A method for producing a free spheroidizing tool steel based on thin strip casting

Abstract

The present application belongs to the field of steel material and short process manufacturing technology, and relates to a method for producing tool steel based on thin strip casting and rolling. The production method comprises the following steps: smelting, obtaining cast strip through thin strip casting and rolling, single-pass hot rolling, air mist cooling to 520-580 DEG C and coiling, and obtaining hot-rolled thin strip with a thickness of 0.6-1.5 mm. The present application utilizes the thin strip casting and rolling sub-rapid solidification and low-temperature coiling process to obtain hot-rolled state organization of extremely fine lamellar troostite with a lamellar spacing of 40-80 nm. The organization has extremely high interface energy, can be used as the initial organization of spheroidizing annealing, can omit the cold rolling process in the traditional process, can significantly shorten the spheroidizing annealing time to 4 hours or less, can greatly improve the production efficiency, and can reduce the energy consumption.

Description

Technical Field

[0001] This invention belongs to the field of steel materials and short-process manufacturing technology, and relates to a method for producing easily spheroidized tool steel based on thin strip casting and rolling, and more specifically to a method for producing easily spheroidized tool steel that does not require cold rolling and has a short spheroidizing annealing time based on thin strip casting and rolling. Background Technology

[0002] Tool steel is a key material for manufacturing various cutting tools, molds, and measuring instruments. Its final service state typically requires a spheroidal pearlitic microstructure to ensure good machinability, wear resistance, and moderate hardness. The traditional production process for high-carbon tool steel is lengthy, generally involving: hot continuous rolling into 2.5~4.0mm thick hot-rolled coils → pickling → multiple cold rolling passes to the target thickness (e.g., 1.0mm) → spheroidizing annealing (usually requiring 10~15 hours). The purpose of cold rolling is not only to reduce thickness but also to introduce a large number of crystal defects through significant plastic deformation, providing a nucleation driving force for the spheroidization of cementite during subsequent spheroidizing annealing.

[0003] However, this traditional approach has significant drawbacks: First, the cold rolling process requires large investments in equipment and consumes a lot of energy. Furthermore, for high-carbon steel, the cold rolling process is prone to problems such as strip breakage and edge cracking, resulting in a low yield. Second, the spheroidizing annealing time is extremely long, severely restricting production efficiency and consuming a large amount of energy. Although the industry has attempted to shorten the time by adjusting the annealing curve (such as using cyclic annealing), the effects have been limited and the problem remains largely unresolved.

[0004] Thin strip casting and rolling technology, as a short-process technology, has demonstrated its advantages in producing thin-gauge products. However, when applied to tool steel, simply pursuing annealing-free processes often fails to achieve the superior processing performance comparable to that of traditional spheroidized annealed products. Therefore, seeking an innovative solution that can utilize the advantages of short and low-cost thin strip casting and rolling processes while optimizing the process to obtain an initial microstructure that is easy to spheroidize quickly, thereby eliminating the need for cold rolling and significantly shortening annealing time, has become an important direction for the innovation of tool steel production processes. This invention is based on this idea, obtaining a special ultra-fine lamellar troostite microstructure through thin strip casting and rolling. This microstructure has extremely high interfacial energy, allowing it to be rapidly and uniformly spheroidized during subsequent annealing without the deformation energy stored in cold rolling. Summary of the Invention

[0005] In view of the problems existing in the prior art, the purpose of the present invention is to provide an easily spheroidized tool steel based on thin strip casting and rolling and its production method, so as to solve the technical problem that the traditional tool steel production process must rely on cold rolling process and the spheroidizing annealing time is too long.

[0006] Specifically, the technical solution adopted in this invention is as follows:

[0007] According to a first aspect of the present invention, a method for producing easily spheroidizing tool steel based on thin strip casting and rolling is proposed. The key lies in preparing an ideal initial microstructure for rapid spheroidizing annealing through thin strip casting and rolling combined with controlled cooling processes. The method includes the following steps:

[0008] (1) The molten steel obtained by smelting has the following chemical composition, expressed as a percentage by mass:

[0009] C: 0.60%~0.95%, Si: 0.17%~0.40%, Mn: 0.6%~1.2%,

[0010] Cr: 0.10~0.40%, P: ≤0.020%, S: ≤0.005%,

[0011] Mo: 0~0.30%, V: 0~0.20%, W: 0~0.50%, balance being Fe and unavoidable impurities;

[0012] (2) The molten steel is continuously cast through a twin-roll thin strip casting and rolling equipment to obtain a strip with a thickness of 1.3~2.4mm;

[0013] (3) The cast strip is hot rolled in a single pass to obtain a thin strip with a thickness of 0.6~1.5mm, wherein the reduction rate of the single pass hot rolling is 20%~60%, and the hot rolling exit temperature is controlled at 860~980℃;

[0014] (4) The hot-rolled strip is cooled to the winding temperature by an air mist cooling system, wherein the cooling rate of the air mist cooling is controlled at 55~80℃ / s;

[0015] (5) The thin strip cooled to the winding temperature is directly wound to obtain a hot-rolled thin strip coil. After winding, it is directly air-cooled to room temperature without online heat preservation. The winding temperature is controlled at 520-580℃.

[0016] The hot-rolled metallographic structure of the hot-rolled thin strip coil is extremely fine lamellar troostite without proeutectoid ferrite, with a troostite lamellar spacing of 40~80 nm; and

[0017] The aforementioned easily spheroidizing tool steel is a medium-high carbon low-alloy tool steel; and

[0018] The method described above does not require a cold rolling process; spheroidizing can be completed within 4 hours through direct spheroidizing annealing.

[0019] The aforementioned easily spheroidized tool steel is cold-stamped or cold-bent in the spheroidized annealed state.

[0020] According to the method for producing easily spheroidized tool steel based on thin strip casting and rolling of the present invention, preferably, in step (1), steel is produced by electric furnace or converter, and the steel is subsequently subjected to VD vacuum decarburization and deoxidation and LF furnace refining to obtain the molten steel.

[0021] According to a second aspect of the invention, an easily spheroidized tool steel is provided, which is produced using a method having one or more of the aforementioned features.

[0022] According to the present invention, the easily spheroidizing tool steel is preferably subjected to spheroidizing annealing directly without cold rolling, at an annealing temperature of 680℃~720℃ and a holding time of 1~4 hours. After annealing, the material can meet the requirements for direct cold bending and other processing.

[0023] According to the present invention, the easily spheroidizing tool steel is preferably first cold rolled and then subjected to spheroidizing annealing at a temperature of 680℃~720℃ for a holding time of 0.2~1 hour. After annealing, the material can meet the requirements for direct cold stamping, cold bending, and other processing.

[0024] Easy-spheroidizing tool steels contain the above-mentioned amounts of Mo, V, and W to further enhance their hardenability, wear resistance, or red hardness.

[0025] The core innovation of the method described in this invention lies in the fact that, through the sub-rapid solidification of thin strip casting and rolling combined with the specific hot rolling and controlled cooling process (especially relatively low-temperature coiling at 520~580℃), the hot-rolled microstructure is made into extremely fine lamellar troostite with a lamellar spacing of 40~80nm. This extremely fine microstructure implies a huge phase interface area and extremely high interfacial energy, placing it in a thermodynamically unstable state, providing an extremely strong driving force for the dissolution, splitting, and spheroidization of cementite during subsequent spheroidizing annealing. Therefore, the hot-rolled coil can be directly subjected to spheroidizing annealing without the traditional cold rolling process; or, after cold rolling, spheroidizing annealing can be completed in a very short time.

[0026] Beneficial technical effects

[0027] Compared with the prior art, the features and beneficial effects of the present invention include:

[0028] (1) Eliminating the cold rolling process and shortening the process: It completely avoids the problems of high carbon steel cold rolling difficulty and easy defects, simplifies the process from molten steel to thin-gauge tool steel, and reduces equipment investment and production costs.

[0029] (2) The spheroidizing annealing time is greatly shortened: the high interfacial energy of the ultrafine lamellar troostite structure is used as the driving force for spheroidization. Without cold rolling, the spheroidizing annealing time is shortened from more than 10 hours to less than 4 hours. After cold rolling, the spheroidizing annealing time can be shortened to less than 1 hour, which significantly improves production efficiency and greatly reduces energy consumption.

[0030] (3) Excellent product performance: The final obtained tissue surface is lightly decarburized, the spheroidized tissue is uniform and fine, and the processing performance is excellent, comparable to or even better than traditional long-process products.

[0031] (4) Green and environmentally friendly: The shortening of the process directly leads to a significant reduction in energy consumption (expected to be reduced by more than 40%), while reducing carbon dioxide and pollutant emissions, which is in line with the direction of green manufacturing. Attached Figure Description

[0032] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings of the embodiments will be briefly described below. Obviously, the drawings described below only relate to some embodiments of the present invention and are not intended to limit the present invention.

[0033] Figure 1 The metallographic structure of the easily spheroidizing tool steel produced in the hot-rolled state according to Example 1 of the present invention.

[0034] Figure 2 The metallographic structure of the hot-rolled coil of easily spheroidizing tool steel produced according to Embodiment 1 of the present invention after short-time spheroidizing annealing.

[0035] Figure 3 The metallographic structure of tool steel produced by conventional hot rolling process according to Comparative Example 1 is shown in the hot-rolled state.

[0036] Figure 4 The metallographic structure of the easily spheroidizing tool steel produced according to Example 2 of the present invention after cold rolling and short-time spheroidizing annealing. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments that can be obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0038] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0039] The present invention will be further illustrated below through specific embodiments 1-2 and comparative examples:

[0040] Example 1

[0041] (1) Smelting: Electric furnace smelting is adopted, followed by VD vacuum treatment and LF furnace refining to obtain molten steel with qualified composition.

[0042] The chemical composition of molten steel (by mass percentage) is as follows:

[0043] C: 0.82%, Si: 0.25%, Mn: 0.65%, Cr: 0.25%, P: 0.016%, S: 0.002%, balance Fe and unavoidable impurities.

[0044] (2) Thin strip casting and rolling: Qualified molten steel is injected into a twin-roll thin strip continuous casting machine and cast under nitrogen protection. The casting and rolling speed is set to 60m / min to obtain a casting strip with a thickness of 2.0mm.

[0045] (3) Hot rolling: The cast strip immediately enters the single-stand hot rolling mill for one-pass rolling with a reduction rate of 45% to roll the strip to a thickness of 1.1 mm. The mill exit temperature is 900℃.

[0046] (4) Cooling and coiling: After hot rolling, the strip enters the controlled cooling section and is cooled to 540°C at a cooling rate of 70°C / s before being coiled. The coiled steel coil is then air-cooled to room temperature.

[0047] Metallographic analysis of the obtained hot-rolled coil revealed its microstructure to be extremely fine lamellar troostite with an interlamellar spacing of approximately 65 nm. (See [link to relevant documentation]). Figure 1 .

[0048] (5) Spheroidizing annealing: The hot-rolled coils are directly loaded into the annealing furnace, heated to 720°C, held for 3.5 hours, and then slowly cooled to 600°C at a rate of 25°C / h. The coils are then removed from the furnace and air-cooled.

[0049] Properties after annealing: Hardness 86 HRB. Microstructure is a uniform, fine-grained spheroidized structure. See [reference needed]. Figure 2 As shown, the cold bending and stamping tests were passed.

[0050] Comparative Example 1 (Traditional Process)

[0051] In Comparative Example 1, molten steel with a composition similar to that of Example 1 was used and produced using a conventional thick slab continuous casting and rolling process: hot-rolled into 3.0 mm hot-rolled coils, the microstructure of which consisted of lamellar pearlite with a lamellar spacing of approximately 380 nm. (See [link to relevant documentation]). Figure 3 As shown.

[0052] The hot-rolled coil is first pickled, then cold-rolled to 1.7 mm, then annealed at 720°C, cold-rolled again to 1.1 mm, and then spheroidized annealed at 740°C. To obtain a qualified spheroidized structure (hardness ≤90 HRB), a total holding time of 15 hours is required. The entire process is much more time-consuming and energy-intensive than that of this invention.

[0053] Example 2

[0054] (1) Steel composition: C: 0.90%, Si: 0.30%, Mn: 0.55%, Cr: 0.35%, P: 0.018%, S: 0.003%, balance Fe.

[0055] (2) Thin strip casting and rolling: Qualified molten steel is injected into a twin-roll thin strip continuous casting machine and cast under nitrogen protection. The casting and rolling speed is set to 53m / min to obtain a casting strip with a thickness of 2.2mm.

[0056] (3) Hot rolling: The cast strip immediately enters the single-stand hot rolling mill for one-pass rolling with a reduction rate of 36% to roll the strip to a thickness of 1.4 mm. The mill exit temperature is 920℃.

[0057] (4) Cooling and coiling: After hot rolling, the strip enters the controlled cooling section and is cooled to 570°C at a cooling rate of 55°C / s before coiling. The coiled steel coil is then air-cooled to room temperature. Metallographic analysis of the obtained hot-rolled coil shows that its microstructure is extremely fine lamellar troostite with a lamellar spacing of approximately 80 nm.

[0058] (5) Cold rolling: The hot-rolled coils are pickled and cold-rolled to 0.5 mm.

[0059] (6) Spheroidizing annealing: The above cold-rolled coils are loaded into an annealing furnace, heated to 720°C, held for 0.5 hours, and then slowly cooled to 600°C before being removed from the furnace and air-cooled.

[0060] Results: Hardness after annealing was 88 HRB, and the microstructure showed complete spheroidization. (See [reference]). Figure 4 It meets the usage requirements.

[0061] The above embodiments and comparative examples demonstrate that the present invention, through the combination of thin strip casting and rolling with precise controlled cooling, successfully prepares an initial microstructure that is extremely easy to spheroidize, achieving the core objective of eliminating cold rolling and shortening annealing time, resulting in significant economic benefits.

[0062] The above description is only a specific embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for producing easily spheroidizing tool steel based on thin strip casting and rolling, characterized in that, The method includes the following steps: (1) The molten steel obtained by smelting has the following chemical composition, expressed as a percentage by mass: C: 0.60%~0.95%, Si: 0.17%~0.40%, Mn: 0.6%~1.2%, Cr: 0.10~0.40%, P: ≤0.020%, S: ≤0.005%, Mo: 0~0.30%, V: 0~0.20%, W: 0~0.50%, balance being Fe and unavoidable impurities; (2) The molten steel is continuously cast through a twin-roll thin strip casting and rolling equipment to obtain a strip with a thickness of 1.3~2.4mm; (3) The cast strip is hot rolled in a single pass to obtain a thin strip with a thickness of 0.6~1.5mm, wherein the reduction rate of the single pass hot rolling is 20%~60%, and the hot rolling exit temperature is controlled at 860~980℃; (4) The hot-rolled strip is cooled to the winding temperature by an air mist cooling system, wherein the cooling rate of the air mist cooling is controlled at 55~80℃ / s; (5) The thin strip cooled to the winding temperature is directly wound to obtain a hot-rolled thin strip coil. After winding, it is directly air-cooled to room temperature without online heat preservation. The winding temperature is controlled at 520-580℃. The hot-rolled metallographic structure of the hot-rolled thin strip coil is extremely fine lamellar troostite without proeutectoid ferrite, with a troostite lamellar spacing of 40~80 nm; and The aforementioned easily spheroidizing tool steel is a medium-high carbon low-alloy tool steel; and The method described above does not require a cold rolling process; spheroidizing can be completed within 4 hours through direct spheroidizing annealing. The aforementioned easily spheroidized tool steel is cold-stamped or cold-bent in the spheroidized annealed state.

2. The method for producing easily spheroidized tool steel based on thin strip casting and rolling according to claim 1, characterized in that: In step (1), steel is produced by electric furnace or converter and then successively undergoes VD vacuum decarburization and deoxidation and LF furnace refining to obtain the molten steel.

3. A type of easily spheroidizing tool steel, characterized in that, The easily spheroidizing tool steel is produced using the method described in any one of claims 1 to 2.

4. The easily spheroidizing tool steel according to claim 3, characterized in that: The easily spheroidized tool steel is directly subjected to spheroidizing annealing without cold rolling, with an annealing temperature of 680℃~720℃ and a holding time of 1~4 hours.

5. The easily spheroidizing tool steel according to claim 3, characterized in that: The easily spheroidized tool steel is first cold rolled, and then spheroidized annealed at a temperature of 680℃~720℃ for a holding time of 0.2~1 hour.

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