A pre-hardened plastic die steel plate and a method for manufacturing the same

Abstract

The present application relates to a kind of pre-hardening plastic die steel plate and its preparation method, the steel plate includes the following mass fraction of chemical components: C:0.35~0.45%, Si:0.20~0.40%, Mn:1.20~1.60%, P≤0.025%, S≤0.015%, Als:0.020~0.045%, Cr:1.70~2.00%, Mo:0.10~0.20%, Ti:0.015~0.025%, B:0.0010~0.0020%, the balance is Fe and inevitable impurities;And above-mentioned element content must simultaneously satisfy the following relationship: 3.0%≤Mn+Cr≤3.2%;Its preparation method includes converter smelting→LF refining→vacuum degassing→pouring→casting blank hot sending heating→rolling→stacking cold;The steel plate finished product prepared by the present application is 30~60mm, center segregation C1.0 level below, steel plate hardness 30~36HRC, same plate hardness fluctuation≤3HRC, without heat treatment can satisfy pre-hardening processing requirement.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of die steel manufacturing, in particular to a pre-hardened plastic die steel plate and a preparation method thereof. BACKGROUND

[0002] Mold is an important basic process equipment in manufacturing industry, which is widely used due to high production efficiency, good product quality, low material consumption and low production cost, and is essential process equipment in modern industry, especially in automobile, aviation, instrument, medical equipment, household appliances, daily necessities and other industrial fields. Mold steel includes cold work, hot work and plastic mold, among which plastic mold is the most widely used, accounting for about 80% of the market. Plastic products have facilitated people's daily life and are closely related to people's livelihood, so it is particularly important to develop plastic mold steel.

[0003] Plastic mold steel has high requirements for steel quality. According to the different purposes and grades of mold parts, the steel plate is required to have good hardness uniformity, easy cutting, mirror polishing and pattern etching, etc. In the 1990s, alloy mold manufacturing in China was mainly imported, and domestic mold started late. Up to now, there are not many domestic steel enterprises that can supply production, and the product quality is uneven. There is still a big gap in high-end mold technology field compared with foreign countries.

[0004] With the increasingly fierce competition in the steel market, the prices of nickel-iron and molybdenum-iron bulk alloys fluctuate greatly, and nickel and molybdenum are important alloy elements of mold steel. This seriously affects the production cost and market competitiveness of mold steel. How to produce low-cost and suitable mold is an important research direction in the future and the survival way of steel enterprises.

[0005] Pre-hardened alloy plastic mold steel first requires hardness to meet standard requirements. Through reasonable design of strong and hard chemical elements such as C, Mn, Cr, Mo and B, the matrix structure hardness can meet the standard requirements. At the same time, the steel plate cross-section core hardness uniformity is also required to be good, and the inclusion content and center segregation degree of the steel need to be reasonably controlled. Because the alloy content of mold steel is generally high, the solute is easily extruded and accumulated to the middle liquid core during the pouring and condensation of molten steel. The center segregation degree of low alloy steel is serious, which easily leads to uneven hardness in the thickness direction of the cross-section. On the other hand, in order to fine-tune the hardness performance of mold steel, heat treatment process is generally needed to achieve.

[0006] In the prior art, domestic pre-hardened plastic mold steel generally adopts normalizing + tempering or (normalizing rolling + tempering) process. The same type of grade is 32CrMo or 40CrMo, etc., mainly with a hardness value of 28-34HRC. There are few studies on 30-36HRC grade individualized high-hardness plastic mold steel. There is also a technical gap in the economic type of pre-hardened plastic mold steel which does not need heat treatment after direct rolling. The present application has certain reference value in terms of market individualized demand and technology cost reduction.

[0007] The Chinese patent application with the application number CN201510870739.2 discloses "an economic plastic mold steel plate and manufacturing method", which adopts 0.40-0.50% C, 0.40-0.70% Si, 1.10-1.50% Mn, 0.60-0.80% Cr, 0.10-0.20% Mo, 0.001-0.0012% B components, adopts LF and vacuum RH furnace refining outside, the casting billet opening rolling temperature is 1080-1150℃, the final rolling temperature is 840-910℃, the opening cooling temperature is 750-800℃, the red temperature is 500-550℃, the water cooling to room temperature is at 450-600℃ tempering, the holding time is 1.5-2.5min / mm, the prepared steel performance is HRC 28-33, the maximum cross section hardness difference is 5HRC. The shortcoming of the application is that the ACC forced water cooling is needed after rolling, the final cooling temperature is low, the steel plate stress cracking risk exists, and the tempering process is needed to adjust the hardness performance, which increases the production cost.

[0008] The application number CN201711277657.2 "a high hardness and high hardenability pre-hardened plastic mold steel and its preparation method", the components are 0.40-0.50% C, 0.20-0.50% Si, 0.60-1.00% Mn, 0.80-1.50% Ni, 0.60-2.00% Mo, 1.60-2.50% Cr, 0.10-0.50% V, through smelting, refining, electroslag remelting pouring into steel ingot, the steel ingot is slowly cooled or annealed, forged into Ф20mm round steel, the opening forging temperature is 1200℃, the final forging temperature is 850℃, the annealing heat treatment is carried out after forging, then the pre-hardening treatment is carried out, the steel hardness reaches HRC 45-50 after 860-900℃ solid solution and 500-650℃ tempering. The application adopts more valuable elements Ni and V, and the steel is controlled cooling after casting and forging, and the forged pre-hardening heat treatment is carried out, the process flow is long, and the production cost is high.

[0009] It can be seen that the pre-hardened plastic mold steels mentioned in the above applications are obtained by adding a certain content of Mn, Cr, Mo and Ni elements to improve the hardenability of the steel, controlling the rolling temperature + ACC forced cooling, and then heat treating by tempering, the production cost is high, and the cycle is long. SUMMARY

[0010] The purpose of the present application is to solve the problems of high production cost and long cycle in the prior art plastic mold steels which are basically obtained by adding a certain content of Mn, Cr, Mo and Ni elements to improve the hardenability of the steel, controlling the rolling temperature + ACC forced cooling, and then heat treating by tempering, and provide a pre-hardened plastic mold steel plate and a preparation method thereof.

[0011] A pre-hardened plastic mold steel plate of the present application comprises the following mass fractions of chemical components: C: 0.35-0.45%, Si: 0.20-0.40%, Mn: 1.20-1.60%, P≤0.025%, S≤0.015%, Als: 0.020-0.045%, Cr: 1.70-2.00%, Mo: 0.10-0.20%, Ti: 0.015-0.025%, B: 0.0010-0.0020%, and the balance of Fe and inevitable impurities; and the element contents must simultaneously satisfy the following relationship: 3.0%≤Mn+Cr≤3.2%.

[0012] The hardness of the steel plate is 30-36 HRC, and the same-plate hardness fluctuation is ≤3 HRC.

[0013] A preparation method of a pre-hardened plastic mold steel plate of the present application comprises the following steps: converter smelting→LF refining→vacuum degassing→pouring→hot charging of cast blank→rolling→stack cooling, wherein:

[0014] (1) Converter smelting: 1-2 minutes at the end of blowing, switching to strong stirring mode at the bottom of the converter, controlling [C] 0.08-0.14%, [P]≤0.022% at the end, the tapping temperature is 1620-1640℃, 4.0-5.3 kg / t of active lime is added in the ladle before tapping, and alloy deoxidation is performed during the tapping process;

[0015] (2) LF refining: the inlet temperature is ≥1510℃, 1.5 kg / t of aluminum pellets is added for forced deoxidation, fluorite balls and lime are used for slagging, submerged arc heating is adopted, alloy fine adjustment is performed when the temperature of the molten steel is ≥1560℃, the white slag retention time is ≥10 min, and the slag basicity is controlled at 5-8;

[0016] (3) Vacuum degassing: under the condition of RH treatment≤67Pa high vacuum, the pressure maintaining time is ≥15 min / oven, Ti is added during the degassing process, and the outlet [H] is ≤1.5PPm;

[0017] (4) Pouring: electromagnetic stirring and dynamic soft reduction are turned on, weak cold slow drawing mode is adopted to draw the steel, the straightening temperature is preferably controlled at 890-900℃, the outlet temperature of the horizontal section of the casting machine is 810-820℃, and the blank is cut and timely hot charged into the heating furnace;

[0018] (5) Hot charging of cast blank: the cast blank inlet temperature is 450-700℃; the soaking section temperature is 1220-1270℃, the soaking time is controlled at H+10 min, the heating rate is 9-12 min / cm, and the outlet temperature is 1160-1200℃, wherein H is the thickness value of the cast blank in centimeters;

[0019] (6) Rolling: high temperature micro-control rolling process is adopted, single pass reduction rate of rough rolling is 10-20%, the thickness of the intermediate blank is the thickness of the finished product + 40 mm, the two-stage rolling temperature is less than or equal to 1000℃, the final rolling temperature is 930±30℃, and the total compression ratio of the blank is greater than or equal to 4.0;

[0020] (7) Pile cooling: after the steel plate is flattened by a straightening machine, the steel plate is air-cooled to 680±20℃ on a cooling bed, is quickly lifted by a clamp hanger, is stacked nearby, is slowly cooled by a holding cover at 600-650℃, and the holding cover time is greater than or equal to 48h.

[0021] Further, during the LF refining, fluorite balls, lime and batches of slag are adopted, specifically, the first batch of lime 6-7.5 Kg / t, fluorite balls 2.5-3.5 Kg / t, and carbide 1.2-1.8 Kg / t are added, the bottom blowing strong stirring mode is started, and power is sent for 3-5 minutes, the second batch of lime 2.0-3.5 Kg / t and fluorite balls 0.5-1.5 Kg / t are added, and it is ensured that the slag forming materials are added within 6 minutes of power sending.

[0022] Further, during pouring, electromagnetic stirring and dynamic soft reduction are started, and the steel is poured in a slow pulling and weak cooling mode, specifically, taking the cross section of the casting blank as 300×2000mm as an example, the electric stirring parameters are set as 450A, 6Hz, the first pulling speed is 0.70-0.75 m / min, the first cooling water is arranged as wide / narrow 4500 / 560L / min, the second cooling water is arranged as weak cooling mode, and the soft reduction 6.5mm is completed at the fan-shaped section 7-8 area.

[0023] The center segregation of the steel plate is less than or equal to C1.0 level, after the steel plate is slowly cooled by a holding cover after rolling, the stress is fully eliminated, the hardness of the steel plate is 30-36HRC, the hardness fluctuation of the same plate is less than or equal to 3HRC, and the steel plate can meet the pre-hardening processing requirements without heat treatment.

[0024] The finished product thickness of the steel plate is 30-60mm, the casting blank thickness is 250-300mm, and the casting blank thickness is greater than or equal to 4.0 times of the finished product steel plate thickness.

[0025] The reasons for limiting the amount of the main chemical components in the present application are described below:

[0026] In the present application, Mn and Cr are both strong toughening and quenching elements, which can improve the stability of austenite. Mn is an important strong toughening element and an austenite stabilizing element, which can expand the austenite area in the iron-carbon phase diagram and promote the medium temperature structure transformation; Cr has a strong solid solution strengthening effect and can effectively improve the structure stability and significantly improve the hardenability of the steel plate. Meanwhile, Mn and Cr have low alloy prices and are equivalent to Fe in atomic weight, which can be well dissolved in the Fe matrix to improve the strength and hardness of the steel. Preferably, 3.0%≤Mn+Cr≤3.2%, which can ensure the hardness and strength of the steel plate matrix under the condition of appropriate low Mo.

[0027] Ti: Ti element can play the role of dispersion strengthening and grain refinement, can inhibit the grain growth in high temperature zone, hinder the austenite coarsening when heated at high temperature, and fix the stable TiN generated by N in the steel, reduce the risk of cracking during pouring, and the Ti content in the application is controlled at 0.015-0.025%.

[0028] B: B exists in the form of interstitial atoms in the steel, and the solubility in the steel is very small, B can delay the formation of ferrite and pearlite, and has very strong hardenability, when the B content is too high, the grain boundary energy is too low to form boron brittleness. The content of B in the application is controlled in the range of 0.0010-0.0020%.

[0029] The process of the mold steel plate of the application and the reasons for setting the process parameters are described below:

[0030] (1) Converter smelting, end point control [C] 0.08-0.14%, [P]≤0.022%, tapping temperature 1620-1640℃. At the end of blowing, the C concentration in the steel is reduced, the C-O reaction of the molten pool is weakened, and the stirring of the steel plate is weakened. Open the strong argon blowing mode at the bottom of the furnace 1-2 minutes before tapping to strengthen the post-stirring, which is beneficial to uniform molten pool reaction, discharging of excess O, reduction of O content, reduction of oxidized inclusion content, and improvement of alloy yield. The setting of the tapping temperature mainly considers the temperature drop of the pre-tapping slag and the alloy.

[0031] (2) LF refining treatment, inlet temperature ≥1510℃, add 1.5kg / t aluminum pellets for forced deoxidation, provide a good reducing atmosphere for desulfurization, use fluorite balls, lime and other batched slag, for example, for 150t tapping, add lime 1000kg, fluorite ball 400kg, calcium carbide 300kg, open the bottom blowing strong stirring mode, add lime 500kg, fluorite 200kg in batches for 3-5 minutes, ensure that the slag material is added within 6 minutes of power transmission. Buried arc heating to steel temperature ≥1560℃, sampling for alloy fine adjustment, small argon flow is adjusted, converted to medium-weak stirring mode, keep uniform and slow heating, observe the slagging situation, white slag keeping time ≥10min, the slag basicity is preferably controlled at 5-8.

[0032] (3) Vacuum degassing, RH treatment ≤67Pa under high vacuum conditions, holding time ≥15min / furnace, [H]≤1.5PPm at outlet, add Ti iron for forced deoxidation and nitrogen fixation at the end of vacuum treatment, further improve the purity of the steel.

[0033] (4) Pouring, electromagnetic stirring and dynamic soft reduction are used in pouring, slow pulling and weak cooling mode is used to pour steel, taking 300x2000mm section as an example, the electric stirring parameters are set to 450A 6Hz, the first cooling is set to 0.70-0.75m / min, the second cooling is set to 4500 / 560L / min, the soft reduction is 6.5mm at the fan-shaped section 7-8 area, the center segregation degree is further reduced, the straightening temperature is controlled to 890-900℃, the casting machine outlet temperature is 810-820℃, the feedback data is avoided, the high-temperature brittle pocket area is avoided, and the crack occurrence rate is reduced.

[0034] (5) Hot charging and heating, according to the casting blank temperature, the total heating time is reduced by 1min for every 10℃ increase in the blank temperature in the range of 450-700℃, the soaking section temperature is 1220-1270℃, the soaking time is controlled according to (blank thickness / cm)+10min, the heating rate is 9-12min / cm, and the discharge temperature is 1160-1200℃; the present application fully considers the influence of the heating rate on hot charging, and plays the role of Ti element in the high-temperature soaking section to hinder the austenite grain coarsening, accelerates the solid solution rate of other elements, and realizes the purpose of achieving soaking and steel heating in a short time.

[0035] (6) Rolling, high-temperature batch micro-controlled rolling process is adopted, the single pass reduction rate is 10-20%, the intermediate blank waiting thickness is the finished product thickness+40mm, the two-stage rolling temperature is ≤1000℃, the finish rolling temperature is 930±30℃, and the total compression ratio of the blank is ≥4.0; the rolling of the present application is through large reduction two-stage high-temperature micro-controlled rolling, the intermediate blank waiting thickness is set mainly considering the control of austenite grain refinement and elongation in the finishing stage, and the matrix strength is appropriately improved; the two-stage rolling temperature is set mainly according to the austenite recrystallization temperature T nr and the hit finish rolling temperature, to avoid mixed crystal rolling; the finish rolling temperature is set mainly to adjust the austenite grain size, the rolling state steel plate hardness should not be too high, the cold state hardness after rolling is controlled in the range of 33-38HRC without buffer cooling, to create certain conditions for the cover stack cooling performance adjustment. In addition, a certain proportion of the total compression ratio of the casting blank is designed, which can ensure that the rolling force is fully transmitted to the core, the austenite grain size after rolling is sufficiently refined, and the size difference of the core organization grain in the cross section is reduced, and the cross section hardness fluctuation is reduced.

[0036] (7) buckling stack cold, the steel plate is flattened after the straightening machine, and is air-cooled to 680±20 DEG C on the cooling bed, and is quickly lifted by the clamp hanger to stack nearby, and is slowly cooled by buckling the heat preservation cover at 600-650 DEG C, and the buckling time is greater than or equal to 48h. The steel plate is heat straightened, the hardness of the steel plate is low in the high temperature state, the load of the straightening equipment can be reduced, the steel plate is easily flattened, and the unevenness is controlled to be less than or equal to 5 / 1000m. The design of the steel plate lifting and buckling temperature of the cooling bed mainly considers that the time interval of the crane lifting the steel plate to the stacking is convenient for the site operation, and the function of the medium temperature tempering at about 500 DEG C is simulated, the slow cooling of the steel plate is controlled, the stress is fully released, the transformation of austenite to sorbite and tempered sorbite is promoted, the organization form is controlled, the hardness of the cold steel plate is 30-36HRC, the hardness deviation of the cross section thickness is less than or equal to 3HRC, and the use requirements are met.

[0037] Compared with the existing 32CrMo equivalent pre-hard plastic mold steel plate and its production process, the present application has the following advantages:

[0038] (1) the inexpensive high hardenability component combination design of Mn+Cr is adopted, the noble alloy Mo is added less, the atomic weight of Mn, Cr and Fe is equivalent, the atomic radius is similar, the good displacement solid solution strengthening effect is ensured, the austenite organization stability and the iron-based organization hardness are improved, and the high hardness performance under the high temperature rolling mode is still guaranteed.

[0039] (2) the casting blank is heated and sent to the furnace by the hot charging and hot sending process, the hot sending temperature into the furnace is 450-700 DEG C, in the hot sending range, the steel plate in the furnace time is reduced by 1min for every increase of 10 DEG C, the casting blank preheating is effectively utilized, and the heating furnace energy consumption cost can be reduced.

[0040] (3) the high temperature micro-control rolling process is combined with buckling slow cooling, the stress of the steel plate is fully eliminated, the hardness performance is moderate, the medium temperature tempering heat treatment function can be replaced, the heat treatment cost is reduced, and the product production process is simplified.

[0041] (4) the hardness of the plastic mold steel plate of the present application is 30-36HRC, the hardness is slightly improved compared with the commonly used hardness 28-34HRC mold steel on the market, the mold service life is longer, and the individualized needs of users can be better met.

[0042] The mold steel plate has the advantages of simplified production process, low process cost, short delivery period, upgraded steel plate hardness and the like, and has a good application prospect.

[0043] This invention utilizes a method that rationally designs the chemical composition of steel, controls refining and continuous casting processes to obtain high-quality billets with low-magnification center segregation (C1.0 or less), appropriately controls the rolling process to maintain an austenitic structure for a period of time after rolling, achieves a uniform microstructure, controls appropriate grain size, and slow cooling under a shroud at a suitable temperature after rolling to obtain a uniform microstructure dominated by tempered sorbite. The steel plate has a hardness of 30–36 HRC, a cross-sectional hardness deviation of ≤3 HRC, and sufficient stress relief, making it suitable for direct sawing and use as a plastic mold steel plate. Attached Figure Description

[0044] Figure 1 This is a metallographic image of the mold steel plate produced in Embodiment 1 of the present invention. Detailed Implementation

[0045] To better explain the technical solution of the present invention, the technical solution of the present invention will be further described below with reference to specific embodiments. The following embodiments are merely illustrative of the technical solution of the present invention and do not limit the present invention in any way. The sequence numbers of the following embodiments are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0046] Table 1 below is a list of chemical composition values ​​for each embodiment and comparative example of the present invention;

[0047] Table 2 below lists the main process parameter values ​​for each embodiment and comparative example of the present invention;

[0048] Table 3 below lists the test results of the main mechanical properties of the embodiments and comparative examples of the present invention.

[0049] A method for preparing a pre-hardened plastic mold steel plate according to various embodiments of the present invention includes converter smelting → LF refining → vacuum degassing → casting → hot charging of cast billet → rolling → stacking cooling, wherein:

[0050] (1) Converter smelting: 1-2 minutes after the blowing end point, switch the converter bottom blowing argon to strong stirring mode, control the end point [C] 0.08-0.14%, [P] ≤0.022%, tapping temperature 1620-1640℃, add 4.0-5.3kg / t of active lime to the ladle before tapping, and deoxidize the alloy during the tapping process;

[0051] (2) LF refining: the temperature at the station is ≥1510℃, 1.5kg / t of aluminum shot is added for forced deoxidation, fluorite balls and lime are used for slag making, submerged arc heating is used, the temperature of molten steel is ≥1560℃, alloy fine adjustment is performed, the white slag holding time is ≥10min, and the slag basicity is controlled at 5~8.

[0052] (3) Vacuum degassing: Under high vacuum conditions of RH treatment ≤67Pa, the holding time is ≥15min / furnace, Ti is added during the degassing process, and the outlet [H] is ≤1.5PPm;

[0053] (4) Pouring: turn on electromagnetic stirring and dynamic soft reduction, adopt weak cold and slow drawing mode to draw the steel, control the straightening temperature to be 890-900 DEG C, the temperature at the exit of the horizontal section of the casting machine is 810-820 DEG C, after the billet is cut, the billet is timely hot sent to the heating furnace for loading;

[0054] (5) Hot charging of the casting billet: the temperature of the billet entering the furnace is 450-700 DEG C; the temperature of the soaking section is 1220-1270 DEG C, the soaking time is controlled according to H+10 min, the heating rate is 9-12 min / cm, and the temperature of the billet out of the furnace is 1160-1200 DEG C, wherein H is the thickness value of the casting billet in centimeter;

[0055] (6) Rolling: adopt high-temperature micro-control rolling process, the single pass reduction rate of rough rolling is 10-20%, the waiting thickness of the intermediate billet is the finished product thickness+40 mm, the two-stage opening rolling temperature is less than or equal to 1000 DEG C, the final rolling temperature is 930±30 DEG C, and the total compression ratio of the billet is greater than or equal to 4.0;

[0056] (7) Pile cooling: after the steel plate is flattened by the straightening machine, the steel plate is air-cooled to 680±20 DEG C on the cooling bed, the steel plate is quickly lifted by using the clamp hanger and is stacked nearby, the steel plate is slowly cooled by using the heat preservation cover at 600-650 DEG C, and the cover time is greater than or equal to 48 h.

[0057] Table 1 lists the chemical components of each embodiment and the comparative example (wt, %)

[0058]

[0059] Table 2 lists the main process parameter value list of each embodiment and the comparative example of the application

[0060]

[0061] Table 3 lists the performance detection analysis result list of each embodiment and the comparative example of the application

[0062]

[0063] From the above Table 3, in terms of the surface hardness and core hardness of the five examples, the six HRC values measured according to the test standard for hardness testing, the surface and cross-section core hardness range 32-35.6 HRC, the same plate hardness difference 1.0-2.2 HRC, the fluctuation is small, and the hardness target is hit. The hardness range of Comparative Example 1 is 27.6-32.4 HRC, and the hardness fluctuation is 4.0 HRC. Among them, the C control is below the lower limit, the Cr+Mn content does not reach the requirement of ≥3.0, the strong hardening element is insufficient, the C2.0 center segregation of the casting blank deteriorates the core performance uniformity, the two-stage finishing temperature is 976℃, which is relatively high, the refined austenite grains are coarsened and grown, and the hardness performance is further reduced; the hardness range of Comparative Example 2 is 34.9-37.8 HRC, and the hardness fluctuation is 3.9 HRC. Among them, the Mo and (Cr+Mn) content is relatively high, the hardenability is stronger, the hardness contribution is too large, the cover is slowly cooled for 24h, the steel plate is not sufficiently slowly cooled, which leads to the hardness and the whole plate hardness deviation exceeding the standard; after rolling, Comparative Example 3 is directly air-cooled without cover, the cooling rate is large, the matrix structure is sorbite structure transformation, the whole hardness of the steel plate is high, and the stress relief is insufficient, which is not conducive to sawing processing.

[0064] Referring to Figure 1 , Figure 1 is a pre-hardened plastic mold steel plate prepared according to Example 1 of the present application. As can be seen from the figure, the matrix structure of the steel is mainly tempered sorbite, the structure is relatively uniform, and the edges and corners of the tempered sorbite are relatively blurred, indicating that the steel has good uniformity and sufficient stress relief.

[0065] The pre-hardened plastic mold cavity is complex, and the size precision and surface roughness are strictly required. The steel plate requires moderate hardness, stable structure along the cross-section direction, and uniform hardness distribution. The plastic mold steel of the present application does not need heat treatment, and the process cost is relatively low. The hardness range of the steel plate is 30-36 HRC, the maximum cross-section hardness difference is ≤3 HRC, the hardness distribution of the same plate is uniform, and it is suitable for making plastic molds with strict requirements on size precision and surface roughness and long service life.

[0066] The above examples are merely specific examples for explaining the present application and do not limit the present application in any form. Any non-substantial changes made by anyone according to the above content and form without deviating from the protection scope of the claims of the present application shall be considered to fall within the protection scope of the claims of the present application. The present application is not limited to the above specific examples.

Claims

1. A pre-hardened plastic mold steel sheet characterized by The steel plate contains the following mass fractions of chemical components: C: 0.35-0.45%, Si: 0.20-0.40%, Mn: 1.20-1.60%, P ≤0.025%, S ≤0.015%, Als: 0.020-0.045%, Cr: 1.70-2.00%, Mo: 0.10-0.20%, Ti: 0.015-0.025%, B: 0.0010-0.0020%, the balance being Fe and inevitable impurities; meanwhile, the following relationship is met: 3.0% ≤Mn+Cr ≤3.2%; The preparation method of the pre-hardened plastic mold steel plate comprises the steps of converter smelting, LF refining, vacuum degassing, pouring, casting blank hot sending heating, rolling and stack cooling. (1) Converter smelting: 1-2 minutes at the end of blowing, switching to strong stirring mode at the bottom of the converter, controlling [C] 0.08-0.14%, [P] ≤0.022% at the end, the tapping temperature is 1620-1640 ℃, 4.0-5.3 kg / t of active lime is added in the ladle before tapping, and alloy deoxidation is carried out during the tapping process; (2) LF refining: the inlet temperature is ≥1510 ℃, 1.5 kg / t of aluminum pellets is added for forced deoxidation, fluorite balls and lime are used for batch slagging, buried arc heating, alloy fine adjustment is carried out when the temperature of molten steel is ≥1560 ℃, the white slag retention time is ≥10 min, and the slag basicity is controlled according to 5-8; (3) Vacuum degassing: under the condition of RH treatment ≤67 Pa high vacuum, the pressure maintaining time is ≥15 min / furnace, Ti is added during the degassing process, and [H] ≤1.5 PPm at the outlet; (4) Pouring: electromagnetic stirring and dynamic soft reduction are started, weak cold slow drawing mode is used to draw the steel, the straightening temperature is preferably controlled at 890-900 ℃, the outlet temperature of the horizontal section of the casting machine is 810-820 ℃, and the blank is cut and sent to the heating furnace in time after the blank is cut; (5) Casting blank hot sending heating: the casting blank inlet temperature is 450-700 ℃, the soaking section temperature is 1220-1270 ℃, the soaking time is controlled according to H+10 min, the heating rate is 9-12 min / cm, and the outlet temperature is 1160-1200 ℃, wherein H is the thickness value of the casting blank in centimeters; (6) Rolling: high-temperature micro-control rolling process is used, the single-pass reduction rate of rough rolling is 10-20%, the intermediate blank waiting thickness is the finished product thickness+40 mm, the two-stage opening rolling temperature is ≤1000 ℃, the final rolling temperature is 930±30 ℃, and the total compression ratio of the blank is ≥4.0; (7) Stack cooling: after being flattened by the straightening machine, the blank is air-cooled to 680±20 ℃ on the cooling bed, then the blank is quickly lifted by the clamp hanger and stacked nearby, the blank is slowly cooled by buckling the heat preservation cover at 600-650 ℃, and the buckling time is ≥48 h.

2. A pre-hardened plastic mold steel sheet according to claim 1, characterized in that: During the LF refining, fluorite balls, lime and calcium carbide are used for batch slagging, specifically, 6-7.5 Kg / t of lime, 2.5-3.5 Kg / t of fluorite balls and 1.2-1.8 Kg / t of calcium carbide are added in the first batch, the strong stirring mode is started, and the power is sent for 3-5 minutes, then 2.0-3.5 Kg / t of lime and 0.5-1.5 Kg / t of fluorite balls are added in the second batch, and the power is ensured to be sent within 6 minutes.

3. A pre-hardened plastic mold steel sheet according to claim 1, characterized in that: The electromagnetic stirring and dynamic soft reduction are used during pouring, and the steel is poured by slow pulling and weak cooling mode, specifically: when the cross section of the casting blank is 300*2000mm, the electric stirring parameters are set as 450A, 6Hz, the first drawing speed is 0.70-0.75m / min, the water distribution of the first cooling is 4500 / 560L / min, the water of the second cooling is used in weak cooling mode, and the soft reduction of 6.5mm is completed at the 7-8 area of the fan-shaped section.

4. A pre-hardened plastic mold steel sheet according to claim 1, wherein: The center segregation of the steel plate is below C1.0 level, the hardness of the steel plate is 30-36HRC, the hardness fluctuation of the same plate is less than or equal to 3HRC, and the pre-hardening processing requirement can be met without heat treatment.

5. A pre-hardened plastic mold steel sheet according to claim 1, wherein: The finished thickness of the steel plate is 30-60mm, the casting blank thickness is 250-300mm, and the casting blank thickness is greater than 4.0 times of the finished steel plate thickness and above.

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