A heat treatment method for eliminating segregation band structure in high alloy steel

Abstract

The application discloses a heat treatment method for eliminating segregation banded structure in high alloy steel, comprising the following steps: (1) preparing medium-carbon high alloy steel: preparing medium-carbon high alloy steel hot-rolled plate according to the formula of medium-carbon high alloy steel; and finally cold-rolling and annealing into finished plate; (2) banded structure content grade determination: analyzing the microstructure of the high alloy steel cold-rolled and annealed plate prepared in step (1) to determine the banded structure content grade thereof; (3) heat treatment: performing heat treatment according to the banded structure content grade determined in step (2), and when the mixed crystal structure content is greater than 3, performing normalizing water cooling + tempering heat treatment. On the basis of the thin plate continuous casting and rolling process, the microstructure of the medium-carbon high alloy steel is creatively analyzed, and according to the banded structure grade in the steel, a heat treatment process is specifically adopted to reduce the banded structure, the banded structure content in the steel strip is significantly reduced, and the quality and competitiveness of the product are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel material heat treatment, and particularly relates to a heat treatment method for eliminating segregation banded structure in high alloy steel. BACKGROUND

[0002] The medium-carbon high alloy steel is difficult to produce and has high technical content, which is due to the high Cr content. When the Cr content is high (≥5%), the medium-carbon high alloy steel is more likely to form the first type of carbide such as Cr23C6 and Cr7C3 with complex lattices. Compared with the second type of carbide such as TiC, VC, W2C and Mo2C with simple lattices, the first type of carbide has relatively low hardness, melting point and poor stability, and is easy to dissolve into austenite when heated. If the dissolution temperature of the carbide is in the austenite-ferrite two-phase region, more carbon will enter the austenite to promote the growth of the austenite, and the ferrite is poor in carbon and relatively slow in growth, so that the grain growth presents a double-peak structure, which is not conducive to the uniformization of the structure and significantly reduces the quality and performance of the cold-rolled steel strip.

[0003] When the high alloy steel is hot-rolled or annealed, the segregation of alloying elements or the segregation of carbides is easy to occur. If the element segregation exists in the hot-rolled plate, the segregation of carbides is more likely to occur in the subsequent annealing, because the alloying elements will adsorb the diffusing carbon to form carbides, and a part of the carbides will be precipitated during the annealing in the two-phase region to form the segregation band of carbides, so that the nucleation position of the austenite is not uniform. The nucleation position is more in the dense area of the carbides, the pinning effect is strong, the austenite interface migration rate is low, the nucleation position of the ferrite boundary austenite is less in the local area, the pinning of the carbides is weak, the ferrite interface migrates fast, and the inconsistency of the interface migration occurs, and then the mixed crystal phenomenon occurs. Therefore, the selection of a reasonable heat treatment process determines the structure of the product, and then affects the performance of the product.

[0004] Through retrieval, the document with the Chinese patent application number CN 110565018 A discloses a control method for improving the banded structure of low-carbon high alloy gear steel in the annealed state. The method is suitable for the gear steel with the following composition: C: ≤0.15-0.20, Mn: 0.50-0.70, Si: 0.20-0.40, S: ≤0.005, P: ≤0.015, Cr: 1.50-1.70, Mo: 0.25-0.35, Ni: 1.35-1.65, Cu: 0.01-0.05, Al: 0.02-0.05, and the balance of Fe and incidental impurities. The document has the following disadvantages: it only aims at weakening the banded structure of the low-carbon high alloy gear steel, can stably control the banded structure of the low-carbon high alloy gear steel in the annealed state at level 2.5, and does not completely eliminate the banded structure, and does not involve the banded structure of the medium-carbon high alloy steel.

[0005] Through retrieval, the document with Chinese patent application number CN 113265513 A discloses an annealing process for eliminating banded structure of 25 steel cold-rolled strip. The method is suitable for 25# carbon steel with C: 0.22-0.29, Mn: 0.50-0.80, Si: 0.17-0.37, S: ≤0.035, P: ≤0.035, Cr: ≤0.25, Ni: ≤0.30, Cu: ≤0.25, and the rest being Fe and other trace impurity elements. 1) The 25 steel cold-rolled coil heated to a process temperature of 630-670℃ is annealed under the protection of nitrogen and hydrogen for 9-11 hours; 2) The 25 steel cold-rolled coil is covered for 3 hours and cooled to 600℃ under the protection of nitrogen and hydrogen; 3) The wind cooling is stopped, the 25 steel is lifted away from the heating cover, and the 25 steel is naturally cooled to 105℃ and discharged from the furnace in 16-18 hours. The document has the following disadvantages: the process temperature is directly given without indicating that the temperature is within the range of AC1 and AC3, and it is not instructive. The content of alloying elements is low, and the alloying element segregation is less, which can be seen from the 3 hours of cover annealing. The document does not involve the elimination of banded structure of medium-carbon high-alloy steel.

[0006] Through retrieval, the document with Chinese patent application number CN 115198063 A discloses a method for reducing banded structure of medium-high carbon tool steel. The method is suitable for C: 0.6%, Si: 0.25%, Mn: 0.7%, S: ≤0.008%, P: ≤0.015%, Cr: 0.8%, Mo: 0.25%, and the rest being Fe and unavoidable impurities. The normalizing heat treatment method is as follows: the thin plate medium-high carbon tool steel is heated to 870-900℃ and kept for 50-60min, and then cooled at a speed of 0.032-0.035℃ / s. The quenching + tempering heat treatment method is as follows: the thin plate medium-high carbon tool steel is heated to 900-930℃ and kept for 60-70min, and then cooled to room temperature at a speed of 2-4℃ / s, and finally tempered at 400-450℃ for 30-40min. The document has the following disadvantages: the annealing time of the two elimination methods is short, which is suitable for carbon segregation, and it is difficult to eliminate alloying element segregation, and the content of alloying elements in the document is low, which is not suitable for alloying element segregation in high-alloy steel. SUMMARY

[0007] In order to solve the problems existing in the prior art, the purpose of the present application is to provide a heat treatment method for eliminating segregation banded structure in high-alloy steel.

[0008] To solve the above technical problems, the present application adopts the following technical scheme:

[0009] The heat treatment method for eliminating segregation banded structure in high-alloy steel comprises the following steps:

[0010] (1) preparing the thin plate of medium carbon high alloy steel: preparing the hot-rolled plate of medium carbon high alloy steel according to the formula of the medium carbon high alloy steel; and finally cold-rolling and annealing into the finished plate;

[0011] (2) determining the content grade of banded structure: analyzing the microstructure of the high alloy steel cold-rolled and annealed plate prepared in step (1) to determine the content grade of banded structure contained therein;

[0012] (3) heat treatment: performing heat treatment according to the content grade of banded structure determined in step (2), when the content of mixed crystal structure is greater than 3, performing normalizing water cooling + tempering heat treatment;

[0013] The chemical components and weight percentage of the medium carbon high alloy steel are as follows: C: 0.30-0.32%, Si: 0.28-0.30%, Mn: 2.02-2.05%, P≤0.012%, S≤0.003%, Cu:≤0.02%, Ni: 0.67-0.75%, Cr: 4.09-4.12%, Mo: 1.22-1.3%, V: 0.35-0.37%, and the balance is Fe and incidental inclusions.

[0014] Further, in step (1), the thin plate continuous casting and rolling includes continuous casting, cooling, soaking treatment, hot rolling, annealing and cold rolling in sequence.

[0015] Further, the continuous casting speed is 4-6 m / min, and the continuous casting thin slab with a thickness of 57 mm is obtained.

[0016] Further, the continuous casting thin slab is put into a soaking furnace at 1000-1200℃ and is kept for 20-40 min.

[0017] Further, the starting temperature of hot rolling is 1150-1200℃, and the final rolling temperature is 880-920℃.

[0018] Further, the final thickness of the hot-rolled plate is 4.0 mm, the first annealing temperature is 760℃, and the holding time is 30 hours; the final thickness of the first cold-rolled plate is the second annealing temperature is 760℃, the holding time is 18 hours, and the final thickness of the cold-rolled plate is 1.70 mm.

[0019] Further, in step (3), the method of heat treatment is as follows: heating the cold-rolled plate of medium carbon high alloy steel to 830-850℃, keeping for 18 hours, then water cooling, and tempering at 400-450℃ for 1 hour.

[0020] Compared with the prior art, the present application has the following beneficial technical effects:

[0021] The application provides a method for reducing segregation banded structure of medium-carbon high-alloy steel, based on a thin-plate continuous casting and rolling process, creatively analyzes microstructure of the thin-plate medium-carbon high-alloy steel, and according to the banded structure grade of the steel, adopts a heat treatment process to reduce the banded structure, so that the content of the banded structure in the steel strip can be significantly reduced, and the quality and competitiveness of the product are improved. BRIEF DESCRIPTION OF DRAWINGS

[0022] The application will be further described below in combination with the drawings.

[0023] Figure 1 Example 1: Schematic diagram of the whole production process of high-alloy steel

[0024] Figure 2 Example 1: Microstructure grain boundary diagram of a thickness of 4.0 mm before annealing

[0025] Figure 3 Example 1: Microstructure grain boundary diagram of a thickness of 4.0 mm after cold rolling and annealing

[0026] Figure 4 Example 1: Microstructure grain boundary diagram of a thickness of 2.8 mm after cold rolling and annealing

[0027] Figure 5 Example 1: Microstructure grain boundary diagram of a thickness of 1.70 mm after cold rolling and annealing

[0028] Figure 6 Example 1: Microstructure grain boundary diagram of a thickness of 1.70 mm after heat treatment DETAILED DESCRIPTION

[0029] The application will be further described below in combination with the drawings.

[0030] Example 1

[0031] (1) For the medium-carbon high-alloy steel, the formula thereof is designed to include the following chemical components in percentage by weight: C: 0.30-0.32, Si: 0.28-0.30, Mn: 2.02-2.05, P≤0.012, S≤0.003, Cu:≤0.02, Ni: 0.67-0.75, Cr: 4.09-4.12, Mo: 1.22-1.3, V: 0.35-0.37, and the balance is Fe and incidental impurities.

[0032] The thin-plate continuous casting and rolling includes continuous casting, cooling, soaking treatment, hot rolling, annealing and cold rolling in sequence.

[0033] (2) the continuous casting speed is 4-6 m / min, and a continuous casting thin slab with a thickness of 50-60 mm is obtained; the soaking process conditions are that the continuous casting thin slab is put into a soaking furnace at 1000-1200 ℃, and the holding time is 20-40 min; and the starting temperature of rolling is 1150-1200 ℃, and the final rolling temperature is 880-920 ℃.

[0034] (3) the final thickness of the hot-rolled plate is 4.0 mm, the first annealing temperature is 760 ℃, and the holding time is 15 hours; the final thickness of the first cold-rolled plate is 2.8 mm, the second annealing temperature is 760 ℃, and the holding time is 18 hours; and the final thickness of the cold-rolled plate is 1.70 mm.

[0035] In the step (3), the normalizing heat treatment method is that the second annealing temperature of the medium-carbon high-alloy steel cold-rolled plate is heated to 830-850 ℃ which is 30-50 ℃ higher than AC3, the holding time is 18 hours, and then water cooling and tempering at 400-450 ℃ for 1 hour.

[0036] (4) the strip-shaped segregation and mixed crystal problems in the cold-rolled annealing process can be solved by using the above normalizing heat treatment method; the hot-rolled microstructure is bainite structure, the microstructure after annealing and cold-rolling is ferrite and granular carbide, the microstructure of the coarse grain zone after cold-rolling and annealing with a thickness of 2.8 mm is ferrite zone and granular bainite, the microstructure of the fine grain zone is ferrite matrix with dense carbide distributed for pinning grain boundary movement, and the grain is fine and uniform; the thickness of the cold-rolled and annealed plate is 1.70 mm, obvious segregation bands appear, dense carbide particles are distributed on the segregation bands, and the size of the granular bainite structure is uneven; the segregation bands are completely eliminated in the microstructure of the cold-rolled and annealed plate with a thickness of 1.70 mm, and the size of the granular bainite structure is uniform and well-distributed.

[0037] The full-process production process of the medium-carbon high-alloy steel in the above embodiment 1 is shown in Figure 1 The final thickness of the hot-rolled plate is 4.0 mm, and the microstructure diagram before annealing is shown in Figure 2 The microstructure grain boundary diagram of the thickness of 4.0 mm after annealing is shown in Figure 3 The microstructure grain boundary diagram of the thickness of 2.8 mm after cold-rolling and annealing is shown in Figure 4 The microstructure grain boundary diagram of the thickness of 1.70 mm after cold-rolling and annealing is shown in Figure 5 The microstructure grain boundary diagram of the thickness of 1.70 mm after cold-rolling and annealing is shown in Figure 6 The microstructure grain boundary diagram of the thickness of 1.70 mm after cold-rolling and annealing is shown in

[0038] The above-described embodiments are only used to describe the preferred modes of the present application, and do not limit the scope of the present application, and various modifications and improvements to the technical solutions of the present application made by those skilled in the art without departing from the design spirit of the present application shall fall within the protection scope of the present application defined by the claims.

Claims

1. A heat treatment method for eliminating segregation banding in high alloy steels, characterized in that: The method comprises the following steps: (1) preparing a medium-carbon high-alloy steel sheet: preparing a medium-carbon high-alloy steel hot-rolled sheet according to a formula of the medium-carbon high-alloy steel; and finally cold-rolling and annealing the hot-rolled sheet into a finished sheet; the thin sheet continuous casting and rolling comprises continuous casting, drawing, cooling, soaking treatment, hot rolling, annealing, cold rolling, annealing and cold rolling in sequence; the final thickness of the hot-rolled sheet is 4.0 mm, the first annealing temperature is 760 DEG C, and the soaking time is 30 hours; after the first cold rolling, the second annealing temperature is 760 DEG C, the soaking time is 18 hours, and the final thickness of the cold-rolled sheet is 1.70 mm; (2) determining the content grade of banded structure: analyzing the microstructure of the high-alloy steel cold-rolled and annealed sheet prepared in step (1) to determine the content grade of banded structure contained therein; (3) heat treatment: performing heat treatment according to the content grade of banded structure determined in step (2); when the content of banded structure is greater than grade 3, normalizing and water cooling + tempering heat treatment is performed; Specifically, the process of "second annealing temperature 760 DEG C, soaking time 18 hours" in step (1) is replaced by "normalizing and water cooling + tempering heat treatment, heating to 830-850 DEG C, soaking for 18 hours, then water cooling, and tempering at 400 DEG C-450 DEG C for 1 hour". The chemical components and weight percentages of the medium-carbon high-alloy steel are as follows: C: 0.30-0.32%, Si: 0.28-0.30%, Mn: 2.02-2.05%, P: ≤0.012%, S: ≤0.003%, Cu: ≤0.02%, Ni: 0.67-0.75%, Cr: 4.09-4.12%, Mo: 1.22-1.3%, V: 0.35-0.37%, and the balance being Fe and incidental inclusions.

2. The heat treatment method for eliminating segregation band structure in high alloy steel according to claim 1, characterized by: The continuous casting drawing speed is 4-6 m / min, and a continuous casting thin slab with a thickness of 57 mm is obtained.

3. The heat treatment method for eliminating segregation band structure in high alloy steel according to claim 1, characterized by: The continuous casting thin slab is put into a soaking furnace at 1000-1200 DEG C and is soaked for 20-40 min.

4. The heat treatment method for eliminating segregation band structure in high alloy steel according to claim 1, characterized by: The starting temperature of hot rolling is 1150-1200 DEG C, and the final rolling temperature is 880-920 DEG C.

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