A composite die steel plate and a manufacturing method thereof

Abstract

The present application provides a kind of composite die steel plate and its manufacturing method, the composite die steel plate includes core layer, upper surface layer and lower surface layer, core layer includes: C:0.25~0.4, Si:0.1~0.8, Mn:0.4~1, Cr:1.5~2.5, Mo:0.1~0.8, Al:0.01~0.05, Ti:0.001~0.06, and Ni:0.01~2, RE:0.01~0.1, Nb:0.01~0.05, V:0.01~0.5 at least one kind;Upper surface layer and lower surface layer include: C:0.4~0.55, Si:0.1~0.5, Mn:0.4~1, Al:0.01~0.05 respectively.The composite die steel plate of the embodiment of the present application reduces the content of valuable alloy elements, reduces production cost.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, specifically to a composite mold steel plate and its manufacturing method. Background Technology

[0002] Die steel is a type of steel used to manufacture molds such as cold stamping dies, hot forging dies, and die casting molds. The development of die steel has driven industrial products towards higher quality, greater diversity, personalization, and higher added value. Die steel should possess high hardness, strength, wear resistance, sufficient toughness, and high hardenability, quenchability, and other processing properties.

[0003] In order to meet the requirements of mold steel, Cr and Mn are usually added during production. When manufacturing high-carbon steel parts with high hardness and high toughness, a certain amount of alloying elements such as Mn, W, Al, Ni, and Si are usually added to the high-carbon steel.

[0004] Related Patent 1: Patent Application No. 200410010656.8, Hot Work Die Steel. This patent discloses a hot work die steel with the following chemical composition: C: 0.2%–0.35%, Cr: 7%–12.224%, Mo: 0.8%–2%, Ni: 0.6%–1.5%, V: 0.3%–1.2%, Mn: 0.2%–0.6%, Si: 0.7%–1.5%, S≤0.04%, P≤0.04%, N: 0.005%–0.1%, Ti: 0.05%–0.2%, Ca: 0.001%–0.05%, with the balance being Fe. However, this composition has a very high alloy content, especially for Cr. Although Cr can increase the hardenability of die steel and improve its strength and hardness, an excessively high Cr content makes the steel prone to secondary hardening during heat treatment and reduces its toughness and impact resistance.

[0005] In addition, composite methods can be used to produce mold steel. Compared with traditional ingots, composite methods can improve the internal structure and properties of rolled products, and the range of rolled billets is wide, which is conducive to mass production and can improve the yield.

[0006] Related Patent 2: Patent Application No. 201410337336.7, a rolling method for mold steel composite steel plate. This patent discloses that the content of alloying elements other than C, Si, Mn, P and S in the mold steel composite steel plate is greater than 9%. That is to say, the alloying element content of the mold steel composite steel plate is greater than 9%. The high content of alloying elements is not conducive to the diffusion between atoms. For the new type of composite mold steel, it is not conducive to the diffusion and recrystallization of each composite layer, and the processing is difficult. Summary of the Invention

[0007] In view of this, the purpose of the present invention is to provide a composite mold steel plate with low alloy element content and high strength and high hardness.

[0008] The present invention also provides a method for manufacturing composite mold steel plates.

[0009] According to a first aspect of the present invention, the composite mold steel plate includes a core layer and an upper surface layer and a lower surface layer respectively laminated to the upper and lower sides of the core layer, wherein the composition and thickness of the upper surface layer and the lower surface layer are independent of each other;

[0010] The core layer, by weight percentage, comprises at least one of the following: C: 0.25%–0.40%, Si: 0.10%–0.80%, Mn: 0.40%–1.00%, Cr: 1.50%–2.50%, Mo: 0.10%–0.80%, P≤0.030%, S≤0.010%, Al: 0.010%–0.050%, Ti: 0.001%–0.060%, and Ni: 0.01%–2.00%, RE: 0.01%–0.10%, Nb: 0.01%–0.050%, and V: 0.01%–0.50%.

[0011] By mass percentage, the upper and lower surface layers respectively comprise: C: 0.40%–0.55%, Si: 0.10%–0.50%, Mn: 0.40%–1.00%, P ≤ 0.030%, S ≤ 0.010%, and Al: 0.010%–0.050%.

[0012] According to another embodiment of the present invention, the composite mold steel plate includes a core layer and an upper surface layer and a lower surface layer respectively laminated to the upper and lower sides of the core layer, wherein the composition and thickness of the upper surface layer and the lower surface layer are independent of each other;

[0013] The core layer comprises, by weight percentage, at least one of the following: C: 0.25%–0.40%, Si: 0.10%–0.80%, Mn: 0.40%–1.00%, Cr: 1.50%–2.50%, Mo: 0.10%–0.80%, P≤0.030%, S≤0.010%, Al: 0.010%–0.050%, Ti: 0.001%–0.060%, and Ni: 0.01%–2.00%, RE: 0.01%–0.10%, Nb: 0.01%–0.050%, and V: 0.01%–0.50%, with the balance being Fe and unavoidable impurities;

[0014] The upper and lower surface layers, by mass percentage, are composed of: C: 0.40%–0.55%, Si: 0.10%–0.50%, Mn: 0.40%–1.00%, P≤0.030%, S≤0.010%, Al: 0.010%–0.050%, with the balance being Fe and unavoidable impurities.

[0015] Furthermore, the microstructure of the core layer is bainite and retained austenite, with the volume percentage of retained austenite ≤ 5%.

[0016] Furthermore, the Rockwell hardness of the core layer is 27HRC to 37HRC.

[0017] Furthermore, the thickness of the core layer is 20mm to 200mm.

[0018] Furthermore, the microstructure of the upper and lower surface layers is tempered troostite, respectively.

[0019] Furthermore, the Brinell hardness of the upper and lower surface layers is 200–300 HBW, respectively.

[0020] Furthermore, the thicknesses of the upper and lower surface layers are 2mm to 20mm, respectively.

[0021] Furthermore, the upper and lower surface layers are continuously cast billets with the same chemical composition and the same thickness.

[0022] A method for manufacturing a composite mold steel plate according to another embodiment of the present invention includes the following steps:

[0023] S1 provides core steel billet, upper surface steel billet and lower surface steel billet respectively;

[0024] S2, perform surface treatment on the surfaces to be contacted of the core steel billet, the upper surface steel billet and the lower surface steel billet in step S1 respectively;

[0025] S3, the core steel billet, upper surface steel billet and lower surface steel billet from step S2 are sequentially assembled and rolled to obtain a precast steel plate;

[0026] S4, the precast steel plate in step S3 is tempered to obtain the composite mold steel plate;

[0027] In step S4, the tempering temperature is 400–600°C. Given a holding time of t minutes at this tempering temperature and a thickness of d millimeters for the composite mold steel plate, the holding time and the thickness of the composite mold steel plate satisfy the following relationship:

[0028] t = nd, where n is 2 to 4.

[0029] Furthermore, in step S2, the rust and oxide layers on each of the surfaces to be contacted are removed by machining to a depth of 5mm to 10mm.

[0030] Further, step S3 includes:

[0031] S31, by machining, multiple bevels are formed on the four sides of each of the contact surfaces of the core steel billet, the upper surface steel billet and the lower surface steel billet in step S2, and then multiple right-angle holes are formed at each of the bevels by machining, and the multiple right-angle holes are connected to form a vacuum channel.

[0032] S32, the upper surface steel billet, core steel billet and lower surface steel billet from step S31 are stacked in order from top to bottom, and then a three-layer continuous casting billet is formed by sealing welding.

[0033] S33, Vacuum treatment is performed on the three-layer continuous casting billet in step S32;

[0034] S34, Seal the vacuum channel on the three-layer continuous casting billet in step S33;

[0035] S35, place the three-layer continuous casting billet from step S34 in a heating furnace and heat it to 1100-1250℃, and hold it for 1-3 hours;

[0036] S36, the three-layer continuous casting billet in step S35 is subjected to vacuum hot rolling. In the vacuum hot rolling step, the initial rolling temperature is 1000-1150℃, the final rolling temperature is 800-950℃, and the reduction rate is ≥50%.

[0037] The above-described technical solution of the present invention has at least one of the following beneficial effects:

[0038] 1. According to an embodiment of the present invention, a composite mold steel plate includes a core layer and an upper surface layer and a lower surface layer respectively composited to the upper and lower sides of the core layer. The composition and thickness of the upper surface layer and the lower surface layer are independent of each other. The core layer is carbon steel with a carbon content of 0.25% to 0.40%, and at least one alloying element selected from Si, Mn, Cr, Mo, Al, Ti, Ni, RE, Nb, and V is added. The core layer has high strength, high hardness, and excellent mechanical properties, and can be used to manufacture the cavity mold body to ensure that the inner layer of the cavity mold of the composite mold steel plate part has the specified mechanical properties and hardness. The upper surface layer and the lower surface layer are carbon steel with a carbon content of 0.40% to 0.55%, and Si, Mn, and Al are added respectively. They can be used as the outer surface layer structure of the cavity mold to connect the composite mold steel part with other components, further reducing the amount of alloying elements added during the manufacture of the composite mold steel plate. In other words, by reducing the amount of alloying elements added to the outer layer of the cavity mold, composite mold steel plates can combine the advantages of low cost and excellent mechanical properties.

[0039] 2. This invention, through the compositional design of the composite mold steel plate and the optimization of the manufacturing process, specifically by designing the composition of the core layer of the composite mold steel plate, can form bainite and retained austenite, and the Rockwell hardness of the core layer is 27HRC~37HRC, ultimately obtaining a core layer with high strength, high hardness and excellent mechanical properties. Furthermore, by designing the composition of the upper and lower surface layers of the composite mold steel plate, tempered troostite can be formed, and the Brinell hardness of the upper and lower surface layers is 200~300HBW respectively. This invention effectively improves the strength, hardness and mechanical properties of the composite mold steel plate with low alloying element addition. Combined with optimized manufacturing methods and process parameters, the upper and lower surface layers are respectively bonded to the upper and lower sides of the core layer by vacuum hot rolling, resulting in a composite mold steel plate that combines low cost and excellent performance. Attached Figure Description

[0040] Figure 1 This is a metallographic image of the core layer of the composite mold steel plate according to Embodiment 1 of the present invention;

[0041] Figure 2 This is a metallographic image of the upper surface layer of the composite mold steel plate according to Embodiment 1 of the present invention. Detailed Implementation

[0042] 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 with reference to the accompanying drawings. 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 are within the scope of protection of the present invention.

[0043] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Similarly, the terms "an" or "a" and similar terms do not indicate a quantity limitation, but rather indicate the presence of at least one. The terms "connected" or "linked" and similar terms are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. "Up," "down," "left," "right," etc., are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship also changes accordingly.

[0044] The composite mold steel plate of the present invention will be described in detail below.

[0045] The composite mold steel plate includes a core layer and upper and lower surface layers respectively laminated to the upper and lower sides of the core layer. The composition and thickness of the upper and lower surface layers are independent of each other. By mass percentage, the core layer comprises: C: 0.25%–0.40%, Si: 0.10%–0.80%, Mn: 0.40%–1.00%, Cr: 1.50%–2.50%, Mo: 0.10%–0.80%, P≤0.030%, S≤0.010%, Al: 0.010%–0.050%, Ti: The upper and lower surface layers comprise, by mass percentage, at least one of the following: Ni: 0.001%–0.060%, RE: 0.01%–0.10%, Nb: 0.01%–0.050%, and V: 0.01%–0.50%; and, respectively, the upper and lower surface layers comprise, by mass percentage: C: 0.40%–0.55%, Si: 0.10%–0.50%, Mn: 0.40%–1.00%, P ≤ 0.030%, S ≤ 0.010%, and Al: 0.010%–0.050%.

[0046] It should be noted here that the phrase "the composition and thickness of the upper and lower surface layers are independent of each other" means that the composition of the upper and lower surface layers can be the same or different, and similarly, the thickness can be the same or different.

[0047] In other words, the core layer is located in the middle of the composite mold steel plate. The core layer is carbon steel with a carbon content of 0.25% to 0.40%, with the addition of at least one of Si, Mn, Cr, Mo, Al, Ti, Ni, RE, Nb, and V. This core layer possesses high strength, high hardness, and excellent mechanical properties, and can be used to manufacture the cavity mold body to ensure that the inner layer of the cavity mold of the composite mold steel plate part has the specified mechanical properties and hardness. The upper and lower surface layers are carbon steel with a carbon content of 0.40% to 0.55%, with the addition of Si, Mn, and Al respectively. These layers can be used to process and form the outer surface layer structure of the cavity mold, allowing the composite mold steel part to connect with other components. In other words, by reducing the amount of alloying elements added to the outer surface layer of the cavity mold, the composite mold steel plate achieves both low cost and excellent mechanical properties.

[0048] Further, according to another embodiment of the present invention, the composite mold steel plate includes a core layer and an upper surface layer and a lower surface layer respectively laminated to the upper and lower sides of the core layer, wherein the composition and thickness of the upper surface layer and the lower surface layer are independent of each other; the composition of the core layer, by mass percentage, is: C: 0.25%–0.40%, Si: 0.10%–0.80%, Mn: 0.40%–1.00%, Cr: 1.50%–2.50%, Mo: 0.10%–0.80%, P≤0.030%, S≤0.010%, Al: 0.010%–0.050%, Ti: 0.001%. The composition of the upper and lower surface layers, by mass percentage, is as follows: C: 0.40%–0.55%, Si: 0.10%–0.50%, Mn: 0.40%–1.00%, P≤0.030%, S≤0.010%, Al: 0.010%–0.050%, with the balance being Fe and unavoidable impurities.

[0049] In other words, Fe is the main component of the core layer, the upper surface layer, and the lower surface layer. By adjusting the mass percentage of beneficial elements and controlling the content of unavoidable impurities, the amount of alloying elements added can be reduced and the cost can be lowered while ensuring the comprehensive mechanical properties of the core layer, the upper surface layer, and the lower surface layer.

[0050] Specifically, the design principles of each chemical element in the core layer, upper surface layer, and lower surface layer of this invention are as follows:

[0051] C: C can improve the strength and hardness of steel through solid solution strengthening and precipitation strengthening. If the C content in steel is too low, it cannot guarantee that the steel will obtain a bainitic structure and the required mechanical and wear-resistant properties; if the C content in steel is too high, it will increase the tendency of segregation in the steel during continuous casting or ingot casting, resulting in severe segregation of the steel plate, reducing the toughness of the steel plate, and causing the mechanical properties to fail to meet the requirements.

[0052] Therefore, in this invention, for obtaining a core layer with a microstructure consisting mostly of bainite and retained austenite, the mass percentage of C in the core layer is controlled at 0.25% to 0.40%; for obtaining an upper and lower surface layer with a microstructure consisting of tempered troostite, the mass percentage of C in the upper and lower surface layers is controlled at 0.40% to 0.55%.

[0053] Si: Appropriate Si is a beneficial deoxidizer in steel. It can form calcium aluminum silicate inclusions that are easy to float together with Ca and Al in steel, thereby improving the purity of steel. Furthermore, the solid solution strengthening effect of Si in ferrite and austenite can improve hardness and strength. However, excessive Si content will lead to a sharp decrease in the toughness of steel.

[0054] Therefore, in this invention, for a core layer with a Rockwell hardness of 27 HRC to 37 HRC, the mass percentage of Si in the core layer is controlled at 0.10% to 0.80%; for an upper and lower surface layer with a Brinell hardness of 200 to 300 HBW, the mass percentage of Si in the upper and lower surface layers is controlled at 0.10% to 0.50%.

[0055] Mn: Mn strongly increases the hardenability of steel, reduces the steel transformation temperature and critical cooling rate. However, when the Mn content is high, it tends to coarsen the grains and increase the steel's temper brittleness sensitivity. It also easily leads to segregation and cracks in the billet, reducing the performance of the steel plate.

[0056] Therefore, in this invention, for a core layer with a Rockwell hardness of 27 HRC to 37 HRC, the mass percentage of Mn in the core layer is controlled at 0.40% to 1.00%; for an upper and lower surface layer with a Brinell hardness of 200 to 300 HBW, the mass percentage of Mn in the upper and lower surface layers is controlled at 0.40% to 1.00%.

[0057] Cr: Cr can improve the hardenability of steel, as well as its strength and hardness. During tempering, Cr can prevent or slow down the precipitation and aggregation of carbides, thus improving the tempering stability of steel. Cr can also significantly improve corrosion resistance. However, excessive Cr content can impair the low-temperature toughness, impact load fracture characteristics, and bending cold workability of steel plates, especially their weldability.

[0058] Therefore, in this invention, to obtain a core layer with a Rockwell hardness of 27 HRC to 37 HRC, the mass percentage of Cr in the core layer is controlled at 1.50% to 2.50%.

[0059] Mo: Mo can refine grains and improve strength and toughness. Mo is an element that reduces temper brittleness and can improve tempering stability.

[0060] Therefore, in this invention, to obtain a core layer with a Rockwell hardness of 27 HRC to 37 HRC, the mass percentage of Mo in the core layer is controlled to be 0.10% to 0.80%.

[0061] Ti: Ti is one of the strong carbide-forming elements, forming fine TiC particles with C. These small TiC particles are distributed at grain boundaries, achieving a grain refinement effect. The harder TiC particles also increase the hardness of the steel.

[0062] Therefore, in this invention, for obtaining a core layer with a Rockwell hardness of 27HRC to 37HRC, the mass percentage of Ti in the core layer is controlled to be 0.001% to 0.060%.

[0063] Al: Al can combine with nitrogen in steel to form fine, insoluble AlN particles, refining the steel grain structure. Al can refine the steel grain structure, fix nitrogen and oxygen in the steel, reduce the steel's sensitivity to notches, reduce or eliminate aging phenomena in the steel, and improve the steel's toughness.

[0064] Therefore, in this invention, for obtaining a core layer with a Rockwell hardness of 27HRC to 37HRC, and for obtaining an upper and lower surface layer with a Brinell hardness of 200 to 300HBW, the mass percentage of Al in the core layer, upper surface layer, and lower surface layer is controlled to be 0.010% to 0.050%, respectively.

[0065] P and S: P is an impurity introduced into steel from pig iron. P can completely dissolve in ferrite, reducing the plasticity and toughness of steel. S is an impurity introduced into steel from pig iron and fuel. Sulfides formed during steel production reduce the mechanical properties of steel, and hot-working fibrous structures are formed during rolling. Therefore, in wear-resistant steel, both P and S are harmful elements, and their content must be strictly controlled.

[0066] Therefore, in this invention, the mass percentage of P in the upper surface layer, core layer, and lower surface layer is controlled to be ≤0.030%, and the mass percentage of S is controlled to be ≤0.010%.

[0067] Ni: Ni has the effect of significantly reducing the brittle transition temperature, but if the content is too high, it will make it difficult to remove the oxide scale on the surface of the steel plate, and the cost will increase significantly.

[0068] Therefore, in this invention, for obtaining a core layer with a microstructure consisting mostly of bainite and retained austenite, Ni can be added to the core layer. Preferably, the mass percentage of Ni in the core layer can be controlled, for example, between 0.01% and 2.00%.

[0069] Rare earth elements (REs) (composed of one of La, Ce, and Nd) improve the fluidity of steel, reduce non-metallic inclusions, make the steel structure denser and purer, and have good deoxidation and desulfurization effects, improving anisotropy. REs are particularly effective in high-carbon steel: they can essentially eliminate MnS and Al2O3 inclusions, forming circular or elliptical rare earth sulfides, rare earth oxysulfides, and composite inclusions with Al2O3 as the core and rare earth elements surrounding them. Rare earth elements have low solid solubility in steel and readily segregate at grain boundaries, thus suppressing the segregation of S, P, and their low-melting-point inclusions at grain boundaries. Rare earth elements combine with these inclusions to form high-melting-point composite inclusions, eliminating the harmful effects of low-melting-point inclusions agglomerating at grain boundaries. Rare earth elements can also inhibit columnar crystal growth, refine the as-cast structure, reduce dendritic and regional segregation, and improve the uniformity of the steel's chemical composition. In summary, adding RE can significantly reduce the brittleness of high-carbon steel and improve the toughness and plasticity of steel plates.

[0070] Therefore, in this invention, to obtain a core layer with a microstructure consisting mostly of bainite and retained austenite, RE can be added to the core layer. Preferably, the mass percentage of RE in the core layer can be controlled, for example, between 0.01% and 0.10%.

[0071] Nb: Nb can improve the strength and toughness of steel through grain refinement.

[0072] Therefore, in this invention, to obtain a core layer with a Rockwell hardness of 27 HRC to 37 HRC, Nb can be added to the core layer. Preferably, the mass percentage of Nb in the core layer can be controlled, for example, between 0.01% and 0.050%.

[0073] V: V exists mainly in steel in the form of carbides. Its main function is to refine the steel's microstructure and grain size, thereby reducing the steel's strength and toughness.

[0074] Therefore, in this invention, to obtain a core layer with a Rockwell hardness of 27 HRC to 37 HRC, V can be added to the core layer. Preferably, the mass percentage of V in the core layer can be controlled, for example, between 0.01% and 0.50%.

[0075] Furthermore, the microstructure of the core layer consists of bainite and retained austenite, with the volume percentage of retained austenite ≤5%. In other words, through the above-mentioned composition design of the core layer, bainite and retained austenite with a volume percentage ≤5% can be obtained, thereby improving the comprehensive mechanical properties of the composite mold steel plate.

[0076] Furthermore, the Rockwell hardness of the core layer is 27HRC to 37HRC. In other words, through the above-mentioned compositional design of the core layer, it possesses excellent comprehensive mechanical properties, thereby further improving the overall mechanical properties of the composite mold steel plate.

[0077] Furthermore, the core layer thickness is 20mm to 200mm. In other words, a core layer thickness of 20mm to 200mm can further improve the overall mechanical properties of the composite mold steel plate.

[0078] Furthermore, the microstructure of the upper and lower surface layers is tempered troostite, respectively. In other words, through the above compositional design of the upper and lower surface layers, tempered troostite can be obtained, thereby improving the overall mechanical properties of the composite mold steel plate.

[0079] Furthermore, the Brinell hardness of the upper and lower surface layers is 200-300 HBW, respectively. In other words, through the above composition design of the upper and lower surface layers, the comprehensive mechanical properties of the upper and lower surface layers can basically serve as the outer surface layer structure of the composite mold steel plate to connect other components.

[0080] Furthermore, the thicknesses of the upper and lower surface layers are 2mm to 20mm respectively. In other words, the thicknesses of the upper and lower surface layers are 2mm to 20mm, which can further meet the comprehensive mechanical performance requirements when the outer surface layer structure of the composite mold steel plate is connected to other components.

[0081] Furthermore, the upper and lower surface layers are continuously cast billets with the same chemical composition and thickness specifications. In other words, the upper and lower surface layers have the same chemical composition and thickness specifications, which allows the same manufacturing methods and process parameters to be used to composite the upper and lower surface layers onto the upper and lower surfaces of the core layer, offering the advantage of convenient processing and manufacturing.

[0082] This invention, through the composition design of the aforementioned composite mold steel plate, controls the total mass percentage of alloying elements added in the core layer to be between 2.151% and 7.9%, which can form bainite and retained austenite in the core layer and ensure its Rockwell hardness is 27HRC to 37HRC. The total mass percentage of alloying elements added in the upper and lower surface layers is controlled to be between 0.51% and 1.59%, which can form tempered troostite in the upper and lower surface layers respectively and ensure their Brinell hardness is 200 to 300 HBW. In other words, this invention effectively improves the strength, hardness, and mechanical properties of the composite mold steel plate while reducing production costs, even with a mass percentage of alloying elements added to carbon steel of <8%.

[0083] This invention also provides a method for manufacturing a composite mold steel plate, comprising the following steps: S1, according to the components of the core layer, upper surface layer, and lower surface layer, respectively, the components are proportioned, smelted, and cast to obtain core steel billets, upper surface layer steel billets, and lower surface layer steel billets; S2, the surfaces to be contacted of the core steel billet, upper surface layer steel billet, and lower surface layer steel billet in step S1 are respectively subjected to surface treatment; S3, the core steel billet, upper surface layer steel billet, and lower surface layer steel billet in step S2 are sequentially assembled and rolled to obtain a precast steel plate; S4, the precast steel plate in step S3 is tempered to obtain a composite mold steel plate; in step S4, the tempering temperature is 400-600℃, the holding time at the tempering temperature is t minutes, and the thickness of the composite mold steel plate is d millimeters, then the holding time and the thickness of the composite mold steel plate satisfy the following relationship: t = nd, where n is 2-4.

[0084] In other words, the comprehensive mechanical properties of composite mold steel plates can be further improved by optimizing manufacturing process parameters. Specifically, tempering at 400–600℃ can homogenize the microstructure, improve plasticity, and eliminate internal stress. Setting the tempering holding time to t = nd, where n is 2–4, helps the composite mold steel plate to be fully heated evenly, eliminates internal stress, improves the uniformity of microstructure properties, and is beneficial for controlling the plate shape.

[0085] Furthermore, in step S2, the rust and oxide layers on each surface to be contacted are removed by machining to a depth of 5mm to 10mm. In other words, before assembling the billet, the rust and oxide layers on its surface are removed by machining to prevent impurities from being mixed into the three-layer continuous casting billet and affecting the overall performance of the composite mold steel plate.

[0086] Further, step S3 includes: S31, forming multiple bevels on the four sides of each contact surface of the core steel billet, upper surface steel billet, and lower surface steel billet in step S2 by machining, and then forming multiple right-angle holes at each bevel by machining, with the multiple right-angle holes interconnected to form a vacuum channel; S32, stacking the upper surface steel billet, core steel billet, and lower surface steel billet in step S31 in a top-to-bottom order, and then forming a three-layer continuous casting billet through sealing welding; S33 In step S32, the three-layer continuous casting billet is subjected to vacuum treatment; in step S34, the vacuum channel on the three-layer continuous casting billet in step S33 is sealed; in step S35, the three-layer continuous casting billet in step S34 is placed in a heating furnace and heated to 1100-1250℃, and held for 1-3 hours; in step S36, the three-layer continuous casting billet in step S35 is subjected to vacuum hot rolling, wherein the initial rolling temperature is 1000-1150℃, the final rolling temperature is 800-950℃, and the reduction rate is ≥50%.

[0087] In other words, firstly, the three-layer continuously cast billet is rolled using a vacuum hot rolling method. This method facilitates complete metallurgical bonding of the upper, core, and lower surface steel billets, resulting in strong bonding and excellent comprehensive mechanical properties. Furthermore, controlling the process parameters of vacuum hot rolling further enhances the comprehensive mechanical properties of the composite mold steel plate. Specifically, controlling the heating temperature between 1100 and 1250°C allows carbon and alloying elements to fully diffuse at the composite interface, achieving metallurgical bonding and promoting the homogenization of carbon and alloying elements. A staged controlled rolling method with an initial rolling temperature of 1000–1150°C and a final rolling temperature of 800–950°C fully utilizes the recrystallization and non-recrystallization effects, achieving grain refinement and improving the strength and toughness of the steel. Finally, a high reduction rolling method with a reduction rate ≥50% further enhances the controlled rolling effect, achieving a grain refinement and strengthening effect.

[0088] This invention, based on a rational chemical composition design and by optimizing the manufacturing method and process parameters of the composite mold steel plate, effectively improves the strength, hardness, and mechanical properties of the composite mold steel plate while maintaining a mass percentage of alloying elements less than 8% in carbon steel. This results in a composite mold steel plate that combines low cost and excellent performance. The Rockwell hardness of the core layer of this composite mold steel plate is 27 HRC to 37 HRC, and this core layer can be used to manufacture the cavity mold body. The Brinell hardness of the upper and lower surfaces of the composite mold steel plate are 200 to 300 HBW, respectively. These upper and lower surfaces can be used as the outer surface structure of the cavity mold, enabling the composite mold steel part to connect with other components, further reducing the amount of alloying elements added during the manufacturing of the composite mold steel plate. This invention, by reducing the amount of alloying elements added to the outer surface layer of the cavity mold and combining it with optimized manufacturing methods and process parameters, enables the composite mold steel plate to combine the advantages of low cost and excellent mechanical properties.

[0089] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below.

[0090] Examples 1-3 and Comparative Examples 1-2

[0091] The composite mold steel plates of Embodiments 1-3 of the present invention are obtained by the following steps:

[0092] (1) Calculate the margin of the iron billet according to the components and component content shown in Table 1. The content of unavoidable impurities in the iron billet should be considered during the calculation. Then, smelting and casting are carried out to obtain the core steel billet, the upper surface steel billet and the lower surface steel billet respectively.

[0093] (2) Remove the rust and oxide layers from each contact surface of the core steel billet, the upper surface steel billet and the lower surface steel billet by mechanical processing, with a processing depth of 8mm;

[0094] (3) The upper surface steel billet, core steel billet and lower surface steel billet in step S2 are stacked in order from top to bottom, and then sealed and welded to form a three-layer continuous casting billet;

[0095] Precast steel plates were obtained by rolling three-layer continuously cast billets using vacuum hot rolling according to the parameters shown in Table 2.

[0096] (4) The precast steel plate after rolling is tempered according to the parameters shown in Table 2 to obtain the composite mold steel plate.

[0097] The steel plates of Comparative Examples 1-2 were rolled and heat-treated according to the composition and composition content shown in Table 1 and the manufacturing process parameters in Table 2. The difference is that Comparative Examples 1-2 are single-layer steel billets.

[0098] Table 1 lists the composition and total amount of corresponding alloying elements of the upper surface layer, core layer, and lower surface layer of the composite mold steel plates in Examples 1-3, as well as the steel plates of Comparative Examples 1-2.

[0099] Table 1. Composition and total amount (wt.%) of the core layer, upper surface layer, and lower surface layer of the composite mold steel plates in Examples 1-3 and the steel plates in Comparative Examples 1-2.

[0100]

[0101] Table 2 lists the main process parameters of vacuum hot rolling and tempering treatment of composite mold steel plates in Examples 1-3, as well as the main process parameters of manufacturing methods in Comparative Examples 1-2. It is worth noting that the thickness of the steel plate in Comparative Examples 1-2 is 100 mm.

[0102] Table 2 shows the main process parameters of the vacuum hot rolling and tempering treatment of the composite mold steel plates in Examples 1-3, and the main process parameters of the manufacturing method in Comparative Examples 1-2.

[0103]

[0104] Samples were taken from the core layer, upper surface layer, and lower surface layer of the composite mold steel plates obtained in Examples 1-3, as well as the steel plates of Comparative Examples 1-2. Hardness tests were then conducted on the composite mold steel plates of Examples 1-3 and the steel plates of Comparative Examples 1-2. The test results for Examples 1-3 and Comparative Examples 1-2 are shown in Table 3.

[0105] Table 3 lists the hardness test results of the upper surface layer, lower surface layer, and core layer of the composite mold steel plates of Examples 1-3, as well as Comparative Examples 1-2.

[0106] Table 3 shows the hardness test results of the upper surface layer, lower surface layer, and core layer of the composite mold steel plates in Examples 1-3, and in Comparative Examples 1-2.

[0107]

[0108] As shown in Table 3, the Rockwell hardness of the core layer of the composite mold steel plate in Examples 1-3 of the present invention is 27HRC to 37HRC, and the Brinell hardness of the upper and lower surface layers is 200 to 300HBW, which meets the requirements for strength, hardness and mechanical properties of mold steel for cavity molds.

[0109] In comparison, the total alloy element content of the upper surface layer, lower surface layer, and core layer of Embodiment 1 of the present invention is 1.448, 1.448, and 4.443, respectively. That is, the total alloy element content of the composite mold steel plate of Embodiment 1 is <4.443; while the alloy element content of Comparative Example 1 is 4.443. In other words, the alloy element content of Comparative Example 1 is greater than that of Embodiment 1. In the hardness test, the hardness of the core layer of Embodiment 1 used for processing the forming cavity mold is comparable to the hardness of the steel plate of Comparative Example 1. That is, compared to Comparative Example 1, the embodiments of the present invention reduce the addition of alloy elements to the composite mold steel plate, saving costs.

[0110] In comparison, the total alloy element content of the upper surface layer, lower surface layer, and core layer of Example 2 of the present invention is 1.183, 1.389, and 3.865, respectively. That is, the total alloy element content of the composite mold steel plate of Example 2 is <3.865. The alloy element content of Comparative Example 2 is 1.389, which means that the alloy element content of Comparative Example 2 is less than that of Example 1. In the hardness test, the hardness of the core layer of Example 2 used for processing the forming cavity mold is significantly better than that of the steel plate of Comparative Example 2. In other words, compared with Comparative Example 2, the present invention improves the strength, hardness, and mechanical properties of the composite mold steel plate.

[0111] As can be seen from Tables 1-3, by designing the composition of the composite mold steel plate in the embodiments of the present invention and optimizing the manufacturing process, a composite mold steel plate with both low cost and high strength and high hardness can be obtained.

[0112] The present invention separately observed the microstructure of the core layer and the upper surface layer of the composite mold steel plate of Example 1, and the results are as follows: Figure 1 and 2 As shown. Figure 1 As shown, the microstructure of the core layer of the composite mold steel plate in Embodiment 1 of the present invention includes bainite and retained austenite, with a bainite volume percentage of 95% and a retained austenite volume percentage of 5%; Figure 2 As shown, the microstructure of the upper surface layer of the composite mold steel plate in Embodiment 1 of the present invention is tempered troostite.

[0113] In summary, by designing the composition of the composite mold steel plate and optimizing the manufacturing process, this invention can obtain a composite mold steel plate that combines low cost and excellent performance, even when the mass percentage of alloying elements added to carbon steel is less than 8%.

[0114] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles 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 composite mold steel plate, characterized in that, It includes a core layer and an upper surface layer and a lower surface layer respectively laminated to the upper and lower sides of the core layer, wherein the composition and thickness of the upper surface layer and the lower surface layer are independent of each other; The core layer, by mass percentage, comprises at least one of the following: C: 0.25%–0.40%, Si: 0.10%–0.80%, Mn: 0.40%–1.00%, Cr: 1.50%–2.50%, Mo: 0.10%–0.80%, P≤0.030%, S≤0.010%, Al: 0.010%–0.050%, Ti: 0.001%–0.060%, and Ni: 0.01%–2.00%, RE: 0.01%–0.10%, Nb: 0.01%–0.050%, and V: 0.01%–0.50%, with the balance being Fe and unavoidable impurities. The microstructure of the core layer is bainite and retained austenite, with the retained austenite having a volume percentage ≤5%. The composition of the upper and lower surface layers, by mass percentage, is as follows: C: 0.40%–0.55%, Si: 0.10%–0.50%, Mn: 0.40%–1.00%, P≤0.030%, S≤0.010%, Al: 0.010%–0.050%, with the balance being Fe and unavoidable impurities; The core layer has a Rockwell hardness of 27 HRC to 37 HRC, the microstructure of the upper and lower surface layers is tempered troostite, and the Brinell hardness of the upper and lower surface layers is 200 to 300 HBW.

2. The composite mold steel plate according to claim 1, characterized in that, The thickness of the core layer is 20mm to 200mm.

3. The composite mold steel plate according to claim 1, characterized in that, The thicknesses of the upper and lower surface layers are 2mm to 20mm, respectively.

4. The composite mold steel plate according to claim 1, characterized in that, The upper and lower layers are continuously cast billets with the same chemical composition and thickness.

5. A method for manufacturing the composite mold steel plate according to any one of claims 1-4, characterized in that, Includes the following steps: S1, according to the components of the core layer, upper surface layer and lower surface layer respectively, are proportioned, smelted and cast to obtain core steel billet, upper surface layer steel billet and lower surface layer steel billet. S2, perform surface treatment on the surfaces to be contacted of the core steel billet, the upper surface steel billet and the lower surface steel billet in step S1 respectively; S3, the core steel billet, upper surface steel billet and lower surface steel billet from step S2 are sequentially assembled and rolled to obtain a precast steel plate; S4, the precast steel plate in step S3 is tempered to obtain the composite mold steel plate; In step S4, the tempering temperature is 400–600°C. Given a holding time of t minutes at this tempering temperature and a thickness of d millimeters for the composite mold steel plate, the holding time and the thickness of the composite mold steel plate satisfy the following relationship: t=nd, where n is 2 to 4.

6. The manufacturing method according to claim 5, characterized in that, In step S2, the rust and oxide layers on each of the surfaces to be contacted are removed by mechanical processing to a depth of 5mm to 10mm.

7. The manufacturing method according to claim 5, characterized in that, Step S3 includes: S31, by machining, multiple bevels are formed on the four sides of each of the contact surfaces of the core steel billet, the upper surface steel billet and the lower surface steel billet in step S2, and then multiple right-angle holes are formed at each of the bevels by machining, and the multiple right-angle holes are connected to form a vacuum channel. S32, the upper surface steel billet, core steel billet and lower surface steel billet from step S31 are stacked in order from top to bottom, and then a three-layer continuous casting billet is formed by sealing welding. S33, Vacuum treatment is performed on the three-layer continuous casting billet in step S32; S34, Seal the vacuum channel on the three-layer continuous casting billet in step S33; S35, place the three-layer continuous casting billet from step S34 in a heating furnace and heat it to 1100-1250℃, and hold it for 1-3 hours; S36, the three-layer continuous casting billet in step S35 is subjected to vacuum hot rolling. In the vacuum hot rolling step, the initial rolling temperature is 1000-1150℃, the final rolling temperature is 800-950℃, and the reduction rate is ≥50%.

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