Plastic mold steel occupies a high proportion in the amount of mold steel, which is mainly used to manufacture molds for production of plastic products. Working characteristics of plastic mold are: it must not only bear erosion and wear of hot plastic melt, but also withstand corrosion of harmful gases such as chlorine and fluorine. Its working environment is complex, there are many types of alloy elements, and internal influence mechanism is diverse. In order to improve quality of plastic products and expand application fields, plastic mold steel is developing in the direction of high precision, high efficiency and long service life. At present, there are two main ways to develop new plastic mold steels: ① According to design principle of alloying composition of steel, combined with facts of China's resources and metallurgical technology, develop high-performance, low-cost new steel grades; ② Introduce some steel grades with excellent performance from abroad, and develop them locally to replace expensive imported steels. China clearly stated in "Thirteenth Five-Year Plan": In terms of plastic mold steel, for 718 plastic mold steel, it is necessary to achieve stable quality of large-section mold steel forging materials and provide mold steel production enterprises with high-temperature homogenization, forging, and pre-heat treatment technology output process parameters. In addition, based on study of relationship between composition, process, organization and service performance, large cross-section, high uniformity, energy-saving bainite pre-hardened plastic mold steel and highly polished, easy-cutting, corrosion-resistant mirror plastic mold steel are developed.

"Modern industry, leading mold", current steel powers such as Sweden, Germany, Japan, France, etc., with advanced technology, complete range of mold steel, has formed several world-renowned brands, in addition to familiar Sweden ASSAB, Japan Greatly equivalent companies, such as Germany's Griez, Austria's Bole, Japan's High Frequency and Hitachi Metals, and Sweden's Scanner Group, are also well-known in the industry.
Compared with European and American countries, China's mold steel research started late. At the beginning, there was no special steel series for plastic mold steel. Steel types used were more and mixed, including carbon steels such as 20 steel and 45 steel, T8 steel and T10. Carbon tool steels such as steel, alloy structural steels such as 40Cr and 38CrMoAl, alloy tool steels such as Cr12, and even general-purpose mold steels such as H13, because they are not special steels, have short tool life, high consumption, and low product quality. Although, since 1990s, China's plastic mold steel has been catching up at a high speed, there are still following three main problems: ① Supply of low-end product market exceeds demand. Due to low entry barrier for low-end product market, domestic manufacturers have entered into vicious competition for market; ②Similar high-end products rely heavily on imports, and price of imported products is 3-6 times that of domestic similar products; ③Most domestic special steel enterprises cannot achieve long-term stable control of products in production equipment and technology application, quality of different batches of products varies.

P20 steel, a plastic mold steel used as a large-section mold, was originally proposed by United States. It is a medium-carbon Cr-Mo plastic mold steel. It contains 1.85%Cr, 1.30%Mn, 0.40%Mo and other alloy elements to ensure hardenability. At present, large plastic products include home appliance shells, car center consoles, etc., whose maximum size is more than 600mm. In order to make overall hardness of such a large module uniform, hardenability must be further improved. Traditional P20 and alloy structural steel can no longer meet requirements of large plastic products. Therefore, mold steel manufacturers continue to improve on the basis of P20 steel based on my country's standards to form their own brand, specific brand and composition are shown in Table 1.

 

KPM30 is a P20 improved plastic mold steel introduced by high frequency. Compared with P20 steel, its hardness, cutting performance and hardenability are better. Under same cutting length, tool wear can be reduced by 50%. Hardness uniformity from surface to core of 500mm thick specimen is significantly improved.
ASSAB's 718 series includes 718 steel, 718H steel, 718HH steel, etc., of which 718 steel is added with 0.8%~1.2% Ni on the basis of P20 steel, so that its toughness, plasticity and corrosion resistance can be significantly improved. Further research by FBEAUDET found that adding 0.35% V to P20 + 0.5% Ni steel can improve hardenability of module. Under same organizational conditions, size of module can be increased by 88%. Test results are also applicable to 718 steel .
1.2738 steel of Glitz is same in composition as 718 steel, and its factory hardness is 32~36HRC, which can be applied to large mold parts such as bumpers, instrument panels, and large trash cans. Chemical composition of 738HH steel is 0.27%C, 0.30%Si, 1.55%Mn, 1.35%Cr, 1.00%Ni. It is pre-hardened at 34~38HRC. It is suitable for mold parts of large automobile covering parts such as bumpers, instrument panels, door panels with higher surface requirements.
During "Eleventh Five-Year Plan" period in China, first, Shanghai University Advanced Mould Material and Application Technology R&D team and Baosteel Special Steel, Hongsheng Heavy Industries and other companies successfully trial-produced large-section pre-hardened plastic mold steel SDP2738. Based on 718 steel, SDP2738 reduces Ni content and increases Mn content, which reduces production cost. It adopts precise air-cooling and water-cooling quenching technologies to make maximum thickness of module reach 1250mm and cross-section hardness fluctuation≤4HRC. It is used in automobile front and rear bumpers, large household appliances, etc. It can replace imported 718HH and other large-section pre-hardened plastic mold steels. Another large cross-section bainite pre-hardened plastic mold steel SDP1 (patent authorization: ZL201210167042.5) developed by Shanghai University has a production cycle shortened by half compared with 718 steel, with a thickness of up to 1100mm, and a finished product hardness fluctuation of ≤3HRC, which can replace imported 718, 2738, P20 and other steels. It is suitable for large-section plastic molds of household appliances and automotive interior molds. Jiang Laizhu invented a bainite large-section plastic mold steel optimized by alloy composition, adding 0.005%~0.008%N and trace Ti elements, Ti/N=3/4, and controlling carbon content to 0.20%~0.30 %, after rolling/forging, no need of quenching and tempering treatment to obtain granular Béné Body and good mechanical properties, maximum thickness of module can reach 500mm.

Reducing cost of plastic mold manufacturing, shortening mold manufacturing cycle, and improving mechanical properties will become main trend of product research and development. In response to above problems, in addition to improving cutting technology and cutting tool quality, processing performance of mold material itself must also be optimized. In order to improve cutting performance of pre-hardened steel, researchers added free cutting elements such as S, Sn, Pb, Bi, Ca, Cu, Ti, Mg to steel. Research on free-cutting steel abroad started earlier. In 1920, United States successfully developed and produced free-cutting steel for the first time. It has more than 90 years of development history. At present, more than half of international free-cutting steel market is occupied by Japan and United States, especially in the field of free cutting plastic mold steel, Japan has an absolute advantage. Representative brands and chemical compositions are shown in Table 2.

 

Japanese high-frequency steel mill currently promotes following two types of free cutting plastic mold steel: ①KAP90F is an H13 improved Cu-based free-cutting steel, with a Cu content of 0.23%~0.28%, and other alloy element contents of 0.39%C, 1.00%Si, 0.45% Mn, 5.30%Cr, 1.10%Mo, 0.55%V, factory hardness is 38~42HRC, mainly used as push rod and mold standard parts; ②KAP65 (unpublished composition) is Ni-Al compound precipitation hardened high-grade plastic mold steel , with a hardness of 40HRC, mainly used for precision molding products.
NAK55 of Japan Datong is S-Cu series easy-cutting age-hardening plastic mold steel. Factory hardness is 37~43HRC. Addition of S and Cu makes it have excellent hardness and cutting performance, and can be used for high-quality, high-hardness precision molds.
CorroplastFM steel was developed by ThyssenKrupp Steel Plant in Germany. It added 0.1%S on the basis of 1.2085 steel to increase Mn content and reduce C content, thereby improving hardening under condition of ensuring corrosion resistance. DFRIE study found that tool for processing 1.2085 steel showed serious damage after processing 22 grooves and could not be used; and tool for processing CorroplastFM steel was still in good condition after processing 52 grooves, and there was no obvious damage. Cutting performance of CorroplastFM steel is better than 1.2085 steel.
1.2312 steel of Griez Steel Plant is added with about 0.06% S on the basis of P20, thus becoming a sulfur-based free cutting plastic mold steel. Delivery state is 29~34HRC, maximum cross-sectional size is about 400mm, and hardness uniformity is good. It is mainly used for production of slightly higher mold bases and mold cores with good processing performance.
China began to study free-cutting steel in 1953. The first to develop free-cutting steel was S series, followed by Ca series and Ti series, and then developed rare earth free-cutting steel. Cr2MnWMoVS steel is a free-cutting precision mold steel jointly developed by Huazhong University of Science and Technology and Shougang Special Steel. Sulfur content is 0.08% to 0.15%. When S is present in the steel as MnS type non-metallic inclusions, it can not only reduce cutting force during cutting, but also lubricate tool and prolong service life of tool. However, if a low melting point FeS-Fe eutectic or a solid solution of MnS and FeS is formed, hot brittleness may occur. Research by Ye Yuehua and others found that by controlling manganese-sulfur content ratio ≥4, hot brittleness can be avoided. At this time, sulfur and manganese in the steel all form MnS. In addition, presence of silicon degrades cutting performance and increases decarburization tendency, and its content should be controlled below 0.4%.
P20S, P20BS, P20SRe, and P20BSCa are all developed by Huazhong University of Science and Technology in the 1990s, and all belong to improved steel grades of P20. Among them, alloying elements of P20BSCa are 0.37%C, 0.78%Si, 1.43%Mn, 0.99%Cr, 0.22%Mo, 0.09%V, 0.0023%B, 0.075%S, 0.0002%Ca, alloying feature is add Cr, V and trace B as strong and toughening elements, which can effectively ensure hardenability. Proper amount of Mo can play a role in purifying grain boundary, thereby suppressing occurrence of temper brittleness. S-Ca composite system is based on S-type free-cutting steel. With addition of an appropriate amount of Ca, it changes size and shape of MnS inclusions, and MnS inclusions are changed from strips to spindles, which further improves cutting performance and lubrication.
SDPCu is a plastic mold steel developed by Shanghai University. Its heat treatment process is quenching 1030℃ + tempering 500~600℃, quenching and tempering hardness is 35~45HRC. Plastic mold steel improves cutting performance of 4Cr13 by adding Cu element, reaches level of 4Cr13S free-cutting steel, while corrosion resistance and dermatoglyph etching performance are also improved, and it can replace imported M333 steel. Geng Hongming and other studies found that reason why Cu can improve cutting performance is that copper-graphite composite phase is produced. Composite phase can act as a solid lubricant to lubricate tool, and form a solid lubricating film on the blade, reduce cutting heat during cutting and tool wear.

Plastic products contain a large amount of synthetic resins and fillers, fillers, stabilizers, lubricants, colorants and other additives. These additives will precipitate corrosive gases containing chlorine, fluorine and other elements in molten state, even in high humidity In the air, mold will also be corroded. At present, main way to improve corrosion resistance of plastic mold steel is to add Cr, N and other elements to steel.
Foreign mold makers usually use martensitic stainless steel as a molding material for plastic products. Among them, 420 martensitic stainless steel is a classic steel type, which is a typical Cr13 type martensitic stainless steel, that is, 2Cr13 martensitic stainless steel. Cr13 type martensitic stainless steel refers to stainless steel with a Cr content of about 13% and a low carbon content (≤0.45%). It also includes martensitic stainless steels with other alloying elements added to improve stainless steel performance. At present, most of steel grades occupying high-end stainless corrosion-resistant steel market are 420 stainless steel and its improved types. Specific grades and chemical compositions are shown in Table 3.

 

On basis of 420 steel and combined with its own advantages, Griez Steel Plant has successively developed 1.2083 series and 1.2316 series. 1.2083 steel is generally delivered in annealed state. Alloy elements are mainly 0.40%C and 13%Cr, which are mainly used for corrosion resistance plastic mold with excellent polishing performance. 1.2083VictoryESR is basically same as 1.2083 steel in alloy composition, but adopts more advanced electroslag remelting technology in metallurgy, so as to obtain a cleaner, uniform and dense steel ingot. Application is similar to 1.2083 steel, but it can be applied to molds with higher roughness requirements. 1.2083VictoryESR+N adds 0.25%V to 1.2083VictoryESR, reduces 0.02%C, and adds appropriate amount of N to further improve corrosion resistance of steel. 1.2085 steel is a corrosion-resistant pre-hardened plastic mold steel, its factory hardness is 28~33HRC, alloy elements include 0.33%C, 16%Cr, 0.25%Ni, 0.07%S, addition of S improves processing performance of steel , is mainly used for inserts and mold bases.
G-STAR of Datong Special Steel is an improved version of 1.2085 steel. Its alloy elements include 0.35%C, 16%Cr, 1%Mo and an appropriate amount of S. Pre-hardened state 31~34HRC is mainly used for corrosion-resistant mold base.
Hitachi Metal's HPM38 steel is a 13Cr series Mo-containing corrosion-resistant steel. State of delivery is quenched and tempered, with a hardness of 50~55HRC. It is mainly used as a mold for tape shells, medical devices, dishes, etc. CENAl pre-hardened stainless corrosion-resistant steel achieves perfect combination of corrosion resistance and polishing performance. Cr, Al, Ni and other elements are precipitated in the form of CrNiAl, Cr, NiAl, etc., which plays a role of precipitation hardening, and is often used as corrosion resistance, high mirror requirements, high gloss and traceless molding.
ASSAB MIRRAX40 is a corrosion-resistant pre-hardened die steel. Factory hardness can reach 360~400HB, and alloy element content is 0.21%C, 0.9%Si, 0.45%Mn, 13.5%Cr, 0.2%Mo, 0.25%V , 0.6% Ni, appropriate amount of NoN and Ni can effectively improve corrosion resistance, suitable for injection molds and blow molds of corrosive plastics. Carbon content of CORRAX is only 0.03%, nickel content is 9.2%, and aluminum content is 1.6%. It can be age-hardened to 40~51HRC and has excellent corrosion resistance. STAVAXESR is a plastic mold steel material with excellent corrosion resistance and polishing properties. Studies have shown that the higher tempering temperature, the worse corrosion resistance; corrosion resistance of slow cooling samples is worse than that of fast cooling.
In China, Cr-based mold steel is currently the most widely used corrosion-resistant plastic mold steel. Common ones are: 2Cr13, 3Cr13, 3Cr17, 4Cr13, 9Cr18, 1Cr17Ni2, 9Cr18Mo, X36CrMo17, 0Cr16Ni4Cu3Nb (PCR), etc., of which 4Cr13 belongs to medium carbon high chromium stainless steel, its alloy composition is 0.36%~0.45%C, Si≤0.60%, Mn≤0.80%, 12%-14%Cr, 0.60%Ni, S≤0.03%, P≤0.04%, Cr can improve hardenability of iron-chromium alloys and significantly improve corrosion resistance, because Cr has a lower corrosion potential than iron and has a stronger passivation ability than iron. In Fe-Cr alloys, increase in Cr content will cause corrosion potential and critical blunt potential of alloy to move to negative potential. Passivation current density gradually decreases, which means that the higher chromium content in the alloy, the easier it is to passivate and the better corrosion resistance.
PCR steel is a maraging precipitation hardening stainless steel developed by Shanghai Institute of Materials. About 0.3% Zr is added to it, which can effectively refine grain. After high temperature solid solution and aging treatment, steel has good corrosion resistance. Its corrosion resistance is much better than that of American 17-4PH steel.
As a new type of steel, high nitrogen steel has no unified standard definition. Mandley defines high-nitrogen steel as steel in which dissolution of nitrogen in steel reaches saturation under certain conditions; BERNS defines it as "stainless steel with 0.9% nitrogen or tool steel with 2% nitrogen". SW333 is a nitrogen-containing corrosion-resistant plastic mold steel independently developed by Baosteel Special Steel. Its composition is characterized by reducing carbon content and increasing nitrogen content. Improvement of its composition reduces precipitation of M23C6 type high chromium carbides and increases precipitation of Cr2N type nitrides [32,33], which increases chromium content in matrix, thereby changing corrosion potential and improving corrosion resistance. Research by Chen Zhuo et al. showed that corrosion rate of SW333 steel is lower than that of 4Cr13 steel, and self-corrosion potential is higher than that of 4Cr13 steel, that is, corrosion resistance of SW333 steel is better than that of 4Cr13 steel. SDP333 (3Cr13N) steel was developed by Shanghai University on the basis of 3Cr13 steel. Its quenching and tempering hardness is 35~45HRC. Nitrogen element replaces part of carbon element, which improves corrosion resistance, machining performance and comprehensive performance. Its overall performance is better than 4Cr13 steel and can replace imported M333 steel. Zhang Yongjun and others found that main reason for corrosion resistance of high nitrogen martensitic stainless steel 3Cr13N is that number of M23C6 type inclusions is less than that of martensitic stainless steel 3Cr13.

With improvement of people's quality of life, quality requirements for plastic products are getting higher and higher, and surface roughness value of mold cavity is mainly reduced by polishing to ensure quality of molded plastic parts. In production process, smooth mold cavity surface not only makes molded product more beautiful, but also facilitates demoulding and shortens production cycle. Relevant studies have found that good mirror polishability of mold steel (surface roughness value of molding surface Ra≤0.11μm) mainly depends on hardness, purity, grain size, inclusion morphology, microstructure compactness and uniformity of steel. According to needs of customers, foreign manufacturers have introduced a large number of high mirror or even super high mirror plastic mold steels. Specific grades and chemical compositions are shown in Table 4.

 

Japan’s Datong Steel Plant currently promotes following 4 types of plastic mold steels for mirror polishing: ①PAT868 (composition not disclosed) is the latest multi-purpose plastic mold steel developed. After hardening, it can reach 46~50HRC, polishing performance can reach mirror level, it has excellent toughness; ②S-STAR steel alloy element content is 0.38%C, Si≤1%, 13.5%Cr, 0.1%Mo, 0.3%V, can be pre-hardened to 31~34HRC. After polishing, mirror finish can reach 12000# or more, which can be used for ultra-mirror corrosion-resistant precision molds; ③NAK80 is a high-mirror pre-hardened steel promoted by Datong, which belongs to Ni-Al-Cu series alloy, and composition is 0.15%C, 0.3%Si, 1.5%Mn, 0.3% Mo, 3% Ni, 1% Al, 1% Cu. After polishing with whetstone, sandpaper and diamond paste, mirror finish can reach 5000# or more, mainly used for high-performance, precision plastic molds, transparent plastic products and other molds that value mirror grinding; ④NAK-PRM is a pre-hardened steel with super mirror surface, high toughness and corrosion resistance, and mirror surface degree can reach above 8000# after polishing.
Hitachi Metal’s HPM-PRO (composition not disclosed) is an improved pre-hardened steel of P21, 40HRC precision processing steel, mirror finish can reach 8000# or more, toughness and precision etching, can be used for mobile phone frames, automobile lights.
High-frequency KAP88 (composition not disclosed) has a high hardness of 40HRC. It is a high-grade plastic mold steel that is hardened by precipitation of Ni-AI compound. After grinding and polishing, its mirror surface can reach more than 8000#. It is mainly used for precision plastic molds and structural parts.
ASPM and ASPM-VIP of Scana Group of Sweden are both mirror-grade pre-hardened plastic mold steels. Two components are basically same, both are 0.27%C, 0.25%Si, 1.50%Mn, 1.40%Cr, 1.1%Ni, 0.55 %V, but ASPM-VIP adopts electroslag remelting technology, which has higher purity than ASPM. The former is suitable for transparent plastic parts, household appliances and automotive plastic molds with high polishing, high etching requirements and dimensional stability, and the latter can be used for transparent parts with higher polishing and etching requirements.
ASSAB’s POLMAX has excellent polishability, good workability, good corrosion resistance and wear performance. Its factory hardness is 200HB. After austenitizing at 1000~1050℃, hardness can reach 46~52HRC. It is mainly used for molding lens and CD mold parts.
Domestic mold steel for mirror polishing can be divided into low nickel and high nickel series according to difference of Ni content. 25CrNi3MoAl is a low-nickel age-hardening high-precision plastic mold special steel developed by Huazhong University of Science and Technology. Original design composition of this steel is S≤0.03%, but when it is included in YB/T094-1997 standard, it is changed to contain S≤0.1% to improve its cutting performance. 06Ni steel and 10Ni3MnCuAl-Mo (PMS) steel from Shanghai Institute of Materials are also domestic excellent low-nickel age-hardening mirror plastic mold steels.
18Ni(250), (300), (350) series are relatively common high-nickel maraging steels. According to different Ti content, they are divided into five grades of 200, 250, 300, 350 and 400ksi. After solution treatment, an ultra-low-carbon Fe-Ni lath martensite matrix is produced. During aging treatment, a dispersed phase will be precipitated to strengthen steel. However, due to high nickel content, price is expensive, and actual use is not much. .
SDP80 is an age-hardening high-mirror plastic mold steel developed by Shanghai University. By adjusting microstructure, addition of Ni, Al, Cu and other alloying elements is controlled to improve machining performance of material and grain etching performance. It is suitable for molds with high polishing or skin etching requirements, and can replace imported NAK80.

During "Eleventh Five-Year" and "Twelfth Five-Year" period, with strong support of state, a number of large-scale special steel enterprises with advanced technology have emerged in China, such as Baosteel Special Steel, CITIC Pacific, Great Wall Special Steel, Northeast Special Steel, etc . On the one hand, these companies have made considerable progress in composition design, alloying principles and steelmaking technology, and some of their products have reached or approached international advanced level; on the other hand, in terms of plastic mold steel grades, through introduction of foreign excellent steel grades and localized designs, types of plastic mold steels in national standard have been relatively complete.
While making great progress, domestic plastic mold steel market also has some shortcomings: ① Quality of high-end mold steel is far from that of similar foreign products, because these steel companies have not fully grasped qualitative, quantitative relationship between mold failure law, organization and performance; ②Quality of similar products of different domestic companies is uneven. Current domestic plastic mold steel market is relatively chaotic. Different steel companies have no unified product standards and no standardized production processes.
In view of above problems, a simple forecast of future development direction of domestic plastic mold steel. In terms of plastic mold steel with large cross-section and high uniformity, following trend of large-scale development of plastic molds, its overall demand continues to increase. In view of shortcomings of quenched and tempered hardened steel in production process, such as complex technology, high energy consumption and long production cycle, non-quenched and tempered plastic mold steel that directly reaches required hardness after forging or rolling will have broad development prospects.
In terms of free cutting plastic mold steel, there is a large gap between domestic steel and foreign excellent products, mainly because of unstable metallurgical quality and poor control of types, quantities and shapes of inclusions. Foreign experience shows that production of free-cutting steel through continuous casting process can effectively reduce existence of segregation and improve shape and distribution of inclusions. Therefore, our country should strengthen research on continuous casting process and further improve quality of existing free-cutting steel.
In terms of corrosion resistant plastic mold steel, reducing production costs is the key. Increasing Mn, Cu, C, N and other elements to reduce content of Ni in corrosion resistant steel will be an important research direction. My country is still in a backward position in the research of high nitrogen steel. Strengthening research on high nitrogen steel will enhance overall competitiveness of my country's corrosion-resistant plastic mold steel.
In terms of plastic mold steel for mirror polishing, control of inclusions and development of stable polishing technology will be the key. While improving metallurgical quality, mechanical polishing process, electrochemical polishing process, ultrasonic polishing process, electrodeposition process and spraying ceramic process are organically combined, finally steel achieves the best mirror effect when composition and structure are determined.
To sum up, my country should continue to strengthen development and research of high-end die steel, combining its own energy and technology advantages, strive to develop into a powerful die country. Domestic plastic mold steel market should gradually be standardized, improve plastic mold steel application system, establish unified testing and production standards, gradually form its own series (such as P series in United States), so that production of plastic mold steel in China can be standardized and refined .