RU2462532C1 - Steel with structure of low-carbon martensite - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: steel contains, in wt %: carbon 0.182 - 0.272, chromium 1.2 - 4.0, nickel 0.3 - 4.0, manganese 1.0 - 3.0, molybdenum no more than 3.0, vanadium no more than 0.3, copper no more than 2.5, titanium no more than 0.1, niobium no more than 0.15, silicium no more than 0.6, nitrogen 0.001 - 0.25, calcium no more than 0.15, cerium no more than 0.15, rare-earth metals no more than 0.03, iron - the rest. After thermohardening with a strain heating or after austenitising with cooling at smooth air and after subsequent release it has a rack and globular martensitic structure.

EFFECT: steel has increased impact hardness and strength, high hardenability and processability in the manufacture of welded structures and metallurgical semi-finished products.

2 dwg, 4 tbl

Description

The invention relates to the field of metallurgy, namely to structural steels hardened in air, the use of which is possible in heat-strengthened structures and large-sized products.

Known low-carbon bainitic steel (Japanese application No. 53-6613, class C22C 38/38, 1978), containing, wt.%:

Carbon  0.03-0.05 Chromium  0.3-3.0 Manganese  0.1-0.8 Calcium  0.01-0.03 Lanthanum  0.005-0.1 Niobium  0.01-0.15 Vanadium  0.01-0.20 Iron  rest

The bainitic structure is not capable of providing sufficient strength and toughness in wide temperature ranges and especially at low temperatures due to the low hardenability with a minimum content of carbon and alloying elements and the relatively low viscosity of bainite.

The disadvantages of bainitic structure can be eliminated using low-carbon martensitic steels. As an analogue (RF Patent No. 20099260 dated March 15, 1994, High-strength weldable steel // L.M. Kleiner, I.V. Tolchina, L.D. Pilikina, AMMolganov, V.M. Arkhipov) steel with the following the ratio of components, wt.%:

Carbon  0.06-0.12 Chromium  1.8-2.5 Manganese  0.8-2.5 Vanadium  0.01-0.13 Niobium  0.02-0.10 Nitrogen  0.001-0.25 Rare earth elements  0.01-0.03 Iron  rest.

The disadvantage of this steel is its low mechanical properties. It is possible to increase the mechanical properties by changing the composition, when, at the above-mentioned content of elements, Ni is additionally introduced - not more than 4.0%, Cu - not more than 2.5%, N - 0.001-0.25%, Ca - not more than 0, 15%, REM - not more than 0.03%, Ce - 0.005-0.15%. As a prototype (RF Patent No. 2314361 of January 10, 2008. High-strength, welded steel with increased hardenability // L.M. Kleiner, I.V. Tolchina, A.A. Shatsov) steel of the following composition was selected, wt.%:

Carbon  0.10-0.18 Silicon  0.12-0.60 Chromium  2.0-3.0 Nickel  1.0-2.0 Manganese  2.0-2.4 Molybdenum  0.4-0.6 Vanadium  0.08-0.12 Titanium  no more than 0,01 Niobium  0.05-0.1 Cerium and / or calcium no more  no more than 0,15 Iron  rest.

With the selected ratio of components, hardenability in air with the formation of a martensitic structure is provided in cross sections up to 200 mm. A further increase in the strength and / or viscosity characteristics is possible in the case of the formation of a rack-globular structure with a different ratio of high-temperature morphological types of martensite (both the rack and globular components are high-temperature forms of martensite). This type of structure with improved properties can be obtained by heat treatment of the prototype and / or by changing the composition of the steel.

In those cases when the compositions corresponding to the prototype were used, a special thermocyclic treatment (TCO) was used to obtain a rack-globular structure (Fig. 1), with a higher carbon content a similar structure can be obtained without a TCO, by heat treatment of 20Kh2G2NMFB steel, including quenching with cooling in calm air (figure 2).

The structure consists of rack 1 and globular 2 morphological types of martensite.

Thus, the well-known low-carbon steels make it possible to obtain the rack-globular structure of martensite only after specially selected TTZ modes, while new compositions make it possible to obtain this type of structure only by heat treatment, including quenching from deformation heating or after austenitization with cooling in still air and subsequent vacation time.

The aim of the invention is the development of low carbon steels with rack and globular martensite structure and high hardenability in calm air. In addition, NMSs must be highly technological in the manufacture of welded structures and metallurgical semi-finished products, heat-strengthened by combining hot forming with quenching (a combined process) or austenitization, followed by cooling in air and tempering, and provide increased strength and toughness.

This goal is achieved by the use of heat treatment and the fact that carbon is additionally introduced into the steel containing carbon, silicon, chromium, manganese, nickel, molybdenum, vanadium, titanium, niobium, calcium, copper, nitrogen, cerium and rare earth elements, with the total content carbide-forming elements not more than 9% and in the following ratio of ingredients, wt.%:

Carbon  0.182-0.272 Chromium  1.2-4.0 Nickel  0.3-4.0 Manganese  1.0-3.0 Molybdenum  no more than 3.0 Vanadium  no more than 0,3 Copper  no more than 2.5 Titanium  no more than 0.1 Niobium  no more than 0,15 Silicon  no more than 0.6 Nitrogen  0.001-0.25 Calcium  no more than 0,15 Cerium  no more than 0,15 Rare earth the elements  no more than 0,03 Iron  rest

Thus, obtaining a rack-globular structure provides a carbon content of from 0.182 to 0.272% at given concentrations of dopants and subsequent heat treatment.

According to the search results, no steels were found in the patent and scientific literature with the same qualitative and quantitative composition of components in combination with the same heat treatment, providing a rack-globular structure, on the basis of which we can conclude that the proposed steel meets the criterion of "significant differences "

The ratio of the components in the selected combination in the proposed compositions due to the formation of the structure of rack-globular martensite provides an increase in the set of characteristics of mechanical properties and manufacturability. The necessary properties are obtained due to the fact that carbon, chromium, nickel and manganese within the specified limits provide high stability of supercooled low-carbon austenite and, therefore, hardenability upon cooling in air with the formation of a rack-globular structure. To ensure operability after cooling in wide temperature ranges, for example after welding, steel should contain molybdenum. Molybdenum additionally increases hardenability, provides weldability and reduces the tendency to temper brittleness. Vanadium, and / or titanium, and / or niobium, introduced into steel alloyed with chromium, manganese, nickel and molybdenum with a carbon content of 0.182-0.272%, bind part of the carbon to carbides, causing additional hardening of the low-carbon alloy base, quenched by cooling in air with the formation of a martensitic structure; cerium and / or calcium in steel form a favorable form of non-metallic inclusions; titanium, and / or vanadium, and / or niobium, and / or cerium, and / or calcium within the specified limits contribute to the grinding of grain, providing the necessary toughness at high strength, and increase the tempering resistance of steel.

So, for the formation of the structure of rack-globular martensite, it is sufficient that the steel contains chromium, manganese and nickel in the indicated proportions, in addition, molybdenum and / or vanadium and / or titanium, and / or niobium, and / or cerium, and / or calcium, and / or REM in the indicated proportions.

The carbon content in the range of 0.182-0.272% is optimal within the specified alloying limits to ensure strength in the specified range of values. A carbon content of less than 0.182% reduces some characteristics of structural strength and does not allow to obtain a rack-globular structure without the use of specially selected thermal cycling modes.

The set of characteristics of the mechanical properties is due to the guaranteed provision of the structure of rack and globular martensite during quenching by slow cooling in air. A distinctive feature of steels with such a martensite structure is the possibility of carrying out a combined process of hot forming with quenching by cooling in air, since high toughness is ensured. Such a process becomes possible, since the size of the characteristic elements of the martensite structure, weakly dependent on the parameters of the hot pressure treatment, is responsible for the viscosity. temperature range and values of degrees of deformation.

Thus, the proposed composition and heat treatment conditions provide, when quenching in air, yield strength σ _0.2 = 1100-1400 MPa, impact strength KCV = 70 J / cm ² or more.

Based on the foregoing, it can be concluded that the proposed composition meets the criteria of "novelty" and "positive effect".

For the study, 50 kg ingots were made, steel was smelted in induction furnaces. Bars of ⌀ 19 mm were made from the steel of each heat to study the mechanical properties and stability of austenite.

Table 1, 2 shows the chemical composition of the studied steels.

Mechanical properties were determined on specimens discontinuous type Sh-7k GOST 1497-84 and shock type 13 GOST 9454-78 after thermal hardening them according to the regimes optimal for prototype steel and steel of the proposed composition, Tables 3, 4.

Table 1 The chemical composition of steels Steel designation Chemical composition, % FROM Si Mn Cr Ni Mo Permissible amount of microalloying elements and impurities 18X2G2N1 0.182 0.30 1.84 2.46 1,08 0.04 Cu - no more than 2.5 19X2G1N2.5 0.192 0.45 0.99 2.60 2,39 0.05 Ti - no more than 0.1 19X2G2NM0.3 0.191 0.28 1.90 2.28 1,52 0.32 Nb - no more than 0.15 19X4G2NM0.6 0.193 0.46 2.05 4.21 1.49 0.55 V - no more than 0.3 19X2G2N4 0.192 0.45 1.99 2.60 4.03 0.05 N - 0.001-0.25 22X2G2NM3 0.221 0.37 1.68 2,31 1.45 3.04 Ca - no more than 0.15 22X2G2NM1 0.221 0.37 1.68 2,31 1.45 0.84 Ce - no more than 0.15 24X2G3NM0.5 0.242 0.28 3.08 2.27 1,50 0.46 REM - no more than 0.03 27X2GZN4M0.5 0.272 0.19 3.01 2.02 4.03 0.52 Al - not more than 0.05 29X2G2NM0.5 0.29 0.17 1.97 1.98 1.40 0.53 Si - no more than 0.6 S and P no more than 0,025

Steel, which did not include molybdenum, vanadium, niobium, titanium, copper, calcium, cerium and rare-earth metals, are presented in table 2.

table 2 The chemical composition of the experimental steel melts Steel designation FROM Si Mn Cr Ni Fe and related impurities 19X2G2N0.3 0.194 0.34 1.82 2.40 0.30 Rest 18X2G2N2.5 0.183 0.39 1.98 2,57 2,38 20X2G2N4 0.201 0.42 1.92 2.63 3.89

The mechanical properties of steels are presented in table 3.

The mechanical properties of steels, which were not included molybdenum, vanadium, niobium, titanium, copper, calcium, cerium and rare-earth metals, are presented in table 4.

Thus, at close strength values with the prototype, the formation of a rack-globular structure provides a higher viscosity (KCV) compared with steels with a lower carbon content (prototype) and steels with a high carbon content that do not have a rack and globular structure.

Brief Description of the Drawings

Figure 1 - River and rack-globular structure of the NMS (prototype), subjected to a TTZ:

and - rack, × 30000;

b - rack-globular, × 20,000.

The fine structure of low-carbon martensitic steel, which is a prototype, is subjected to thermocyclic processing. 1 - rack component, 2 - globular component of the structure.

Figure 2 - The rack-globular structure of the NMS 20X2G2NMFB, × 37000. The fine structure of the developed low-carbon martensitic steel hardened in calm air is presented. 1 - rack component, 2 - globular component of the structure.

Claims (1)

Steel containing carbon, silicon, chromium, manganese, nickel, molybdenum, vanadium, titanium, niobium, calcium, cerium and iron, characterized in that it additionally contains copper, nitrogen and rare-earth metals in the following ratio, wt.%:
carbon 0.182-0.272 chromium 1.2-4.0 nickel 0.3-4.0 manganese 1.0-3.0 molybdenum no more than 3.0 vanadium no more than 0.3 copper no more than 2.5 titanium no more than 0.1 niobium no more than 0,15 silicon no more than 0.6 nitrogen 0.001-0.25 calcium no more than 0,15 cerium no more than 0,15 REM no more than 0,03 iron rest
Moreover, after quenching from deformation heating or after austenitization with cooling in still air and subsequent tempering, it has a rack-globular martensitic structure.

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