RU2760140C1 - Method for producing low-carbon martensitic steel - Google Patents

Abstract

FIELD: metallurgy.SUBSTANCE: invention relates to the field of metallurgy, namely, to production of workpieces from low-carbon martensitic steel containing 0.12 to 0.27% wt. of carbon. A workpiece is smelted from steel including 0.1 to 0.5% wt. silicon, 1.8 to 2.6% wt. manganese, 2.1 to 2.8% wt. chromium, 1.0 to 1.6% wt. nickel, up to 0.15% wt. vanadium and up to 0.15% wt. niobium as components. Rolling heating of the workpiece, subsequent double quenching and tempering thereof are executed. Full quenching is executed as the first quenching after rolling heating from a temperature of 950°C, medium- or high-temperature tempering is executed after the first quenching, and the second quenching is executed from the intercritical temperature range of 800 to 810°C.EFFECT: mechanical properties of low-carbon martensitic steel are increased with the ensured high cold resistance thereof.1 cl, 3 tbl, 1 ex

Description

The invention relates to the field of metallurgy, in particular, to the production of high-strength weldable structural steels. It can be used to obtain steel for instrument making, mechanical engineering, mining, processing, cable and other industries.

Known steel with the structure of low-carbon martensite under the RF patent for invention No. 2462532, С22С 38/58, 2012. Steel contains, 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 metals no more than 0.03, iron the rest. After quenching from deformation heating or after austenitization with cooling in still air and subsequent tempering, it has a lath-globular martensitic structure. The disadvantage is the low physical and mechanical characteristics, in particular, the impact strength.

Known high-strength, weldable steel with increased hardenability according to RF patent for invention No. 2314361, 2008. Steel contains, wt%: carbon 0.10-0.18, silicon 0.12-0.60, chromium 2.0-3, 0, manganese 2.0-2.4, nickel 1.0-2.0, molybdenum 0.4-0.6, cerium and / or calcium up to 0.15, vanadium 0.08-0.12, titanium less 0.01, niobium 0.05-0.10, iron the rest. Steel after quenching from rolling heating or after austenitization at a temperature of 950-1050 ° C with accelerated cooling and subsequent tempering at a temperature not exceeding 550 ° C, it has a structure of packet martensite. The disadvantages of this steel are in a small range of guaranteed values of mechanical properties, in low limiting values of hardenability, impact toughness, which can cause a significant change in properties in the heat-affected zone during welding.

Known steel with the structure of packet martensite according to the RF patent for invention No. 2507297, С22С 38/58, 2014. Steel contains, in wt%: carbon from 0.04 to 0.099, chromium to 7.00, manganese from 0.15 to 2 , 5, nickel no more than 4, molybdenum no more than 1.0, vanadium no more than 0.30, titanium no more than 0.06 and / or niobium no more than 0.15, nitrogen no more than 0.25, copper no more than 2, 00, rare earth elements or calcium no more than 0.15, iron and inevitable impurities - the rest. The steel has a packet-lath structure of martensite when the ratio, wt%: Cr / C is not less than 20. In the hardened state or after low-temperature tempering, the structure of the steel almost entirely consists of packet martensite with strength up to 1200 MPa, and has a high toughness. The disadvantage is the preservation of the packet structure only in narrow ranges of variation of the concentrations of carbon and alloying elements. Steel has relatively low values of strength, impact toughness and cold resistance combinations.

As the closest analogue to the claimed technical solution, a method of heat treatment of oil country tubular goods made of corrosion-resistant steel was selected according to the RF patent for invention No. 2635205, C21D 9/08, 2017. The pipe is made of corrosion-resistant steel of the martensitic class, containing, wt%: carbon 0.12-0.17, silicon 0.15-0.50, manganese 0.30-0.90, chromium 12.00-14.00, nickel 1.80-2.20, copper no more than 0, 25, aluminum 0.02-0.05, sulfur no more than 0.010, phosphorus no more than 0.020, nitrogen no more than 0.020, iron - the rest. The pipe was quenched from 920 to 1020 ° C, the second quenching from the intercritical temperature range from 700 to 830 ° C and tempered in the temperature range from 560 to 690 ° C. The disadvantage is the insufficiently high mechanical properties for using the pipe at low temperatures in the northern regions.

The technical result of the claimed invention is to improve the mechanical properties of low-carbon martensitic steel while ensuring its high cold resistance.

The technical result is achieved due to the fact that in the method of producing low-carbon martensitic steel, including rolling heating with air cooling and subsequent double quenching with tempering, of which the second quenching is carried out at a temperature corresponding to the temperature of the intercritical temperature range, characterized in that steel with carbon content 0.12-0.27 wt.%, as the first quenching after rolling heating, complete quenching is carried out in air or in liquid cooling media from a temperature of 950 ° C, then medium or high-temperature tempering is carried out and the second quenching is carried out from the intercritical temperature range 800 -810 ° C.

The technical result is provided by obtaining steel with a two-phase martensitic structure, due to which the steel has the required properties. The structure of martensite-martensitic steel is obtained with a carbon content of 0.12-0.27 wt% in steel in combination with a special heat treatment technology. The carbon content is up to 0.1% wt.%, Provides an improvement in the structure of steel due to the formation of packet martensite. However, high mechanical properties of steel in combination with cold resistance are provided due to the formation of a martensite - martensitic structure with a carbon content of 0.12-0.27 wt.%. Moreover, with a given steel structure, with an increase in carbon content, the values of mechanical characteristics improve. The lowest carbon content - 0.12 wt%, was determined experimentally, proceeding from the implementation of a special type of structural inheritance, which manifests itself in the preservation of the morphology of martensite when heated to Ac3. With the implementation of such heredity, the complex of mechanical properties of the claimed steel is higher than that of analogous steels. The maximum carbon content is 0.27 wt.%, Determined from the condition of the formation of a martensite – martensite structure with predominantly packet martensite after quenching, since with the appearance of a lamellar component in the structure of steel, the properties of steel, affecting reliability, deteriorate sharply. With a further increase in carbon in a solid solution, the Ms value decreases, the proportion of the lamellar component increases, and the incubation period of pearlite and bainitic transformations decreases. Where Ms (Mn) and Mf (Mk) are the start and end points of the martensitic transformation. The best properties of low-carbon martensitic steel with a carbon content of 0.12-0.27 wt.% Are provided by heat treatment after rolling heating, including full quenching at temperatures above Ac3, medium- or high-temperature tempering and quenching from the intercritical temperature range (ICI). In order to design the carbide system, tempering is carried out before quenching from MCI, this allows creating a given, close to ordered, dislocation distribution, since the number and specified distribution of crystal structure defects largely determines the properties of the resulting steel. Carrying out, as the final stage of heat treatment, quenching from MCI between the critical temperatures Ac1 and Ac3 promotes the formation of the martensite-martensite structure of the material. This structure is characterized by the presence of two α-phases with martensite morphology: the “parent” one, which retained the morphology of the heating field in the MCI, and the “fresh” one, transformed from austenite upon cooling from the interphase region. The optimum temperatures for complete hardening, tempering, and hardening from MCI were determined experimentally, based on the required combination of mechanical properties of steel and the required limit of cold resistance. High values of impact toughness, strength and cold resistance characteristics after the specified sequence of heat treatment operations are achieved due to the formation of a martensite-martensitic structure of low-carbon martensitic steel. High impact strength at a temperature of minus 40 ° C is confirmed by a large proportion of the dimple component in fractures. The claimed steel with the specified carbon content is prone to structural inheritance. Thermal action, aimed at the formation of a two-phase martensitic structure, is effective for low-carbon martensitic steels, prone to structural heredity, alloyed with strong carbide-forming elements due to the formation of special carbides that dissolve little in austenite, retard the growth of austenite grain and contribute to the preservation of the morphology of the alpha phase to Ac3 ...

The method of obtaining low-carbon martensitic steel is as follows.

Use low carbon steel with the content of the components indicated in the table:

Carbon 0.12-0.27; Copper up to 0.8; Vanadium up to 0.15; Silicon 0.1-0.5; Aluminum up to 0.005-0.05; Niobium up to 0.15; Manganese 1.8-2.6; Titanium up to 0.02; Cerium up to 0.06; Chromium 2.1-2.8; Calcium up to 0.06; The rest of the iron. Nickel 1.0-1.6; Zirconium up to 0.1;

Steel of the proposed composition is melted in an induction furnace, poured into ingots and hot rolled into a circle. The heating temperature for rolling is maintained within 1220 ÷ 1100 ° C. Rolling completion temperature 900 ° C. After hot forming, the workpieces are cooled in air. The mechanical properties of the blanks are determined on mechanically cut samples. Next, heat treatment is carried out, including, as the main operations, quenching in air followed by tempering and quenching in air from the intercritical temperature range at the following temperature conditions:

- hardening at a temperature of 950-980 ° C in calm air or in liquid cooling media;

- vacation at a temperature of 450-660 ° C (medium or high temperature) with subsequent cooling in calm air;

- hardening from the intercritical temperature range in the range of 800-810 ° C.

After both of these quenches, air cooling is carried out, cooling in water, in oil or in special quenching liquids is also possible. The tempering temperature is selected depending on the composition of the steel. An increase in the carbon content and the presence of strong carbide-formers cause a transition from medium-temperature tempering to high-temperature tempering.

Example. We used castings of steels of grades 12Х2Г2НМФБ, 15Х2Г2НМФБ, 27Х2Г2НМФБ of the following chemical composition:

Element content, wt. % steel grade C Si Mn Cr Ni Moe Nb V Cu S P 12Х2Г2НМФБ 0.12 0.40 2.24 2.39 1.38 0.45 0.070 0.1 - 0.007 0.015 15Х2Г2НМФБ 0.15 0.27 2.07 2.10 1.23 0.42 0.063 0.09 - 0.01 0.02 27X2G2NMFB 0.27 0.43 2.45 2.37 1.48 0.53 0.14 0.14 0.19 0.008 0.016

Castings of steels designated 12Х2Г2НМФБ, 15Х2Г2НМФБ, 27Х2Г2НМФБ were deformed at a temperature of 1250-900 ° C with air cooling. From the rods obtained after deformation by mechanical methods, samples were made with their subsequent heat treatment according to the following scheme:

Bars of 12Kh2G2NMFB steel were hardened from a temperature of 980 ° C with cooling in calm air, steel 15Kh2G2NMFB - from a temperature of 950 ° C with cooling in calm air, steel 27Kh2G2NMFB - from a temperature of 950 ° C with cooling in calm air. Then tempering was carried out at temperatures for steel 12Kh2G2NMFB - 660 ° C, for steel 15Kh2G2NMFB - 450 ° C, for steel 27Kh2G2NMFB - 660 ° C, followed by cooling in quiet air. Quenching was carried out from the intercritical temperature range for steel 12Kh2G2NMFB at a temperature of 810 ° C, for steel 15Kh2G2NMFB at a temperature of 800 ° C, for steel 27Kh2G2NMFB at a temperature of 800 ° C. The obtained values of mechanical properties are presented in the table

Steel σВ, MPa σ0.2, MPa δ,% Ψ,% KCV + 20, MJ / m ² KCV-40, MJ / m ² KCV-60, MJ / m ² KCV-50, MJ / m ² 12X2G3MFT-1 1440 1190 fourteen 55 1.28 0.98 0.36 - 12X2G3MFT-2 1390 1120 fourteen 55 0.82 1.18 0.46 - 12X2G3MFT-3 1450 1190 13 51 0.70 0.45 0.26 - 15X2G3MFB-1 1460 1190 12 62 1.3 0.9 0.3 0.6 15X2G3MFB-2 1320 1190 13 63 1,2 0.6 0.2 0.78 15X2G3MFB-3 1310 1010 16 62 0.7 0.45 0.3 - 27X2G3MFB-1 1800 1190 12 59 1.5 1.1 0.2 - 27X2G3MFB-2 1650 1200 nine 46 1.1 0.65 0.2 - 27X2G3MFB-3 1650 1220 15 62 0.35 0.2 0.2 -

Legend:

1 - full hardening, high-temperature tempering, hardening from MKI;

2 - full hardening, medium temperature tempering, hardening from MKI;

3 - full hardening, hardening 950 ° С, low tempering.

Analysis of the data presented shows that at temperatures up to "-40 ° C" the value of the impact toughness of the samples of all steels subjected to complete hardening, tempering and quenching from MKI significantly exceeds the impact toughness of the samples indicated in the table under number 3, which underwent a different heat treatment, which was used previously. Fractograms obtained by testing the impact bending of specimens with a V-shaped notch confirm good cold resistance, since at a test temperature of "- 40 ° C" a high proportion of the dimple component of the fracture remains. As can be seen from the table, the best combination of strength and reliability characteristics, as well as cold resistance, was obtained after a mode including full hardening, high- or medium-temperature tempering, and quenching from ICI. The obtained low-carbon martensitic steels are weldable steels with increased values of strength, toughness and cold resistance characteristics. The use of these steels will provide a high range of mechanical properties in parts and structural elements.

Thus, the claimed invention improves the mechanical properties of low-carbon martensitic steel while ensuring its high cold resistance.

Claims (1)

A method of producing a billet from low-carbon martensitic steel containing 0.12-0.27 wt.% Carbon, including smelting the billet, rolling heating of the billet, subsequent double quenching and tempering, characterized in that the billet is smelted from low-carbon martensitic steel, the components of which included 0.1-0.5 wt.% silicon, 1.8-2.6 wt.% manganese, 2.1-2.8 wt.% chromium, 1.0-1.6 wt.% nickel, up to 0.15 wt.% Vanadium, up to 0.15 wt.% Niobium, as the first quenching after rolling heating, complete quenching is carried out from a temperature of 950 ° C, after the first quenching, medium- or high-temperature tempering is carried out, and the second quenching is carried out from the intercritical interval temperatures 800-810 ° C.