RU2703085C1 - Structural steel with bainitic structure, obtained from it forged parts and method of forged part production - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, particularly, to heat treatment of steel. To ensure technical result structural steel contains, wt%: up to 0.25 C, up to 0.45 Si, 0.20–2.00 Mn, up to 4.00 Mn, 0.6–3.0 Mo, 0.004–0.020 N, up to 0.40 S, 0.001–0.035 Al, 0.0005–0.0025 B, up to 0.015 Nb, up to 0.01 Ti, up to 0.10 V, up to 1.5 Ni and up to 2.0 Cu, iron and unavoidable impurities – the rest, with the following condition: %Al/27+%Nb/45+%Ti/48+%V/25>%N/3.75. Structural steel has yield strength of at least 750 MPa, tensile strength of at least 950 MPa and a structure containing at least 80 vol% bainite and maximum total amount of residual austenite, ferrite, perlite and/or martensite of 20 vol%. Steel is used, in particular, for production by means of forging of forged parts, having along their length significant changes in cross section. Invention also discloses a method of making forged parts.

EFFECT: high strength and low tendency to deformation.

15 cl, 2 tbl

Description

This invention relates to structural steel with high strength, the structure of which contains at least 80 vol. % bainite.

The invention also relates to forged parts made from such structural steel.

Finally, the invention relates to a method for the production of forged parts from structural steel according to the invention.

Wherever descriptions of alloys or other steel compositions are further provided using the symbol “%”, in each case, unless explicitly stated otherwise, it refers to the mass.

All the mechanical properties presented in this text of the steel according to this invention and any other steels mentioned for comparison purposes are defined in accordance with DIN EN ISO 6892-1, unless otherwise indicated.

hochbeanspruchte Schmiedebauteile” («Разработка высокопрочной, ковкой бейнитной стали (HDB) для высоконапряженных кованых компонентов»), появившейся в Schmiede-Journal, выпуск от сентября 2010 г, публикация lndustrieverband Massivumformung e.V., в кузнечном производстве существует особенно высокая потребность в касающихся стальных материалов решениях, способных предложить возможности достижения высокой прочности и пластичности с обеспечением в то же самое время коротких технологических цепочек их производства. В статье также заявляется, что в этой связи многообещающими выглядят материалы с бейнитной структурой, в которых сочетаются хорошие прочностные свойства и свойства пластичности без необходимости в дополнительной термической обработке и которые отличаются прочностью при растяжении более 1 200 МПа, пределом текучести более 850 МПа и относительным удлинением при разрыве более 10% с работой разрушения при испытаниях на ударную вязкость с надрезом, составляющей 27 Дж при комнатной температуре. В качестве примера концепций легирования, предлагающих такие свойства, статья представляет сталь с содержанием в масс.%: 0,18% C, 1,53% Si, 1,47% Mn, 0,007% S, 1,30% Cr, 0,07% Мо, 0,0020% B, 0,027% Nb, 0,026% Ti, 0,0080% N с остальным, состоящим из железа и неизбежных примесей, и сталь с 0,22% C, 1,47% Si, 1,50% Mn, 0,006% S, 1,31% Cr, 0,09% Мо, 0,0025% B, 0,035% Nb, 0,026% Ti, 0,0108% N с остальным, представленным железом и неизбежными примесями.According to Chartered Engineer Christoph Keul et al. In the article “Entwicklung eines hochfesten duktilen bainitischen (HDB) Stahls

hochbeanspruchte Schmiedebauteile ”(“ Development of high-strength, forged bainitic steel (HDB) for high-tension forged components ”), published in the Schmiede-Journal, September 2010 edition, publication of lndustrieverband Massivumformung eV, forging there is a particularly high demand for steel materials able to offer the possibility of achieving high strength and ductility while ensuring at the same time short technological chains of their production. The article also states that in this regard, materials with a bainitic structure look promising, which combine good strength and ductility properties without the need for additional heat treatment and which are characterized by tensile strength of more than 1,200 MPa, yield strength of more than 850 MPa and elongation at a break of more than 10% with the work of destruction during tests for impact strength with a notch of 27 J at room temperature. As an example of alloying concepts offering such properties, the article presents steel with a mass content of: 0.18% C, 1.53% Si, 1.47% Mn, 0.007% S, 1.30% Cr, 0, 07% Mo, 0.0020% B, 0.027% Nb, 0.026% Ti, 0.0080% N with the rest consisting of iron and inevitable impurities, and steel with 0.22% C, 1.47% Si, 1, 50% Mn, 0.006% S, 1.31% Cr, 0.09% Mo, 0.0025% B, 0.035% Nb, 0.026% Ti, 0.0108% N with the rest represented by iron and inevitable impurities.

Another development, which is similarly aimed at producing steel for the production of die-forged parts, which without additional heat treatment have high strength and at the same time high ductility, is described in EP 1,546,426 B1. The steel known from this patent description contains (in mass%): 0.12 - 0.45% C, 0.10 - 1.00% Si, 0.50 - 1.95% Mn, 0.005 - 0.060% S, in each case 0.004 - 0.050% Al and Ti, in each case up to 0.60% Cr, Ni, Co, W, Mo and Cu, up to 0.01% B, up to 0.050% Nb, 0.10 - 0.40% V, 0.015 - 0.04% N and the rest represented by iron and inevitable impurities, provided that the product of the contents in steel V and N leaves between 0.0021 and 0.0120, that the S content represented by the% indicator S, the Al content represented by% Al, the Nb content represented by% Nb, and the Ti content represented by% T i, meet the condition 1.6 x% S + 1.5 x% Al + 2.4 x% Nb + 1.2 x% Ti = 0.040 - 0.080% and the Mn content represented by% Mn, the Cr content represented by % Cr, Ni content represented by% Ni, Cu content represented by% Cu and Mo content represented by% Mo satisfy 1.2 x% Mn + 1.4 x% Cr + 1.0 x% Ni + 1.1 x% Cu + 1.8 x% Mo = 1.00 - 3.50%.

As essential here, it is considered that the necessary improvement in ductility is achieved by reducing the carbon content in the steel. A significant loss of strength according to the prior art is compensated by conventional alloying elements, the contents of which are coordinated so that hardening occurs due to the formation of a solid solution.

In addition, from DE 697 28 076 T2 (EP 0 787 812 B1) there is a known method for the production of forged steel parts, in which steel with a mass content of 0.1 to 0.4% C, 1 to 1.8% Mn, 0.15 - 1.7% Si, up to 1% Ni, up to 1.2% Cr, up to 0.3% Mo, up to 0.3% V, up to 0.35% Cu and in each optionally 0.005-0.06% Al, 0.0005-0.01% B, 0.005-0.03% Ti, 0.005-0.06% Nb, 0.005-0.1% S, up to 0.006% calcium, up to 0.03% Te, up to 0.05% Se, up to 0.05% Bi and with the rest represented by iron and unavoidable impurities, it is cast into molds to produce blanks that are hot-worked in a standard way with the aim of receiving forged parts. The forged part is then subjected to heat treatment, including cooling at a speed Vr of more than 0.5 ° C / s from the temperature at which the steel is austenitic to a temperature Tm between Ms + 100 ° C and Ms -20 ° C. The forged part is then aged for at least two minutes at a temperature between the temperature Tm and the temperature Tf, such that Tf> Tm -100 ° C. Thus, the aim is to obtain a steel component with a substantially bainitic structure containing at least 15% lower bainite and preferably at least 20% bainite formed at temperatures between Tm and Tf.

Practical tests performed with steel materials of the type described above showed that such bainitic steels are unsuitable for components that have significant changes in their cross section due to their tendency to deformation and significant fluctuations in mechanical properties.

In this regard, the object of the invention is to provide high-strength steel without the need for heat treatment by complex methods, exhibiting a slight tendency to deformation and which is therefore particularly suitable for the production of forged products with significant changes in the cross section along their length.

The objective of the invention is also to create a forged part having an optimal combination of properties without complicated methods of heat treatment.

Finally, the challenge is to propose a method of manufacturing a forged part, allowing simple means to create forged parts with an optimized combination of properties.

Regarding such steel, the invention solves the above problem with the structural steel specified in paragraph 1 of the claims.

As for the forged part, the solution to the above problem is to create a steel part made of steel according to the invention.

Finally, with regard to the method, this problem is solved in such a way that in the manufacture of the forged part the steps of the method described in paragraph 13 of the claims are implemented.

Preferred embodiments of the invention are presented in the dependent claims and will be further explained in detail together with the main idea of the present invention.

Structural steel according to the invention has a yield strength of at least 750 MPa, tensile strength of at least 950 MPa and at least 80 vol. % bainitic structure with the remaining 20 vol. % of the structure may be residual austenite, ferrite, perlite or martensite.

In this case, the steel according to the invention is characterized by a high elongation at break A of at least 10%, in particular at least 12%, while in practice it has been shown that the steels according to the invention usually achieve elongation at break of at least at least 15%.

Therefore, according to the invention, structural steel contains (in mass percent) up to 0.25% C, up to 1.5% Si, in particular up to 1% Si or up to 0.45% Si, 0.20 - 2.00% Mn, up to 4.00% Cr, 0.7 - 3.0% Mo, 0.004 - 0.020% N, up to 0.40% S, 0.001 - 0.035% Al, 0.0005 - 0, 0025% B, up to 0.015% Nb, up to 0.01% Ti, up to 0.10% V, up to 1.5% Ni, up to 2.0% Cu and the rest consisting of iron and inevitable impurities , despite the fact that the content in structural steel Al represented by% Al, the Nb content represented by% Nb, the Ti content represented by% Ti, soda zhanie V, representable exponent% V, and the content of N, represented by index% N, in each case correspond to the following condition:

% Al / 27 +% Nb / 45 +% Ti / 48 +% V / 25>% N / 3.75.

The inevitable impurities that appear during production include all elements that, in terms of the properties sought here, are present in quantities that do not affect the alloying method, and turn out to be in steel in connection with the method of its production or in accordance with the selected starting material ( scrap). Inevitable impurities also include, in particular, P in content up to 0.0035 mass. %

The steel according to the invention and the forged parts obtained from it can differ, in particular, by a uniform distribution of properties, even if, due to the changing dimensions of the components viewed in the transverse direction of the volume of the forged part, strongly prevail during cooling from the forging temperature in any specific locations different cooling conditions. This immunity to cooling conditions is achieved due to the fact that the structural steel according to the invention has a homogeneous, as much as possible exclusively bainitic structure with slight changes in hardness. This homogeneous microstructure at the same time has low internal stresses, which has a positive effect on the deformation behavior.

Accordingly, the steel according to the invention is particularly suitable for the production of forged parts in which sections with significantly different volumes and diameters collide with each other. Examples of such forged parts, for the production of which the use of steel forging technology according to the invention is particularly suitable, are represented by crankshafts, piston rods and other similar, intended, in particular, for internal combustion engines parts.

In addition, the details of the chassis and suspension components of the wheels with significantly different cross sections can be reliably made of steel according to the invention without post-processing by grinding, while providing specified signs of strength.

As can be understood from the attached as FIG. 1 of the temperature-time diagram of the steel according to the invention, from the material science point of view this means that in the case of structural steel according to the invention, a particularly wide window for bainitic processing can be used when the structural steel according to the invention is continuously cooled from the forging temperature. Thus, the alloying of structural steel according to the invention is selected so that during cooling in the structure neither martensite, nor ferrite, nor perlite are formed, affecting its properties. Structural steel according to the invention is thus characterized in that it is advantageous, i.e. up to at least 80 vol. %, has a bainitic structure, despite the fact that the content of non-bainitic structural components in the steels according to the invention is typically minimized to such an extent that the steel according to the invention in the technical sense has a completely bainitic structure.

Here, in the case of structural steel according to the invention, an almost constant hardness develops in bainite as much as possible regardless of the cooling rate. This constant hardness is the result of an almost complete transformation into bainite of what was previously austenite, preferably at the stage of bainitic transformation.

The restriction of the content of C to a maximum value of 0.25 mass. % means, on the one hand, that the structural steel according to the invention, despite its maximized strength, has good elongation and ductility properties. The low content in the steel according to the invention C is also involved in accelerating the bainitic transformation in order to avoid the development of undesirable structural components.

At the same time, however, a certain amount of carbon in the structural steel according to the invention can also contribute to strength. In this regard, a content in steel C of at least 0.09 masses can be provided. % Thus, the optimized effect of the presence in the steel according to the invention C can be achieved when the content of C is adjusted to a value of 0.09 - 0.25 mass. %

The content in the steel according to the invention Si is limited to 1.5 mass. %, in particular, 1 mass. % or 0.75 mass. %, in order to allow the earliest possible passage of bainitic transformation. For complete confidence regarding the achievement of this effect, the Si content can be limited to a maximum value of 0.45 mass. %

Mo is present in the structural steel according to the invention in an amount of 0.6 to 3.0 mass. % for the purpose of restraining the transformation of the structure into ferrite or perlite. This effect is manifested, in particular, if at least 0.7 masses are present in the steel. %, in particular, more than 0.70 mass. % Mo When the content is above 3.0 mass. % no further economically feasible increase in the positive effect of Mo in the steel according to the invention does not occur. In addition, when the amounts of Mo more than 3.0 mass. % there is a danger of the formation of a carbide phase enriched in molybdenum, which can adversely affect the plasticity properties. The optimal effect of the presence of Mo in the steel according to the invention can be expected if the Mo content is at least 0.7 mass. % Especially effective here was a maximum Mo content of 2.0 mass. %

Manganese is present in the steel according to the invention in an amount of 0.20 - 2.00 mass. % for the regulation of tensile strength and yield strength. To increase the strength, it is necessary to ensure a minimum Mn content of 0.20 mass. % If the goal is a particularly reliable achievement of this effect, then an Mn content of at least 0.35 mass can be provided. % An excessively high Mn content inhibits the bainitic transformation and, thus, the predominant martensitic transformation. Therefore, the Mn content is limited to a maximum of 2.00 mass. %, in particular, 1.5 mass. % The negative influence of the presence of Mn can, in particular, be reliably avoided by limiting the Mn content in the steel according to the invention to a maximum value of 1.1 mass. %

To maintain the workability of steel, the sulfur content in the steel according to the invention can be up to 0.4 mass. %, in particular, a maximum of 0.1 mass. % or a maximum of 0.05 mass. %

The fine-tuning of alloying technologies with respect to the mechanical properties and microstructure of the structural steel according to the invention takes place in accordance with the concept of alloying according to the invention through combined microalloying of elements such as boron in a concentration of 0.0005-0.0025 mass. %, nitrogen in an amount of 0.004 to 0.020 mass. %, in particular at least 0.006 mass. % N or up to 0.0150 mass. % N, aluminum with a content of 0.001 to 0.035 mass. %, Nb with a content up to 0.015 mass. %, titanium in an amount up to 0.01 mass. % and vanadium in an amount up to 0.10 mass. %

The contents of% Al,% Nb,% Ti,% V,% N and Al, Nb, Ti, V, N are related by the condition

% Al / 27 +% Nb / 45 +% Ti / 48 +% V / 25>% N / 3.75,

as a result, the nitrogen contained in the structural steel, due to the presence in the corresponding amounts of Al and the necessary amounts of the added Nb, Ti and V, is completely bound, and boron can thus exhibit its conversion-retarding effect. At the same time, the content of trace elements provided according to the invention and balanced with each other and with the N content, contribute to the increase in the stability of the fine-grained structure and strength.

The binding according to the invention N also allows boron to efficiently act as the element dissolved in the matrix and inhibits the formation of ferrite and / or perlite.

To ensure the reliable implementation of the advantages of using microalloying elements and aluminum, it may be advisable to set the Al content to at least 0.004 mass. %, Ti content equal to at least 0.001 mass. %, the content of V equal to at least 0.02 mass. % or Nb content of at least 0.003 mass. % Here, the microalloying elements V, Ti, and Nb, on the one hand, and Al, on the other hand, can in each case be present individually or in combination with one or more elements of the group Al, V, Ti, Nb in amounts exceeding the indicated minimum contents.

For a particularly effective manifestation of the effect of the presence of these elements in the structural steel according to the invention, they can be used in amounts up to 0.008 mass. % Ti, up to 0.01 mass. % Nb, up to 0.075 mass. % V or up to 0.020 mass. % Al. At the same time, the resulting nitrides or carbonitrides lead to an increase in strength and contribute to the stability of the fine-grained structure. Also, in order to achieve the optimum effect of this alloying element, in all cases, one should adhere to the indicated upper limits for the contents of Ti, Nb, V, or Al, individually or in combination with each other.

Optionally present Cr in concentrations up to 4.00 mass. %, in particular, up to 3 mass. % or up to 2.5 wt. % contributes to the durability and corrosion resistance of the steel according to the invention. In this regard, as an example, it is possible to provide at least 0.5 mass. % or at least 0.8 mass. % Cr.

Similarly, optionally present in concentrations up to 1.5 mass. % Ni can also contribute to the steel's ability to take quenching.

Alloying elements entering the steel according to the invention through the starting materials, or deliberately added alloying elements also include Cu, the content of which is limited to a maximum concentration of 2.0 mass in steel according to the invention in order to avoid negative effects. % The beneficial effect of the optionally present copper upon alloying the structural steel according to the invention consists in the formation of the thinnest films of residual austenite and in this connection a substantial increase in the level of ductility. This effect can be achieved when at least 0.3 mass. % Cu, in particular, more than 0.3 mass. % Cu is present in structural steel according to the invention. An optimized positive effect of the presence of copper can be achieved by limiting the Cu content to a maximum value of 0.9 mass. %

If the steel according to the invention is heated to heating temperatures typical of hot working and at least 100 ° C higher than the corresponding Ac3 temperature, in particular to a heating temperature of more than 900 ° C, then subjected to hot processing and final cooling in a controlled or uncontrolled mode in conditions of stationary or air flow to a temperature below 200 ° C, in particular, to room temperature, as a result a homogeneous bainitic structure is formed in an extremely wide range of cooling rates next after passing the conversion. The temperature Ac3 of the steel can be determined in a known manner based on its corresponding composition. The upper limit of the heating temperature range is typically 1300 ° C, in particular 1250 ° C or 1200 ° C.

The time t8 / 5, that is, the time required to cool the corresponding hot-worked part from a temperature of 800 ° C to 500 ° C, can be used as a parameter for estimating the range of cooling rates. To cool the components obtained from steel according to the invention, it is necessary that this time t8 / 5 be from 10 to 1,000 s.

The cooling time selected in each case should be selected based on the corresponding heating temperature. The effect of the heating temperature can be estimated from that shown in FIG. 2 is a temperature-time diagram showing the corresponding cooling time position of the corresponding bainitic range for heating temperatures of 900 ° C (solid line), 1100 ° C (dashed line) and 1300 ° C (dotted line). Accordingly, at lower heating temperatures of 900 ° C, a shorter time t8 / 5 should be chosen to achieve the desired bainitic structure, while at higher heating temperatures, cooling may be slower. High confidence that during the cooling of steel according to the invention, the range of bainite will be achieved regardless of the corresponding heating temperature, exists for steels according to the invention at heating temperatures in the range from 900 to 1300 ° C and, accordingly, if t8 / 5 is 100 - 800 s

Therefore, the alloying concept according to the invention makes it possible to have high hot working temperatures exceeding 1150 ° C, as a result of which the forces used during deformation during hot processing can be reduced without undesired manifestations of grain growth.

The method according to the invention, intended for the production of forged parts with a yield strength of at least 750 MPa, tensile strength of at least 950 MPa and at least 80 vol.% Bainitic structure, which may contain up to 20 vol.% Residual austenite, ferrite, perlite or martensite, contains the following process steps:

a) providing a forging intermediate containing structural steel with the composition according to the invention described above;

b) heating the forging intermediate to a forging temperature at least 100 ° C higher than the Ac3 temperature of the respective structural steel, while the Ac3 temperature is determined in a standard way depending on the composition of the corresponding structural steel;

c) forging into a forged part intended for forging a semi-finished product heated to the forging temperature;

d) cooling the forged part from the forging temperature to a temperature below 500 ° C, while the cooling time t8 / 5 is 10 -1000 s.

To reduce the deforming forces during the process according to the invention in terms of minimizing the necessary forging forces, it may be preferable if the corresponding intermediate product representing the starting point of the forging is heated to a forging temperature of more than 1150 ° C.

Further control of the mechanical properties, in particular, the strength and ductility of the hot-worked components, in particular, the forged steel components according to the invention, can be carried out by tempering, during which the corresponding part is kept for 0.5 - 2 hours at a temperature in the range from 180 to 375 ° C.

In practice, with a steel according to the invention, tensile strength of at least 950 MPa, yield strength of at least 750 MPa and elongation at break A of at least 15% can be reliably achieved, while in practice it is shown that even higher elongation A values of at least 17%. This combination of features in forged parts containing steel according to the invention is currently achievable, in particular if they are created by the method according to this invention.

The invention is further explained in more detail using examples of its implementation.

The steel melts E1 to E6 according to the invention and the comparative melt V1 with the compositions shown in Table 1 were smelted and cast as intermediates, which included blocks typically provided for further processing using forging techniques.

Such intermediates are heated to deform by forging to a temperature of Tw, then they are hot worked in a standard way using forging in dies to obtain forged parts and then cool them to room temperature in air. Some of the obtained forged parts are then tempered by heat treatment.

Table 2 shows the heating temperatures Tw used in the data in the examples, the time t8 / 5 required in each case to pass through the critical temperature range from 800 to 500 ° C, the temperature and duration of tempering by heat treatment, in cases where it was actually carried out, and the proportion of bainite content in the structure, tensile strength Rm, yield strength Re, elongation A, and fracture work during impact testing with notch W of the forged part obtained after such forging.

These examples show that, for the description of the present invention, forged parts can be obtained that allow for variation of the operating parameters set when they are created over a wide range of values and, at the same time, allow to obtain hot deformed components with optimized mechanical properties.

Claims (22)

1. Structural steel for the production of forged parts, having a yield strength of at least 750 MPa, tensile strength of at least 950 MPa and a structure consisting of at least 80 vol.% Bainite and a total of a maximum of 20 vol.% Residual austenite, ferrite, perlite and / or martensite, and the steel contains, wt.%: C 0.09-0.25 Si 0-1.5 Mn 0.20-2.00 Cr 0-4.00 Mo 0.6-3.0 N 0.004-0.020 S 0-0.40 Al 0.001-0.035 B  0.0005-0.0025 Nb 0-0.015 Ti 0-0.01 V 0-0.10 Ni 0-1.5 Cu 0-2.0 iron and inevitable impurities rest wherein the Al content expressed as% Al, the Nb content expressed as% Nb, the Ti content expressed as% Ti, the V content expressed as% V and the N content expressed as% N in this structural steel in each case correspond to the following condition: % Al / 27 +% Nb / 45 +% Ti / 48 +% V / 25>% N / 3.75. 2. Structural steel according to claim 1, characterized in that the content of C in it is at least 0.09 wt.%. 3. Structural steel according to claim 1 or 2, characterized in that the content of Al therein is at least 0.004 wt.%. 4. Structural steel according to any one of paragraphs. 1-3, characterized in that the maximum Al content in it is 0.020 wt.%. 5. Structural steel according to any one of paragraphs. 1-4, characterized in that the content of Nb is at least 0.003 wt.%. 6. Structural steel according to any one of paragraphs. 1-5, characterized in that the maximum content of Nb in it is 0.01 wt.%. 7. Structural steel according to any one of paragraphs. 1-6, characterized in that the content of Ti is at least 0.001 wt.%. 8. Structural steel according to any one of paragraphs. 1-7, characterized in that the Ti content in it is at most 0.008 wt.%. 9. Structural steel according to any one of paragraphs. 1-8, characterized in that the content of V in it is at least 0.02 wt.%. 10. Structural steel according to any one of paragraphs. 1-9, characterized in that the maximum content of V in it is 0.075 wt.%. 11. Structural steel according to any one of paragraphs. 1-10, characterized in that its elongation at break A is at least 10%. 12. Forged steel part made of structural steel according to any one of paragraphs. 1-11 and having a yield strength of at least 750 MPa, a tensile strength of at least 950 MPa and a structure consisting of at least 80 vol.% Bainite and a total of 20 vol.% Residual austenite, ferrite, perlite and / or martensite. 13. A method of manufacturing a forged steel part having a yield strength of at least 750 MPa, a tensile strength of at least 950 MPa and a structure consisting of at least 80 vol.% Bainite and a total of 20 vol.% Residual austenite, ferrite, perlite and / or martensite, comprising the steps of: a. obtaining an intermediate from structural steel according to any one of paragraphs. 1-11, b. heating the intermediate to a forging temperature of at least 100 ° C higher than the temperature Ac _{3 of} structural steel, c. forging a semi-product heated to a forging temperature, d. cooling the forged part from the forging temperature to a temperature below 200 ° C, and the cooling time t8 / 5 is 10-1000 s. 14. The method according to p. 13, characterized in that the forging temperature exceeds 1150 ° C. 15. The method according to p. 13 or 14, characterized in that after cooling the forged part is subjected to tempering at a temperature of 180-375 ° C with exposure for from 0.5 to 2 hours

Images