Method for refining and homogenizing crystal grains of 9%-12% Cr steel thick-wall large forge piece

Abstract

The invention discloses a method for refining and homogenizing crystal grains of a 9%-12% Cr steel thick-wall large forge piece, and belongs to the field of ferrous metallurgy. According to the method, the large forge piece is heated to be close to the critical point of the complete austenitizing temperature in a stepped mode, the large forge piece is subjected to heat treatment by regulating and controlling dispersive precipitation of carbides in an initial structure and uniform nucleation of austenite and / or introducing a small amount of undissolved ferrite, residual precipitated phases and multi-phase interfaces and controlling the cooling speed; nucleation particles of pearlite transformation in the vibration heat preservation process are remarkably increased, uniform distribution of granular pearlite in the supercooled austenite is promoted, structural inheritance is cut off and eliminated, and the purpose of grain refining and homogenizing after performance heat treatment is achieved. According to the method, the grains of the thick-wall large forge piece are refined to 5 grades or above, the cross section grain size difference is within 2 grades, a series of problems that in the production process, grass waves exist in ultrasonic flaw detection, the grain size is unqualified, and the low-temperature impact toughness does not reach the standard can be solved, the product percent of pass and the production efficiency are remarkably improved, and meanwhile the production energy consumption is reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of steel metallurgy and relates to a method for grain refinement of 9%-12% Cr steel thick-wall heavy forgings. BACKGROUND

[0002] In the production process of heavy forgings, due to the large size of the original ingot, the slow crystallization process, the abnormally coarse as-cast structure, the long forging process, the few heating times, the difficulty in guaranteeing the forging ratio and the uneven distribution of deformation, the slow heating speed and the long residence time in the high temperature zone, the heavy forging steel has a large genetic tendency, so that the internal structure of the heavy forging after forging is very coarse and unevenly distributed. The grain refinement after forging and performance heat treatment can be used to solve a series of problems such as unqualified low-temperature impact toughness or drop hammer impact, and the presence of grass-like waves in ultrasonic flaw detection affecting the quality of flaw detection.

[0003] 9%-12% Cr steel has good strength and toughness, heat resistance, corrosion resistance and excellent processability, and the main alloying elements are chromium, molybdenum, vanadium and other alloying elements. At present, it is widely used in heavy forgings for nuclear power, thermal power and hydroelectric power, such as F91 steel, FB2 and 10Cr9Mo1V, etc. The coarse grain or mixed grain tendency of this type of steel is large, which is a common problem in the heavy machinery industry. This is because the alloying element content of 9%-12% Cr steel is high, and the stability of the austenite undercooling state is high. Under the condition of slow quenching cooling speed, the heart of the forging can obtain martensite or lower bainite, which is beneficial to guarantee the consistency of the tensile strength and hardness of the cross section of the heavy forging. However, the thick-wall heavy forging of 9%-12% Cr steel is prone to form martensite or lower bainite structure after forging and cooling. Due to the certain crystallographic orientation relationship between the newly formed alpha ferrite phase and the gamma austenite parent phase, the organization genetic phenomenon is prone to occur during the subsequent preparation heat treatment or performance heat treatment austenitizing process. At present, the methods for eliminating the organization genetic phenomenon and their shortcomings are as follows: (1) adopting multiple normalizing methods, which requires multiple high-temperature heating and cooling cycles of grains, resulting in a large amount of energy waste and mixed grain phenomenon caused by improper control; (2) accelerating the heating rate of the heavy forging, using the austenite reverse process to form granular austenite to eliminate the organization genetic phenomenon, which has practical problems such as the power of industrial heating equipment and the heat transfer of the heart of thick-wall heavy forgings; (3) adopting traditional isothermal annealing method to obtain pearlite and other near equilibrium state organizations, which has problems such as high energy consumption, high comprehensive cost and unstable actual effect. Based on the pseudo-segregant eutectoid principle, a small amount of dispersed unsolved ferrite or carbide is constructed in the austenitizing process to increase the nucleation sites of pearlite transformation, promote the uniform distribution of granular pearlite structure in the undercooled austenite, cut off and eliminate the organization genetic phenomenon, and realize the grain refinement after performance heat treatment. SUMMARY

[0004] The purpose of the present application is to provide a method for grain refinement and homogenization of 9%-12% Cr steel thick-wall large forgings, which not only refines the grain of 9%-12% Cr steel thick-wall large forgings (wall thickness ≥ 450 mm) to 5 or more, and controls the difference in cross-sectional grain size to within 2, but also solves a series of problems such as grass wave, unqualified grain size, and substandard low-temperature impact toughness in ultrasonic flaw detection during production, greatly improves product qualification rate and production efficiency, and significantly reduces production energy consumption.

[0005] The technical scheme adopted by the present application is as follows:

[0006] A method for grain refinement and homogenization of 9%-12% Cr steel thick-wall large forgings, the forged large forgings are heated to T1 in steps, and a small amount of dispersed ferrite or carbide is introduced; then, the temperature is lowered to T2 at a certain cooling rate, and oscillation and heat preservation are carried out, the heat preservation time is calculated according to the time required to obtain more than 35% granular pearlite, and then the temperature is cooled to below 500℃, and then reheated for austenitizing to realize recrystallization and again refine and homogenize the grains.

[0007] The alloy composition of the 9%-12% Cr steel is as follows in terms of mass percentage: C: ≤0.15%, Si: ≤0.60%, Mn: 0.15%-0.90%, Cr: 8.5%-13.5%, Ni: ≤4.50%, Mo: 0.50%-4.50%, W: ≤2.50%, Co: ≤3.50%, Al: ≤0.06%, Nb: ≤0.15%, V: ≤0.50%, N: ≤0.15%, B: ≤0.020%; and the balance is Fe and unavoidable impurity elements.

[0008] The forged large forgings refer to workpieces with a single weight ≥ 5 t or a cross-sectional size ≥ 2000 mm 2 , including but not limited to forged large forgings for preliminary (forged) heat treatment, and are also applicable to secondary repair of large forgings due to coarse grains after quenching and tempering;

[0009] T1 is the critical point Ac3±30℃ of complete austenitization temperature, wherein Ac3 refers to the critical temperature point at which the forged state organization is completely converted into austenite organization;

[0010] Ensure that uniform and dispersed ferrite or carbide is left over during the austenitization process, the temperature of the large forgings before being loaded into the furnace is not higher than 500℃, then the large forgings are heated to Ac1-30℃-50℃ below at a temperature rising rate of not higher than 100℃ / h, the heat preservation time is calculated as 0.5h / 100mm-3h / 100mm (referring to the effective wall thickness of the forgings, the same below), after the heat preservation is completed, the temperature is raised to T1 at a furnace temperature rising rate of higher than 15℃ / h, the heat preservation time is calculated as 1.5h / 100mm-5h / 100mm, and if residual ferrite is introduced, the content should be not more than 10%;

[0011] The cooling speed is controlled in the range of 5 ℃ / h-30 ℃ / h, which is beneficial to in-situ precipitation and growth of carbides in supercooled austenite;

[0012] The T2 is a pearlite transformation nose temperature T0±30℃. Wherein T0 refers to, under the above austenitizing conditions, the shortest time corresponding to the temperature at which the thick-walled heavy forgings steel begins to occur pearlite transformation in the isothermal phase transition process, usually higher than the traditional isothermal annealing (austenitizing temperature above Ac3 80℃~150℃) by 10℃~50℃ than the pearlite phase transition nose temperature T0;

[0013] The oscillation fluctuation holding range is T2±35℃, and the wall thickness section temperature difference is ensured to be ≤25℃;

[0014] The re-heating austenitizing refers to heating the heavy forgings to Ac3 temperature above 100℃~180℃, and the holding time is calculated according to 1h / 100mm~3h / 100mm;

[0015] The heavy forgings wall thickness is ≥450mm.

[0016] The main grain refinement principle and design idea of the present application are completely different from the prior art, which is introduced as follows:

[0017] Currently, the main method for grain refinement of low-carbon medium-high alloy steel including 9%-12% Cr is to use post-forging heat treatment or pre-heat treatment before quenching and tempering, which usually includes multiple normalizing, continuous / isothermal (pearlite phase transformation region long time isothermal) annealing, etc., mainly based on multiple recrystallization or obtaining near-equilibrium state pearlite structure, cutting off and eliminating the principle of organizational inheritance. For example, multiple normalizing is adopted, and thick-walled large forgings need multiple temperature rising and falling processes. Due to the initial organizational inhomogeneity and stress difference of forgings under the wall thickness effect, although grain refinement can be achieved under certain conditions, there are problems of unstable grain refinement effect, serious coarse grain and mixed grain phenomenon for thick-walled large forgings. Using continuous slow cooling annealing method, due to the high content of Cr and Mo alloying elements in 9%-12% Cr steel, the undercooled austenite is particularly stable, even if a slower cooling rate is used, a certain proportion of pearlite structure can be obtained, but there will be a considerable proportion of martensite structure or bainite structure in the surface and even the core, which makes it difficult to ensure the consistency of the surface and core structure before quenching and tempering, and brings challenges to the recrystallization again during the subsequent quenching and tempering process; if the conventional isothermal annealing method is used, the forged structure is heated to above the Chernofer b point temperature to obtain recrystallized austenite, most of the carbides in the austenite have completely dissolved, and when cooled to the pearlite nose temperature and kept for a long time, the carbides are mainly precipitated in the form of ex situ precipitation, with small size, which is difficult to serve as the nucleation site for subsequent pearlite transformation, resulting in long pearlite transformation time (only the incubation period of the beginning of pearlite transformation is more than 10 5 days, and more than 1 month is needed for long-term. At present, considering the energy consumption and efficiency of isothermal annealing, the amount of pearlite or pseudo-pearlite obtained by traditional isothermal annealing is generally not more than 30%, and the distribution is uneven. After the recrystallization during the subsequent quenching process, a relatively good grain refinement effect can be obtained, but for thick-walled large forgings, the grain refinement is generally not more than 5 levels, and the grain uniformity of thick-walled forgings above 450 mm is difficult to control within 2 levels. At the same time, due to the composition inhomogeneity and uneven temperature distribution of thick-walled forgings, there is a certain deviation in the nose temperature T0 of pearlite transformation at different positions, resulting in a large difference in the degree of pearlite transformation at constant temperature T0 in large forgings, and introducing oscillation holding near T0 can avoid the above problems.

[0018] Therefore, the process of the present application is firstly to heat the large forging to the critical point near the complete austenitizing temperature (Ac3), to control the dispersion of carbides in the initial structure, uniform nucleation of austenite, and / or introduce a small amount of undissolved ferrite (isolated austenite region), residual precipitated phase and multi-phase interface, by controlling the subsequent cooling rate, the carbide fully occurs in-situ precipitation transformation, significantly increases the nucleation of pearlite transformation in the oscillation holding process, promotes the uniform distribution of granular pearlite structure in the undercooled austenite, cuts off and eliminates the genetic organization, and realizes the purpose of grain refinement after heat treatment. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 : Schematic diagram of heat treatment process of different examples or comparative examples; wherein, (a) Example 1, (b) Comparative Example 1, (c) Comparative Example 2;

[0020] Figure 2 : SEM photos of annealed structure obtained by heat treatment process of different examples 1 and comparative example 1; wherein, (a) Example 1; (b) Comparative Example 1;

[0021] Figure 3 : Grain size of forgings obtained after heat treatment of different examples or comparative examples; wherein, (a) Example 1; (b) Comparative Example 1, (c) Comparative Example 2. DETAILED DESCRIPTION

[0022] Example 1

[0023] In this embodiment, a vacuum induction + electroslag remelting method is used to smelt more than 6 tons of 9%Cr steel casting billets (diameter φ600mm), which are forged into shaft thick-walled large forgings (forging ratio ≥3) with a size of about φ700x3800mm by three-die three-drawing forging. The alloy composition (mass percentage) is as follows: C: 0.11%, Si: 0.35%, Mn: 0.45%, Cr: 8.65%, Ni: 0.20%, Mo: 0.92%, Al: 0.01%, Nb: 0.10%, V: 0.25%, N: 0.05%, B: 0.001%, and the rest is Fe and impurity elements. According to the method of GB / T 6394-2017, the grains in different parts of the large forging are rated, and the surface layer and the core are 0 level and 2.5 level respectively, and the grain inhomogeneity is greater than 2 level. The phase transition points Ac1 and Ac3 of the forging are measured by thermal expansion, which are 800℃ and 890℃ respectively, and the nose temperature T0 of the pearlite transformation of the material is 692℃. According to the process of the application, the large forging is cooled to 500℃ and then heated to 760℃ at a heating rate of 50℃ / h for 6 hours, and then heated to 880℃ at a furnace temperature of 50℃ / h for 15 hours, about 5% of dispersed ferrite and a small amount of MC carbide are introduced; then slowly cool to 690℃ at a speed of 20℃ / h, and oscillate at a temperature of 660℃-720℃ (the furnace temperature fluctuation is controlled within 20℃ / h, and the temperature difference of the forging is controlled within 15℃), the total oscillation time is 20 hours, about 65% of granular pearlite is obtained, and then cooled to 300℃ at a speed of 10℃ / h, and then heated to 1030℃ for 15 hours, and then air-cooled to room temperature, and the heat treatment process is as shown in Figure 1 .

[0024] Comparative Example 1

[0025] The process conditions of the comparative example are the same as those of example 1, except that in the comparative example, a traditional isothermal annealing process is used, the large forging is heated to 1030℃ at a rate of 50℃ / h for austenitizing and holding for 20 hours, and then isothermal annealing at 692℃ for 120 hours, and then air-cooled to room temperature to obtain the final product.

[0026] Comparative Example 2

[0027] The process conditions of the comparative example are the same as those of example 1, except that in the comparative example, a three-time normalizing process is used, the large forging is heated to 1070℃ at a rate of 50℃ / h for austenitizing and holding for 15 hours, and then air-cooled to room temperature to obtain the final product.

[0028] Figure 2The transformation of pearlite structure of the process and the comparative example 1 is shown in the figure. As can be seen from the figure, the transformation of pearlite is about 65% by using the process of the present application for isothermal transformation for 20 hours, the carbide in the pearlite is fully spheroidized, and the Vickers hardness is only 134 HV. However, by using the traditional annealing process, even if the isothermal time is as long as 120 hours, the transformation of pearlite is less than 30%, and the Vickers hardness is still about 170 HV. It can be seen that the process of the present application can significantly improve the transformation rate and spheroidization effect of pearlite.

[0029] Figure 3 The grain size of the forged heat treatment by using the process in the present application, the comparative example 1 and the comparative example 2 of (a), (b) and (c) is 8, 6.0 and 1.5 respectively. It can be seen that the grain size is finer under the condition of shortening the heat treatment process cycle by using the process of the present application. The yield strength of the F91 steel thick wall heavy forging is ≥425 MPa, the tensile strength is ≥585 MPa, the average impact toughness of C is ≥54 J, the minimum value is ≥48 J, and the non-ductile transition temperature (NDT) is ≤-25℃. o C average impact toughness ≥54 J, minimum value ≥48 J and non-ductile transition temperature (NDT) ≤-25℃ requirements.

[0030] Example 2

[0031] In this embodiment, the thick wall heavy forging of the intermediate pressure rotor of the steam turbine, the rotor length is 6 m, the diameter is 1200 mm, the grade is X12CrMoWVNb10-1-1 steel, and the alloy composition (mass percentage) is as follows: C: 0.13%, Si: 0.09%, Mn: 0.45%, Cr: 10.50%, Ni: 0.80%, Mo: 1.02%, Al: 0.008%, Nb: 0.055%, W: 0.98%, V: 0.18%, N: 0.05%, the rest is Fe and impurity elements. According to the method of GB / T 6394-2017, the grain of different parts of the heavy forging is rated, the surface layer, T / 8 wall thickness and core are 0, 1.5 and 2.0 respectively, and the grain inhomogeneity is more than 2. The phase transition point Ac1 and Ac3 of the forging are measured by thermal expansion, which are 808℃ and 910℃ respectively, the nose temperature T0 of the pearlite transformation of the material is 698℃. According to the process of the present application, the above rotor forging cooled to room temperature is heated to 775℃ at a heating rate of 25℃ / h, the heavy forging is heated to 895℃ at a furnace temperature rising rate of 30℃ / h for 25h, about 3% of dispersed ferrite and a small amount of (Fe, Cr) 23C6 and (Nb, V) (C, N) carbides, and then slowly cooled to 670℃ at 5℃ / h, then heated to 678-718℃ at 20℃ / h, and oscillated for 48h (the furnace temperature fluctuation is controlled within 20℃ / h, and the temperature difference of the forging is controlled within 15℃), about 55% granular pearlite is obtained, and then cooled to 300℃ at 8℃ / h, and then heated to 1030℃ for 28h, and then air cooled to room temperature.

[0032] According to the GB / T 6394-2017 method, the grain size of the above steam turbine intermediate pressure rotor thick wall large forgings is rated, and the above application process can refine the surface layer, T / 8 wall thickness and the core grain of the thick wall large forgings to 5.5, 6.5 and 6.0 levels respectively, which meets the requirements of grain size ≥ 5 levels and mixed grain size ≤ 2 levels at different positions of the thick wall large forgings.

[0033] Example 3

[0034] In this embodiment, the FB2 steel thick wall large forging of the ultra-supercritical unit is taken as an example, the rotor diameter is more than 1200mm, the length is 7500mm, and the alloy composition (mass percentage) is as follows: C: 0.13%, Si: 0.05%, Mn: 0.38%, Cr: 9.10%, Ni: 0.15%, Mo: 1.52%, Co: 1.22%, Al: 0.006%, Nb: 0.050%, V: 0.18%, N: 0.05%, B: 0.010%, and the rest is Fe and impurity elements. The phase transition points Ac1 and Ac3 of the rotor large forging material are measured by thermal expansion, which are 812℃ and 920℃ respectively, and the nose temperature T0 of the pearlite transformation of the material is 690℃. According to the process of the application, the large forging cooled to 150℃ after forging is loaded into the furnace, and then the large forging is heated to 775℃ at a heating rate of 15℃ / h for 12h, and then the temperature is increased to 935℃ at a rate of 30℃ / h for 30h, and a small amount of Laves phase or (Nb, V) (C, N) and the like is introduced into the austenite; then slowly cooled to 665℃ at 8℃ / h, then heated to 690℃ at 15℃ / h, ±25℃ oscillation for 60h (the temperature rising and falling rate is controlled within 10℃ / h, and the temperature difference of the forging is controlled within 20℃), about 35% granular pearlite is obtained, and then cooled to 300℃ at 12℃ / h, and then heated to 1090℃ for 16h, and then oil cooled to room temperature.

[0035] According to the GB / T 6394-2017 method, the grain size of the above steam turbine intermediate pressure rotor thick wall large forgings is rated, and the above application process can refine the surface layer, T / 8 wall thickness and the core grain of the thick wall large forgings to 5.5, 6.5 and 6.0 levels respectively, which meets the requirements of grain size ≥ 5 levels and mixed grain size ≤ 2 levels at different positions of the thick wall large forgings.50 ）。

Claims

1. A method for grain refinement and homogenization of 9-12% Cr steel thick wall heavy forgings, characterized by, The 9%-12% Cr steel large forging after forging is stepwise heated to T1 to introduce diffuse ferrite or carbide; then cooled to temperature T2 at a set cooling speed, oscillation holding, holding time being according to the time for obtaining more than 35% granular pearlite; after the holding ends, the forging is cooled to below 500℃, then reheated for austenitizing to realize recrystallization and again homogenize grains; Wherein, T1 is the critical point Ac3±30℃ of complete austenitizing temperature, and T2 is the pearlite transformation nose temperature T0±30℃.

2. The method for grain refinement and homogenization of 9-12% Cr steel thick wall forgings according to claim 1, characterized in that, The stepwise heating process is that the large forging is heated to 30℃-50℃ below Ac1 at a heating rate not higher than 100℃ / h, the holding time being calculated according to 0.5h-3h per 100mm effective wall thickness; after the holding ends, the temperature is raised to T1 for austenitizing at a heating rate higher than 15℃ / h, the holding time of this stage being calculated according to 1.5h-5h per 100mm effective wall thickness. If residual ferrite is introduced, the content is not more than 10%.

3. The method for grain refinement and homogenization of 9%-12% Cr steel heavy section forgings according to claim 2, characterized in that, The 9%-12% Cr steel has the following component composition by mass percentage: C: ≤0.15%, Si: ≤0.60%, Mn: 0.15%-0.90%, Cr: 8.5%-13.5%, Ni: ≤4.50%, Mo: 0.50%-4.50%, W: ≤2.50%, Co: ≤3.50%, Al: ≤0.06%, Nb: ≤0.15%, V: ≤0.50%, N: ≤0.15%, B: ≤0.020%; the balance being Fe and inevitable impurity elements.

4. The method for grain refinement and homogenization of 9-12% Cr steel heavy section forgings according to claim 2, characterized in that, The post-forged heavy forgings are workpieces with a single-piece weight ≥ 5 t or a cross-sectional size ≥ 2000 mm 2 The post-forged heavy forgings are workpieces with a single-piece weight ≥ 5 t or a cross-sectional size ≥ 2000 mm 5. The method for grain refinement and homogenization of 9-12% Cr steel heavy section forgings according to claim 2, characterized in that, The temperature of the large forging before the stepwise heating is not higher than 500℃.

6. The method for grain refinement and homogenization of 9-12% Cr steel heavy section forgings according to claim 2, characterized in that, The set cooling speed is 5℃ / h-30℃ / h.

7. The method according to claim 2, wherein the 9-12% Cr steel thick-walled heavy forge piece is grain refined and homogenized. The temperature range of the oscillation holding is T0±35℃, and the wall thickness section temperature difference is ensured to be ≤25℃.

8. The method according to claim 2, wherein the 9-12% Cr steel thick-walled heavy forge piece is grain refined and homogenized. The reheating for austenitizing process is that the large forging is heated to 100℃-180℃ above Ac3 temperature for holding, the holding time being calculated according to 1h-3h per 100mm effective wall thickness.

9. The method according to claim 2, wherein the 9-12% Cr steel thick-walled heavy forge piece is grain refined and homogenized, characterized in that, The wall thickness of the large forging is ≥450mm.

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