EP-4130316-B1 - PRE-HARDENED MIRROR MOLD STEEL PLATE AND MANUFACTURING METHOD THEREFOR

Inventors

• HE, Guangxia
• MENG, Yu
• WU, YANG
• LI, Guozhong
• XU, XIAOHONG
• BAI, YUN
• MIAO, Pifeng
• HUANG, JUN
• GE, Henggui
• DI, Menglong

Dates

Publication Date

20260506

Application Date

20210107

Claims (2)

1. A pre-hardened steel plate for a mirror-surface mold, whose chemical constituents by mass percentages are C: 0.28 to 0.36 %, Si: 0.50 to 0.80 %, Mn: 1.20 to 1.50 %, P ≤ 0.020 %, S ≤ 0.010 %, Cr: 1.30 to 1.80 %, Mo: 0.30 to 0.50 %, Ni: 0.65 to 0.90 %, V: 0.15 to 0.30 %, H ≤ 1.5 ppm, and the balance is Fe and inevitable impurities; the steel plate is not quenched-and-tempered, and the steel plate has a thickness of 20-180 mm, has a hardness of 38-42 HRC, and has a hardness fluctuation over the entire thickness ≤ 2 HRC, and a flaw-detection level meeting the requirement of level I in GB/T2970.
2. A method of manufacturing the pre-hardened steel plate for a mirror-surface mold according to claim 1, comprising following steps: (1) smelting of molten steel: molten iron is pretreated and smelted in an oxygen converter, followed by refining in a ladle furnace and then vacuum degassing; wherein the vacuum degassing lasts for at least 30 minutes, soft blowing of argon for at least 20 minutes, so as to reduce the content of H in the molten steel and to make non-metallic inclusions in the steel to float; (2) continuous casting: continuous casting of molten steel into a continuous casting slab with a thickness of 370-450 mm and a width of 1600-2600 mm; wherein the continuous casting process is cooperated with a ladle slag detection, and a pouring without a protection against oxidation; a superheat degree in the pouring is ≤ 25 °C; in the continuous casting process, using a dynamic soft reduction technology to reduce center segregation and center porosity of the slab; (3) rolling: heating the continuous casting slab to 1150 °C - 1250 °C, and a time in the furnace is calculated by multiplying the thickness of the casting slab in cm with 7-14 min/cm; in an initial rolling stage, using a high-temperature rolling, and the rolling temperature is controlled to be not lower than 1020 °C, in a single pass a reduction amount is 40-55 mm and the highest reduction rate is ≥ 18 %, so that a rolling force penetrates into the core of the slab, so as to roll for recrystallization zones; in a second stage which is finish rolling, rolling a 80-200 mm thick steel plate obtained from the initial rolling stage into a target thickness, and finishing a shape of the plate, so as to achieve the target thickness which is 20-180 mm thick, after rolling, the steel plate is moved off an assembly line, and is stacked and is slowly cooled to room temperature; (4) heat treatment: including two steps, i.e. normalizing and tempering, that is, after the stacked cooling, heating the steel plate to 870-910 °C, with a holding time of 2-2.5 min/mm, removing the steel plate out of furnace, air cooling to room temperature, so that bainite is formed in the steel plate; after normalizing, tempering the steel plate according to the thickness of the steel plate; wherein the tempering is performed according to different thickness of steel plate: in case of 20-100 mm thick steel plate, the tempering temperature is 590-620 °C, in case of the steel plate thicker than 100 mm, the tempering temperature is 580-610°C, and a holding time is calculated by multiplying plate thickness in mm with 3-4 min/mm.

Description

TECHNICAL FIELD The invention belongs to the technical field of iron-based alloys, and in particular relates to a pre-hardened steel plate for a mirror-surface mold, and the manufacture method thereof. BACKGROUND As manufacturing enterprises upgrade towards an automatic production, the manufacture of products gradually shifts from relying on manual-intensive labors to relying on industrial equipment, so it is necessary to improve the level of automation in manufacturing. The large-scale and automatic production equipment promotes our country's plastic mold manufacturing industry to develop in the direction of a larger module, a higher uniformity and a better mirror surface. This requires the steels for plastic molds to have a thicker gauge, a higher hardenability and more uniform properties. This is mainly shown as a larger thickness, a higher and more uniform hardness, and higher density requirements. At the same time, in order to ensure good market competitiveness, the manufacturing plant requires a mold steel having a short production cycle, a low cost, and a high cost performance. The widely used pre-hardened mold steels are e.g. 3Cr2Mo and 3Cr2MnNiMo, and its hardness ranges from 28-36 HRC, which can no longer meet the requirements in high dimensional accuracy and in high polishing performance for mirror-surface molds. In order to obtain a better polishing effect, the pre-hardened hardness of the mold steel requires 38-42 HRC. This type of steel plate is mainly NAK80 pre-hardened steel from Daido, Japan. However, this steel has a high content of alloy elements and is mostly produced by steel ingot + forging + quenching and tempering process. The production cycle lasts long, the cost is high, and the price is expensive. In recent years, some domestic steel mills have added Cr, Mo, V and micro-alloy elements on the basis of 3Cr2Mo to obtain a higher-quality steel for plastic molds. The patent publication CN102650021 A discloses adding Mn on the basis of 3Cr2Mo, and adding micro-alloy element Nb, so as to obtain a pre-hardened steel of a bainite steel with a high hardenability. Its hardness is 30.6-33.6 HRC, and does not belong to a mold steel of high hardness. Besides, the used manufacturing process is steel ingot + forging, the yield is low, the production cycle lasts long, and the production cost is high. The patent publication CN109136736 A discloses a vanadium-containing steel plate for plastic molds, which is made by continuous casting and rolling, has a short production time and a simple process. Compared with the process of steel ingot + forging, the production cost is lower. However, the thickness of the steel plate is 15-80 mm, and the hardness is 280-330 HBW (equivalent to 29-35 HRC). There is still no higher breakthrough in hardness. The patent publication CN107699801 A produces a V-containing steel for plastic molds, having fine grains and uniform hardness. However, the production of the steel plate requires high-temperature forging, quenching and tempering treatment, and secondary tempering. The production cycle lasts long, the manufacture process is relatively complicated, and a complex heat treatment also significantly increases the production cost. Besides, the hardness of the core of the steel plate in this publication is only higher than 30 HRC, but cannot be higher. The patent publications CN107974636 A and CN109706397 A disclose some high-hardness steels for plastic molds, having good hardenability and high hardness. However, they all adopt induction furnace, electroslag remelting, casting steel ingots, and forging into materials. The production cycle lasts long and the production cost is high. EP 3 502 296 A1 discloses a thick plate with high and homogenous hardness. CONTENT OF THE INVENTION The technical problem to be solved by the present invention is to aim at the above-mentioned prior arts and to provide a pre-hardened steel plate for a mirror-surface mold. The steel plate not only has high hardness, but also has good uniformity in hardness along the direction of thickness of the steel plate, and a curve fluctuation of hardness along the thickness is ≤ 2 HRC, and its flaw-detection level meet the requirements of level I according to GB/T2970. The invention is set out in the appended set of claims. The pre-hardened steel plate for a mirror-surface mold in the present invention is based on 3Cr2Mo, wherein the contents of easily-segregated elements like carbon and Cr are reduced, which improves the hardness uniformity of the steel plate, wherein the contents of relatively cheap elements like Si and Mn are increased and an appropriate amount of alloy elements are added, which refines grains and increase the hardenability of the steel plate and improve the strength and hardness of the steel. Thereby, a pre-hardened steel for a mirror-surface mold, with excellent comprehensive performance and high cost performance, is obtained. The chemical constituents are C: 0.28 to 0.36 %, Si: 0.50