RU2158320C1 - Low-hardening structural steel - Google Patents

Abstract

FIELD: structural steel with low hardenability applicable in use of steel bulk-surface hardening method developed and found industrial application in Russia for hardening of critical, heavily loaded machine parts. SUBSTANCE: claimed steel contains carbon, manganese, silicon, chrome, nickel, titanium and iron and additionally contains copper, aluminum and vanadium with the following amounts of components, wt.%: carbon 0.40- 0.85; manganese not above 0.20; silicon not above 0.20l chrome not above 0.10; nickel not above 0.10; copper not above 0.10; aluminum 0.03-0.10; titanium 0.06-0.12; vanadium not above 0.40; the balance, iron. EFFECT: guaranteed stability of low hardenability and obtained superfine grain of hardened steel at level of 11-12 points with hardening from temperatures from Ac₃ to Ac₃+100 C. 1 tbl, 4 ex

Description

 The invention relates to the development of compositions of structural steels with reduced hardenability, which are used when using the method of volume-surface hardening. The method of volumetric-surface hardening of steel (the OPZ method) was developed and received industrial application in Russia for hardening critical, heavily loaded machine parts.

The SCR method has significant technical and economic advantages over the well-known thermal hardening methods used in industry:
1. The SCR method allows to obtain surface, contour hardening and at the same time hardened core on parts of complex shape: gears, crosses and others with achieving record high strength and service properties compared to other hardening methods - cementation, nitrocarburizing, thermal improvement, etc.

 2. The SCR method allows you to successfully replace the specified long and labor-intensive processes of heat treatment, while instead of alloy steels use special carbon steels of low hardenability (PP steel), which allows the production of structural steels to reduce the consumption of alloying elements (ferroalloys) in 3-7 times and reduce their cost by 30-50%. In addition, compared to cementation and thermal improvement, savings are achieved by reducing energy consumption and replacing quenching of parts in oil by quenching with fast moving water.

 3. For a number of metal-consuming parts, the SCR method makes it possible to reduce their mass (weight), for example for springs, by 25-30%.

 4. When using the SCR method, complete ecological purity of the process is achieved, since only electric heating and process water are used without any additives.

 The SCR method is based on the use of steels specially created for this process, the hardenability of which is consistent with the dimensions of the working section of the parts so that the surface layer of the part when quenched by rapidly moving water is quenched on the martensite structure (HRC≈60), and the core on the structure of trostite and sorbitol ( HRC = 30 - 45).

 For parts with small working section sizes - with a diameter or thickness of up to 20 mm (middle module gears, crosses, springs and others), the steel for the SCR method should have a hardenability lower than that of standard carbon structural steels (which are known to have the most low hardenability of all structural steels). Therefore, such steels for the OPZ process are called "steels of low hardenability" (steel "PP").

 The hardenability of such steels, characterized by the value of the "ideal critical diameter", the value of which is in the range from 8 to 16 mm.

Such steels are used to perform the SCR process on the gears of the middle module (from 4 to 12 mm), crosspieces, springs, springs and other details with a small size of their working section. An example of such steels is steel 58 (55PP), which according to GOST 1050-74 contains components in the following ratio in wt.%:
carbon - 0.55 - 0.63
silicon - 0.10 - 0.30
Manganese - not more than 0.20
chrome - no more than 0.15
sulfur - no more than 0,040
phosphorus - not more than 0.035
copper - no more than 0.25
nickel - no more than 0.25
arsenic - not more than 0.08
iron - the rest
(Marochnik of steels and alloys. P / r V.G. Sorokina, M., Engineering, 1989, pp. 78-79).

After quenching by the SCR method, the hardness of the surface layer is HRC 58-59, and the core HRC is 26-28 with a critical diameter of 6.5 - 19.0 mm. However, to achieve higher strength and service properties of parts hardened by the SCR process, it is extremely important to achieve in heat-treated parts the so-called ultrafine hardened steel grains of 11-12 points on a standard scale (with an average grain area of 60-30 μm ² ; while with traditional methods of heat treatment, a grain of 7-8 points is achieved, that is, an area of 1000-500 microns ² ). Thus, in the first case, the grain is finer on average 15-20 times.

The closest analogue of the invention is structural steel of low hardenability according to the copyright certificate of the USSR N 128482 from 10.31.59, containing components in the following ratio in wt.%:
carbon - 0.4 - 1.2
Manganese - not more than 0.20
silicon - not more than 0.3
chrome - no more than 0.3
nickel - no more than 0.25
titanium - not more than 0.5
iron - the rest
Experience shows that in such steel the properties of reduced hardenability and a tendency to grain growth have a spread from smelting to smelting, which requires adjustment of the heating mode for hardening and heat treatment for each smelting.

 These disadvantages are eliminated by the present invention.

The technical result of the invention is to achieve guaranteed stability of low hardenability properties and to obtain "ultrafine" grain of hardened steel with a value of 11-12 points when hardening from temperatures in the range from Ac ₃ to Ac ₃ + 100 ^o C.

The technical result is achieved by the fact that the proposed structural steel of low hardenability contains components in the following ratio in wt.%:
carbon - 0.40 - 0.85
Manganese - not more than 0.20
silicon - not more than 0.20
chrome - not more than 0.10 - nickel - not more than 0.10
copper - no more than 0.10
aluminum - 0.03 - 0.10
titanium - 0.06 - 0.12
vanadium - not more than 0.40
iron - the rest
The invention is illustrated by the following examples.

Example 1
Steel of the claimed composition was smelted in three ranges, conventionally designated as 62PP1, 62PP2 and 62PP3, to produce steels intended for spring sheets with a thickness of 11 and 14 mm. The ratio of components in steel 62PP1, 62PP2 and 62PP3 are given in table 1.

 When hardening the obtained samples, the hardening depth was from 0.12 to 0.2 of the sheet thickness.

 Melting 62PP1, carried out without silicon, potentially makes it possible to obtain a finer and more stable grain during hardening and a larger viscosity reserve of hardened steel than steel with silicon.

Example 2
Melting 62PNP (extremely low hardenability) has the lowest possible content of manganese and silicon, as well as impurity elements - chromium, nickel and copper. This steel is intended for testing spring sheets with a thickness of 8-10 mm, on which a hardening depth of 0.15-0.20 of the sheet thickness is provided. The resulting steel can also be tested on other parts with a thin working section, for example, on gears of a module from 4 mm. See table 1.

Example 3
Melting 62PP4 yielded steel having a higher level of hardenability than all previous options. It is intended for spring sheets with a thickness of 16 and 18 mm. This option gives a hardening depth of 0.12 to 0.17 of the sheet thickness. The composition is shown in table 1.

Example 4
A variant of steel 80PP1 with a carbon content of 0.8% is intended for testing thin spring sheets with a thickness of 10 mm or more, in order to achieve higher fatigue strength, which was shown during tests at KAMAZ with trial melts of this type and with the same carbon content. The composition of steel 80PP1 is given in table 1.

 A general remark that applies to all the examples of high hardenability steel given in the examples is that these PP steel options can be successfully applied not only to spring sheets, but also to a wide range of critical heavy-loaded machine parts: spur and bevel gears and gears, crosses, piston fingers, spherical fingers, rolling bearings and others.

 Depending on the dimensions of the working sections of the parts, the necessary hardenability level and steel option can be selected. Corresponding calculations of the hardenability of steel according to its chemical composition and the values of the hardening depth obtained in hardened parts are developed in detail and applied in practice.

Claims (1)

Structural steel of low hardenability containing carbon, manganese, silicon, chromium, nickel, titanium and iron, characterized in that it additionally contains copper, aluminum and vanadium in the following ratio, wt.%:
Carbon - 0.40 - 0.85
Manganese - Not more than 0.20
Silicon - Not more than 0.20
Chrome - No more than 0.10
Nickel - Not more than 0.10
Copper - Not more than 0.10
Aluminum - 0.03 - 0.10
Titanium - 0.06 - 0.12
Vanadium - Not more than 0.40
Iron - Else

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