Economical mirror mold steel plate and production method thereof

Abstract

The invention discloses an economical mirror surface die steel plate and a production method thereof. The economical mirror surface die steel plate comprises the following chemical components in percentage by mass (wt%): 0.37-0.43% of C, 0.2-0.4% of Si, 1.10-1.40% of Mn, less than or equal to 0.025% of P, less than or equal to 0.005% of S, 1.8-2.1% of Cr, 0.8-1.0% of Ni, 0.2-0.4% of Mo, 0.0015-0.0023% of B and the balance of Fe and residual elements. The hardness of the steel plate is 38-42 HRC, the thickness of the steel plate is 80-150 mm, and continuous casting production is adopted. In the aspect of component design, compared with NAK80 mirror surface die steel, the precious alloy Ni element is greatly reduced, by increasing the content of C and Cr elements and reasonably proportioning Mn, Cr, Mo and B elements, the ACC water-cooling hardenability and hardenability of a rolled steel plate are improved, meanwhile, the Cu element is omitted, and copper cracks in the casting blank heating process are avoided. A steel plate with the thickness of 80-150 mm is obtained through rolling, water cooling, air cooling and tempering heat treatment, the hardness of sections of all the thicknesses is uniform, and the requirement of the market for high-performance mirror surface die steel is completely met.

Description

Technical Field

[0001] The invention relates to the field of steel plate production, and in particular to an economical mirror-finished die steel plate and a production method thereof. Background Art

[0002] The mold industry is known as the mother of industry. It is the foundation for the development of other industries and an important indicator of a country's industrial level. In recent years, my country's mold steel industry has developed rapidly, especially the development of the plastics industry, which has promoted the overall development of the plastic mold steel industry. However, there is still a certain gap between the overall level and that of European and American companies.

[0003] High mirror mold steel is a material with high requirements in plastic molds. It is widely used in automobile, mobile phone, home appliance and high-end electronic products industries, and the market demand is increasing. Generally, the mirror bright performance of steel is mainly controlled by the following factors: the material is a matrix structure mainly composed of martensite or bainite, requiring uniform and fine grains, high cleanliness, and an ideal hardness of 39-45HRC. At present, due to the level of process equipment and the characteristics of product requirements, a matrix structure mainly composed of martensite has not yet appeared, and high mirror plastic mold steel mainly composed of bainite structure is the majority. Europe, the United States, Japan and other countries have invested a lot of money to develop plastic mold steel with mirror performance mainly composed of bainite structure, and the typical representative is the Japanese grade NAK80. This steel is an aging hardening steel. It uses a low carbon content and Ni-Cu-Al ternary alloy in a certain proportion to improve hardenability, while making the matrix obtain the maximum precipitation hardening effect. After aging treatment, extremely fine intermetallic compounds AlNi and Cu-rich phases are dispersed. In order to ensure the good hardenability of this steel and obtain uniform bainite structure, alloy elements such as Mn, Mo, and Ni are usually added. The composition of NAK80 is: C: 0.06-0.16%, Si: 0.05-0.2%, Mn: 1.4-1.7%, P≤0.02%, S≤0.01%, Alt: 0.7-1.3%, Mo: 0.2-0.5%, Ni: 2.5-3.5%, Cu: 0.8-1.25%, and the rest is Fe and residual elements. Its composition design adopts low carbon content, but a large amount of precious Ni alloy is added, the production cost is high, it cannot meet market demand, and it is not suitable for the current sustainable development of energy conservation and emission reduction.

[0004] In view of this, the present invention is proposed. Summary of the invention

[0005] In order to meet the above technical requirements, the purpose of the present invention is to provide an economical mirror mold steel plate with low production cost, which can meet the field of view requirements and adapt to the current sustainable development of energy conservation and emission reduction.

[0006] Another object of the present invention is to provide an economical method for producing mirror-finished mold steel plates, which adopts continuous casting to facilitate production organization in the field of metallurgical technology for extra-thick plates, and at the same time enables its comprehensive performance to meet the use requirements.

[0007] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is: an economical mirror mold steel plate, the thickness of the steel plate is 80-150 mm, and it contains the following chemical components in mass percentage (unit: wt%): C: 0.37-0.43%, Si: 0.2-0.4%, Mn: 1.10-1.40%, P: ≤0.025%, S: ≤0.010%, Cr: 1.2-1.5%, Ni: 0.3-0.5%, Mo: 0.2-0.4%, B: 0.0015-0.0023%, and the rest are Fe and residual elements; the hardness of the steel plate surface and thickness section is 38-42HRC, the roughness is ≤0.05μm, and the organization is uniform and fine tempered bainite.

[0008] Regarding the composition design, it should be noted that, compared with the NAK80 composition, the present invention greatly reduces the precious alloy Ni element, increases the content of C and Cr elements, and adds B element to improve the hardenability and hardenability of the steel plate during water cooling. At the same time, the Cu element is eliminated to avoid the occurrence of "copper cracks" during the heating process of the casting.

[0009] Specifically, carbon (C): carbon is the main alloying element of steel materials. The main functions of carbon in steel are: forming solid solution structure to improve the strength of steel, such as ferrite and austenite structures, which both contain dissolved carbon elements; forming carbide structure to improve the hardenability and hardenability of steel. Fine carbides are the main precipitation strengthening phases in steel, which can improve the hardness of pre-hardened mirror plastic mold steel and are beneficial to improving mirror performance. Therefore, the C content of the present invention is limited to 0.37-0.43%.

[0010] Silicon (Si): It has a strong solid solution strengthening effect in steel, can improve the hardenability of steel, prevent ferrite nucleation and growth, and shift the "C" curve to the right. However, too high Si content can easily cause the risk of cracks in the ingot and steel plate. In this designed alloy system, the Si content is limited to 0.2-0.4%.

[0011] Manganese (Mn): Manganese is second only to nickel in its ability to stabilize austenite. It also strongly increases the hardenability of steel. When combined with carbon, it can increase the work hardening rate of steel and improve the hardenability and hardenability of steel. However, when the manganese content is high, it is easy to cause the center segregation of the continuous casting billet structure, the tendency of quenching cracks and the amount of residual austenite will increase. Therefore, the Mn content in the steel of the present invention is limited to 1.1% to 1.4%.

[0012] Chromium (Cr): Increases the hardenability of steel and increases and refines excess carbides to increase the wear resistance of steel. Cr can slow down the decomposition rate of austenite and significantly improve the hardenability of steel, but it will also increase the temper brittleness tendency of steel. Cr can improve the hardenability of steel and ensure that the mold steel has a certain corrosion resistance. For thick steel plates, a certain amount of Cr needs to be added to improve the hardenability to compensate for the strength and hardness loss caused by the weakened core cooling capacity when the thickness is large; Cr is also a medium-strength carbide-forming element, which can form carbides in steel to improve the strength and hardness of steel, but too high Cr and Mn are added to steel at the same time, which is easy to form surface cracks during the steel rolling process, so the Cr content is limited to 1.2-1.5%.

[0013] Nickel (Ni): It has a very strong ability to stabilize and expand austenite in steel, can improve the toughness of steel, improve the thermal fatigue performance of molds, and can also improve the hardenability of steel; but too much Ni increases the production cost. my country's mirror mold steel can be divided into two major series: low-nickel and high-nickel according to the different Ni content. The solution adopted by the present invention is a low-Ni type, and the Ni content is controlled at 0.3-0.5%.

[0014] Molybdenum (Mo): Molybdenum has a solid solution strengthening effect on ferrite, and also improves the stability of carbides, thereby improving the strength of steel. Molybdenum plays a favorable role in improving the ductility, toughness and wear resistance of steel; because molybdenum increases the softening and recovery temperature and recrystallization temperature after deformation strengthening, and strongly improves the creep resistance of ferrite, effectively inhibits the aggregation of cementite at 450-600°C, and promotes the precipitation of special carbides, it becomes the most effective alloy element for improving the thermal strength of steel. In addition, molybdenum is beneficial to improve the hardness and thermal stability of this steel, increase the temper brittleness temperature, and avoid the occurrence of temper brittleness. In the present invention, the Mo content is limited to 0.2-0.4%.

[0015] Boron (B): The main function of boron in steel is to increase the hardenability of steel, thereby saving other rare and precious metals, such as nickel, chromium, and molybdenum. For this purpose, its content is generally specified in the range of 0.001% to 0.005%. It can replace 1.6% of nickel, 0.3% of chromium or 0.2% of molybdenum, but as the boron content increases, there is a tendency to promote temper brittleness, so the boron content should not be too high. Therefore, the boron content in the steel of the present invention is controlled at 0.0015 to 0.0023%.

[0016] C, Mn, Cr, and Mo elements can all improve the hardenability and hardenability of steel, and are the guarantee elements for the uniformity of cross-sectional hardness. Because pre-hardened mirror plastic mold steel is mostly made of extra-thick plates (thickness ≥ 80mm), if the hardenability of the steel plate is poor during cooling, the surface layer will cool quickly, while the core will cool slowly, resulting in uneven surface and core structures, which will bring about uneven hardness in the cross-sectional direction, and fail to achieve the use effect of pre-hardened mirror plastic mold steel. Therefore, in pre-hardened mirror plastic mold steel, under the composition design with a higher C content, the elements Mn, Cr, and Mo that improve the hardenability of steel need to be reasonably proportioned. B is an element that strongly improves the hardenability of steel. If only B is used to improve the hardenability of steel, more B elements need to be added. As mentioned above, when the content of B element is high, there is a tendency to promote temper brittleness. Therefore, it is necessary to play the role of C, Cr, Mo, and Mn in improving the hardenability of steel, and limit the addition amount of B element on the basis of ensuring the hardenability of steel.

[0017] The production method of the economical mirror mold steel plate comprises the following steps: molten steel smelting, continuous casting, billet heating, rolling, ACC water cooling, air cooling, and tempering heat treatment:

[0018] The rolling process requires that the starting rolling temperature is 1000℃~1100℃, and the pass reduction rate is ≥15% to ensure that the rolling force penetrates into the core of the steel plate, improve the original loose defects of the ingot, and fully break the grains to prevent grain growth. The final rolling temperature is 880~960℃, and the steel plate is cooled by ACC water after rolling.

[0019] The ACC water cooling process requires that the steel plate start cooling temperature is ≥860℃, and the steel plate is cooled in multiple times according to the thickness of the steel plate. The 80-110mm thick steel plate is cooled 3 times, and the 111-150mm thick steel plate is cooled 5 times. The water cooling time for each time is 30-40S. After each cooling, the steel plate is air-dried and returned to red for 5-6 minutes before the next water cooling. After the last cooling and returning to red for 5 minutes, the surface temperature of the steel plate is controlled at 350-450℃. The multiple cooling + multiple red-returning process is adopted. On the one hand, the core of the steel plate is cooled down. At the appropriate cooling rate and temperature, the cross-sectional microstructure transformation is mainly bainite, while avoiding the overcooling cracking of the steel plate caused by the high cooling rate. On the other hand, due to the excessive core temperature, in the subsequent air cooling process, the 1 / 8-1 / 4 area of ​​the thickness section produces a self-tempering of >600℃ due to the high temperature diffusion of the core, resulting in low hardness in the 1 / 8-1 / 4 area of ​​the section and uneven cross-sectional hardness.

[0020] The air cooling process requires that the steel plate be cooled in a wind field on the cooling bed after water cooling, and the air cooling termination temperature is 200-300°C; the advantage of wind field cooling is that it is higher than the cooling rate of the steel plate naturally cooled in the air, and lower than the cooling rate of water cooling, which mainly promotes the transformation of the core of the steel plate to bainite structure. At the same time, the tissue stress generated at a lower temperature will not cause the steel plate to crack.

[0021] The tempering heat treatment process requires that the steel plate be quickly put into the furnace for tempering after air cooling. The temperature of the steel plate entering the furnace is greater than 150°C, and the tempering insulation temperature is 540-560°C. The insulation time for 80-110mm thick steel plates is 3.5-3.7min / mm, and the insulation time for 111-150mm thick steel plates is 3.7-4.0min / mm.

[0022] The invention provides an economical mirror-finished mold steel plate and a production method thereof, and its beneficial effects include:

[0023] In terms of composition design, the present invention greatly reduces the precious alloy Ni element compared to NAK80, and improves the hardenability and hardenability of the steel plate after rolling by ACC water cooling by increasing the content of C and Cr elements and reasonably matching Mn, Cr, Mo and B elements. At the same time, the Cu element is eliminated to avoid the occurrence of "copper cracks" during the heating process of the casting billet; the present invention uses continuous casting billets to produce 80-150mm thick extra-thick plates. Through reasonable production organization, it is ensured that each thickness section obtains a consistent structure. Its flaw detection can meet the requirements of GB / T 2970I level. The hardness of the steel plate surface and each thickness section is 38-42HRC, and the structure is uniform and fine tempered bainite. After cutting, the roughness is ≤0.05μm, which meets the use requirements of the mirror die steel NAK80 hardness and surface roughness after processing, thereby obtaining a mirror die steel that meets market demand. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0025] Figure 1 It is a metallographic structure diagram of the 132 mm thick economical mirror mold steel plate of the present invention at 1 / 4 of the thickness.

[0026] Figure 2 It is a metallographic structure diagram of the 132 mm thick economical mirror mold steel plate at 1 / 2 thickness of the present invention. DETAILED DESCRIPTION

[0027] In order to better describe the technical features and performance of the present invention, the present invention takes the production of steel plates with thicknesses of 81 mm, 96 mm, 111 mm, 132 mm and 150 mm as five embodiments. When implemented specifically, the production is carried out according to the following chemical compositions (unit, wt%) by mass percentage:

[0028] C: 0.37~0.43%, Si: 0.2~0.4%, Mn: 1.10~1.40%, P: ≤0.025%, S: ≤0.010%, Cr: 1.2~1.5%, Ni: 0.3~0.5%, Mo: 0.2~0.4%, B: 0.0015~0.0023%, and the rest are Fe and residual elements.

[0029] This embodiment provides a method for producing an economical mirror-finished mold steel plate, including converter smelting, LF refining, VD vacuum refining, continuous casting, billet heating, rolling, water cooling, air cooling, tempering heat treatment and other process controls. The implementation steps are as follows:

[0030] (1) Converter smelting: at the end of smelting, the C content is controlled to be ≤0.35%, P ≤0.01%, the tapping temperature is: T = 1560 ~ 1600 ° C, and the slag is blocked by a slag cone at the end of tapping;

[0031] (2) LF refining: A 120-ton LF refining furnace is used, and the lime consumption per furnace is 900-1000 kg. The white slag retention time is 30-40 minutes, and the S content is controlled to be ≤0.005%. Boron iron is added in the second heating process after the white slag is formed. Calcium carbide is used as the process deoxidizer, and the amount of calcium carbide is controlled at 50-80 kg. The white slag retention time is 15-18 minutes.

[0032] (3) VD vacuum refining: The holding time at ≤67Pa is controlled to be 12-15min to ensure that the N content in the molten steel is ≤50ppm, the O content is ≤20ppm, and the H content is ≤1.2ppm. After breaking the vacuum, soft blowing is performed for 3-5min, and then the ladle is lifted and left the station.

[0033] (4) Continuous casting: The continuous casting process is protected by argon sealing, and the pouring superheat is controlled at 10-20℃. During the continuous casting and steel drawing process, the electromagnetic stirring operates stably, with a current intensity of 500A and a current frequency of 5HZ. When the superheat of the continuous casting molten steel is greater than 20℃, the columnar crystals of the ingot are well developed, which is easy to cause serious center segregation; when the superheat of the continuous casting molten steel is less than 10℃, although the center segregation is reduced, the superheat is too low, which is easy to cause nozzle blockage and serious crusting of the ladle and the tundish.

[0034] (5) Billet heating: the billet temperature is 150-450°C, the first heating temperature is ≤950°C, the second heating temperature is 1210-1230°C, the third heating temperature is 1200-1220°C, and the heating time is 11-13 min / cm.

[0035] (6) Rolling: The starting rolling temperature is 1000℃~1100℃, the pass reduction rate is ≥15%, the final rolling temperature is 880~960℃, and the steel plate is water-cooled after rolling.

[0036] (7) ACC water cooling: The steel plate cooling temperature is ≥ 860℃. The steel plate is cooled in multiple times according to the thickness of the steel plate. The 80-110mm thick steel plate is cooled 3 times and the 111-150mm thick steel plate is cooled 5 times. The water cooling time for each time is 30-40 seconds. After each cooling, the steel plate is allowed to dry and return to red for 5-6 minutes before the next water cooling. After the last cooling and return to red for 5 minutes, the surface temperature of the steel plate is controlled at 350-450℃.

[0037] (8) Air cooling: After water cooling, the steel plate is cooled by air field on the cooling bed, and the air cooling termination temperature is 200-300℃.

[0038] (9) Tempering heat treatment: After air cooling, the steel plate is quickly put into the furnace for tempering. The temperature of the steel plate entering the furnace is greater than 150°C, and the tempering insulation temperature is 540-560°C. The insulation time for 80-110 mm thick steel plates is 3.5-3.7 min / mm, and the insulation time for 111-150 mm thick steel plates is 3.7-4.0 min / mm.

[0039] Example

[0040] Through the process control of converter smelting, LF refining, VD vacuum refining, continuous casting, ingot heating, ingot rolling, water cooling, air cooling, tempering heat treatment, etc., the key points of process control are shown in Table 1-4, and the 80mm-150mm thick economical mirror mold steel with the chemical composition described in Table 5 is obtained. The mechanical properties are shown in Table 6 below.

[0041] Table 1 80mm~150mm thick economic mirror die steel smelting process data

[0042]

[0043] Table 2 80mm~150mm thick economic mirror die steel plate rolling process data

[0044]

[0045] That is: the reduction rate of each pass is ≥15%.

[0046] Table 3 Post-rolling cooling process data of 80mm~150mm thick economic mirror die steel plate

[0047]

[0048]

[0049] Table 4 Tempering process data of 80mm~150mm thick economic mirror die steel plate

[0050] Example Thickness(mm) Tempering temperature(℃) Tempering insulation coefficient (min / mm) 1 81 550～560 3.5 2 96 550～560 3.6 3 111 540～550 3.8 5 132 540～550 3.9 6 150 540～550 4.0

[0051] Table 5 Chemical composition of 80mm~150mm thick economical mirror mold steel plate (wt%)

[0052]

[0053] Table 6 Mechanical properties of 80mm to 150mm thick economical mirror mold steel plates

[0054]

[0055] The thickness of the economical mirror mold steel plate produced by the above process can reach 150mm, the internal quality of the steel plate is excellent, and the flaw detection can meet the requirements of GB / T 2970I level. According to the GB / T 10561A method, the total level of inclusions is not more than 2.5. The hardness of the surface and each thickness section of the steel plate after tempering is 38-42HRC, and the organization is uniform and fine tempered bainite. After cutting, the roughness is ≤0.05μm, which meets the processing needs of users.

[0056] In summary, the economical mirror mold steel provided by the present application has a reasonable combination of chemical composition, production process, and internal organization, and the hardness of each thickness section is uniform, which fully meets the market requirements for mirror mold steel.

[0057] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. An economical mirror-finished mold steel plate, characterized in that: The steel plate has a thickness of 80 to 150 mm and contains the following chemical components in mass percentage (unit: wt%): C: 0.37 to 0.43%, Si: 0.2 to 0.4%, Mn: 1.10 to 1.40%, P: ≤0.025%, S: ≤0.010%, Cr: 1.2 to 1.5%, Ni: 0.3 to 0.5%, Mo: 0.2 to 0.4%, B: 0.0015 to 0.0023%, and the rest are Fe and residual elements; the hardness of the steel plate surface and thickness section is 38 to 42 HRC, the roughness is ≤0.05 μm, and the structure is uniform and fine tempered bainite.

2. A method for producing an economical mirror-finished mold steel plate, characterized in that: The steel production method comprises the following steps: molten steel smelting, continuous casting, billet heating, rolling, ACC water cooling, air cooling, and tempering heat treatment; The rolling process requires that the starting rolling temperature is 1000℃~1100℃, the pass reduction rate is ≥15%, the final rolling temperature is 880~960℃, and the steel plate is cooled by ACC water after rolling; The ACC water cooling process requires that the steel plate start cooling temperature is ≥860℃, and the steel plate is cooled several times according to the thickness of the steel plate. The 80-110mm thick steel plate is cooled 3 times, and the 111-150mm thick steel plate is cooled 5 times. The water cooling time for each time is 30-40S. After each cooling, the steel plate is dried and returned to red for 5-6 minutes before the next water cooling. After the last cooling and returning to red for 5 minutes, the surface temperature of the steel plate is controlled at 350-450℃. The air cooling process requires that the steel plate be cooled by air field on the cooling bed after water cooling, and the air cooling termination temperature is 200-300℃; Tempering heat treatment process requirements: After air cooling, the steel plate is quickly put into the furnace for tempering. The temperature of the steel plate entering the furnace is> 150℃, tempering holding temperature 540~560℃, the holding time of 80~110mm thick steel plate is 3.5~3.7min / mm, and the holding time of 111~150mm thick steel plate is 3.7~4.0min / mm.

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