversion, increased hardness protection, impact viscosity and crack resistance at maintaining high strength level.

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Description

The invention relates to the field of metallurgy, in particular to the field of research of bainitic class steels, and can be used in the manufacture of large-sized products operating under conditions of significant impact, pressure vessels, cutting tools, in special equipment.

Known steel (and.with. No. 836190, IPC C22C 38/24, 04.16.79,) containing carbon, manganese, chromium, silicon, molybdenum, vanadium, iron in the following ratio of ingredients, in wt.%:

Carbon 0.1-0.2 Manganese 3.2-4.0 Chromium 3.2-4.0 Silicon 0.17-0.36 Molybdenum 0.5-1.0 Vanadium 0.3-0.5 Iron rest

This composition is adopted as a prototype.

Known steel contains a large amount of manganese, chromium, and especially vanadium and molybdenum, which leads to the formation of special carbides in austenite, which do not allow increasing the stability of supercooled austenite, therefore, with delayed cooling, even in air, the structure of upper carbide bainite is formed. Hence - a decrease in strength, crack resistance and toughness. In addition, the hardenability of steel at the level of 110-160 mm, which limits its use for large products with a cross section of more than 160 mm. To obtain high-strength steel of this composition, quenching after austenitization at 900 ° C in a liquid medium (oil, salt bath) and tempering at 250 ° C are necessary, which complicates the process of obtaining a given structure and properties, costly, environmentally disadvantageous. The composition is not systemically alloyed.

Signs of the prototype, coinciding with the features of the claimed invention is carbon in an amount of 0.1-0.2 wt.%, Manganese, chromium, silicon, molybdenum, vanadium, iron.

The objective of the invention is to obtain economically-system-alloyed steel for working under the impact conditions of large-sized products with increased stability of supercooled austenite in the field of bainitic transformation, increased hardenability, toughness, crack resistance while maintaining a high level of strength.

The problem was solved due to the fact that the known steel containing carbon, manganese, chromium, silicon, molybdenum, vanadium, iron, contains the ingredients in the following ratio, wt.%:

Carbon 0.10-0.20 Manganese 2.0-3.0 Chromium 2.0-3.0 Silicon 1.0-1.5 Molybdenum 0.4-0.6 Vanadium 0.08-0.12 Iron rest

As impurities, the steel may contain, wt.%: Sulfur up to 0.009; phosphorus up to 0.20; copper up to 0.19; titanium up to 0.004; nickel up to 0.16. Steel after austenitization with rolling heating is cooled in air.

A distinctive feature of the claimed steel composition is the quantitative ratio of the ingredients used, wt.%: Manganese - 2.0-3.0; chrome 2.0-3.0; silicon - 1.0-1.5; molybdenum - 0.4-0.6; vanadium - 0.08-0.12; iron is the rest.

The composition has both weak (manganese, chromium), medium (molybdenum) and strong (vanadium) groups of carbide formers, introduced according to the scheme of maintaining the continuity of the chain of bonds of carbide formers Mn-Cr-Mo-V. The preservation of such a system of component bonds, their ratio in the claimed concentration range, with a decrease in concentration from weak to strong carbide former, but maintaining the ratio of Cr to Mn as 1: 1, avoids the formation of special carbides that impair impact strength, crack resistance of products and reduce hardenability. This is especially true for large products.

Get the composition of the steel at the following ratios of carbide formers relative to chromium, in wt. parts: Cr: Mn 1: 1; Cr: Mo 1: 0.2; Cr: V 1: 0.04, and relative to each other in a chain from chromium to vanadium, in parts by weight: Mn: Cr 1: 1; Cr: Mo 1: 0.2; Mo: V 1: 0.2. When the carbon content in steel is up to 0.2%, the structure of packet martensite is obtained in air from rolling heating.

The composition of the steel in the claimed range of component concentrations, the combination and the ratio of the concentrations of carbide formers with each other and with silicon provide: upon heating, obtaining a fine uniform austenite grain, and upon slow cooling in air from rolling heating, increased stability of supercooled austenite and, as a result, obtaining a low-carbon structure batch martensite.

Experimental studies of the structure and properties of steel showed that the silicon content in the amount of 1.0-1.5 wt.% In the economically-systemically alloyed steel composition increases the thermodynamic activity of carbon, and the activity of iron decreases. As a result, during isothermal exposure in the region of lower bainite formation or in the process of slow cooling (in air and at even lower speeds) in this region, carbon atoms accumulate in austenite, but the lack of iron does not allow cementite to precipitate. As a result, a specific structure of carbide-free bainite is formed.

Silicon in the inventive system-economically alloyed steel homogenizes the structure, neutralizes the process of carbide formation in the field of bainitic transformation, allowing alloying elements and carbon to remain in austenite, thereby increasing its stability during cooling and deformation stability during operation of products, especially large ones .

Silicon in the composition of system-alloy steel was first introduced in an amount of 1.0-1.5 wt.%. According to the obtained values of impact strength and crack resistance, the result was unexpected. It is known that silicon, dissolving in ferrite, increases hardness and strength, but reduces the viscosity of steel (AS Samokhodsky. Technology of heat treatment of metals. M. "Mashgiz", 1962, p.194). For example, (Destruction. M. Metallurgy, translated from English, edited by G. Libowitz, 1976, p. 234, 241, 263) it is stated that the silicon content is from 0.27 to 1.55% viscosity in steel does not increase. The inventive steel, having high strength, acquired at the same time high impact strength, crack resistance, hardenability. Obviously, an unexpected result was obtained in connection with the total effect, including the systemic doping, the ratio of components and their concentration, the addition of fibrous doping silicon in the optimal range, which is able to organize surface or volume duplication of its structure in the process of interaction with other alloying components of the claimed composition during formation new phase.

Steel of the claimed composition is prepared as follows.

Silicon, chromium, molybdenum, manganese and vanadium are introduced into the furnace in the form of ferroalloys. Test ingots are cast according to GOST 7832-65.

The study was carried out on cylindrical specimens with a diameter of 3 mm and a height of 10 mm. Metallographic studies were carried out using a light microscope on thin sections prepared on cross sections of impact samples. Olympus-GX-51 microscope was used as a light microscope. Impact strength was determined on specimens with a sharp notch, KCV and on specimens with additional side notches, and KCU with a U-shaped notch. The specific work of the crack, KST * and dynamic crack resistance, KST * was determined in accordance with GOST 9454-78. Heat treatment of the samples was carried out in Nakal furnaces at a temperature of 930 ° C for quenching, a holding time of 1 hour, and cooling in air. The cooling rate was estimated using a chromel-alumel thermocouple milled into a sample connected to a Thermodat digital device.

The inventive chemical composition of steel and its properties are presented in the table.

For the experimental verification of the claimed composition, eight alloys were prepared: from the claimed composition (claims 4-6), the composition of the prototype (claims 1-3) and the composition outside the claimed composition (claims 7-8). Of these, three compositions showed optimal results (paragraphs 4-6 of the table).

The inventive steel composition and the ratio of the content of its components make it possible to obtain a strength level from 1300 MPa (see table. P. 4) to 1380 MPa (see table. P. 5) and then to 1500 MPa (see table. with high characteristics of ductility, toughness and dynamic crack resistance (see table. pp. 4-6).

To ensure high stability of supercooled austenite, carbide formers of all three groups are introduced into the composition, organizing a continuous chain of them in order to avoid the formation of special carbides, redistributing carbon between groups in the chain of bonds. From the group of weak carbide formers there is chromium and manganese, taken in a 1: 1 ratio and concentration: Cr and Mn (2.0-3.0 and 2.0-3.0, respectively). In relation to the group of weak carbide formers, a medium-strength carbide former, Mo (molybdenum) and strong V (vanadium), were introduced in a decreasing, but in different concentration ratios (Mo - 0.4-0.6%, V - 0.08-0 ,12%).

The content of silicon, as an alloying component, in an amount of 1.0-1.5 wt.% In the composition of the inventive steel provides an increase in carbon activity and at the same time a decrease in iron activity. As a result, with isothermal exposure slightly higher than the temperature of the onset of martensitic transformation, it becomes possible to obtain a very promising carbide-free bainite structure with a high level of mechanical properties.

The introduction of a smaller, in comparison with the claimed composition, amount of carbon and alloying elements, including silicon, does not allow for a sufficiently high stability of supercooled austenite. As a result, the structure of the upper carbide bainite is obtained and, as a consequence, a reduced level of properties (see table, item 7).

The introduction of a larger, in comparison with the claimed composition, amount of carbon and alloying elements, including silicon, although it provides the structure of martensite, or carbide-free bainite, but leads to embrittlement of steel (see table. P. 8).

From the data of the table it can be seen that, in comparison with the prototype, the claimed composition has a higher hardenability (critical diameter is 110-160 and 420-490 mm for the prototype (paragraphs 1-3 of the table) and the claimed composition (paragraphs 4-6) respectively); slightly higher strength (tensile strength is 1300-1360 and 1300-1500 MPa for the prototype and the claimed composition, respectively), as well as higher impact strength (KCU - 73-85 and 106-108 J / cm ² and KCV - 24- 27 and 30-35 J / cm ² in the prototype and the claimed composition, respectively). In this case, KST and KST * dynamic viscosity is 12-15 J / cm ² .

In addition, the inventive steel composition, compared with the prototype, is economically alloyed, since carbide-forming - chromium and manganese are used in much lower concentrations, and the non-carbide-forming component of silicon, which is cheap, in high concentrations; systemically alloyed, since carbide-forming compounds are arranged in a row with a decrease in concentration from chromium and manganese taken in the ratio 1: 1 to vanadium in order to avoid the formation of special carbides that worsen the properties of steel.

Thus, the use of the proposed steel composition will allow:

1. To increase the hardenability of large-sized products to 420-490 mm by achieving high stability of supercooled austenite.

2. It is guaranteed to avoid the formation of special carbides, to neutralize the process of carbide formation to the bainitic region of transformation of supercooled austenite, and to unexpectedly obtain a “carbide-free bainite” structure. Moreover, the structure of carbide-free bainite is obtained in large sections (up to 360-370 mm) using conventional thermal equipment (there is no need for salt and nitrate bath furnaces).

3. Increase toughness and crack resistance.

4. Simplify heat treatment - quenching after austenitization and rolling heating of the billet is carried out in air, and not in liquid media, as in the prototype, which saves quenching material (oil, nitrate, etc.), improves the ecology of thermal production.

Due to its high strength properties, high toughness and high hardenability, the proposed steel can be used in products of new technology in the manufacture of highly loaded products of complex configuration and large cross section.

The inventive composition is able to replace steels - chrome-nickel-molybdenum type 12X2G2NMFT, especially with a high nickel content (3-5%) while maintaining the same high level of strength, toughness, crack resistance, increasing the stability of supercooled austenite in the bainitic region while maintaining stability in the pearlite region and significantly reducing the cost of steels due primarily to the availability of cheap silicon and the absence of expensive nickel.

Table No. p / p The content of chemical elements,% mass. Mechanical properties and hardenability C Mn Cr Mo V Si σ _B , MPa KCU, J / cm ² KCV, J / cm D mm Prototype one 0.1 3.2 3.2 0.5 0.3 0.17 1300 85 27 160 2 0.15 3.6 3.6 0.8 0.1 0.25 1330 83 25 130 3 0.2 4.0 4.0 1,0 0.5 0.36 1360 73 24 110 The inventive composition four 0.10 2.0 2.0 0.4 0.08 1,0 1300 108 35 490 5 0.15 2,5 2,5 0.5 0.10 1,2 1380 107 31 450 6 0.20 2.7 2.7 0.55 0.12 1.3 1500 106 thirty 420 7 0.09 <2.0 <2.0 0.4 0.08 <1.0 1220 92 23 75 8 0.21 > 3.0 > 3.0 > 0.6 > 0.12 > 1.5 1470 64 16 110

Claims (1)

A large-sized product made of steel with high hardenability containing carbon, manganese, chromium, silicon, molybdenum, vanadium and iron, characterized in that the steel contains components in the following ratio, wt.%:
Carbon 0.10-0.20 Manganese 2.0-3.0 Chromium 2.0-3.0 Silicon 1.0-1.5 Molybdenum 0.4-0.6 Vanadium 0.08-0.12 Iron rest
in this case, the product after heating under quenching to a temperature of 930 ° C, holding for 1 hour and cooling in air has the structure of lower carbide-free bainite.