RU2735308C1 - Thermomechanical processing method - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, particularly, to thermomechanical treatment of sheet and section rolled stock from low- and medium-carbon structural steels. To increase mechanical and functional properties of articles and structures, method includes controlled non-recrystallization rolling and post-deformation cooling at arbitrary speed to ambient temperature, as well as repeated induction heating at a rate of more than 90 °C/s to temperature t = Ac₃ − (20–40) °C, holding at a given temperature for at least 10 minutes and subsequent accelerated cooling at a rate greater than 5 °C/s.

EFFECT: level of mechanical properties of the article can be adjusted by varying duration of holding at selected temperature and rate of subsequent cooling.

1 cl, 7 dwg

Description

The invention relates to metallurgy, in particular to a method for thermomechanical processing of sheet and long products made of low and medium carbon structural steels, and can be used to increase the structural strength of products (structures) made of them.

In the last decade, an increasing volume of metal products is made from new generation high-strength ultra-low-carbon building steels. Due to the high purity of harmful impurities and non-metallic inclusions, ultradispersed structure (d _s = 3 ... 5 microns), the absence of pearlite, balanced mechanisms of dispersion (due to precipitation of special MeC carbides) and aggregate (carbon-free bainite / martensite) strengthening of the ferrite matrix, such steels have complex high mechanical properties: σ _m> 550 MPa, σ _B> 650 MPa, δ> 25%, KCV ^-40> 2,5 MJ / m ² (Efron LI metallurgy in the "large" Tubular steel metallurgy. m. .: Metallurgy, 2012.696 p.).

Achievement of increased strength of these steels in combination with high ductility and cold resistance is possible due to the dispersion of the ferrite matrix and strengthening structural components, which is determined by the choice of optimal parameters of deformation-heat treatment (controlled rolling). In controlled rolling (CC), the temperatures of the beginning and end of hot deformation, its degree, fractionality, distribution over temperature regions, etc. are regulated.

The greatest dispersion of the structure occurs at non-recrystallization CP, when deformation ends in the austenitic region in the temperature range 900 ° C ... Ar ₃ , where Ar ₃ is the critical temperature corresponding to the onset of ferrite precipitation upon cooling. In this case, strong carbide-forming elements (Nb + Ti + V ≤ 0.15%) inhibit the recrystallization of austenite due to the release of carbonitride particles. Under such conditions, only the polygonization process takes place in austenite, which leads to the formation, upon cooling, of a large number of places for the formation of ferrite nuclei and strengthening structural components (bainite, martensite), and, accordingly, to the maximum refinement of the final structure.

A known method of thermomechanical processing of a thick sheet of low-carbon microalloyed steels (Pat. 2502820. Russian Federation, IPC C22C 38/14, C21D 8/02, C21D 9/46. Thick-walled high-strength hot-rolled steel sheet with excellent low-temperature impact toughness and the method of its production / Kami T., Nakata H., Nakagawa K. JFE STEEL CORPORATION, publ. 20.09.2013, bul. No. 26), according to which a high set of mechanical properties of sheets after controlled rolling is determined by the composition of steel and the parameters of accelerated post-deformation cooling (rate and temperature of the end of cooling).

However, a thick sheet or structural steel made from such steels has a number of disadvantages inherited from non-recrystallization CP:

- a high ratio of the yield strength to the ultimate strength σ _t / σ _at > 0.95, contributing to a reduced deformability of steels (a low level of relative elongation δ and narrowing ψ, an index of work hardening n, etc.);

- an increased tendency to deformation aging, which can cause embrittlement of the metal during installation and operation of the structure and, as a result, to its premature destruction;

- the presence of a layered structure responsible for the anisotropy of properties;

- the formation of splits in the metal of the products.

The reliability of structures during operation is determined to a large extent by the crack resistance of the metal, which is assessed by the results of special tests, for example, for static crack resistance, or by the margin of viscosity (KCV) and the temperature of ductile-brittle transition (t _xp ) during impact bending tests.

The known method of thermomechanical processing (Pat. 2519343. Russian Federation, IPC B21B 1/26, C21D 9/46, C21D 8/02, B21B 45/00, C21D 1/42. The method of thermomechanical treatment / Grill R., Egger R., Stingeder K. FESTALPINE GROBBLEH GMBH, publ. 10.06.2014, bul. No. 16), in which, to increase the toughness, a sheet of low-carbon steel is first subjected to controlled rolling at t> Ar₃ and subsequent accelerated cooling to a temperature slightly below Ar₁, and then re-induction heating in the austenitic region to t> Ac₃where Ac₃ - the critical temperature corresponding to the end of dissolution of ferrite during heating, and deformation in it.

The proposed technology was tested for thick sheets of low-carbon pipe steels of the 06G2M and 03G2S types, as a result of which a decrease in the ductile-brittle transition temperature of the metal by ~ 40 ° C was achieved. However, the presented results do not contain information on other mechanical properties and their anisotropy, as well as the tendency of the metal to strain aging and the appearance of splitting. In addition, the introduction of additional thermomechanical treatment (repeated CP) complicates the technological process - it is necessary to control the temperature of the beginning and end of deformation, its degree and fraction, distribution over temperature regions, and the method described above does not indicate the effect of these parameters on the complex of mechanical properties of the treated steels.

Thus, the tendency to deformation aging and the formation of splits, the anisotropy of mechanical properties due to the formation of a strip structure during CP led to the need to find methods for heat treatment of hot-rolled sheet in the mill line, the most promising of which seems to be heating to the region of elevated temperatures up to Ac ₃ , followed by accelerated cooling. ...

The closest in technical essence to the proposed method is a method of thermomechanical processing of heavy plate low-carbon steels, developed by JFE Steel Corporation (Pat. 2502820. Russian Federation, IPC C22C38 / 14, C21D8 / 02. Plate steel, characterized by a low ratio between the yield strength and ultimate strength, high strength and high uniform elongation, and the method of its manufacture / D. Simamura, N. Ishikawa, N. Shikanai JFE STEEL CORPORATION, publ. 12/27/2013, bul. No. 36), which proposes a method for reducing the ratio σ_t/ σ_in up to 0.85 and below.

The method consists in performing the following operations:

- heating the workpiece (sheet, pipe, etc.) to temperatures of 1000 ... 1300 ° C;

- controlled rolling with a temperature of its end in the range of 900 ° C ... Ar ₃ ;

- post-deformation cooling at a rate of more than 5 ° С / s up to temperatures of 680 ... 500 ° С;

- reheating at a rate of more than 2 ° C / s in the temperature range 550 ... 750 ° C;

- holding at a given temperature for τ ≤ 30 min;

- post-deformation air cooling.

The patent says that as a result of thermomechanical treatment according to the proposed scheme, sheets of low-carbon microalloyed steels of the 05G2S type have a low tendency to deformation aging. Thus, before and after heating at 250 ° C for 30 minutes, the yield stress level for all proposed formulations and processing conditions was over 517 MPa, the ratio of σ _r / σ _to not exceed 0.85, uniform elongation is not less than 6% a sufficiently high level of impact toughness of both the base metal and the heat-affected zone of the pipe.

However, the use of this method of thermomechanical treatment does not eliminate the following negative aspects:

- anisotropy of mechanical properties along and across the rolling direction;

- layered structure, leading to the possible formation of secondary (focal) cracks in the metal of products - splitting, and, consequently, to a decrease in resistance to stress cracking;

- to obtain products of a higher strength class without loss of deformability.

The technical problem solved by the proposed invention consists in the use of thermomechanical processing of metal products from low and medium-carbon steels according to such a technological scheme that will allow the formation of a homogeneous heterophase ultradispersed mixture of structural components in the metal and, thereby, will eliminate the anisotropy of mechanical properties and layer structure, and also the tendency to deformation aging, that is, to significantly increase the complex of its mechanical and functional properties.

The solution to this problem is ensured by using, after controlled rolling and subsequent cooling at an unregulated rate to a temperature not lower than the temperature M _n , where M _n is the critical temperature corresponding to the onset of martensitic transformation during cooling, repeated induction heating at a rate of more than 90 ° C / s to temperature t = Ac ₃ - (20 ... 40) ° C, holding at a given temperature for at least 10 minutes and subsequent accelerated cooling at a rate of more than 5 ° C / s. In this case, the level of mechanical properties of the product can be regulated by varying the holding time at t = Ac ₃ - (20 ... 40) ° C and the rate of subsequent cooling.

The invention is illustrated in the following drawings.

FIG. 1 shows a diagram of the proposed thermomechanical treatment, which includes heating 1 to temperatures of 1000 ... 1300 ° C, non-recrystallization controlled rolling 2 with an end temperature in the range of 900 ° C ... Ar ₃ , post-deformation cooling 3 in an ad hoc mode to a temperature not lower than the temperature M _n , repeated induction heating 4 at a rate of more than 90 ° C / s to a temperature t = Ac ₃ - (20 ... 40) ° C, holding 5 at a given temperature for at least 10 minutes and subsequent accelerated cooling 6 at a rate of at least 5 ° C / s.

Post-deformation cooling 3 of the product is carried out in an ad hoc mode, that is, at an arbitrary speed, for example, in calm air and / or by means of spray air cooling in order to accelerate the technological mode.

The temperature of the end of post-deformation cooling should not be lower than the temperature of the beginning of the martensitic transformation M _n , since in the structure, in addition to quasi-polygonal ferrite and bainite, a certain amount of unconverted austenite should be present.

The reheating temperature in the upper area of the intercritical temperature range t = Ac ₃ - (20 ... 40) ° С was selected based on the following considerations:

- heating below the temperature Ac ₁ , where Ac ₁ is the critical temperature corresponding to the transformation of pearlite into austenite upon heating, due to the absence of recrystallization does not lead to the removal of the layer structure, and, consequently, to the elimination of the anisotropy of mechanical properties; In addition, persisting in ferrite carbonitride particles - dislocations detents define high level σ _m / σ _in and do not contribute to the complete elimination of the effect of strain aging;

- heating to the lower region of the intercritical temperature range up to t = Ac ₁ + (20 ... 40) ° C leads to incomplete recrystallization of steel and a partial decrease in the anisotropy of properties, however, it is accompanied by significant embrittlement of the metal due to the grain-boundary effect: at the grain boundaries, carbon-saturated first austenitic grains that, upon subsequent cooling, at any rate turn into martensite; the appearance of such brittle components at the boundaries of the original grains leads to a change in the mechanism of destruction from intragranular to intergranular and a decrease in the energy intensity of the propagation of the main crack;

- heating above the temperature Ac ₃ leads to complete recrystallization and the absence of anisotropy of properties, but does not allow to eliminate the effect of strain aging.

Exposure 5 at a given reheating temperature should be at least 10 minutes in order to equalize the temperature across the thickness of the rolled product, as well as to form a certain amount of austenite and "old" ferrite refined with respect to carbon in the metal structure. An increase in the holding time leads to the coagulation of carbonitride particles, which makes it possible to regulate the level of strength properties of the final product by changing the contribution of precipitation hardening.

The rate of subsequent cooling 6 should be at least 5 ° C / s and is chosen such in order to obtain the highest strength class of rolled products at the required level of visco-plastic characteristics by varying the type and proportion of low-temperature decomposition products of supercooled austenite (bainite and / or martensite).

The action of the proposed method of thermomechanical treatment is shown on the example of low-carbon microalloyed steels of two manufacturers, type 08G2B, used for the production of high-pressure welded pipes with a diameter of 1420 mm, strength class X80. Sheets of these steels with a thickness of 27.7 mm were subjected to non-recrystallization controlled rolling with accelerated cooling in industrial conditions, and the blanks cut out of them were reheated at rates of 0.3-90 ° C / s in laboratory conditions to temperatures of 500-1000 ° C, holding within 30 minutes, followed by accelerated cooling (in oil).

Requirements for the mechanical properties of pipes of strength class X80 are regulated by the API-5L standard (API Spec 5L-2018, 46th edition, 2018. Pipes for pipelines. Specifications). In addition, more stringent consumer requirements are imposed on the viscosity of such steels. Thus, the level of impact toughness of a pipe with a diameter of 1420 mm of strength class X80, according to the standard, should be KCV ≥ 68 J / cm ² at a test temperature of 0 ° C, while at the request of OAO Gazprom - KCV ≥ 250 J / cm ² at t = –40 ° C (Technical requirements for the Bovanenkovo - Ukhta main gas pipeline. Approved by the Deputy Chairman of the Management Board of JSC GAZPROM A. G. Ananenkov on 23.05.2007).

FIG. 2-7 show the mechanical properties of steels X80 depending on the reheating temperature. Dark icons indicate the mechanical characteristics of steel from manufacturer 1, light icons indicate manufacturer 2, the requirements for mechanical properties are shown by dashed lines, the intercritical temperature range is highlighted by bold lines, while the critical temperatures Ac ₁ = 770 ° C and Ac ₃ = 915 ° C were determined with using dilatometric analysis for a heating rate of 90 ° C / s, which corresponded to the heating rate in the inductor.

FIG. 2 shows the dependence of the yield point σ _t0.5 on the heating temperature. According to API Spec 5L-2018, the yield point of X80 steels should be in the range 555 MPa ≤ σ _t0.5 ≤ 705 MPa. Steel 2 (σ _t = 660 MPa) meets these requirements, while steel 1 (σ _t = 730 MPa) is above the upper limit of this range. Heating up to 450 ° C does not change this picture. With an increase in the heating temperature, the value of σ _t0.5 falls and at t = 730 ° C it goes beyond the lower limit of the requirements, reaching a minimum at t = Ac ₁ + (20 ... 30) ° C. At a temperature of t = Аc ₃ - (20 ... 40) ° С, the level of σ _0.5 = 565-590 MPa with a small margin complies with the requirements of API Spec 5L-2018.

FIG. 3 shows the dependence of the ultimate strength σ _in on the heating temperature. According to API Spec 5L-2018, the tensile strength of X80 steels should be in the range of 625 MPa ≤ σ _to ≤ 825 MPa. In the state after CP, both steels have a corresponding level of σ _c . Heating up to temperature Ac ₁ does not change the situation. In this case, an increase in the value of σ _in by ~ 80-100 MPa at t ≥ 730 ° C does not lead to going beyond the required range. When the temperature t = Ac ₃ - (20 ... 40) ° C _in level σ = 740-770 MPa with a good margin meets API Spec 5L-2018.

FIG. 4 shows the dependence of the ratio of the yield stress to the ultimate strength σ _t0.5 / σ _in on the heating temperature. The value of σ _t0.5 / σ _in is one of the main indicators for assessing the tendency of steel to deformation aging. The appearance of this effect at σ _т0.5 / σ _в > 0.9 leads to significant strengthening, especially at the yield point, and a decrease in the visco-plastic properties of steels. According to the standard API Spec 5L-2018 steels X80 _t0,5 ratio σ / σ _a ≤ 0,93. From FIG. 4 it can be seen that in steels X80 in the state after CP, σ _t0.5 / σ _in → 1.0 is observed. Heating up to 700 ° C does not lead to a significant drop in this value, and only a further increase in temperature eliminates this effect. When the temperature t = Ac ₃ - (20 ... 40) ° C _t0,5 level σ / σ _in = 0,73-0,74 both steels corresponds request API Spec 5L-2018 and indicates a low tendency to strain aging and high deformability.

FIG. 5 shows the dependence of the relative elongation δ on the heating temperature. According to API Spec 5L-2018, the value of the relative elongation of pipe steels is determined by the cross-sectional area of the corresponding tensile test specimen and the pipe strength class and is δ ≥ 16% for X80 steels (during tension, standard cylindrical specimens with a working section diameter d _о = 10 mm). The relative elongation of both steels in the state after CP is 7-8% higher than the required level, and subsequent heating does not reduce this plasticity margin. At temperature t = Аc ₃ - (20 ... 40) ° С, the level of δ = 22-23% with a good excess corresponds to the API-5L requirement.

FIG. 6 shows the dependence of the impact strength KCV^-40 on the heating temperature. According to the technical requirements of Gazprom, the value of the impact strength at the test temperature t = –40 ° С should be KCV^-40 ≥ 250 J / cm²... Steel X80 in the state after KP have a high margin of viscosity (KCV^-40 = 290-300 J / cm²), which for both steels corresponds to the required minimum. Heating up to a temperature of 700 ° C practically does not change the KCV level^-40... At temperatures slightly above or below Ac₁ a sharp drop in impact toughness is observed, and with a further increase in the heating temperature, the values of KCV^-40 are restored, and at a temperature t = Аc₃ - (20 ... 40) ° С KCV level^-40 = 265-270 J / cm² meets API-5L requirements with a small margin.

FIG. 7 shows the dependence of the ovality d / D, which characterizes the anisotropy of properties, on the heating temperature, where d is the minor axis of the oval, D is the major axis of the oval, estimated in the neck of the specimen destroyed by tension. A low d / D value indicates a large ovality of the neck and, therefore, a significant anisotropy of properties along the rolling direction: the major axis D is along the rolling direction, and the small d axis is perpendicular to it. It can be seen that both steels in the state after CP have high anisotropy (d / D = 0.77-0.81). Heating up to a temperature of 700 ° C does not lead to anisotropy correction - the ovality d / D remains at approximately the same level. An increase in d / D corresponding to a decrease in anisotropy begins in the intercritical temperature range, and at a temperature t = Аc ₃ - (20 ... 40) ° С d / D → 1, which indicates the elimination of anisotropy of properties.

Thus, the proposed method for thermomechanical processing of sheet and long products made of low and medium-carbon structural steels makes it possible to improve the following functional properties of products (structures):

- to eliminate the effect of deformation aging, which increases the reliability of the structure;

- to minimize the anisotropy of the structure and mechanical properties;

- to regulate in a wide range the level of structural strength of steels - the ratio of strength and viscous-plastic characteristics, by varying the holding time during reheating and the rate of subsequent cooling;

- to simplify the technological process by eliminating the regulation of the cooling rate after controlled rolling.

Claims (1)

A method of thermomechanical processing of metal products from structural steels, including heating the billet to a temperature of 1000-1300 ° C, controlled rolling with a temperature of its end in the range of 900 ° C ... Ar ₃ , where Ar ₃ is the critical temperature corresponding to the onset of ferrite precipitation during cooling, and post-deformation cooling the product, and then repeated accelerated heating at a rate of more than 90 ° C / s, holding and cooling, characterized in that the post-deformation cooling of the product is carried out to a temperature not lower than M _n , where M _n is the critical temperature corresponding to the onset of martensitic transformation upon cooling, and its reheating is carried out to a temperature t = Ac ₃ - (20-40) ° C, where Ac ₃ is the critical temperature corresponding to the end of the dissolution of ferrite when heated, with holding at this temperature for at least 10 minutes and subsequent cooling at a rate of at least 5 ° C / s.

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