Method for producing a non-heat-treated tool steel for light pressings based on thin strip casting

Abstract

The present application belongs to the field of steel material and short process near-net shape manufacturing technology, and relates to a method for producing heat treatment free tool steel for shallow stamping based on thin strip casting and rolling. The production method comprises the following steps: smelting, obtaining 1.5-2.5 mm cast strip through thin strip casting and rolling, single pass hot rolling, air mist cooling to 680-740 DEG C and coiling, and then placing the steel coil in a holding pit for more than 4 hours. The present application utilizes the sub-rapid solidification characteristics of thin strip casting, combines controlled cooling and holding process, and directly obtains a mixed structure containing ferrite, fine lamellar sorbite with a lamellar spacing of 100-160 nm and not less than 30% granular carbide in the hot rolled state, so that the material has high strength and excellent plasticity, and can be directly used for shallow stamping forming without subsequent spheroidizing annealing, thereby greatly shortening the process, reducing energy consumption and cost.

Description

Technical Field

[0001] This invention belongs to the field of steel materials and short-process near-net-shape manufacturing technology, and relates to a method for producing heat-free tool steel for shallow stamping based on thin strip casting and rolling. Background Technology

[0002] Tool steel is widely used in the manufacture of stamping and cutting tools, requiring high strength and good cold-working plasticity. The traditional production process for tool steel is lengthy and energy-intensive. A typical process is: steel smelting → continuous casting into thick slabs → slab reheating → multi-pass hot rolling into coils → cold rolling → spheroidizing annealing. The microstructure obtained after hot rolling is mostly coarse lamellar pearlite (lamellar spacing often above 400 nm) or contains network cementite, resulting in high hardness and poor plasticity and toughness. This microstructure makes it difficult to perform any form of cold stamping (whether shallow or deep drawing).

[0003] Therefore, traditional processes must incorporate spheroidizing annealing before cold working to transform lamellar cementite into uniformly distributed granular carbides, thereby reducing hardness and increasing plasticity to facilitate subsequent cold working. However, spheroidizing annealing typically takes tens of hours or even longer, consumes a huge amount of energy, has high production costs, and easily leads to oxidation and decarburization on the steel surface, affecting product quality and yield.

[0004] Thin strip casting and rolling technology is a revolutionary short-process production technology that enables the direct casting of molten steel into thin strips of 1-3 mm thickness, followed by online hot rolling and coiling. This technology boasts advantages such as sub-rapid solidification (cooling rates up to 103°C / s), an extremely short process (only tens of seconds from molten steel to hot-rolled coil), and low energy consumption. The sub-rapid solidification characteristics of thin strip casting and rolling effectively refine the as-cast microstructure and reduce elemental segregation. The extremely short thermal history also helps suppress austenite grain growth and surface decarburization at high temperatures, while facilitating the subsequent acquisition of sorbite with extremely fine lamellar spacing. This extremely fine lamellar microstructure implies a large interfacial area and extremely high interfacial energy, providing a powerful driving force for the subsequent dissolution, splitting, and spheroidization of cementite.

[0005] However, how to utilize the aforementioned process characteristics of thin strip casting and rolling technology to directly obtain a microstructure suitable for a certain degree of cold stamping processing in the hot-rolled state through composition and process design, that is, to achieve spheroidizing annealing-free processing to replace the traditional long process of hot rolling + cold rolling + spheroidizing annealing, remains a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] In view of the problems existing in the prior art, the purpose of this invention is to overcome the shortcomings of the prior art and provide a heat-free tool steel for shallow stamping based on thin strip casting and rolling and its production method, so as to solve the technical problems of long production process, high energy consumption and reliance on spheroidizing annealing process for cold working of traditional tool steel, and realize short process and low cost manufacturing of tool steel for shallow stamping.

[0007] Specifically, the technical solution adopted in this invention is as follows:

[0008] According to a first aspect of the present invention, a method for producing heat-free tool steel for shallow stamping based on thin strip casting and rolling is provided, the method comprising the following steps:

[0009] (1) The smelting yields molten steel with the following chemical composition, expressed as a percentage by mass:

[0010] C: 0.45%~0.70%, Si: 0.20%~0.35%, Mn: 0.5%~1.0%,

[0011] Cr: ≤0.40%, P: ≤0.020%, S: ≤0.005%, 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 1.5~2.5mm;

[0013] (3) The cast strip is hot rolled in a single pass to obtain a thin strip with a thickness of 0.8~1.6 mm, wherein the reduction rate of the single pass hot rolling is 20%~50% and the hot rolling exit temperature is controlled at 860~950℃;

[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 10~30℃ / 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 680~740℃.

[0016] (6) The hot-rolled thin strip coil is placed in a heat preservation pit, wherein the temperature of the heat preservation pit is controlled at 250~400℃ and the placement time is controlled at not less than 4 hours. The metallographic structure of the hot-rolled thin strip coil is a mixture of ferrite, fine lamellar sorbite with a lamellar spacing of 100~160nm and granular carbides. The area percentage of the granular carbides is not less than 30%. The hot-rolled thin strip coil does not need to undergo subsequent heat treatment and is used for shallow stamping to produce tool parts.

[0017] According to the method of producing heat-free tool steel for shallow stamping based on thin strip casting and rolling according to the present invention, preferably, in step (1), steelmaking is carried out by converter steelmaking, and the steel is successively subjected to VD vacuum decarburization and deoxidation and LF furnace refining to obtain the molten steel.

[0018] According to the method of producing heat-free tool steel for shallow stamping based on thin strip casting and rolling according to the present invention, preferably, in step (5), the coiling temperature is controlled at 700~740℃.

[0019] According to the method of producing heat-free tool steel for shallow stamping based on thin strip casting and rolling according to the present invention, preferably, in step (6), the ambient temperature of the insulation pit is 300~400℃.

[0020] According to the method of producing heat-free tool steel for shallow stamping based on thin strip casting and rolling according to the present invention, preferably, in step (6), the yield strength of the hot-rolled thin strip coil is not higher than 650 MPa, the tensile strength is not higher than 850 MPa, and the elongation after fracture is not lower than 15%.

[0021] According to a second aspect of the invention, a heat-free tool steel for shallow stamping based on thin strip casting and rolling is provided, said heat-free tool steel for shallow stamping being produced using a method having one or more of the aforementioned features.

[0022] The core of the production method described in this invention lies in utilizing the sub-rapid solidification characteristics of thin strip casting and rolling to obtain fine initial austenite grains, which transform into lamellar sorbite with extremely fine cementite lamellae (100~160nm) during the subsequent cooling process. Simultaneously, by controlling the cooling rate after hot rolling and the coiling temperature (especially the relatively high-temperature coiling at 680~740℃), a certain amount of ferrite appears in the microstructure (ensuring the material matrix possesses a certain degree of plasticity). During the subsequent placement in the heat-insulating pit, some of the extremely fine lamellar cementite (meaning a larger phase interface area and interfacial energy) fragments and spheroidizes, ultimately forming a mixed microstructure composed of ferrite, fine lamellar sorbite, and a certain proportion of granular carbides directly in the hot-rolled state. This microstructure maintains high strength while exhibiting significantly better plasticity than traditional fully lamellar pearlite, allowing the material to possess a certain degree of cold stamping performance without traditional spheroidizing annealing treatment, and can be directly applied to shallow stamping applications of parts.

[0023] Beneficial technical effects

[0024] Compared with the prior art, the features and beneficial effects of the present invention include:

[0025] (1) The process is extremely simple and the cost is significantly reduced: This invention eliminates the cold rolling and spheroidizing annealing processes in the traditional process, and simplifies the production process to: thin strip casting and rolling → single-pass hot rolling → controlled cooling coiling → heat preservation pit placement. The production cycle is shortened from several days to hours, which greatly reduces equipment investment, energy consumption and labor costs.

[0026] (2) Excellent microstructure and properties, achieving heat treatment-free processing: Through the sub-rapid solidification of thin strip casting and rolling and the precise controlled cooling and heat preservation process, an ideal mixed microstructure of ferrite, fine lamellar pearlite and sufficient granular carbides is obtained directly in the hot rolled state, so that the material has both high strength and good plasticity, and can meet the requirements of shallow stamping without subsequent heat treatment.

[0027] (3) High product dimensional accuracy and good surface quality: The thin strip casting and rolling process is short and the thermal process is less, which effectively inhibits surface oxidation and decarburization, resulting in excellent product surface quality and high precision in plate shape and thickness.

[0028] (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

[0029] 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.

[0030] Figure 1 The image shows the metallographic structure of the heat-free tool steel produced according to Embodiment 1 of the present invention.

[0031] Figure 2 The image shows the metallographic structure of the tool steel produced using conventional processes according to Comparative Example 1 in its hot-rolled state. Detailed Implementation

[0032] 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 obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0033] 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.

[0034] The following are embodiments of the present invention. The described embodiments are only a part 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.

[0035] The present invention will be further illustrated below through specific embodiments 1-2 and comparative examples:

[0036] Example 1

[0037] (1) Smelting: The steel is smelted in a converter, and then refined in a VD vacuum treatment and LF furnace to obtain molten steel with qualified composition.

[0038] The chemical composition of molten steel (by mass percentage) is as follows:

[0039] C: 0.55%, Si: 0.25%, Mn: 0.70%, Cr: 0.15%, P: 0.015%, S: 0.002%, balance Fe and unavoidable impurities.

[0040] (2) Thin strip continuous casting: Qualified molten steel is injected into a twin-roll thin strip continuous casting machine, and the casting speed is 65m / min to obtain a casting strip with a thickness of 1.8mm.

[0041] (3) Hot rolling: The cast strip is immediately rolled in a single pass with a reduction rate of 47% to a thickness of 0.95 mm. The mill exit temperature is 870℃.

[0042] (4) Cooling and coiling: After hot rolling, the strip is cooled to 705°C at a cooling rate of 15°C / s and then coiled.

[0043] (5) Insulation treatment: The coiled steel coil is hoisted to an insulation pit (ambient temperature of about 350℃) and kept for 8 hours.

[0044] The obtained hot-rolled coils were subjected to performance testing and metallographic analysis. Their mechanical properties were: yield strength 540 MPa, tensile strength 770 MPa, and elongation after fracture 21%.

[0045] Metallographic structure such as Figure 1 As shown, the microstructure consists of a mixture of 16% ferrite, sorbite with a lamellar spacing of approximately 140 nm, and approximately 40% granular carbides. This strip can be directly subjected to 90° bending and shallow stamping tests without cracking.

[0046] Example 2

[0047] (1) Smelting: molten steel with qualified composition is obtained, with the following composition: C: 0.64%, Si: 0.28%, Mn: 0.80%, Cr: 0.20%, P: 0.018%, S: 0.003%, and the balance being Fe and unavoidable impurities.

[0048] (2) Thin strip continuous casting: to obtain a casting strip with a thickness of 2.0 mm.

[0049] (3) Hot rolling: single-pass rolling with a reduction rate of 40% to a thickness of 1.2 mm and a mill exit temperature of 860℃.

[0050] (4) Cooling and winding: Cool to 690°C at a cooling rate of 25°C / s and then wind up.

[0051] (5) Insulation treatment: The steel coil is kept in an insulation pit (ambient temperature of about 380℃) for 6 hours.

[0052] Performance test results: Yield strength 590 MPa, tensile strength 820 MPa, elongation after fracture 18%. Metallographic observation shows that the microstructure is a mixture of 20% ferrite, sorbite with an interlamellar spacing of approximately 115 nm, and approximately 35% granular carbides. It exhibits good stamping performance.

[0053] Comparative Example 1

[0054] Using molten steel with a composition similar to that of Example 1, the steel was produced via a conventional process: continuous casting into a 230mm thick slab → reheating to 1200℃ → rough rolling and finish rolling into a 3mm hot-rolled coil → air cooling after coiling. Its hot-rolled microstructure consisted of coarse lamellar pearlite. (See [reference needed]). Figure 2 As shown, the interlamellar spacing is approximately 380 nm, exhibiting high hardness but poor ductility and toughness, making direct cold stamping impossible. Two cold rolling processes and a total of over 24 hours of spheroidizing annealing are required to obtain a spheroidized structure suitable for stamping. The entire process takes 3-5 days, with energy consumption and costs far exceeding those of this invention.

[0055] 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 heat-free tool steel for shallow stamping based on thin strip casting and rolling, characterized in that, The method includes the following steps: (1) The smelting yields molten steel with the following chemical composition, expressed as a percentage by mass: C: 0.45%~0.70%, Si: 0.20%~0.35%, Mn: 0.5%~1.0%, Cr: ≤0.40%, P: ≤0.020%, S: ≤0.005%, 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 1.5~2.5mm; (3) The cast strip is hot rolled in a single pass to obtain a thin strip with a thickness of 0.8~1.6 mm, wherein the reduction rate of the single pass hot rolling is 20%~50% and the hot rolling exit temperature is controlled at 860~950℃; (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 10~30℃ / 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 680~740℃. (6) The hot-rolled thin strip coil is placed in a heat preservation pit, wherein the temperature of the heat preservation pit is controlled at 250~400℃ and the placement time is controlled at not less than 4 hours. The metallographic structure of the hot-rolled thin strip coil is a mixture of ferrite, fine lamellar sorbite with a lamellar spacing of 100~160nm and granular carbides. The area percentage of the granular carbides is not less than 30%. The hot-rolled thin strip coil does not need to undergo subsequent heat treatment and is used for shallow stamping to produce tool parts.

2. The method for producing heat-free tool steel for shallow stamping based on thin strip casting and rolling according to claim 1, characterized in that: In step (1), steelmaking is carried out in a converter, and the steel is then subjected to VD vacuum decarburization and deoxidation and LF furnace refining in sequence to obtain the molten steel.

3. The method for producing heat-free tool steel for shallow stamping based on thin strip casting and rolling according to claim 1, characterized in that: In step (5), the winding temperature is controlled at 700~740℃.

4. The method for producing heat-free tool steel for shallow stamping based on thin strip casting and rolling according to claim 1, characterized in that: In step (6), the ambient temperature of the insulation pit is 300~400℃.

5. The method for producing heat-free tool steel for shallow stamping based on thin strip casting and rolling according to claim 1, characterized in that: In step (6), the yield strength of the hot-rolled thin strip coil is not higher than 650 MPa, the tensile strength is not higher than 850 MPa, and the elongation after fracture is not lower than 15%.

6. A heat-free tool steel for shallow stamping based on thin strip casting and rolling, characterized in that, The shallow-drawing, heat-free tool steel is produced using the method described in any one of claims 1 to 5.

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