Plastic mold steel sheet and method for manufacturing the same

Abstract

The present application provides a kind of steel plate for plastic mold and its manufacturing method, in addition to comprising more than 90% Fe and inevitable impurities, it also comprises the following chemical elements by mass percentage: C: 0.25~0.35%; Si: 0.10~0.50%; Mn: 0.20~1.60%; Cr: 0.50~2.00%; Mo: 0.20~0.60%; V: 0.01~0.20%; Ti: 0.01~0.10%, Al: 0.010~0.060%. The alloying component in the steel plate is mainly low-carbon low-alloy, and the refining and grain strengthening effects of Cr, Mo, V, Ti and other alloying elements are utilized, so that the steel plate has excellent mechanical properties and processing performance. The method of the present application adopts controlled rolling and controlled cooling process, and the preparation process is simple, and a steel plate for plastic mold with excellent performance can be obtained.

Description

Technical Field

[0001] This invention relates to steel and its manufacturing method, and more particularly to a steel plate for plastic molds and its manufacturing method. Background Technology

[0002] With the development of industrial production technology and the continuous emergence of new materials, plastic products have become an important industrial material and are widely used in various sectors of the national economy. From spacecraft to ships, from building materials to agricultural production, from household appliances to children's toys, plastic products are indispensable. In many industrialized countries, the output value of plastic molds has already ranked first in the mold manufacturing industry.

[0003] Plastic mold steel can be categorized by manufacturing process into quenched and tempered pre-hardened plastic mold steel and non-quenched and tempered plastic mold steel. Quenched and tempered pre-hardened plastic mold steel is generally forged into modules and pre-heat treated to achieve the required hardness and performance of the mold. It exhibits uniform hardness, good machinability and mechanical properties, and minimal mold distortion. However, it also suffers from drawbacks such as complex manufacturing processes, high energy consumption, and susceptibility to cracking due to significant internal stress under high water cooling rates. Non-quenched and tempered plastic mold steel, without tempering, achieves pre-hardness after forging and rolling, which is beneficial for energy conservation, cost reduction, and shorter production cycles. However, its microstructure and property uniformity differ from quenched and tempered steel, making it difficult to meet the requirements of high-end products. Summary of the Invention

[0004] To address the aforementioned technical problems in the prior art, a first aspect of the present invention provides a steel plate for plastic molds, wherein the steel for plastic molds, in addition to containing more than 90% Fe and unavoidable impurities, also contains the following chemical elements by mass percentage:

[0005] C: 0.25~0.35%; Si: 0.10~0.50%; Mn: 0.20~1.60%; Cr: 0.50~2.00%; Mo: 0.20~0.60%; V: 0.01~0.20%; Ti: 0.01~0.10%, Al: 0.010~0.060%.

[0006] A second aspect of the present invention provides a steel plate for plastic molds, comprising the following chemical elements in mass percentage: C: 0.25-0.35%; Si: 0.10-0.50%; Mn: 0.20-1.60%; Cr: 0.50-2.00%; Mo: 0.20-0.60%; V: 0.01-0.20%; Ti: 0.01-0.10%; Al: 0.010-0.060%, with the balance being Fe and unavoidable impurities.

[0007] This invention provides a steel plate for plastic molds through a rational chemical composition design. Its alloy composition is mainly composed of low-carbon low alloys. At the same time, the steel plate has excellent mechanical and processing properties by utilizing the grain refinement and strengthening effects of alloying elements such as Cr, Mo, V, and Ti.

[0008] Preferably, in the steel plate of the present invention, the content of impurity elements by mass percentage satisfies: P≤0.030%, more preferably P≤0.010%; S≤0.010%, more preferably S≤0.003%.

[0009] Preferably, in the steel plate of the present invention, the content of chemical elements in the steel plate, expressed as a percentage by mass, satisfies one or more of the following:

[0010] C: 0.26–0.35%;

[0011] Si: 0.15–0.35%;

[0012] Mn: 0.85–1.60%, preferably 0.85–1.55%;

[0013] Cr: 0.80–1.70%, preferably 0.85–1.70%;

[0014] Mo: 0.25–0.50%, preferably 0.25–0.37%;

[0015] V: 0.05–0.20%, preferably 0.05–0.16%;

[0016] Ti: 0.017–0.050%, preferably 0.017–0.030%;

[0017] Al: 0.020–0.060%, preferably 0.020–0.055%.

[0018] The design principles of the chemical elements in the steel plate for plastic molds of this invention are as follows:

[0019] Carbon: Carbon is the most basic and important element in steel. It enhances the strength and hardness of steel through solid solution strengthening and precipitation strengthening. Carbon is also crucial for obtaining bainitic and martensitic structures. If the carbon content is too low, the steel cannot guarantee the acquisition of bainitic or martensitic structures and the required mechanical properties, such as wear resistance. If the carbon content is too high, it increases the tendency for segregation during continuous casting or ingot casting, leading to severe segregation in the steel plate, reducing its toughness, and resulting in unqualified mechanical properties. Therefore, in this invention, the C content is controlled at 0.25–0.35%, preferably 0.26–0.35%.

[0020] Silicon: Silicon is a beneficial deoxidizer in steel. It can form easily floating calcium aluminum silicate inclusions with calcium and aluminum in the steel, thereby improving the purity of the steel. Silicon dissolved in ferrite and austenite increases their hardness and strength; however, excessive silicon content can lead to a sharp decrease in the toughness of the steel. Therefore, in this invention, the Si content is controlled at 0.10–0.50%, preferably 0.15–0.35%.

[0021] Manganese: Manganese improves the hardenability of steel, but a high manganese content tends to coarsen grains and increase the steel's temper brittleness sensitivity, and can easily lead to segregation and cracks in the cast billet. Therefore, in this invention, the Mn content is controlled at 0.20–1.60%, preferably 0.85–1.60%, and more preferably 0.85–1.55%.

[0022] Chromium: Chromium can improve the hardenability of steel, as well as its strength and hardness. During tempering, chromium can prevent or slow down the precipitation and aggregation of carbides, thus improving the tempering stability of steel. However, chromium is a precious metal, and adding too much will significantly increase costs. Therefore, in this invention, the chromium content is controlled at 0.50–2.00%, preferably 0.80–1.70%, and more preferably 0.85–1.70%.

[0023] Molybdenum: Molybdenum can refine grains, improving strength and toughness. It is an element that reduces temper brittleness and improves temper stability. Molybdenum can also improve high-temperature resistance. Mo is also a precious metal, and adding too much can significantly increase costs. Therefore, in this invention, the Mo content is controlled at 0.20–0.60%, preferably 0.25–0.50%, and more preferably 0.25–0.37%.

[0024] Vanadium: The addition of vanadium is mainly to refine the grains, preventing the austenite grains in the billet from growing too coarse during the heating stage. This allows for further grain refinement during subsequent multi-pass rolling, improving the steel's strength and toughness. Vanadium is also a precious metal element, and excessive addition can significantly increase costs. Therefore, in this invention, the vanadium content is controlled at 0.01–0.20%, preferably 0.05–0.20%, and more preferably 0.05–0.16%.

[0025] Aluminum: Aluminum and nitrogen in steel can form fine, insoluble AlN particles, refining the steel grain structure. Furthermore, aluminum readily combines with N and O in steel, fixing nitrogen and oxygen, reducing the steel's sensitivity to notches, minimizing or eliminating aging phenomena, and improving the steel's toughness. Excessive Al content is very detrimental to the continuous casting process, easily leading to problems such as nozzle clogging. Therefore, in this invention, the Al content is controlled at 0.010–0.060%, preferably 0.020–0.060%, and more preferably 0.020–0.055%.

[0026] Phosphorus and sulfur: Sulfur and phosphorus are both harmful elements in steel. Their content must be strictly controlled to avoid reducing the quality and service life of the steel plate. In the steel of this invention, P≤0.030%, preferably P≤0.010%; S≤0.010%, preferably S≤0.003%. The fewer impurity elements in the steel, the purer the steel and the better its performance.

[0027] Preferably, the microstructure of the steel plate for plastic molds of the present invention comprises bainite and carbides.

[0028] Preferably, the Rockwell hardness of the steel plate used for the plastic mold of the present invention is 25-35 HRC. Specifically, the near-Rockwell hardness and the half-thickness Rockwell hardness of the steel plate are both 25-35 HRC.

[0029] A third aspect of the present invention provides a plastic mold formed from the aforementioned steel plate.

[0030] A fourth aspect of the present invention provides a method for manufacturing a steel plate for a plastic mold, comprising the following steps performed sequentially:

[0031] 1) The molten steel is smelted and cast to obtain slabs;

[0032] 2) Heat the slab;

[0033] 3) Rolling to obtain rolled steel plates;

[0034] 4) Online quenching; and

[0035] 5) Heat treatment.

[0036] Preferably, in the online quenching step 4), the rolled steel plate is water-cooled at a rate of 5-8°C / s and the water-cooling stop temperature is between the martensitic transformation temperature Ms and the bainitic transformation temperature Bs. Then, it is held at 300-400°C for more than 8 hours and then air-cooled to room temperature.

[0037] More preferably, the rolled steel plate is water-cooled more than once. More preferably, the rolled steel plate is water-cooled only once.

[0038] For thin steel plates with a thickness of less than 200 mm, due to their good hardenability, they can be quenched to the temperature between the martensitic transformation temperature Ms and the bainitic transformation temperature Bs in one quenching at a suitable cooling rate, and a uniform bainitic structure is formed on both the surface and inside of the steel plate.

[0039] In this invention, the martensitic transformation temperature Ms and the bainitic transformation temperature Bs are determined by the composition of the steel plate used for plastic molds, specifically:

[0040] The martensitic transformation initiation temperature Ms is calculated using the following formula: Ms (°C) = 539 - 423°C - 30.4Mn - 12.1Cr - 7.5Mo

[0041] The bainitic transformation onset temperature Bs is calculated using the following formula: Bs (°C) = 830 - 270°C - 90Mn - 70Cr - 83Mo

[0042] In the formula, each chemical element symbol represents the value before the percentage sign of the corresponding element's mass percentage content. The method of this invention, by employing the process parameters for online quenching described in 4) above, can obtain steel plates with excellent overall performance. Under rapid cooling, for example, at a cooling rate of 5–8 °C / s as described in this invention, the core of the steel plate can achieve an ideal cooling rate, forming a bainitic structure. However, if the cooling rate exceeds the above range, the surface of the steel plate will form a martensitic structure due to the excessively rapid cooling, which is prone to cracking and causes workpiece damage.

[0043] The online quenching of this invention solves the above-mentioned problems. After the steel plate surface is water-cooled to a certain temperature, the steel plate is rapidly transferred to air for further cooling. At this time, due to the large temperature gradient between the inside and outside, the heat inside the steel plate is rapidly transferred to the mold surface driven by the temperature gradient. This allows the microstructure formed at the edges and corners during water cooling to undergo sufficient self-tempering, alleviating local stress concentration. Simultaneously, it maintains a relatively fast cooling rate in the core to avoid the formation of ferrite + pearlite.

[0044] Preferably, in step 2), the heating temperature is 1100–1250°C and the holding time is 1–3 hours.

[0045] The heating temperature is controlled above 1100℃ to ensure complete austenitization of the steel and the formation of a uniform steel structure. Simultaneously, the heating temperature needs to be below 1250℃ to facilitate energy conservation.

[0046] Preferably, in step 3), the rolling process includes roughing and finishing, with the roughing finishing temperature being 950–1150°C and the finishing finishing temperature being 850–950°C.

[0047] Preferably, in step 5), the heat treatment includes tempering at a temperature of 450–650°C and a holding time of 30–120 min.

[0048] In summary, the method of the present invention, through controlled rolling and online quenching and water cooling, produces a plastic mold steel plate with excellent mechanical and processing properties, mainly composed of fine bainite and carbides in the metallographic structure. The method of the present invention is simple in process, avoiding the complexity of traditional quenching and tempering (water quenching and oil quenching) processes, and solving the shortcomings of non-quenched and tempered products, making it suitable for industrial application. Detailed Implementation

[0049] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains.

[0050] In this article, "Rockwell hardness (HRC)" is used as an indicator of the hardness value of steel. Rockwell hardness (HRC) is determined as follows: according to GB / T230.1—2018 "Metallic materials Rockwell hardness test - Part 1: Test method".

[0051] Near-Rockwell hardness refers to the surface Rockwell hardness of the steel plate; 1 / 2 thickness Rockwell hardness refers to the Rockwell hardness at 1 / 2 thickness of the steel plate.

[0052] The technical solutions of this disclosure will be further described in detail below with reference to the embodiments. It should be understood that the following embodiments are only used to describe the specific implementation of this disclosure and are not intended to constitute any limitation on the scope of protection of this disclosure.

[0053] Example

[0054] The steel plates of Examples 1-5 were obtained through the following steps:

[0055] 1) According to the formula shown in Table 1, the molten steel is smelted and cast to obtain slabs;

[0056] 2) Heating the slab: The heating temperature is 1100~1250℃, and the holding time is 1~3h.

[0057] 3) Rolling to obtain rolled steel plates: the finishing temperature of rough rolling is 950~1150℃; the finishing temperature of finish rolling is 850~950℃.

[0058] 4) Online quenching: The rolled steel plate is water-cooled at a rate of 5-8℃ / s and the water-cooling stop temperature is Ms-Bs. Then it is held at 300-400℃ for more than 8 hours and then air-cooled to room temperature.

[0059] 5) Heat treatment: Temper the quenched steel plate at a temperature of 450-650℃ and a holding time of 30-120 minutes.

[0060] Comparative Examples 1-2 were prepared using essentially the same steps as described above, except that: some chemical elements of the steel plate in Comparative Example 1 were not within the scope of this invention, the water cooling rate during online quenching of the steel plate in Comparative Example 2 was not within the specified range, the cooling stop temperature was <Ms, and the microstructure at the end of quenching was mainly martensite.

[0061] Table 1 shows the chemical composition of the steel plates in Examples 1-5 and Comparative Examples 1-2, in wt.%, and the corresponding Ms and Bs values, in °C.

[0062] Table 1 (Balance is Fe and unavoidable impurities other than P and S)

[0063] Example 1 0.26 0.15 1.05 0.010 0.003 0.85 0.25 0.05 0.025 0.020 384.94 585.05 Example 2 0.28 0.30 0.85 0.008 0.002 1.25 0.37 0.08 0.017 0.050 368.36 554.29 Example 3 0.30 0.25 1.55 0.007 0.002 1.55 0.33 0.16 0.030 0.035 343.75 473.61 Example 4 0.32 0.35 1.30 0.008 0.002 1.55 0.25 0.10 0.025 0.030 343.49 497.35 Example 5 0.35 0.20 1.50 0.009 0.003 1.70 0.28 0.11 0.030 0.055 323.28 464.9 Comparative Example 1 0.10 0.25 1.30 0.015 0.008 1.35 0.10 0.15 0.020 0.050 440.10 583.2 Comparative Example 2 0.29 0.30 1.45 0.013 0.010 1.55 0.23 0.19 0.010 0.060 351.77 493.61

[0064] Table 2 shows the process parameters and the thickness of the finished steel plates for Examples 1-5 and Comparative Examples 1-2.

[0065] Table 2

[0066]

[0067] Samples were taken from the steel plates of Examples 1-5 and Comparative Examples 1-2, and the metallographic structure, mechanical properties and processing properties of each steel sample were tested. The test results are shown in Table 3.

[0068] Table 3

[0069]

[0070] As shown in Tables 1-3, the metallographic structure of the steel plates for plastic molds in Examples 1-5 of the present invention and the steel in Comparative Example 1 is mainly bainite + carbides. Since the elemental composition used in Comparative Example 1 is not within the scope defined by the present invention, compared with Comparative Example 1, the steel obtained in Examples 1-5 of the present invention has significantly improved hardness through optimization of the chemical composition of the steel. The steel plate in Comparative Example 2 used a high water cooling rate during online quenching, resulting in excessively rapid cooling and a tempered troostite structure, which is prone to cracking. The present invention, through reasonable chemical composition design and optimized preparation process, can obtain steel plates for plastic molds with excellent mechanical and processing properties.

[0071] It should be noted that all technical features described in this application can be freely combined or combined in any way, unless they contradict each other. Various modifications and variations can be made to this invention without departing from its scope, as will be apparent to those skilled in the art. For example, features shown or described as part of one embodiment can be used with another embodiment to produce yet another embodiment. Therefore, this invention is intended to cover these modifications that fall within the scope of the appended claims and their equivalents.

Claims

1. A steel plate for plastic molds, characterized in that, The steel plate contains the following chemical elements by mass percentage: C: 0.25~0.35%; Si: 0.10~0.50%; Mn: 0.20~1.60%; Cr: 0.50~2.00%; Mo: 0.25-0.60%; V: 0.01~0.20%; Ti: 0.01~0.10%; Al: 0.010~0.060%; balance being Fe and unavoidable impurities; the microstructure of the steel plate includes bainite and carbides; The steel plate is manufactured by the following method, which includes sequential rolling and online quenching. The rolling process yields a rolled steel plate. In the online quenching step, the rolled steel plate is water-cooled at a cooling rate of 5~8℃ / s and a water-cooling stop temperature of Ms~Bs. It is then held at 300~400℃ for more than 8 hours and then air-cooled to room temperature. The martensitic transformation start temperature Ms = 539-423℃-30.4Mn-12.1Cr-7.5Mo and the bainitic transformation start temperature Bs = 830-270℃-90Mn-70Cr-83Mo. The units of Ms and Bs are both ℃. In the formula, each chemical element symbol represents the value before the percentage sign of the corresponding element's mass percentage content.

2. The steel plate for plastic molds as described in claim 1, characterized in that, The content of impurity elements, expressed as a percentage by mass, meets the following requirements: P ≤ 0.030%, S ≤ 0.010%.

3. The steel plate for plastic molds as described in claim 1, characterized in that, The chemical element content of the steel plate, expressed as a percentage by mass, meets one or more of the following criteria: C：0.26～0.35%； Si: 0.15~0.35%; Mn: 0.85~1.60%; Cr：0.80～1.70%； Mo: 0.25–0.50%; V：0.05~0.20%； Ti: 0.017~0.050%; Al：0.020～0.060%。 4. The steel plate for plastic molds as described in claim 1, characterized in that, The Rockwell hardness of the steel plate is 25~35HRC.

5. The steel plate for plastic molds as described in claim 1, characterized in that, P≤0.010%, S≤0.003%, Mn 0.85~1.55%, Cr 0.85~1.70%, Mo 0.25~0.37%, V 0.05~0.16%, Ti 0.017~0.030%, Al 0.020~0.055%.

6. A plastic mold, formed from a steel plate for making plastic molds according to any one of claims 1-5.

7. A method for manufacturing a steel plate for a plastic mold according to any one of claims 1-5, characterized in that, The method includes the following steps performed sequentially: 1) The molten steel is smelted and cast to obtain slabs; 2) Heating the slab; 3) Rolling to obtain rolled steel plates; 4) Online quenching; and 5) Heat treatment; In step 4), the rolled steel plate is water-cooled at a rate of 5-8°C / s and the water-cooling stop temperature is Ms-Bs. Then, it is held at 300-400°C for more than 8 hours and then air-cooled to room temperature. The martensitic transformation start temperature Ms = 539-423°C-30.4Mn-12.1Cr-7.5Mo and the bainitic transformation start temperature Bs = 830-270°C-90Mn-70Cr-83Mo. The units of Ms and Bs are both °C. The symbols of each chemical element in the formula represent the values ​​before the percentage sign of the mass percentage content of the corresponding element.

8. The method as described in claim 7, characterized in that, In step 2), the heating temperature is 1100~1250℃ and the holding time is 1~3h.

9. The method as described in claim 7, characterized in that, In step 3), rolling includes roughing and finishing. The final rolling temperature of roughing is 950~1150℃; the final rolling temperature of finishing is 850~950℃.

10. The method as described in claim 7, characterized in that, In step 5), the heat treatment includes tempering at a temperature of 450-650°C and a holding time of 30-120 minutes.

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