RU2641429C1 - Method to increase strength of stable austenitic steel - Google Patents

Abstract

FIELD: metallurgy.SUBSTANCE: stable austenitic steel 02X17H14M3 is subject to austenite tempering, then rolling with cooling in liquid nitrogen at -196°C in several passes to form microtwin structures and subsequent deformation in the temperature range of 20-700°C in several passes to provide propagation of deformation localization bands in the microtwin structure and the substructural hardening by the high density of dislocations. After deformation, final annealing is carried out in the range of 600-800°C with duration from 200 s to 1 h.EFFECT: improved strength properties of steel while preserving plasticity due to production of structure with and submicro-and nano-sized fragments.3 dwg, 1 tbl, 3 ex

Description

The invention relates to the field of iron metallurgy, and more particularly to a change in the mechanical properties of stable austenitic steel by successive plastic deformation with increasing temperature and annealing. The initial deformation is carried out after cooling the samples in liquid nitrogen, the next in the temperature range T = 293 - 973 K (20 - 700 ° C). Final annealing is carried out in the temperature range of 873 - 1073 K (600 - 800 ° C) lasting from 200 s to 1 hour. The proposed method can be used in the pressure treatment of workpieces and parts of highly loaded steel structures.

A known method of thermomechanical processing of steels of austenitic class (RF patent No. 2525006, IPC C21D6 / 00, publ. 08/10/2014). In this method, the steel is subjected to a treatment consisting of a combination of plastic deformation in the temperature range T = 673 - 973 K in two stages and tempering at T = 673 - 873 K. In this case, the total degree of true strain e> 2.5. The disadvantage of this method is the use of relatively high degrees of deformation at high temperatures.

A known method of cryogenic-deformation processing of steel, including hardening, plastic deformation at cryogenic temperatures in several stages with a total degree of deformation of 50-90%, low-temperature tempering after each stage at a temperature of 220 - 270 ° C and high-temperature tempering at the final stage of processing billets (patent RF number 2394922, IPC C21D8 / 00, publ. 07.20.2010). The disadvantage of this processing method is the high (> 30%) volume fraction of martensite in the steel structure.

A known method of processing steels, which consists in grinding the microstructure by plastic deformation, characterized in that the treatment is carried out in the austenitic region in the temperature range of 1000 - 400 ° C in one or more stages with a stage-by-stage regulated decrease in temperature to obtain the final grain size D_cr. One of the applications of this processing method is the deformation of austenitic and austenitic-ferritic stainless steels at temperatures below M_D. Then re-deformation is carried out in the range of 600 - 800 ° C; as well as a new deformation cycle at a temperature below M_D and heating to a temperature of reverse transformation of 600 - 650 ° C (RF patent No. 2181776, IPC C21D8 / 00, publ. 04/27/2002). The disadvantage of this processing method is the use of a significant number of technological operations and a high total total degree of deformation specified by the sample, which imposes a restriction on the dimensions of the workpieces.

The closest in technical essence the solution chosen as a prototype is a method for thermomechanical processing of metastable austenitic steel (RF patent No. 2598744, C21D8 / 00, C21D6 / 04, C21D6 / 00, C21D7 / 13, published on 09.27.2016). In this method, austenitic steel is subjected to low-temperature deformation with cooling in liquid nitrogen, subsequent thermal deformation in the temperature range T = 673 - 973 K (400 - 700 ° С) and annealing in the temperature range 873 - 1073 K (600 - 800 ° С) with a duration from 200 s to 1 h. A significant difference between this technical solution is the use of metastable austenitic steel, in which the ratio of the elements Cr / Ni ≈ 18/10. This steel during the low-temperature deformation undergoes intense martensitic (γ → α ′) - transformation with the formation of ≈ 50-80% α′-martensite. In the process of warm deformation and annealing (at T ≥ 600 ° С), the reverse (α ′ → γ) transformation is realized, which leads to a significant increase in the strength of steel. However, the martensite content under the conditions of the proposed regimes of thermomechanical treatments of metastable austenitic steel is 5-60%. A decrease in the martensite content (less than 5%) leads to a decrease in the strength of metastable austenitic steel.

The objective of the invention is to develop a method of thermomechanical treatment (TMT) of stable austenitic steel, which allows to increase its strength properties at relatively low degrees of deformation while maintaining a high (96-99.5%) volume fraction of austenite.

The technical result of the invention is that:

 - the proposed thermomechanical treatment regimes make it possible to obtain a structure with a high density of deformation microtwins packages (over several twinning systems) and submicro- and nanosized fragments formed during the propagation of deformation localization bands in microtwins in stable austenitic steel (with a Cr / Ni ≈ 17/14 ratio) packages, as well as a developed dislocation substructure with significantly less (up to 4%), compared with the prototype, the content of α′-martensite;

 - high strength properties are achieved through the implementation of mechanisms of structural and substructural hardening;

 - relatively small (e <1) degrees of deformation, a limited number of technological operations and relatively low processing temperatures contribute to more uniformity of the resulting structure and provide high economic efficiency of this method.

To solve this problem, a method for thermomechanical processing of stable austenitic steel is proposed, including plastic deformation by rolling with cooling of the workpiece to a temperature of liquid nitrogen (T = 77 K). Before low-temperature deformation, the preform is austenitized at a temperature of 1373 K (1100 ° С) for 1 h. The first stage of deformation is carried out by low-temperature rolling in several passes until the true deformation is e = 0.1 - 0.4. Between the passes, the workpiece is kept at a temperature of liquid nitrogen (T = 77 K). The second stage of deformation is carried out by rolling in the temperature range T = 293 - 973 K (20 - 700 ° C) with the number of passes from one to several until the true deformation is reached e ≤ 0.6. After deformation at elevated temperatures, the workpiece is cooled into water. To stabilize the obtained structural states and increase ductility, annealing is carried out in the temperature range T = 873–1073 K (600–800 ° C) with a holding time of 200 s to 1 h, followed by cooling in air.

The invention is illustrated by drawings.

FIG. 1. The microstructure of steel after TMO-3. (a) the microdiffraction pattern, the axis of the matrix zone <110>, two families of microdoubles with the axis of the zones <110>, (b) the dark-field image in the matrix reflection g = [200], (c) the dark-field image in the reflection g = [ 11] twins of one system, (g) - dark-field image in the reflection g = [1 1] twins of another system, (e) - bright-field image of a site with a high density of microtwins, (e) - dark-field image in the twin reflex of the same region.

FIG. 2. Steel microstructure after TMT-3: (a) microdiffraction pattern in the region of the strain localization band lying in the microdouble structure, a set of three axis axes of <110> austenite zones and individual reflections of α′-martensite, (b) bright field image ( approximate band boundaries are indicated by hatching), (c), (d) - the combined dark-field images in reflections (110α ′ + 110γ) are indicated by arrows in the diffraction pattern.

FIG. 3. The microstructure of the steel after TMT-3 and annealing at 800 ° C, 200 s: (a), (b) - dark-field images in the 200γ reflection at goniometer tilt angles of -13 ° - (a) and -10 ° - (b), the projection of the tilt axis is perpendicular to the reflex, (c) the microdiffraction pattern k (a), (d) the bright field image of the microdouble structure.

The technical result achieved is confirmed by the data shown in table 1.

Mechanical tensile tests were carried out according to GOST 1497-84 at room temperature. Structural studies were performed by transmission electron microscopy. The phase content in the steel was studied by X-ray phase analysis and specific magnetization measurements.

Studies of the features of the microstructure and phase composition of steel showed that low-temperature deformation leads to intense mechanical twinning with the formation of micro twin packages. After cooling, deformation in liquid nitrogen with a degree of ≥ 10% leads to the formation of α′-martensite, the content of which is 0.5–10% at specified degrees of true deformation e = 0.1 - 0.4, with a higher degree of deformation corresponding to a higher content of martensite. Continued deformation at room temperature promotes more intense twinning with an increase in the density of microtwins, the formation of nanotwins inside primary twins and a developed dislocation substructure. The specified structural state provides an increase in strength properties with satisfactory values of elongation (table 1).

Table 1. Phase content, yield strength, tensile strength and elongation during tensile tests at T = 20 ° C.

Plastic deformation in the temperature range T = 400 - 700 ° C (after low-temperature deformation) contributes to a decrease in the martensite content, which is 0.5-3.7%. In the process of electron microscopic studies, it was shown that under the conditions of such treatments, the micro-twin structure of the steel is retained (Fig. 1), and localization bands of deformation propagating in the micro-twin packets (Fig. 2) were found. As a result of the interaction of deformation localization bands with microtwins, submicro- and nanoscale fragments are formed with small and high-angle disorientation boundaries. A distinctive feature of the strain localization bands is their two-phase structure, represented by fragments of austenite and α′-martensite. In the diffraction pattern (Fig. 2a), martensitic reflections corresponding to nanoscale fragments of this phase are found in the region of the bands. Austenitic fragments are represented by a set of orientations of the three axis axes <110>, which have high-angle misorientations between them ≈ 60-70 ° <110> (Fig. 2a). Inside the localization band, in addition to high-angle, there are many small-angle misorientations of austenite fragments.

Additional annealing after thermomechanical treatments can increase ductility, reduce the volume content of martensite to minimum (0.5%) values while maintaining a high level of strength properties (table 1). During annealing, partial recrystallization of fragments of deformation localization bands is observed, the sizes of which increase slightly, remaining within the submicrocrystalline scale, while the packet micro twin structure is preserved. In FIG. Figure 3 shows dark-field images of submicrocrystalline austenite fragments at various tilt angles of the goniometer, which reveal a small-angle boundary between the fragments (Fig. 3 a, b), and a bright-field image of a microdouble structure.

The proposed TMT options make it possible to increase the yield strength of steel by ≈3-3.8 times in comparison with the initial value. The properties achieved in this case are comparable with the prototype - metastable austenitic steel 08X18H10T after thermomechanical treatments. However, in the prototype, a close level of strength is achieved with a significantly higher 5-60% volumetric content of α′-martensite, a high-strength phase. Approximately 5-15 times lower martensite content allows increasing the degree of specified deformation both during low-temperature rolling (up to e ≤ 0.4) and subsequent deformation in the temperature range T = 20 - 700 ° C (up to e ≤ 0.6). An increase in the strength of stable austenitic steel is achieved due to the formation of a structure with a high density of strain micro twin packages and strain localization bands with a fragmented submicro- and nanoscale structure with a minimum α′-martensite content.

Examples of specific embodiments of the invention are given below.

Example 1

A sheet of 2.5 mm thick steel 02Kh17N14M3 of industrial production was austenitized at 1100 ° C for 1 h. Samples of dimensions 35 × 15 × 2.5 mm were cooled in liquid nitrogen until the “boiling” ceased, after which they were placed on a rolling mill. The strain ε ≈ 10% was achieved in 4 passes; between the passes, the samples were cooled in liquid nitrogen. After low temperature deformation, the samples were warmed to room temperature. Subsequent deformation was carried out at room temperature for 5 passes until the degree of deformation ε ≈ 50% was reached. The phase content and the achieved strength properties are presented in table 1 - TMO-1.

Example 2

A sheet of 11 mm thick industrial steel 02X17H14M3 was austenitized at 1100 ° C for 1 h. Samples 30 × 20 × 11 mm in size were cooled in liquid nitrogen until the “boiling” ceased, after which they were placed on a rolling mill. The strain ε ≈ 20% was achieved in 3 passes; between the passes, the samples were cooled in liquid nitrogen. After low-temperature deformation, the samples were warmed to room temperature and placed in an oven at a temperature of T = 600 ° C. Upon reaching the indicated temperature, the samples were rolled in two passes until the degree of deformation ε ≈ 40% was reached. Upon exiting the mill, the samples were cooled in water. The structural state of the steel is shown in FIG. 1 and 2, while the content of austenite is 98.5-99.5%. The achieved strength properties are presented in table 1 - TMO-3.

Example 3

As in example 2, only after deformation was annealing in an inert gas medium at a temperature of T = 800 ° C for 200 s followed by cooling in air. The structural state of the steel is shown in FIG. 3 and is characterized by packages of microdoubles and fragments of a submicrocrystalline scale, with an austenite content of 99-99.5%. The achieved strength properties are presented in table 1 - TMO-3 + 800 ° C, 200 s.

The advantages of the invention include high strength characteristics achieved without the use of intensive plastic deformation methods with a limited number (up to three) of technological operations and a significantly lower (5-15 times) α′-martensite content compared to the prototype with comparable strength and plastic properties . These results indicate the high efficiency of the proposed method for increasing the strength of stable austenitic steel.

Claims (1)

   A method for thermomechanical processing of stable austenitic steels, including hardening for austenite, plastic deformation and final annealing, characterized in that plastic deformation of the steel is carried out by rolling with cooling in liquid nitrogen at a temperature of -196 ° C and with a degree of deformation e = 0.1 - 0, 4 in several passes with holding between passes at the mentioned temperature, and then deformation is carried out by rolling in the temperature range 20–700 ° C with a degree of logarithmic deformation e ≤ 0.6 in one or more passes with subsequent cooling in water, and the final annealing is carried out at 600–800 ° C for a duration of 200 s to 1 h.

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