CN113388789B - Low-cost plastic die steel with uniform hardness in cross section direction and production method thereof - Google Patents

Low-cost plastic die steel with uniform hardness in cross section direction and production method thereof Download PDF

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CN113388789B
CN113388789B CN202110734040.9A CN202110734040A CN113388789B CN 113388789 B CN113388789 B CN 113388789B CN 202110734040 A CN202110734040 A CN 202110734040A CN 113388789 B CN113388789 B CN 113388789B
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steel
equal
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temperature
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CN113388789A (en
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习天辉
卜勇
张开广
杜涛
杨治争
张欢
王宪军
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Baowu Group Echeng Iron and Steel Co Ltd
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    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21CPROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/0006Adding metallic additives
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21CPROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/10Handling in a vacuum
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226Hot rolling
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/02Ferrous alloys, e.g. steel alloys containing silicon
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/06Ferrous alloys, e.g. steel alloys containing aluminium
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32Ferrous alloys, e.g. steel alloys containing chromium with boron
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21CPROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
    • C21C7/00Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/0006Adding metallic additives
    • C21C2007/0018Boron
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00Microstructure comprising significant phases
    • C21D2211/008Martensite

Abstract

The invention relates to a low-cost plastic die steel with uniform hardness in the cross section direction and a production method thereof, wherein the steel comprises the following chemical components in percentage by mass: c: 0.40 to 0.50%, Si: 0.70-1.00%, Mn: 1.60-2.00%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, Cr: 0.80-1.00%, Als: 0.010-0.050%, Ti: 0.010-0.030%, B: 0.0010-0.0020%, N: 0.0030 to 0.0080 percent, and the balance of Fe and inevitable impurities; meanwhile, the chemical components also need to satisfy the formula: b = -0.001507+0.0455Als + 0.0318Ti + 0.171N; the steel of the invention does not add noble metal elements such as Mo, Cu and the like, only adopts vacuum treatment to refine the molten steel, the rolling process does not need to be limited, and the post-rolling normalizing heat treatment commonly adopted by plastic die steel is saved through the production process path of post-rolling stacking cooling and tempering, so that the plastic die steel is produced at low cost; the steel has high hardenability and uniform hardness distribution along the section, and is suitable for manufacturing plastic molds with strict requirements on dimensional accuracy, surface roughness and the like and complicated cavities.

Description

Low-cost plastic die steel with uniform hardness in cross section direction and production method thereof
Technical Field
The invention relates to the technical field of die steel manufacturing, in particular to low-cost plastic die steel with uniform hardness in the cross section direction and a production method thereof.
Background
The die is an important basic process equipment in the manufacturing industry, is widely applied due to high production efficiency, good product quality, low material consumption and low production cost, and is essential process equipment in modern industry, particularly in the industrial fields of automobiles, motorcycles, aviation, instruments, medical instruments, electronic communication, weapons, household appliances, hardware tools, daily necessities and the like. The mold has high industrial relevance, is one of important guarantees of industrial upgrading and technical progress of various related industries in the manufacturing industry, has become an important mark for measuring the level of the manufacturing industry of one country, and is also one of important guarantees for keeping international competitiveness of industrial products of one country.
The mold industry is more horizontal and material application is critical. Because of improper material selection and use, the die fails prematurely, which accounts for about 45% or more of the failed die. The mold steel is generally classified into three types according to the use: cold work die steel, hot work die steel and plastic die steel, wherein the plastic die steel accounts for more than 70% of the whole die steel, and becomes one of the focuses of steel mill market competition. Because the cavity of the plastic mould is complex, the requirements for dimensional accuracy, surface roughness and the like are strict, and the plastic mould is developing towards large-scale and precision, the plastic mould steel has excellent cutting processability, mirror polishing property and pattern etching property; the hardness of the large-section module is uniformly distributed along the section, and the basic performances of wear resistance, electric spark processing property, weldability, corrosion resistance of plastic products and the like are all good, and the metallurgical essence of the performance requirements is that the hardenability of the plastic die steel is high, and the structure is uniform along the section direction.
Product quality and production costs are the lives of the enterprises. On the premise of ensuring the product quality, reducing the production cost becomes one of the main directions of the efforts of enterprise technicians. There are some reports in the prior art on low cost plastic mold steels and methods for their production.
The Chinese patent application with the application number of CN201710764360.2 discloses a low-cost plastic die steel and a production method thereof, 0.35-0.45% of C, 0.20-0.40% of Si, 1.65-1.85% of Mn, 0.60-0.80% of Cr and 0.025-0.040% of Als are adopted, C + Mn + Cr-Cr/C-C/Cr are limited, normalizing and tempering heat treatment is adopted after hot rolling, and the performance of the steel reaches HRC 28-33. The application limits C + Mn + Cr-Cr/C-C/Cr, increases the production control difficulty and reduces the qualification rate; two times of heat treatment are adopted, so that the heat treatment capacity is occupied, and the production cost is increased.
The Chinese patent application with the application number of CN201711352948.3 discloses 'hot-rolled and tempered bainite steel', which adopts 0.31-0.41% of C, 0.31-0.42% of Si, 0.65-0.95% of Mn, 1.35-1.65% of Cr, 0.12-0.16% of Mo and 0.025-0.040% of Als to limit the ratio of (C + Si)/(Mn + Cr), and the performance of the steel is HRC 28-38 by conventional two-stage hot rolling, air cooling to room temperature and tempering. The application adds the precious metal Mo, so that the production cost is increased; the (C + Si)/(Mn + Cr) is limited, the production control difficulty is increased, and the qualification rate is reduced.
The Chinese patent application with the application number of CN201510870739.2 discloses that the components used in the method for manufacturing the economical plastic die steel plate are 0.40-0.50% of C, 0.40-0.70% of Si, 1.10-1.50% of Mn, 0.60-0.80% of Cr, 0.10-0.20% of Mo and 0.001-0.0012% of B, the steel is refined outside a ladle furnace LF and a vacuum RH furnace, the casting blank is rolled at 1080-1150 ℃, the finish rolling temperature is-910 ℃, the start cooling temperature is 750-800 ℃, the re-reddening temperature is 500-550 ℃, water is cooled to room temperature and then tempered at 450-600 ℃, the heat preservation time is 1.5-2.5min/mm, the performance of the prepared steel is HRC 28-33, and the maximum section hardness difference is 5 HRC. The application has the disadvantages that precious metal Mo needs to be added, so that the production cost is increased; LF and RH external refining are adopted, so that the production time is prolonged, and the production cost is increased; and (3) after rolling, the steel plate with the C content of 0.40-0.50% is subjected to water spraying cooling until the temperature reaches room temperature, so that the risk of cracking of the steel plate is increased.
Chinese patent application No. CN201310654945.0 discloses an economical non-quenched and tempered microalloyed plastic die steel plate and a manufacturing method thereof, which comprises the following components of 0.32-0.40% of C, 0.20-0.50% of Si, 1.00-1.50% of Mn, 1.20-1.70% of Cr, less than or equal to 0.20% of Mo, less than or equal to 0.0025% of B, less than or equal to 0.03% of Ti and less than or equal to 0.03% of Nb, and LF and RH are adopted for external refining; heating the casting blank at 1200-1230 ℃, finish rolling at 900-950 ℃ and finish rolling at 870-920 ℃ to obtain a steel plate with the thickness of 20-80 mm; controlling cooling after rolling, performing cooling with different strengths according to the thickness of the steel plate, controlling the cooling speed to be 3-8 ℃/s, controlling the temperature of red return to be less than 500 ℃, air-cooling to 250 ℃, taking off the line and carrying out cooling in a heaped mode; tempering temperature is 550-600 ℃, and heat preservation time is 250-280 min; the section hardness of the obtained steel plate is 310-340 HB. The application has the disadvantages that LF and RH external refining are adopted, so that the production time is prolonged, and the production cost is increased; the casting blank needs to be heated and installed, so that the production organization difficulty is increased; and after the steel plate with the C content of 0.32-0.40% is rolled, the water-spraying cooling is carried out until the temperature of red return is less than 500 ℃, so that the risk of cracking of the steel plate is increased.
The Chinese patent application with the application number of CN201010289367.1 discloses a production process of a plastic die steel thick plate with uniform section hardness, which adopts the components of 0.30-0.36% of C, 0.40-0.60% of Si, 0.90-1.20% of Mn, 1.40-1.70% of Cr, 0.30-0.50% of Mo, less than or equal to 0.03% of Ti and less than or equal to 0.002% of B, and adopts two-stage controlled rolling, the temperature of a lower line after rolling is below 250 ℃, the steel plate is stacked, cooled for 24 hours and then tempered at 550-580 ℃ to obtain the plastic die steel thick plate with the uniform section hardness of 60-120 mm, and the section hardness of the obtained steel plate is 33-35 HRC, and the difference of the same section hardness is 2 HRC. The application has the disadvantages that precious metal Mo needs to be added, so that the production cost is increased; the heating time of hot-delivery hot-charging casting blanks in the furnace is 6h, the in-furnace production time is increased, the fuel and process cost is increased, and the productivity is also occupied; in terms of performance, the difference of the hardness of the same section is 2HRC, and the hardness uniformity in the section direction is not the same concept.
The publication 'development of air-cooled economical plastic die steel P20' (Fuqingxia, Qiuxihua. air-cooled economical plastic die steel P20) [ J ] Jiangxi metallurgy, 2015 (35), 2: 4-7 ], wherein LF and RH are adopted for external refining of plastic die steel with the components of 0.37-0.42% of C, 0.30-0.50% of Si, 1.40-1.60% of Mn, 1.50-2.00% of Cr, 0.07-0.18% of Mo and less than or equal to 0.25% of Cu, the final rolling temperature in the rolling process is not lower than 950 ℃, air cooling is carried out after rolling, 560 tempering and 600 ℃ is carried out to obtain a bainite tissue with a whole section, the hardness of the steel plate is HRC 29-35, and the difference of the hardness and the plate is not more than 4 HRC. The method has the disadvantages that precious metal Mo needs to be added, so that the production cost is increased; LF and RH external refining are adopted, so that the production time is prolonged, and the production cost is increased. The publication "study on heat treatment and hardness of air-cooled economical P20 plastic die steel" (zekadi. study on heat treatment and hardness of air-cooled economical P20 plastic die steel [ J ]. southern metals, 2013 (194), 10: 18-20.) although a pilot plant solution, requires the addition of precious metal Mo, which increases production costs.
It can be seen that although the above inventions and publications refer to economical plastic die steel, these proposals start with the technical measures of adding precious metal Mo, limiting the composition, adopting LF and RH external refining, cooling the high C content steel plate by water spraying after rolling, etc., to obtain the steel plate with the desired properties.
Disclosure of Invention
The technical problem to be solved by the invention is as follows: the steel for plastic dies needs to have excellent cutting processability, mirror polishing performance and pattern etching performance; the hardness of the large-section module is uniformly distributed along the section, and basic performances such as wear resistance, electric spark processing property, weldability, corrosion resistance of plastic products and the like are all good, the metallurgical essence of the performance requirements is that the hardenability of the plastic die steel is high, the structure is uniform along the section direction, and on the premise of ensuring the two points, the production cost is reduced, so that the market competitiveness is improved, and the low-cost plastic die steel with uniform hardness in the section direction and the production method thereof are provided. The invention aims to produce plastic die steel at low cost by a production process path of stacking cooling and tempering after rolling on the basis of not adding precious metal elements.
The invention relates to a low-cost plastic die steel with uniform hardness in the cross section direction, which comprises the following chemical components in percentage by mass: c: 0.40 to 0.50%, Si: 0.70-1.00%, Mn: 1.60-2.00%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, Cr: 0.80-1.00%, Als: 0.010-0.050%, Ti: 0.010-0.030%, B: 0.0010-0.0020%, N: 0.0030 to 0.0080 percent, and the balance of Fe and inevitable impurities; meanwhile, the chemical components also need to satisfy the formula: b = -0.001507+0.0455Als + 0.0318Ti + 0.171N.
The thickness of the plastic die steel is 10-100 mm, the Rockwell hardness HRC 28-33, the maximum section hardness difference is less than or equal to 4HRC, and the metallographic structure of the die steel is tempered martensite and tempered troostite.
The invention relates to a production method of low-cost plastic die steel with uniform hardness in the cross section direction, which comprises the following steps: molten iron desulfurization, converter smelting, vacuum treatment, continuous casting, casting blank heating, rolling, stacking slow cooling after steel plate rolling and tempering heat treatment, wherein:
(1) and (3) vacuum treatment: the vacuum degree is 80-50 bar, and the vacuum treatment time is not less than 15 min; adding B after Ti treatment and 3-5 min before vacuum treatment;
(2) continuous casting: the superheat degree of the molten steel is 17-25 ℃, and argon sealing protection casting is adopted; the continuous casting blank is sent to a heating furnace after being hot-fed and hot-charged or put into a slow cooling pit for slow cooling, and the temperature of the hot-fed and hot-charged continuous casting blank is more than or equal to 420 ℃; when the casting blank enters the slow cooling pit, slow cooling is required to be carried out for more than 72 hours, and the temperature of the casting blank is less than or equal to 200 ℃;
(3) heating a casting blank: controlling the heating temperature of the casting blank to 1170-1230 ℃, and keeping the temperature for 20-35 min;
(4) stacking and slow cooling after rolling of steel plates: for steel plates with the thickness of more than or equal to 40mm, the stacking temperature is 540-620 ℃, the thickness of a steel plate with the thickness of more than or equal to 50mmQ355 is larger than or equal to the thickness of a steel plate with a lower line of the stacking position, the lower part of the stacking position is paved with a mat, the upper part of the stacking position is covered with a steel plate with the thickness of more than or equal to 50mmQ355, the temperature is not less than 500 ℃, and the steel plate is slowly cooled and stacked below 150 ℃;
(5) tempering heat treatment: the tempering temperature is 600-650 ℃, and the heat preservation time is 1.5-3.0 min/mm.
Preferably, the superheat degree of the molten steel is controlled to be less than or equal to 21 ℃ during continuous casting.
The reasons for the limited amounts of the chemical components in the present invention are detailed below:
c: c exists in the steel in the form of interstitial atoms, can effectively improve the hardness and hardenability of steel, is the most economic hardening element, and has a lower limit of 0.40% in order to ensure that the hardness of the steel reaches the design requirement and embody the low-cost idea; if the C content exceeds 0.50%, the hardness of the steel sheet will exceed the standard. Therefore, the C content of the steel of the present invention is determined to be 0.40 to 0.50%.
Si: the process of using silicon as a deoxidizer in the steelmaking process is mature, and the cost is low; si exists in a solid solution form in the steel, the strength and the hardness of the steel can be obviously improved, and the lower limit of the Si content is 0.70 percent; the high Si content brings the risk of cracks on the casting blank and the steel plate and great inconvenience to the production structure, so the upper limit of the Si content is 1.00%.
Mn: mn is dissolved in ferrite and austenite in a solid manner, so that an austenite region is enlarged, a phase change point in a steel cooling process is reduced, the hardenability of the steel is improved, and the strength and hardness of the steel are obviously improved; mn can also form MnS with a higher melting point with S in steel, so that a FeS film is prevented from being formed on a crystal boundary, and the hot brittleness of the steel is eliminated, so that the lower limit of the Mn content is 1.60%; however, when the Mn content is too high, the fluidity of the cast slab is lowered, and internal quality defects such as shrinkage cavities and bubbles are generated, so the upper limit of the Mn content is 2.00%.
P: p is a harmful element in steel, increases the cold brittleness of the steel, reduces the plasticity, and also causes cold cracks on a steel plate due to the segregation to a crystal boundary, but the reduction of the P content increases the production cost, so that the phosphorus content in the steel is required to be less than or equal to 0.015 percent.
S: s is also a harmful element in steel, so that the steel generates hot brittleness, the ductility and the toughness of the steel are reduced, the steel generates anisotropy, the molten iron S removing technology is mature and has little influence on the production cost, and therefore, the upper limit of the content of S is 0.005%.
Cr: the hardenability of steel can be improved, and particularly, for thick steel plates, a certain amount of Cr needs to be added to improve the hardenability so as to make up for strength and hardness loss caused by thickness; cr is also a medium strength carbide-forming element, which can form carbides in steel to increase the strength of the steel. However, since too high Cr and Mn are added to steel at the same time, surface cracks are easily formed during steel rolling, the Cr content is limited to 0.80 to 1.00%.
B: is an element for strongly improving the hardenability of steel, particularly, the steel plate with thick specification such as plastic die steel is more, and the addition amount of 0.0010% is indispensable. The upper limit of the content of B is limited to 0.0020% because B has a small atomic radius and is easily biased to a grain boundary, and increases the dislocation density of the adjacent region to serve as a hydrogen trap to induce cracking of the grain boundary at the position.
B = -0.001507+0.0455Als + 0.0318Ti +0.171N is also defined, which is a requirement that B function. B is very liable to form BN inclusions with N in molten steel to deteriorate the properties of the steel, and more disadvantageously, BN does not improve the hardenability of the steel. B can only exist in a simple substance form in steel to play a role of B in strongly improving the hardenability of the steel, so that the relation among Als, Ti, N and B content is limited, and the B can be ensured to exist in the simple substance form in the steel to effectively play a role of B in improving the hardenability of the steel.
The performance of the steel of the invention needs to be strictly controlled in production process parameters in addition to the above composition requirements, and the reason for controlling the production process parameters of the invention is detailed below:
the invention needs vacuum treatment, the vacuum treatment time is not less than 15min, because the cavity of the plastic mould is complex, the requirements on dimensional accuracy, surface roughness and the like are strict, the plastic mould is developing towards large-scale and precision, the large-section module needs to be uniformly distributed along the hardness of the section, namely the inherent quality of the plastic mould is strict, the vacuum treatment is required in the smelting process, in order to ensure that the gas in the molten steel is fully removed and the high-purity molten steel is obtained, the vacuum treatment time is not less than 15min, B is added after Ti treatment in the vacuum treatment process and before 3-5 min of vacuum treatment, and the B element is uniformly distributed in the molten steel in a simple substance form through vacuum circulation.
The invention controls the superheat degree of the molten steel to be less than or equal to 25 ℃, preferably the superheat degree of the molten steel to be less than or equal to 21 ℃, and the invention reduces the superheat degree of the molten steel during continuous casting, reduces the segregation of low-melting-point solute atoms and inclusions in the center of a casting blank through rapid and uniform solidification, obtains a homogenized casting blank, and lays a foundation for obtaining the hardness performance which is uniformly distributed along the cross section.
The invention needs the casting blank to be hot-fed and hot-charged or put into the slow cooling pit for slow cooling, the temperature of the hot-fed and hot-charged furnace is more than or equal to 420 ℃, and the casting blank is put into the slow cooling pit for slow cooling for more than 72 hours, because the C content of the steel is 0.40-0.50%, the Si content reaches 0.70-1.00%, the casting blank is easy to crack in the conventional air cooling process, and even the blank breaking accident occurs. If the casting blank is subjected to hot charging, the charging temperature is more than or equal to 420 ℃, because the bainite phase transformation of the steel occurs at the temperature of less than or equal to 400 ℃, the superposition of thermal stress and phase transformation structure stress easily causes the fracture of the casting blank. If the casting blank enters the slow cooling pit, the casting blank needs to be slowly cooled for more than 72 hours, the temperature of the casting blank is lower than 200 ℃, the structure transformation is completely finished at the moment, and the casting blank can be taken out of the slow cooling pit and enters a heating furnace.
The reason why the heating temperature of the casting blank is controlled to 1170-1230 ℃ is that the heating temperature is kept high enough to ensure that elements such as C, Si, Mn, Cr and the like are fully dissolved and uniformly distributed in high-temperature austenite, so that conditions are created for obtaining a homogenized steel plate, and the performance of the invention steel is prevented from being deteriorated due to poor structure caused by excessive growth of the high-temperature austenite.
The invention needs the stacking and slow cooling after the rolling of the steel plate, because if the cooling speed of the rolled steel plate is high, a martensite structure is formed, the process window of the subsequent tempering heat treatment is greatly reduced, the hardness exceeds the standard, and the risk of cracks and even breakage of the steel plate is caused by the superposition of thermal stress and structural stress; the steel plates are stacked and slowly cooled after being rolled, normalizing in a normalizing and tempering process route which is usually adopted by plastic die steel is also omitted, the process cost of normalizing heat treatment is saved, the capacity of a heat treatment production line is also saved, and other heat treatment products with high added values can be produced more to create effects indirectly. In order to achieve the purpose of slow cooling after stacking, for steel plates with the thickness of more than or equal to 40mm, the stacking temperature is 540-620 ℃, the lower padding and upper covering temperature of the stacking position is not less than 500 ℃, and the steel plate Q355 with the thickness of more than or equal to 50mm is just inserted into the lower line, so as to be slowly cooled, and the stacking is carried out below 150 ℃.
The steel needs to be tempered and heat-treated because the structure of the steel plate before the tempering and heat treatment is bainite and martensite, the hardness exceeds standard and is not uniform, and the maximum section hardness difference reaches 7 HRC. The tempering temperature is 600-650 ℃, the heat preservation time is 1.5-3.0 min/mm, the Rockwell hardness HRC of the plastic die steel is 28-33, and the maximum section hardness difference is less than or equal to 4 HRC.
Compared with the prior art, the invention does not add noble metal elements such as Mo, Cu and the like, thereby saving the alloy cost; in the process of refining the molten steel, only vacuum treatment is adopted to fully remove gas in the molten steel, so that high-purity molten steel is obtained, the production time is shortened, and the production cost is reduced; the rolling process is not limited, and the method is suitable for large-scale production operation; the steel plate is stacked and slowly cooled after being rolled, so that the phenomenon that the hardness exceeds the standard due to the fact that the process window of subsequent tempering heat treatment is greatly reduced because the steel plate forms a martensite structure due to high cooling speed is avoided, and the risk that the steel plate cracks and even breaks due to superposition of thermal stress and structural stress is also avoided; after the steel plates are rolled, the steel plates are stacked and slowly cooled without normalizing heat treatment, so that the process cost of one normalizing heat treatment is saved, the saved capacity can be used for producing other heat treatment products with high added values more, and indirect effects are created; the Rockwell hardness HRC 28-33 of the plastic die steel, the maximum section hardness difference of less than or equal to 4HRC, stable tissue and uniform hardness distribution along the section are suitable for manufacturing plastic dies with strict requirements on dimensional accuracy, surface roughness and the like and complex die cavities.
Drawings
FIG. 1 shows a typical metallographic structure (tempered martensite + tempered troostite) of the surface of a plastic mold steel according to the present invention;
FIG. 2 shows a typical metallographic structure (tempered martensite + tempered troostite) of a steel core of a plastic mold according to the present invention.
Detailed Description
In order to better explain the technical solution of the present invention, the technical solution of the present invention is further described below with reference to specific examples, which are only exemplary to illustrate the technical solution of the present invention and do not limit the present invention in any way.
The following table 1 is a list of values of chemical components of each example and comparative example of the present invention;
table 2 below is a list of values of process paths and partial process parameters of each example and comparative example of the present invention;
table 3 below is a list of values of some main process parameters different from the comparative examples according to the present invention;
table 4 below shows the mechanical property test results of the inventive and comparative examples.
Wherein the thicknesses of the products of examples 1-5 are respectively 10mm, 32mm, 67mm, 85mm and 100mm, and the thicknesses of the products of comparative examples are respectively 55mm and 40 mm.
The low-cost plastic die steel with uniform hardness in the cross section direction comprises the following chemical components in percentage by mass: c: 0.40 to 0.50%, Si: 0.70-1.00%, Mn: 1.60-2.00%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, Cr: 0.80-1.00%, Als: 0.010-0.050%, Ti: 0.010-0.030%, B: 0.0010-0.0020%, N: 0.0030 to 0.0080 percent, and the balance of Fe and inevitable impurities; meanwhile, the chemical components also need to satisfy the formula: b = -0.001507+0.0455Als + 0.0318Ti + 0.171N.
The production method of the low-cost plastic die steel with uniform hardness in the cross section direction, provided by the embodiments of the invention, comprises the following steps: molten iron desulfurization, converter smelting, vacuum treatment, continuous casting, casting blank heating, rolling, stacking slow cooling after steel plate rolling and tempering heat treatment, wherein:
(1) and (3) vacuum treatment: the vacuum degree is 80-50 bar, and the vacuum treatment time is not less than 15 min; adding B after Ti treatment and 3-5 min before vacuum treatment;
(2) continuous casting: the superheat degree of the molten steel is 17-25 ℃, and argon sealing protection casting is adopted; the continuous casting blank is sent to a heating furnace after being hot-fed and hot-charged or put into a slow cooling pit for slow cooling, and the temperature of the hot-fed and hot-charged continuous casting blank is more than or equal to 420 ℃; when the casting blank enters the slow cooling pit, slow cooling is required to be carried out for more than 72 hours, and the temperature of the casting blank is less than or equal to 200 ℃;
(3) heating a casting blank: controlling the heating temperature of the casting blank to 1170-1230 ℃, and keeping the temperature for 20-35 min;
(4) stacking and slow cooling after rolling of steel plates: for steel plates with the thickness of more than or equal to 40mm, the stacking temperature is 540-620 ℃, the thickness of a steel plate with the thickness of more than or equal to 50mmQ355 is larger than or equal to the thickness of a steel plate with a lower line of the stacking position, the lower part of the stacking position is paved with a mat, the upper part of the stacking position is covered with a steel plate with the thickness of more than or equal to 50mmQ355, the temperature is not less than 500 ℃, and the steel plate is slowly cooled and stacked below 150 ℃;
(5) tempering heat treatment: the tempering temperature is 600-650 ℃, and the heat preservation time is 1.5-3.0 min/mm.
TABLE 1 tabulated (wt,%) list of chemical compositions for each example of the invention and comparative example
Figure 881283DEST_PATH_IMAGE002
TABLE 2 tabulation of process paths and partial process parameter values for various examples and comparative examples of the present invention
Figure DEST_PATH_IMAGE004
Table 3 list of values of some main process parameters different from comparative examples in each example of the present invention
Figure DEST_PATH_IMAGE006
TABLE 4 mechanical property test results of inventive and comparative examples
Figure DEST_PATH_IMAGE008
Description of the drawings: in Table 4, the average value is shown in parentheses in the two columns of surface hardness and core hardness.
As can be seen from Table 4, the structures of the plastic die steel of the present invention are tempered martensite + tempered troostite, and the corresponding results of the tempered structures are stable and uniform in performance. For the surface hardness of 5 examples, the three HRC values measured according to the test standards of the hardness test were very small in difference from each other. The difference between the maximum and minimum values for the three HRC values of example 1 was only 0.5HRC, and the difference between the maximum and minimum values for the three HRC values of example 5 was 3.3 HRC. Similarly, in the case of the core hardness of 5 examples, the difference between the maximum value and the minimum value was 1.3HRC among the three HRC values of example 1, and the difference between the maximum value and the minimum value was 1.7HRC among the three HRC values of example 3.
The plastic mould cavity is complex and has strict requirements on dimensional accuracy and surface roughness, so the plastic mould steel has excellent cutting processability, mirror polishing property and pattern etching property; the hardness of the large-section module is uniformly distributed along the section, basic performances such as wear resistance, electric spark processing performance, weldability, corrosion resistance of plastic products and the like are all good, the metallurgical essence of the performance requirements is that the hardenability of the plastic die steel is high, the structure is stable along the section direction, the hardness distribution is uniform, and the evaluation index is the maximum section hardness difference HRC. As seen from Table 4, the maximum difference in the sectional hardness between the surface hardness and the core hardness of example 1 was the largest and reached 3.9HRC, and the maximum difference in the sectional hardness between the surface hardness and the core hardness of example 2 was the smallest and reached 2.9 HRC. The Rockwell hardness HRC 28-33 of the plastic die steel, the maximum section hardness difference of less than or equal to 4HRC, stable tissue and uniform hardness distribution along the section are suitable for manufacturing plastic dies with strict requirements on dimensional accuracy, surface roughness and the like and complex die cavities.
The embodiments described above are merely specific examples of the present invention exemplified for explaining the present invention, and do not limit the present invention in any way, and any insubstantial changes from the contents and forms described above, which do not depart from the scope of the claims of the present invention, should be construed as falling within the scope of the claims of the present invention. The invention is not limited to the specific embodiments described above.

Claims (3)

1. A low-cost plastic die steel with uniform cross-sectional direction hardness is characterized by comprising the following chemical components in percentage by mass: c: 0.40 to 0.50%, Si: 0.70-1.00%, Mn: 1.60-2.00%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, Cr: 0.80-1.00%, Als: 0.010-0.050%, Ti: 0.010-0.030%, B: 0.0010-0.0020%, N: 0.0030 to 0.0080 percent, and the balance of Fe and inevitable impurities; meanwhile, the chemical components also need to satisfy the formula: b = -0.001507+0.0455Als + 0.0318Ti + 0.171N;
the production method of the low-cost plastic die steel with uniform hardness in the cross section direction comprises the following steps: the method comprises the following steps of molten iron desulphurization, converter smelting, vacuum treatment, continuous casting, casting blank heating, rolling, stacking slow cooling after steel plate rolling and tempering heat treatment, and specifically comprises the following steps:
(1) and (3) vacuum treatment: the vacuum degree is 80-50 bar, and the vacuum treatment time is not less than 15 min; adding B after Ti treatment and 3-5 min before vacuum treatment;
(2) continuous casting: the superheat degree of the molten steel is 17-25 ℃, and argon sealing protection casting is adopted; the continuous casting blank is sent to a heating furnace after being hot-fed and hot-charged or put into a slow cooling pit for slow cooling, and the temperature of the hot-fed and hot-charged continuous casting blank is more than or equal to 420 ℃; when the casting blank enters the slow cooling pit, slow cooling is required to be carried out for more than 72 hours, and the temperature of the casting blank is less than or equal to 200 ℃;
(3) heating a casting blank: controlling the heating temperature of the casting blank to 1170-1230 ℃, and keeping the temperature for 20-35 min;
(4) stacking and slow cooling after rolling of steel plates: for steel plates with the thickness of more than or equal to 40mm, the stacking temperature is 540-620 ℃, the thickness of a steel plate with the thickness of more than or equal to 50mmQ355 is larger than or equal to the thickness of a steel plate with a lower line of the stacking position, the lower part of the stacking position is paved with a mat, the upper part of the stacking position is covered with a steel plate with the thickness of more than or equal to 50mmQ355, the temperature is not less than 500 ℃, and the steel plate is slowly cooled and stacked below 150 ℃;
(5) tempering heat treatment: the tempering temperature is 600-650 ℃, and the heat preservation time is 1.5-3.0 min/mm.
2. A low-cost plastic die steel of uniform cross-sectional direction hardness according to claim 1, wherein: the thickness of the plastic die steel is 10-100 mm, the Rockwell hardness HRC 28-33, the maximum section hardness difference is less than or equal to 4HRC, and the metallographic structure of the die steel is tempered martensite and tempered troostite.
3. A low-cost plastic die steel of uniform cross-sectional direction hardness according to claim 1, wherein: and controlling the superheat degree of molten steel to be 17-21 ℃ during continuous casting.
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