RU2578873C1 - Steel with bainite structure - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: steel contains the following component ratio, wt %: carbon 0.25-0.75; manganese 2.4-3.0; chrome 2.4-3.0; silicon 1.6-2.5; molybdenum 0.5-0.6; vanadium 0.10-0.16; and iron is the rest. After rolling heating at continuous slow cooling with the rate of 0.3-0.03°C/s and the temperature of transformation in the beinite area of 400-100°C it has the structure of carbide-free lower bainite.

EFFECT: specified hardenability to the depth up to 1000 mm, and the guaranteed creation of the structure of lower carbide-free bainite.

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Description

The invention relates to metallurgy, in particular to the field of finding compositions of system-alloyed steels of the type H3G3FMS, and can be used for the manufacture of various tools, pressure vessels, parts and components of rolling stock, large-sized parts in various installations (rolling mills, backup rolls, etc. .d.), as well as parts and products of small cross-sectional size, i.e. for the manufacture of products with bainitic hardenability of any section up to 1000 mm, used in various engineering industries.

Known steel with bainitic structure (application for invention No. 94006015 dated 06/27/1996), containing, by weight. %:

Carbon 0.15-0.45 Manganese 0.3-2.0 Chromium 0.5-3.0 Silicon 0.15-2.0

And at least one element selected from the group which includes: molybdenum, nickel, copper, niobium, vanadium, titanium, boron.

Steel is obtained with the structure of carbide bainite under cooling conditions, allowing to obtain a product with hardenability of not more than 20 mm (cooling rate from rolling heating to 500-300 ° C - 1-10 ° C / s). The basic composition of steel is supplemented by deficient elements - carbide-forming and non-carbide-forming - in large quantities (up to 0.50% Cu, up to 4% Ni, etc.) to increase the strength of bainitic structures and their stability. Steel is not systemically alloyed in its basic composition, as the elements were introduced at a high carbohydrate content without a system based on the principle of doping with carbide formers in the form of a continuous chain from weak to strong carbide formers with a decrease in the concentration of strong (see patent No. 2477333 of 03/10/2013), and does not guarantee additions to the main composition with other carbide forming agents the possibility of obtaining products with a wide hardenability range, i.e. from 20 mm and above.

Known economically alloyed bainitic steel (and.with. No. 836190 from 06/07/81), containing, by weight. %: 0.1-0.2 C; 3.2-4.0 Cr; 3.2-4.0 Mn; 0.5-1.0 Mo; 0.3-0.5 V; iron the rest.

Steel does not contain expensive nickel and is considered economically alloyed, but it 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 to increase the stability of supercooled austenite, therefore, with slow cooling in air, a structure is formed only upper carbide bainite, which by definition leads to a decrease in strength, crack resistance and toughness. Hardenability at the same time at various cooling rates in the range of 100-160 mm, which limits the use of steel for large products with a larger cross section. In addition, quenching after austenitization at 900 ° C is carried out in a liquid medium (oil, salt bath) and tempering at 250 ° C, which complicates the process of obtaining a given structure and properties, costly, environmentally disadvantageous, and the composition is not systemically doped.

The closest in technical essence and the achieved result is the composition of the system-alloyed steel (Yu.N. Simonov, Simonov M.Yu., Poduzov D.P., Smirnov A.V., Galimova I.A., Transformation, structure and properties of systemically alloyed low-carbon nickel-free steels, MiTOM magazine No. 11, 2012, p. 5-9). Steel corresponds to the type HZGZMFS in the following ratio of components, wt. %:

Carbon 0.10 Manganese 2,51 Chromium 2.75 Silicon 1.25 Molybdenum 0.4 Vanadium 0.12 Iron rest

As impurities, the steel may contain, by weight. %: 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 from rolling heating is cooled in air at speeds of 50-0.050 ° C / s. Bainitic steel; contains carbide formers from weak (Cr, Mn), medium (Mo) to strong groups (V); iron is the basis. Cr and Mn are introduced in a 1: 1 ratio to each other. Steel is economical in terms of the concentration of alloying elements; system-alloyed, i.e. carbide forming agents are arranged in a continuous chain of bonds: from weak to strong, in the order of decreasing their concentrations, respectively; has an additional alloying element - silicon. 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, wt. hours: 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 wt. hours: Mn / Cr 1: 1; Cr / Mo 1: 0.2; Mo / V 1: 0.2.

But when the carbon content in the steel is 0.1%, the structure of packet martensite in air is obtained from rolling heating at cooling rates of 50-0.3 ° C / s. With the introduction of silicon into the economically alloyed composition of steel, studies of the structure and properties of steel have shown that the thermodynamic activity of carbon increases and the activity of iron decreases. As a result, during isothermal exposure in the region of formation of lower bainite or during slow cooling (in air and at a rate of 0.050 C / s) in this region, carbon atoms accumulate in austenite, but a lack of iron does not allow cementite to precipitate. At the same time, silicon increases the stability of supercooled austenite, contributing to the hardenability of large billets, products of up to 500-600 mm made of this steel, but in a narrow range (398-375 ° C) of transformation temperatures and at one cooling rate with obtaining mainly lower carbide bainite, and not lower carbide-free bainite, which does not guarantee the reproduction of the process of obtaining an alloy with specified properties from melting to melting, from billet to billet and product properties, respectively, because random factors exist.

The task to which the claimed invention is directed is to develop a composition of medium-carbon system-alloyed steel with regulated hardenability to a depth of 1000 mm and guaranteed formation of the structure of lower carbide-free bainite in this steel, expanding the scope.

The problem was solved due to the fact that steel with a bainitic structure containing carbon, silicon, manganese, chromium, molybdenum, vanadium, iron - the rest, according to the invention contains ingredients in the following proportions, wt. %:

Carbon 0.25-0.75 Manganese 2.4-3.0 Chromium 2.4-3.0 Silicon 1.6-2.5 Molybdenum 0.5-0.6 Vanadium 0.10-0.16 Iron rest,

Moreover, after rolling heating with continuous slow cooling at a rate of 0.3-0.03 ° C / s and a transformation temperature in the bainitic region of 400-100 ° C, it has the structure of carbide-free bainite.

A comparative analysis of the inventive steel with the prototype showed that the claimed composition is different in the quantitative ratio of the ingredients used and the guaranteed formation of carbide-free bainite structure obtained as a result of continuous slow cooling from rolling heating (1000 ° C) at speeds in the range of 0.3-0, 03 ° C / s, depending on the carbon content and the ratio of alloying elements in the system-doped composition of the type X3G3MFS.

Unlike the prototype, the inventive steel contains carbon in the range of 0.25-0.75%, which allows it, with a system-alloyed ratio of components in combination with 1.6-2.5% silicon, to ensure the stability of austenite in the bainitic region of phase transformations and guarantee the presence of lower carbide-free bainite in a wide range of transformation temperatures and, therefore, reproduce a given structure from smelting to smelting, from workpiece to workpiece, excluding the factor or factors of chance.

During the experiments, the authors unexpectedly revealed that with the claimed alloying system, the optimum ratios of carbon to silicon are as follows: for 0.25% carbon C: Si = 1: 10; for 0.45%, C: Si = 1: 5; for 0.75%, C: Si = 1: 2.2. Although logically there should be an inverse relationship: the more carbon, the more silicon should be in order to prevent the formation of special carbides in steel. Thus, the combination of system doping and the optimal ratio of carbon to silicon allows to obtain lower carbide-free bainite in a wide temperature range, as well as to guarantee steel with increased bainitic hardenability in a wide range of continuous slow cooling speeds and, therefore, expand its field of application in various industries - from tools of small section to large-sized products.

When silicon is introduced within the stated limits, the process of cementite carbide precipitation is inhibited, despite the presence of an increased carbon content in steel. Increased carbon and silicon contents in steel are announced for the first time for steels obtained by the principle of systemic alloying and without nickel, which significantly reduces the cost of the composition of steel and its products.

Due to the increase in carbon content (up to 0.25-0.75%), compared with the prototype (0.10%), a system was developed for the production of steel products of the medium-carbon class, which allowed us to solve the problem of obtaining a class of steels constructed on the basis of the system alloying, which is able to obtain lower carbide-free bainite in a wide thermal interval of transformation, which has established itself as a structure that provides a high set of characteristics of mechanical properties.

Studies have shown that for the first time a new composition of steel was developed - medium-carbon medium alloyed according to the principle of introducing alloying elements in the form of a continuous chain of bonds between them from weak carbide-forming (Cr, Mn) to medium (Mo) and strong (V) with the following ratios of carbide formers with respect to chromium, in average wt. hours: Cr / Mn 1: 1; Cr / Mo 1: 0.18; Cr / V 1: 0.04; and relative to each other in a chain of bonds from weak to strong carbide former: Mn / Cr 1: 1; Cr / Mo 1: 0.18; Mo / V 1: 0.20, with hardenability in a wide range of specified product dimensions (from 50 mm, at a cooling rate of 0.3 ° C / s, up to 1000 mm - at a speed of 0.03 ° C / s). Consequently, due to bainitic hardenability, while maintaining high mechanical properties, which, by definition, are ensured by lower carbide-free bainite, steel can be recommended in various engineering industries, and in size - without section limitation (from 50 mm to 1000 mm).

The content in the mixture of silicon in an amount of 1.6-2.5 wt. % provides lower carbide-free bainite.

The content in the mixture of carbon in an amount of 0.25-0.75 wt. % expands the range of bainitic transformation by lowering the temperature of the onset of martensitic transformation.

Low-alloy bainitic-martensitic nickel-free type 14Kh2GMRB became known, but they are not system-alloyed, have hardenability of up to 20 mm, and are cooled in oil. At low cooling rates, carbide bainite is formed at temperatures above 600 ° C, and the end of the transformation is 380 ° C, which reduces the stability of supercooled austenite, the calcination depth and microhardness, which limits its use as a welding material.

Middle alloyed martensitic-bainitic steels are known. They contain carbon 0.25-0.30% without nickel, silicon 1.0-1.2%. Key representatives: 25-30 HGSA (GOST 4543-71), having a tensile strength of 1050-2000 MPs. With nickel, the compositions contain up to 0.42% carbon (http: // msd. Mid-alloyed martensitic-bainitic steels. Musiyachenko VF, Sarzhevsky VA).

But the steel is not system-alloyed, it is cooled in cycles to certain temperatures and get carbide bainite at high speeds, hardness and hardenability are low values.

For the study, 50 kg ingots were made, steel was smelted in induction furnaces. Billets with a diameter of 19 mm and a length of 60 mm were made from the steel of each heat. To assess the possible cooling rates to obtain carbide-free lower bainite and the hardenability of the test steel, the billets were heated to a temperature of 920 ° C for 40 min and cooled: in air, in a box with sand in air, together with the furnace, in a box with sand in air with an oven, which corresponds to cooling rates of 30-1.5; 0.1-0.3; 0,054; 0.03-0.003 ° C / s. Transformations during cooling over a wide speed range were studied using a Linseis 278 RITA quenching dilatometer, the structure was studied using an OLYMPUS GX-51 light microscope, and the microhardness of structural components was measured using a Durascsan-70 microhardness tester.

For experimental verification of the claimed composition were prepared 6 alloys. Table 1 presents the chemical composition of the studied steels: p. 1 - prototype, p. 2-4 - the claimed composition; p. 5-6 - compositions outside the values of the components of the claimed composition.

Studies have shown that the achievement of a technical result and optimization of the properties of the inventive steel are achieved due to the increased content of carbon and silicon (Table 1), systemic alloying of steel and the expansion of the range of possible cooling rates (Table 2, 3). This made it possible to collectively achieve the effect of guaranteeing the production of carbide-free lower bainite (Table 2), and stably obtain it in a wide temperature range of phase transformations. So, if the prototype carbide lower bainite receives in the temperature range - 23 ° C and only at a cooling rate of 0.05 ° C / s, then the claimed - depending on the possible cooling rates (within 0.30 to 0.03 ° C / s) guarantees the production of carbide-free bainite structure in steel in the range of transformation temperatures of 150-320 ° - pp. 2-4, which avoids accidents and reproduces the process of obtaining steel from melting to melting, stably, as well as from heat treatment to heat treatment, from product to product to guarantee the set structural parameters and, accordingly, the properties of the products, because by definition, the structure of carbide-free lower bainite in steel provides high physical and mechanical properties.

The inventive steel due to the guaranteed receipt of lower carbide-free bainite in it allows to expand the number of industries within the engineering industry using the inventive steel composition with a regulated hardenability depth (small section tools, large-sized devices), etc. (tab. 3).

The inventive steel has a hardenability of up to 1000 mm due to the introduction of an increased, compared with the prototype, carbon and silicon contents, which contribute to increasing the stability of the presence of carbide-free bainite and the stability of the state of supercooled austenite in the bainitic region of transformations.

Depending on the ratio of components in the steel during its slow continuous cooling from rolling heating, the temperature range of phase transformations changes. With a carbon content of 0.25%, chromium 2.4%, manganese 2.4%, silicon 2.5%, molybdenum 0.60%, vanadium 0.16%, the bainitic region has a conversion range of 200 ° C, i.e. . from 400 to 200 ° C, and martensitic - 200 ° C, i.e. from 300 ° C to 100 ° C (table. 2, example 2), which allows, in comparison with the prototype 23 ° C, to increase the temperature range of the bainitic transformation by an order of magnitude and, therefore, guarantee in the range of cooling rates of 0.05-0, 03 ° C / s obtaining blanks with a carbide-free lower bainite structure with hardenability of 500-1000 mm, i.e. in a wider range of sections - from minimum to 1000 mm. And this means that from this steel it is possible to manufacture products for various purposes (Table 3), with a regulated hardenability depth.

When the carbon content of 0.45%, manganese 2.7%, chromium 2.7%, molybdenum 0.55%, vanadium 0.13%, silicon 2.0% receive carbide-free bainite in the range of conversions from 400 ° C to 80 ° C (table. 2, example 3). In this case, the hardenability depth reaches 1000 mm, and martensite is formed after bainite - within the temperature range of 250 ° -80 ° C. This composition is the best of the three in terms of reliability of producing carbide-free lower bainite in a wide range of transformations, the range of use of cooling temperatures, with the guarantee of obtaining steel with a carbide-free bainite structure, which allows to obtain products in a wider hardenability range.

The composition containing 0.75% carbon, manganese 3.0%, chromium 3.0%, molybdenum 0.50%, vanadium 0.10%, silicon 1.6%, allows to obtain guaranteed lower carbide-free bainite in the range of transformations in the region bainite from 300 ° C to 150 ° C, and martensite after receiving lower carbide-free bainite - from 180 ° C to 100 ° C (table 2, example 4), thereby allowing guaranteed production of products with hardenability up to 1000 mm (table 3 )

When the carbon content is <0.25%, chromium <2.4%, manganese <2.4%, molybdenum> 0.60%, vanadium> 0.16% and silicon> 2.5%, a sharp decrease in the stability of supercooled austenite occurs. The hardenability in this case drops to 50 mm with an increase in the cooling rate to 0.3 ° C / s (Table 3, Example 5).

When the carbon content> 0.75%, chromium> 3.0%, manganese> 3.0%, molybdenum <0.50%, vanadium <0.10%, silicon <1.6%, lower carbide is obtained in the bainitic region bainite (table. 2, example 6), which affects the hardenability (table. 3), as well as other physical and mechanical properties of steel.

Table 4 presents data on the microhardness of the inventive steel and on the microstructure depending on the composition and cooling rate.

From the data in table 4 it can be seen that when the content of 0.25% carbon, 2.5% silicon, 2.4% chromium and manganese, 0.6% molybdenum, 0.16% vanadium, the rest is iron, at speeds cooling with rolling heating (from 1000 ° C) 3.0-0.3 ° C / s completely martensite is formed. At speeds of 0.05-0.03 ° C / s, mainly carbide-free lower bainite is simultaneously formed (HV, HRC 590-581 (52-52), respectively).

When the carbon content is 0.45%, manganese and chromium - 2.7% each, silicon - 2%, molybdenum - 0.55%, vanadium - 0.13%, mainly carbide-free bainite and martensite are simultaneously formed (HV, HRC 648 -559, (56-50), respectively) at cooling rates of 0.3-0.03 ° C / s. And at cooling rates of 3.0-1.5 ° C / s - completely martensite.

When the carbon content is 0.75%, manganese and chromium - 3.0% each, silicon 1.6%, molybdenum - 0.5%, vanadium 0.16%, lower carbide-free bainite is formed at a cooling rate of 0.3-0, 05 ° C / s with a low perlite content, and at lower speeds, with perlite as an excess phase that degrades the properties of steel (HV, HRC 566-513, (51-48)).

The inventive steel composition has advantages over the prototype:

- Steel provides hardenability to products in a wide range of overall dimensions (up to 1000 mm in cross section).

- The composition allows you to vary the cooling rate, the degree of hardenability, the temperature of the transformations in the bainitic region and below with guaranteed receipt of carbide-free bainite.

- The composition of the steel and its preparation at different cooling rates (in the range of 0.3-0.03 ° C / s), depending on the carbon and silicon contents in it, with systemic alloying of the remaining components (Cr, Mn, Mo, V) lead to a decrease in the critical points of martensite transformations after the formation or during the formation of a bainitic structure.

- Thanks to the effects obtained, it was for the first time possible to obtain a medium-carbon system-alloyed steel with a carbide-free lower bainite structure.

Steel, due to the stable appearance of carbide-free lower bainite before martensitic transformation, created conditions for the hardenability of products over the cross section from 50 mm to 1000 mm, which allowed to expand the field of its use in various branches of mechanical engineering: from instrumental to products used in heavy mechanical engineering - large-sized, large section.

Claims (1)

Steel with a bainitic structure, containing carbon, manganese, chromium, molybdenum, vanadium, silicon and iron, characterized in that it contains ingredients in the following ratio, wt.%:
Carbon 0.25-0.75 Manganese 2.4-3.0 Chromium 2.4-3.0 Silicon 1.6-2.5 Molybdenum 0.5-0.6 Vanadium 0.10-0.16 Iron rest,
Moreover, after rolling heating with continuous slow cooling at a rate of 0.3-0.03 ° C / s and a transformation temperature in the bainitic region of 400-100 ° C, it has the structure of carbide-free bainite.