Plastic mold steel plate and its manufacturing method

Abstract

The present invention discloses a plastic mold steel plate and a manufacturing method thereof. The manufacturing method includes heating a steel billet at a soaking temperature of 1210-1250°C, rolling it into a steel plate at a rolling start temperature of 1060-1140°C and a final rolling temperature of 980-1050°C, transferring the steel plate to a cooling bed, air-cooling it to below 200°C, and then heating it to a normalizing temperature of Ac 3 +60℃~Ac 3 After normalizing, the steel plate is transferred to a cooling bed and B f -50℃~B f Air-cool to -20℃, and finally, cross-stack the steel plate with ferritic-pearlitic steel plate at 450-550℃, and self-temper the steel plate during the stacking period, and the temperature of the steel plate is B again. f -50℃~B f This involves destacking and air cooling after the temperature has dropped to -20°C.

Description

[Technical field]

[0001] The present invention belongs to the technical field of material manufacturing, and relates to a manufacturing method of a plastic mold steel plate, and a plastic mold steel plate manufactured by the manufacturing method. [Background technology]

[0002] With the rapid development of petrochemical processes, the production volume of plastics is increasing rapidly. Many plastic products need to be pressed into molds during the production process, and the mold material is an important factor that affects the quality, performance and service life of the mold.

[0003] The main material of plastic molds is mold steel, which is mainly processed into various mold frame parts such as pouring system, cavity, mold core, etc. Due to the complex structure, the contact between the plastic material and the inner cavity surface of the mold frame is prone to wear and impact, etc., so the mold steel plate is required to have uniform cross-sectional structure and mechanical properties and not deform during processing. However, in order to improve homogeneity, traditional plastic mold steel plates adopt production methods such as die casting, forging, and quenching processes, which have long process flows and high costs, or the homogeneity of the structure between the surface and core is very low, and the difference in Rockwell hardness of the cross section is more than 4HRC. Summary of the Invention

[0004] In order to solve the above technical problems, the present invention aims to provide a plastic mold steel sheet and a manufacturing method thereof, which has a short process route and can improve the homogeneity of the structure.

[0005] In order to achieve the above object of the present invention, one embodiment of the present invention comprises: The steel billet is placed in a heating furnace, and three heating steps are performed: the preheating step temperature is 850-950°C, the preheating step residence time is 60 min or more, the heating step temperature is 1100-1220°C, and the soaking step temperature is 1210-1250°C. The first heating step has a residence time of 240 min or more. A second heating step in which the steel billet discharged from the furnace in the first heating step is reheated to have a soaking stage at a temperature of 1140 to 1170 ° C., and the residence time in the furnace is 200 min or more; a rolling step of rolling the steel billet discharged from the furnace in the second heating step into a steel plate at a rolling start temperature of 1060 to 1140°C and a final rolling temperature of 980 to 1050°C; The steel plate obtained by the final rolling is transferred to a cooling bed and cooled to 200°C or less in air. The steel sheet cooled in the cooling process after rolling is subjected to normalizing at a temperature T N Ac3+60℃≦T N A normalizing process for performing normalizing treatment so that Ac3+90°C or less is satisfied; The steel plate that has been subjected to the normalizing process is transferred to a cooling bed and B f -50℃≦T F ≦B f Temperature T that meets -20℃ F A cooling process after normalizing in which the material is air-cooled; The steel plate is cross-stacked with a ferritic-pearlitic steel plate having a temperature of 450 to 550°C, and the steel plate is self-tempered during the stacking period. When the temperature of the steel plate reaches B again, f -50℃≦T M ≦B f Temperature T that meets -20℃ M and a cross-stacking self-tempering process in which the temperature is lowered to 100° C., the steel is de-stacking, and the steel is naturally cooled to room temperature. The present invention provides a method for manufacturing a steel plate for a plastic mold, in which, in the cross stacking, the bottom layer and the top layer are both ferritic-pearlitic steel plates, and the steel plate and the ferritic-pearlitic steel plate are alternately laminated one layer at a time.

[0006] More preferably, the length L2, width W2 and thickness H2 of the steel plate and the length L1, width W1 and thickness H1 of the ferritic-pearlitic steel plate satisfy L1≧L2+500 mm, W1≧W2+300 mm and H1≧H2.

[0007] More preferably, in the second heating step, the steel billet is heated in three stages, with the furnace temperature being 700°C or higher, the preheating stage temperature being 950-1000°C, and the heating stage temperature being 1100-1150°C.

[0008] More preferably, in the rolling step, the steel billet is rolled into a steel plate having a thickness of 80 mm or more.

[0009] More preferably, either or both of the cooling step after rolling and the cooling step after normalizing are First, the steel plate is transferred to the cooling bed and the temperature of the top surface of the steel plate is B s +15℃≦T A ≦B s Temperature T that meets +35℃ A Cool naturally until the temperature drops to . After that, the temperature of the top surface of the steel plate reaches T F This method includes turning on a fan to agitate the air below the steel plate until the temperature drops to 0.4°C, and controlling the temperature difference between the top and bottom surfaces of the steel plate to be 5°C or less.

[0010] More preferably, the air blowing direction of the fan is parallel to the lower surface of the steel plate or obliquely downward so as to move away from the lower surface of the steel plate.

[0011] More preferably, the unevenness of the obtained steel sheet is 4 mm / 2 m or less.

[0012] More preferably, in the cooling step after the rolling, the final cooling temperature is 100 to 200° C., and the furnace entry temperature in the normalizing step is 100° C. or higher.

[0013] More preferably, the difference in Rockwell hardness between the surface layer and the core of the obtained steel plate is 1.6 HRC or less.

[0014] More preferably, the chemical components of the steel billet used are, in mass percent, C 0.33-0.38%, Si 0.11-0.19%, Mn 0.70-0.90%, P≦0.014%, S≦0.004%, Cr 1.40-1.80%, Ni 0.70-0.90%, Mo 0.16-0.24%, with the Cr / Mn ratio being 2±0.05, the Cr / (Mn+Ni) ratio being 1±0.05, Mn+Cr+Ni+Mo being 3.0%-3.8%, and the balance being Fe and unavoidable impurities.

[0015] More preferably, the obtained steel plate has a yield strength of 700 MPa or more, a tensile strength of 1050 MPa or more, a Charpy V-type impact energy of 15 J or more, and a Rockwell hardness of 31 to 34 HRC.

[0016] In order to achieve the object of the present invention, one embodiment of the present invention relates to a manufacturing method comprising the steps of: The steel billet is placed in a heating furnace, and three heating steps are performed: the preheating step temperature is 850-950°C, the preheating step residence time is 60 min or more, the heating step temperature is 1100-1220°C, and the soaking step temperature is 1210-1250°C. The first heating step has a residence time of 240 min or more. A second heating step in which the steel billet discharged from the furnace in the first heating step is reheated to have a soaking stage at a temperature of 1140 to 1170 ° C., and the residence time in the furnace is 200 min or more; a rolling step of rolling the steel billet discharged from the furnace in the second heating step into a steel plate at a rolling start temperature of 1060 to 1140°C and a final rolling temperature of 980 to 1050°C; The steel plate obtained by the final rolling is transferred to a cooling bed and cooled to 200°C or less in air. The steel sheet cooled in the cooling process after rolling is subjected to normalizing at a temperature T N Ac3+60℃≦T N A normalizing process for performing normalizing treatment so that Ac3+90°C or less is satisfied; The steel plate that has been subjected to the normalizing process is transferred to a cooling bed and B f -50℃≦T F ≦B f Temperature T that meets -20℃F A cooling process after normalizing in which the material is air-cooled; The steel plate is cross-stacked with a ferritic-pearlitic steel plate having a temperature of 450 to 550°C, and the steel plate is self-tempered during the stacking period. When the temperature of the steel plate reaches B again, f -50℃≦T M ≦B f Temperature T that meets -20℃ M and a cross-stacking self-tempering process in which the temperature is lowered to 100° C., the steel is de-stacking, and the steel is naturally cooled to room temperature. In the cross stacking, the bottom layer and the top layer are both ferritic-pearlitic steel plates, and the steel plate and the ferritic-pearlitic steel plate are alternately laminated one layer at a time.

[0017] In order to achieve the above object of the present invention, one embodiment of the present invention comprises: A heating process in which the steel billet is placed in a heating furnace, and three-stage heating is performed, with a preheating stage temperature of 850-950°C, a residence time in the preheating stage of 60 min or more, a heating stage temperature of 1100-1220°C, and a soaking stage temperature of 1210-1250°C, and the residence time in the furnace of 240 min or more; a rolling step of rolling the steel billet discharged from the furnace in the heating step into a steel plate at a rolling start temperature of 1060 to 1140°C and a final rolling temperature of 980 to 1050°C; The steel plate obtained by the final rolling is transferred to a cooling bed and cooled to 200°C or less in air. The steel sheet cooled in the cooling process after rolling is subjected to normalizing at a temperature T N Ac3+60℃≦T N A normalizing process for performing normalizing treatment so that Ac3+90°C or less is satisfied; The steel plate that has been subjected to the normalizing process is transferred to a cooling bed and B f -50℃≦T F ≦B f Temperature T that meets -20℃ F A cooling process after normalizing in which the material is air-cooled; The steel plate is cross-stacked with a ferritic-pearlitic steel plate having a temperature of 450 to 550°C, and the steel plate is self-tempered during the stacking period. When the temperature of the steel plate reaches B again, f -50℃≦TM ≦B f Temperature T that meets -20℃ M and a cross-stacking self-tempering process of destacking and air-cooling after lowering the temperature to 100° C.; The present invention provides a method for manufacturing a steel plate for a plastic mold, in which, in the cross stacking, the bottom layer and the top layer are both ferritic-pearlitic steel plates, and the steel plate and the ferritic-pearlitic steel plate are alternately laminated one layer at a time.

[0018] More preferably, the length L2, width W2 and thickness H2 of the steel plate and the length L1, width W1 and thickness H1 of the ferritic-pearlitic steel plate satisfy L1≧L2+500 mm, W1≧W2+300 mm and H1≧H2.

[0019] More preferably, either or both of the cooling step after rolling and the cooling step after normalizing are First, the steel plate is transferred to the cooling bed and the temperature of the top surface of the steel plate is B s +15℃≦T A ≦B s Temperature T that meets +35℃ A Cool naturally until the temperature drops to . After that, the temperature of the top surface of the steel plate reaches T F This method includes turning on a fan to agitate the air below the steel plate until the temperature drops to 0.4°C, and controlling the temperature difference between the top and bottom surfaces of the steel plate to be 5°C or less.

[0020] More preferably, the air blowing direction of the fan is parallel to the lower surface of the steel plate or obliquely downward so as to move away from the lower surface of the steel plate.

[0021] More preferably, the chemical components of the steel billet used are, in mass percent, C 0.33-0.38%, Si 0.11-0.19%, Mn 0.70-0.90%, P≦0.014%, S≦0.004%, Cr 1.40-1.80%, Ni 0.70-0.90%, Mo 0.16-0.24%, with the Cr / Mn ratio being 2±0.05, the Cr / (Mn+Ni) ratio being 1±0.05, Mn+Cr+Ni+Mo being 3.0%-3.8%, and the balance being Fe and unavoidable impurities.

[0022] More preferably, the obtained steel plate has a yield strength of 700 MPa or more, a tensile strength of 1050 MPa or more, a V-type Charpy impact energy of 15 J or more, a Rockwell hardness of 31 to 34 HRC, and a difference in Rockwell hardness between the surface layer and the core of 1.6 HRC or less.

[0023] In order to achieve the object of the present invention, one embodiment of the present invention relates to a manufacturing method comprising the steps of: A heating process in which the steel billet is placed in a heating furnace, and three-stage heating is performed, with a preheating stage temperature of 850-950°C, a residence time in the preheating stage of 60 min or more, a heating stage temperature of 1100-1220°C, and a soaking stage temperature of 1210-1250°C, and the residence time in the furnace of 240 min or more; a rolling step of rolling the steel billet discharged from the furnace in the heating step into a steel plate at a rolling start temperature of 1060 to 1140°C and a final rolling temperature of 980 to 1050°C; The steel plate obtained by the final rolling is transferred to a cooling bed and cooled to 200°C or less in air. The steel sheet cooled in the cooling process after rolling is subjected to normalizing at a temperature T N Ac3+60℃≦T N A normalizing process for performing normalizing treatment so that Ac3+90°C or less is satisfied; The steel plate that has been subjected to the normalizing process is transferred to a cooling bed and B f -50℃≦T F ≦B f Temperature T that meets -20℃ F A cooling process after normalizing in which the material is air-cooled; The steel plate is cross-stacked with a ferritic-pearlitic steel plate having a temperature of 450 to 550°C, and the steel plate is self-tempered during the stacking period. When the temperature of the steel plate reaches B again, f -50℃≦T M ≦B f Temperature T that meets -20℃ M and a cross-stacking self-tempering process of destacking and air-cooling after lowering the temperature to 100° C.; In the cross stacking, the bottom layer and the top layer are both ferritic-pearlitic steel plates, and the steel plate and the ferritic-pearlitic steel plate are alternately laminated one layer at a time.

[0024] Compared with the prior art, the present invention has the following beneficial effects: by using the process means of heating, controlled rolling and cross-stacking self-tempering, the homogeneity of the structure can be significantly improved with a simple process route, the difference in Rockwell hardness between the core and surface of the obtained steel plate is within 1.6HRC, the process flow of the entire production process is simple, the production cycle is short, the efficiency is high and the cost is low. [Brief description of the drawings]

[0025] [Figure 1] 1A and 1B are metallographic micrographs of the cross section of a steel plate in Example 1 of the present invention, where FIG. 1A is a position of 1 / 4 of the cross section of the steel plate, and FIG. 1B is a position of 1 / 2 of the cross section of the steel plate. [Diagram 2] 2A and 2B are metallographic micrographs of the cross section of a steel plate in Example 2 of the present invention, where FIG. 2A is a position of 1 / 4 of the cross section of the steel plate, and FIG. 2B is a position of 1 / 2 of the cross section of the steel plate. [Diagram 3] 3A and 3B are metallographic micrographs of the cross section of a steel plate in Example 3 of the present invention, where FIG. 3A is a position of 1 / 4 of the cross section of the steel plate, and FIG. 3B is a position of 1 / 2 of the cross section of the steel plate. [Figure 4] 4A and 4B are metallographic micrographs of the cross section of a steel plate in Example 4 of the present invention, where FIG. 4A is a position of 1 / 4 of the cross section of the steel plate, and FIG. 4B is a position of 1 / 2 of the cross section of the steel plate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] As described in the background art, in the conventional manufacturing method of plastic mold steel sheets, in order to improve homogeneity, a production method with a long process flow and high cost, such as die casting, forging, and quenching, is adopted, or the homogeneity of the structure between the surface layer and the core is very low, and the difference in Rockwell hardness of the cross section is 4HRC or more. In other words, it is impossible to achieve both production efficiency and cost and homogeneity of the structure. Therefore, the present invention aims to provide a manufacturing method of plastic mold steel sheets that breaks the conventional process flow of a long process route using processes such as die casting, forging, and quenching, and can improve the homogeneity of the structure with only a short process route.

[0027] In the following, the technical solutions of the present invention are further described by specific embodiments, but the scope of protection claimed is not limited to the following description.

[0028] First Embodiment In this embodiment, a plastic mold steel sheet is provided, the chemical components of which, in mass percent, are C 0.33-0.38%, Si 0.11-0.19%, Mn 0.70-0.90%, P≦0.014%, S≦0.004%, Cr 1.40-1.80%, Ni 0.70-0.90%, and Mo 0.16-0.24%, with a Cr / Mn ratio of 2±0.05, a Cr / (Mn+Ni) ratio of 1±0.05, Mn+Cr+Ni+Mo being 3.0%-3.8%, and the remainder being Fe and unavoidable impurities.

[0029] The role of each element in the chemical composition of the steel sheet of the present invention will be briefly described below.

[0030] Although C is a strengthening element, an increase in C tends to reduce plasticity and toughness. In the present invention, by controlling the mass percentage of C to 0.33 to 0.38%, a good balance between strength and toughness can be achieved.

[0031] Although Si is a deoxidizing element, an increase in Si content may cause fayalite to form on the surface of a continuously cast billet, which may affect the surface quality of the steel sheet. In the present invention, the mass percent of Si is controlled to 0.11 to 0.19%.

[0032] Regarding Mn, Cr, Ni, and Mo, Mn and Cr can delay pearlite transformation, Cr expands the temperature range of pearlite transformation, and Mn reduces the temperature range of pearlite transformation and is prone to central segregation. In the present invention, the Cr / Mn ratio can be controlled to 2±0.05 to promote the core of the steel plate not to undergo pearlite transformation at a slow cooling rate. Mo can delay pearlite transformation and increase the temperature range of pearlite transformation, and Ni can reduce the chemical free energy of austenite and delay bainite transformation. In the present invention, the Cr / (Mn+Ni) ratio is controlled to 1±0.05. Furthermore, the synergistic action of Mn, Cr, Ni, and Mo strongly suppresses ferrite and pearlite transformation, and the steel plate can undergo bainite transformation from the surface layer to the core within a large cooling rate range, resulting in a uniform structure throughout the entire thickness.

[0033] P and S are impurity elements, and in the present invention, the mass percentage of P is controlled to 0.014% or less, preferably 0.008-0.014%, and the mass percentage of S is controlled to 0.004% or less, preferably 0.002-0.004%.

[0034] Compared with the prior art, the steel plate of the present invention, through the above-mentioned optimized design of chemical composition, especially through the cooperation of C, Si, Mn, Cr, Ni, Mo alloying elements, can make the steel plate transform into bainite in a large cooling rate range during production, so that the steel plate, especially the thick steel plate with a thickness of 80 mm or more, can form a uniform structure and ensure the homogeneity of the structure even when there is a large difference in the cooling rate between the surface and the core. In addition, it is favorable to improve the homogeneity of the structure of the steel plate with a gentle process and a large process window.

[0035] In addition, by optimizing the chemical composition design, the present invention can omit the precipitation elements such as Nb, V, Ti, and the high hardenability element B in the prior art, which not only saves alloy costs but also solves the crack defects caused by these elements. For example, in the prior art, the addition of Ti element is likely to generate TiN hard spots that cause cracks, and the addition of B element is likely to cause crack formation during flame cutting of the die steel plate due to the segregation of B to the grain boundaries.

[0036] Furthermore, in this embodiment, the steel plate has a yield strength of 700 MPa or more, a tensile strength of 1050 MPa or more, a V-type Charpy impact energy of 15 J or more, a Rockwell hardness of 31 to 34 HRC, and a difference in Rockwell hardness between the surface and the core of 1.6 HRC or less, and thus has excellent mechanical properties, good hardness, and a uniform structure.

[0037] In this embodiment, the steel plate is manufactured by passing a steel billet through the steps of a heating step, a rolling step, a cooling step after rolling, a normalizing step, a cooling step after normalizing, and a cross-stacking self-tempering step in sequence. That is, the manufacturing method of the steel plate includes the steps of a heating step, a rolling step, a cooling step after rolling, a normalizing step, a cooling step after normalizing, and a cross-stacking self-tempering step, which are performed in sequence. Each step will be described in detail below.

[0038] (1)Heating process The steel billet is placed in a heating furnace and heated in three stages, that is, in the order of preheating stage, heating stage, and soaking stage, the temperature in the preheating stage is 850-950°C, the residence time in the preheating stage is 60 min or more, the temperature in the heating stage is 1100-1220°C, the temperature in the soaking stage is 1210-1250°C, and the residence time in the furnace is 240 min or more.

[0039] In this way, on the one hand, the heating rate of the steel billet is controlled and the steel billet is heated slowly and uniformly, so that the surface quality of the steel billet is ensured and the occurrence of microcracks is avoided, and on the other hand, the high temperature is maintained during the soaking stage, so that the complete solid solution of the alloying elements in the steel billet is promoted, the columnar crystal structure in the steel billet is eliminated, and the core segregation defects are improved.

[0040] Here, the steel billet is preferably a continuous casting billet, but is not limited thereto. It is understood that the chemical composition of the steel billet is the same as that of the steel plate, and similarly, in mass percent, C 0.33-0.38%, Si 0.11-0.19%, Mn 0.70-0.90%, P≦0.014%, S≦0.004%, Cr 1.40-1.80%, Ni 0.70-0.90%, Mo 0.16-0.24%, and the ratio of Cr / Mn is 2±0.05, the ratio of Cr / (Mn+Ni) is 1±0.05, Mn+Cr+Ni+Mo is 3.0%-3.8%, and the balance is Fe and unavoidable impurities. The chemical composition of the steel billet is not limited to these, and can be changed to other chemical compositions suitable for the manufacturing method of the present invention.

[0041] More preferably, the heating step may be performed by performing the above-mentioned three-stage heating as the first heating, and after the first heating is completed, the steel billet discharged from the furnace in the first heating may be subjected to a second heating, with the temperature in the soaking stage being 1140 to 1170°C and the furnace time being 200 minutes or more. In this way, by performing the second heating and controlling the temperature in the soaking stage, energy consumption is reduced, and oxide film and oxidation combustion loss are avoided, and further, complete solid solution and homogenization of alloy components in the steel billet are realized, segregation is improved, and a foundation is laid for obtaining an equiaxed grain structure and refining recrystallized grains in the subsequent rolling.

[0042] In the second heating, the steel billet is preferably heated in three stages, with the furnace entry temperature being 700°C or higher, the preheating stage temperature being 950-1000°C, and the heating stage temperature being 1100-1150°C. In this way, by entering the furnace at a high temperature, the preheating time and heating time of the second heating process are shortened, which has the effect of saving energy and reducing consumption. Of course, in a modified embodiment, the second heating may be performed by directly inputting the steel billet into the soaking stage at a furnace entry temperature of 700°C or higher (i.e., there is no preheating stage or heating stage in the second heating).

[0043] In addition, the first heating is performed in a first heating furnace, and the second heating is performed in a second heating furnace, i.e., the first heating and the second heating are not performed in the same heating furnace, thus enabling rapid production and simplified process operations.

[0044] (2) Rolling process The steel billet discharged from the furnace in the heating process is rolled into a steel plate. At this time, the rolling start temperature is 1060 to 1140° C., and the final rolling temperature is 980 to 1050° C. In other words, after the heating process is completed, the steel billet is rolled into a steel plate by a rolling mill.

[0045] In this way, by controlling the rolling start temperature and the final rolling temperature, the recrystallization zone rolling process is realized in the rolling process, the whole rolling is carried out in the recrystallization zone, and finally equiaxed grains are obtained, band structure is avoided, center segregation is reduced, band structure is eliminated, and the structure optimization of the steel plate is realized. In addition, it can ensure that the load of the rolling mill is small in the rolling process, which on the one hand reduces the damage of the rolling mill, and on the other hand improves the efficiency and rhythm of rolling.

[0046] In the rolling process, specifically, the steel billet can be rolled into a steel plate having a thickness of 80 mm or more. That is, the manufacturing method provided in this embodiment is suitable for manufacturing a thick steel plate for a plastic mold having a thickness of 80 mm or more, and has a more significant advantage over the prior art in manufacturing a thick steel plate for a plastic mold. Preferably, in this embodiment, the rolling process can roll the steel billet into a steel plate having a thickness of 100 to 165 mm, and thus the thickness of the obtained steel plate is 100 to 165 mm.

[0047] (3) Cooling process after rolling The steel plate obtained from the final rolling is transferred to a cooling bed and air-cooled to below 200°C.

[0048] In this embodiment, the cooling step after rolling may be specifically performed by naturally cooling the steel sheet on a cooling bed until the temperature is 200° C. or less. That is, no intervention is performed. Of course, the specific implementation of the cooling step after rolling is not limited to this, and it may be performed, for example, in the second embodiment described later.

[0049] Here, the final cooling temperature (i.e., end temperature) of the cooling step after rolling is 200°C or lower. Specifically, it may be room temperature, and then the subsequent normalizing step is performed. That is, the furnace entry temperature of the steel sheet in the normalizing step is room temperature. Or, preferably, it may be 100 to 200°C, and then the subsequent normalizing step is performed. That is, the furnace entry temperature of the steel sheet in the normalizing step is 100°C or higher, and by normalizing at such a temperature, the time in the normalizing step of the steel sheet can be reduced, and energy consumption can be reduced.

[0050] In this embodiment, the cooling step after rolling may be specifically performed by naturally cooling the steel sheet on a cooling bed until the temperature is 200° C. or less. That is, no intervention is performed. Of course, the specific implementation of the cooling step after rolling is not limited to this, and it may be performed, for example, in the third embodiment described later.

[0051] (4) Normalizing process The steel sheet cooled in the cooling step after rolling is subjected to normalizing treatment. At this time, the normalizing temperature T N Ac3+60℃≦T N ≦Ac3+90℃.

[0052] Here, Ac3 is the temperature at which all ferrite is transformed into austenite during heating, and specifically, it can be calculated from the mass percent contents [C], [Ni], [Si], [V], and [Mo] of C, Ni, Si, V, and Mo in the chemical components of the steel billet. For example, in this embodiment,

number

[0053] In the present embodiment, the normalizing process, particularly the control of the normalizing temperature, is combined with the above-mentioned control of the final rolling temperature, thereby improving the microstructural homogeneity and mechanical properties of the steel sheet, and optimizing the microstructural homogeneity and mechanical properties of the steel sheet.

[0054] As described above, it is preferable to set the furnace temperature in the normalizing step to 100° C. or higher, thereby reducing the time required for the normalizing step of the steel sheet and reducing energy consumption.

[0055] (5) Cooling process after normalizing The steel plate that has left the normalizing process is transferred to a cooling bed and cooled to a temperature of T F Here, B f -50℃≦T F ≦B f -20°C, specifically preferably T F =B f It can be -30°C. f is the temperature at which the bainite transformation is completed during cooling, and specifically, it may be obtained from the continuous cooling transformation curve (i.e., CCT curve) of supercooled austenite, or may be calculated from the element contents of the chemical components in the steel plate.

[0056] In this way, the steel plate is heated to temperature T F (That is, the bainite transformation finish temperature Bf By air-cooling the steel sheet to a temperature 20 to 50°C lower than the normalizing temperature, the steel sheet is completely transformed into bainite from the surface layer to the core after the cooling process after the normalizing, and microcracks on the surface can also be avoided, which is advantageous for further optimization of the subsequent structure homogeneity.

[0057] In this embodiment, the cooling step after normalizing is specifically performed by cooling the steel sheet on a cooling bed. F Natural cooling after normalizing may be performed, i.e., without any intervention. Of course, the specific implementation of the cooling process after normalizing is not limited to this, and it may be performed, for example, in the second embodiment described later.

[0058] (6) Cross-stacking self-tempering Immediately after the cooling process after the normalizing, that is, the steel sheet is cooled to T by the cooling process after the normalizing. F When the steel plate is cooled to 450°C, the steel plate (referring to the steel plate provided / manufactured by the present invention) is cross-stacked with a ferritic-pearlitic steel plate having a temperature of 450-550°C, and the steel plate is self-tempered during the stacking period, and the temperature of the steel plate is again T M After the temperature has dropped, destacking is performed and the container is allowed to cool naturally to room temperature.

[0059] Here, B f -50℃≦T M ≦B f -20°C, T M The specific value of T F and may be the same or different, and preferred values ​​are specifically T M =B f The temperature may be -30°C. In the cross stacking, the bottom and top layers are both ferrite-pearlite steel plates, and the steel plates and ferrite-pearlite steel plates are alternately stacked one layer at a time. In this way, the top surface of each steel plate is covered with the ferrite-pearlite steel plate of the upper layer, and the bottom surface is covered with the ferrite-pearlite steel plate of the lower layer.

[0060] Thus, when the temperature is T F and the temperature is T FFerrite-pearlite steel sheets with a higher temperature than the M In other words, the destacking temperature is T M (Bainite transformation finish temperature B f As a result, during the stacking period, the bainite structure of the steel sheet undergoes stable tempering transformation, MA in the bainite structure is decomposed, and carbides in the bainite ferrite are precipitated. Furthermore, the steel sheet obtained in this embodiment has a uniform structure and performance. In addition, since the stacking time is about 18 to 24 hours, high production efficiency can be ensured.

[0061] Here, the temperature of the steel plate is determined by measuring the temperature of the side edge of the steel plate during the cross stacking period. M Needless to say, it is also possible to determine whether the destacking temperature has been reached by suspending the topmost ferrite-pearlite steel plate and measuring the temperature of the top surface of the uppermost steel plate, and using the result as the temperature of the steel plate.

[0062] Preferably, the length L2, width W2 and thickness H2 of the steel plate and the length L1, width W1 and thickness H1 of the ferritic pearlitic steel plate satisfy L1≧L2+500mm, W1≧W2+300mm and H1≧H2. In this way, since the size of the ferritic pearlitic steel plate is larger than the size of the steel plate, the end of the steel plate can be effectively stacked and tempered, which further ensures the uniformity of the structure and performance.

[0063] As described above, the present embodiment has the following advantageous effects compared to the conventional technology.

[0064] On the one hand, by using the process means of heating, controlled rolling and cross-stacking self-tempering, the homogeneity of the structure can be significantly improved with a simple process route, and the difference in Rockwell hardness between the core and surface of the obtained steel plate is within 1.6HRC. The process flow of the entire production process is simple, the production cycle is short, the efficiency is high and the cost is low.

[0065] On the other hand, the optimization of chemical composition, especially the synergy of C, Si, Mn, Cr, Ni, Mo alloying elements, combined with the improvement of the manufacturing method, allows the bainite transformation to occur within a large cooling rate range, and improves the structural homogeneity of the steel plate with a gentle process and a large process window. The advantages are more obvious in the case of thick steel plates, especially those with a thickness of 80 mm or more. In addition, the precipitation elements such as Nb, V, Ti and high hardenability element B in the conventional technology are omitted, which saves alloy costs and solves the crack defects caused by these elements.

[0066] <Second embodiment> This embodiment also provides a plastic mold steel plate and its manufacturing method. As a further optimization of the first embodiment, the difference between this embodiment and the first embodiment is mainly in the cooling process after the normalizing. Hereinafter, only the difference will be described, and the description of other common parts will be omitted.

[0067] First, in the cooling step after normalizing in the first embodiment, the steel sheet is cooled on a cooling bed. F However, in this embodiment, the cooling process after the normalizing is as follows.

[0068] First, the normalized steel plate is transferred to the cooling bed, and the temperature of the top surface of the steel plate reaches T A Cool naturally until the temperature drops to 0.05°C. In other words, do not take any intervention. Here, B s +15℃≦T A ≦B s +35°C, specifically preferably T A =B s It can be +30℃. s is the temperature at which bainite transformation starts during cooling, and specifically, it may be obtained from the continuous cooling transformation curve (i.e., CCT curve) of supercooled austenite, or may be calculated from the element contents of the chemical components in the steel plate.

[0069] Immediately after that, when the temperature of the top surface of the steel plate reaches T AAfter the temperature of the top surface of the steel plate drops to T F The fan is turned on to circulate the air below the steel plate until the temperature drops to 0.4°C, and the temperature difference between the top and bottom surfaces of the steel plate is controlled to be 5°C or less.

[0070] That is, in the cooling process after normalizing in this embodiment, the temperature of the upper surface of the steel sheet is T A ~T F In this way, the air below the steel plate is stirred by a fan throughout the entire transformation region, so that the temperatures of the upper and lower surfaces of the steel plate are almost the same, and the difference between the two is always kept within 5°C, thereby realizing that the cooling rates, transformation start time, transformation end time, and transformation process of the upper and lower surfaces of the steel plate are all consistent, and furthermore, micro-deformation of the steel plate during the transformation process is avoided, and the unevenness of the finally obtained steel plate is guaranteed to be small. In addition, controlling the temperature by the method of stirring the air with a fan can reduce equipment costs and increase production efficiency, as well as avoid surface cracks of the steel plate, ensure low energy consumption costs and gentle process conditions, and reduce the difficulty of production.

[0071] Furthermore, the cooling bed is provided with multiple fans with adjustable airflow rates located below the steel sheet. In this way, in the cooling process after normalizing, the number of fans turned on and the airflow rates of the fans can be adjusted according to the temperature difference between the top and bottom surfaces of the steel sheet, thereby ensuring that the temperature difference between the top and bottom surfaces of the steel sheet is always kept within 5°C throughout the entire transformation region.

[0072] For example, when the difference between the upper and lower temperatures is greater than 30°C, 10 fans will be turned on and the fan volume will be 80000~100000m 3 / h. When the temperature difference between the top and bottom surfaces is greater than 15℃ and less than 30℃, turn on the seven fans and set the fan volume to 70,000~90,000m 3 / h. When the temperature difference between the top and bottom surfaces is greater than 5℃ and less than 15℃, turn on the three fans and set the fan volume to 70,000~90,000m 3 / h, and the fan may not be turned on when the difference between the upper surface temperature and the lower surface temperature is 5° C. or less. Of course, this is only an example, and in reality, other methods can be used. Basically, the fan's overall air volume is controlled to increase stepwise as the difference between the upper surface temperature and the lower surface temperature increases stepwise. Of course, the specific parameter values ​​of the fan's air volume and the stepwise change in the difference between the upper surface temperature and the lower surface temperature are not limited to these.

[0073] In addition, the fan blows air in a direction parallel to the underside of the steel plate or obliquely downward away from the underside of the steel plate. In this way, the fan does not blow air directly toward the underside of the steel plate, but only accelerates the airflow under the steel plate. This ensures that the temperature of the underside of the steel plate is uniform and not locally low, further optimizing the plate shape and avoiding surface cracks.

[0074] Thus, compared with the prior art, in addition to the beneficial effects of the first embodiment, this embodiment can also improve the plate shape at low cost with a simple process flow, and the obtained plastic mold steel plate has an unevenness degree of 4mm / 2m or less, even 3mm / 2m or less, as detected according to the GB / T 709-2019 standard, and the plate shape quality reaches or even exceeds the quality of the plastic mold steel plate of the prior art.

[0075] <Third embodiment> This embodiment also provides a plastic mold steel sheet and its manufacturing method. As a further optimization of the first or second embodiment, the difference between the first or second embodiment and this embodiment is mainly in the cooling process after rolling. Hereinafter, only the difference will be described, and the description of other common parts will be omitted.

[0076] First, in the cooling step after rolling in the first and second embodiments, the steel sheet is naturally cooled on a cooling bed to 200°C or less. However, unlike this, the cooling step after rolling in this embodiment is similar to the cooling step after normalizing in the second embodiment and is set as follows.

[0077] First, the final rolled steel plate is transferred to the cooling bed, and the temperature of the top surface of the steel plate is T A Cool naturally until the temperature drops to .

[0078] Immediately after that, when the temperature of the top surface of the steel plate reaches T A After the temperature of the top surface of the steel plate drops to T F The fan is turned on to circulate the air below the steel plate until the temperature drops to 0.4°C, and the temperature difference between the top and bottom surfaces of the steel plate is controlled to be 5°C or less.

[0079] Then, with the fan closed (i.e., the fan is stopped from disturbing the airflow), the steel plate is heated to a top surface temperature T F Cool naturally to below 200℃.

[0080] That is, in the cooling process after rolling in this embodiment, the temperature of the upper surface of the steel sheet is T A ~T F In this way, the temperature of the upper surface and the lower surface of the steel plate are made almost the same in the entire transformation region, which prevents the steel plate from being slightly deformed and ensures that the unevenness of the steel plate is small.

[0081] For other fan adjustments and air direction settings, the cooling process after normalizing in the second embodiment can be referred to, and a description thereof will be omitted here.

[0082] Thus, compared with the prior art, in addition to the beneficial effects of the first embodiment, the present embodiment improves the plate shape at a low cost with a simple process flow, and the obtained plastic mold steel plate has an unevenness degree of 4mm / 2m or less, even 3mm / 2m or less, as detected according to the GB / T 709-2019 standard, and the plate shape quality reaches or even exceeds the quality of the plastic mold steel plate of the prior art.

[0083] In the following, several embodiments of the present invention are provided to further illustrate the technical solutions of the present invention.

[0084] First, the steel sheets provided in Examples 1 to 7 are all manufactured using a continuous casting billet cast from the same furnace steel, and the chemical components of the continuous casting billet are, in mass percent, C: 0.35%, Si: 0.15%, Mn: 0.81%, P≦0.014%, S≦0.004%, Cr: 1.60%, Ni: 0.80%, Mo: 0.18%, and the balance is Fe and unavoidable impurities, where the Cr / Mn ratio is 1.98, the Cr / (Mn+Ni) ratio is 0.99, and Mn+Cr+Ni+Mo is 3.39%.

[0085] Thus, the chemical composition of the steel plate of this embodiment is the same as that described above. Based on the mass percent contents of C, Ni, Si, V, and Mo in the chemical composition of the steel billet [C], [Ni], [Si], [V], and [Mo], the formula

number

[0086] The steel sheets of Examples 1 to 7 were all manufactured through a heating process--rolling process--cooling process after rolling--normalizing process--cooling process after normalizing process--cross stacking self-tempering process, specifically as follows.

[0087] (1)Heating process The steel billets used in Examples 1 to 7 were placed in a first heating furnace and subjected to three-stage heating, with the preheating stage temperature being 850 to 950°C, the residence time in the preheating stage being 60 min or more, the heating stage temperature being 1100 to 1220°C, the soaking stage temperature being 1210 to 1250°C, and the residence time in the furnace being 240 min or more.

[0088] After the steel billet was removed from the first heating furnace, it was heated in three stages in the second heating furnace, with the furnace entry temperature being 700°C or higher, the preheating stage temperature being 950-1000°C, the heating stage temperature being 1100-1150°C, the soaking stage temperature being 1140-1170°C, and the furnace residence time being 200 min or longer.

[0089] (2) Rolling process The steel billets used in Examples 1 to 7 were removed from the second heating furnace and then rolled into steel plates. The rolling start temperature was 1060 to 1140° C., and the final rolling temperature was 980 to 1050° C. The thicknesses of the steel plates in Examples 1 to 7 are as shown in Table 1.

[0090] (3) Cooling process after rolling Immediately after the rolling step, the steel sheets of Examples 1 to 5 were transferred to a cooling bed and naturally cooled to 100°C to 200°C.

[0091] On the other hand, the steel plates of Examples 6 to 7 were transferred to a cooling bed and first naturally cooled until the temperature of the upper surface of the steel plate was cooled to 517°C. As a result of detecting the temperature at this time, the temperatures of the lower surfaces of the steel plates of Examples 6 and 7 were 541°C and 549°C, respectively, and the temperature differences between the upper and lower surfaces of the steel plates of Examples 6 and 7 were 24°C and 32°C, respectively. In Examples 6 and 7, 7 and 10 fans were turned on to disturb the air below the steel plate with the fans, and the temperature difference between the upper and lower surfaces of the steel plate was controlled to be reduced to within 5°C. Thereafter, the number of fans turned on and the air volume of the fans were adjusted according to the temperatures of the upper and lower surfaces of the steel plate to maintain the temperature difference between the upper and lower surfaces of the steel plate within 5°C, and the temperature of the upper surface of the steel plate was cooled to 296-326°C, and then the steel plate was naturally cooled to 100°C to 200°C.

[0092] (4) Normalizing process Immediately after the cooling step after rolling, the steel sheets of Examples 1 to 7 were each subjected to normalizing treatment. At this time, the furnace entry temperature was 100°C or higher, and the normalizing temperature was 870°C.

[0093] (5) Cooling process after normalizing Immediately after the normalizing step, the steel sheets of Examples 1 to 4 and 6 were transferred to a cooling bed and naturally cooled to 296 to 326° C., and the step was completed.

[0094] The steel plates of Examples 5 and 7 were transferred to a cooling bed and naturally cooled until the temperature of the upper surface of the steel plate was cooled to 517°C. As a result of detecting the temperature at this time, the temperatures of the lower surfaces of the steel plates of Examples 5 and 7 were 537°C and 546°C, respectively, and the temperature differences between the upper and lower surfaces of the steel plates of Examples 5 and 7 were 20°C and 29°C, respectively. In Examples 5 and 7, seven and seven fans, respectively, were turned on to disturb the air below the steel plate with the fans, and the temperature difference between the upper and lower surfaces of the steel plate was controlled to be reduced to within 5°C. Thereafter, the number of fans turned on and the air volume of the fans were adjusted according to the temperatures of the upper and lower surfaces of the steel plate to maintain the temperature difference between the upper and lower surfaces of the steel plate within 5°C, and the process was completed after the temperature of the upper surface of the steel plate was cooled to 296 to 326°C.

[0095] (6) Cross-stacking self-tempering Immediately after the cooling step after the normalizing, each of the steel plates of Examples 1 to 7 and a ferrite-pearlite steel plate having a temperature of 450 to 550°C were cross-stacked such that the bottom and top layers were both ferrite-pearlite steel plates, and the steel plates and the ferrite-pearlite steel plates were alternately stacked one layer at a time. During the stacking period, the steel plates were self-tempered to increase in temperature, and when the temperature of the steel plates was lowered to 296°C again, they were de-stacked and then naturally cooled to room temperature. Here, the length L2, width W2 and thickness H2 of the steel plate and the length L1, width W1 and thickness H1 of the ferritic-pearlitic steel plate satisfied L1≧L2+500 mm, W1≧W2+300 mm and H1≧H2.

[0096] The steel sheets of Examples 1 to 7 were sampled and examined, and were found to have excellent structure and good structure homogeneity. The metal structure diagrams of Examples 1 to 4 are shown in Figures 1a to 4b, respectively. The thickness, mechanical properties, structure performance, and unevenness (according to the GB / T 709-2019 standard) of the steel sheets obtained in each Example were specifically as shown in Table 1.

[0097] [Table 1] JPEG0007673260000003.jpg101166

[0098] As can be seen from each example, the steel plate of the present invention has excellent structure homogeneity, the difference in Rockwell hardness between the surface layer and the core is 1.6HRC or less, and the mechanical properties and structure performance are good, with a yield strength of 700MPa or more, a tensile strength of 1050MPa or more, a V-type Charpy impact energy of 15J or more, and a Rockwell hardness of 31 to 34HRC. It can also be seen from Examples 5 to 7 that the plate shape can be controlled by controlling the temperature in the transformation region in the cooling process after rolling and / or the cooling process after normalizing, and the unevenness is 2mm / 2m or less.

[0099] This specification is described based on the embodiments, but it should be understood that each embodiment does not include only one independent technical solution. Such description of the specification is merely for clarity, and those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0100] The detailed descriptions recited above are merely specific descriptions of possible embodiments of the present invention, and are not intended to limit the protection scope of the present invention; any equivalent embodiments or modifications that do not depart from the technical spirit of the present invention shall be included in the protection scope of the present invention.

Claims

1. A first heating step in which the steel billet is placed in a heating furnace and heated in three stages, the temperature of the preheating stage is 850-950°C, the residence time of the preheating stage is 60 minutes or more, the temperature of the heating stage is 1100-1220°C, and the temperature of the soaking stage is 1210-1250°C, and the residence time of the first heating step is 240 minutes or more; A second heating step in which the steel billet discharged from the furnace in the first heating step is reheated to have a soaking stage at a temperature of 1140 to 1170°C, and the residence time in the furnace is 200 minutes or more; a rolling step of rolling the steel billet discharged from the furnace in the second heating step into a steel plate at a rolling start temperature of 1060 to 1140°C and a final rolling temperature of 980 to 1050°C; a post-rolling cooling step in which the steel sheet obtained by the final rolling is transferred to a cooling bed and air-cooled to 200°C or less; The steel sheet cooled in the cooling step after rolling is subjected to normalizing at a temperature T Ｎ Ac ３ +60°C≦T Ｎ ≦Ac ３ A normalizing process for performing a normalizing treatment to a temperature of +90°C; The steel plate that has been subjected to the normalizing process is transferred to a cooling bed, and B ｆ -50℃≦T Ｆ ≦B ｆ Temperature T that meets -20°C Ｆ A cooling process after normalizing in which the material is air-cooled; The steel plate is cross-stacked with a ferrite-pearlite steel plate having a temperature of 450 to 550°C, and the steel plate is self-tempered during the stacking period. When the temperature of the steel plate reaches B again, ｆ -50℃≦T Ｍ ≦B ｆ Temperature T that meets -20°C Ｍ and a cross-stacking self-tempering process in which the temperature is lowered to 100° C., the steel is de-stacking, and the steel is naturally cooled to room temperature. The cross-stacking refers to a method for manufacturing steel plates for plastic molds, in which the bottom and top layers are both ferritic-pearlitic steel plates, and the steel plates and the ferritic-pearlitic steel plates are alternately stacked one layer at a time, the stacking period refers to a period during which the cross-stacked state is maintained, and the de-stacking refers to a method for separating the stacked steel plates and the ferritic-pearlitic steel plates one by one.

2. The length L of the steel plate ２ , width W ２ , thickness H ２ and the length L of the ferrite-pearlite steel plate １ , width W １ , thickness H １ L １ ≧L ２ +500mm, W １ ≧W ２ +300mm, H １ ≧H ２ The method for producing a plastic mold steel plate according to claim 1, characterized in that:

3. The method for manufacturing a plastic mold steel plate according to claim 1, characterized in that in the second heating process, the steel billet is heated in three stages, with the furnace temperature being 700°C or higher, the preheating stage temperature being 950-1000°C, and the heating stage temperature being 1100-1150°C.

4. 2. The method for manufacturing a plastic mold steel plate according to claim 1, wherein in the rolling step, the steel billet is rolled into a steel plate having a thickness of 80 mm or more.

5. Either or both of the cooling step after rolling and the cooling step after normalizing are First, the steel plate is transferred to the cooling bed, and the temperature of the top surface of the steel plate is B ｓ +15°C≦T Ａ ≦B ｓ Temperature T that meets +35°C Ａ Cool naturally until the temperature drops to . After that, the temperature of the top surface of the steel plate becomes T Ｆ 2. The method for manufacturing a steel plate for a plastic mold according to claim 1, further comprising: turning on a fan to disturb the air below the steel plate with the fan until the temperature drops to 0.4°C, and controlling the temperature difference between the upper and lower surfaces of the steel plate to be 5°C or less; and the direction of the air blown by the fan is parallel to the lower surface of the steel plate or obliquely downward away from the lower surface of the steel plate.

6. The method for producing a steel plate for a plastic mold according to claim 5, characterized in that the unevenness of the obtained steel plate is 4 mm / 2 m or less.

7. The method for producing a plastic mold steel sheet according to claim 1, characterized in that in the cooling process after rolling, the final cooling temperature is 100 to 200 ° C, and the furnace entry temperature in the normalizing process is 100 ° C or higher.

8. The method for producing the plastic mold steel plate according to claim 1, characterized in that the chemical components of the steel billet used are, in mass percent, C 0.33-0.38%, Si 0.11-0.19%, Mn 0.70-0.90%, P≦0.014%, S≦0.004%, Cr 1.40-1.80%, Ni 0.70-0.90%, Mo 0.16-0.24%, and the ratio of Cr / Mn is 2±0.05, the ratio of Cr / (Mn+Ni) is 1±0.05, Mn+Cr+Ni+Mo is 3.0%-3.8%, and the balance is Fe and unavoidable impurities.

9. The method for manufacturing a plastic mold steel plate according to claim 8, characterized in that the obtained steel plate has a yield strength of 700 MPa or more, a tensile strength of 1050 MPa or more, a V-type Charpy impact energy of 15 J or more, a Rockwell hardness of 31-34 HRC, and a difference in Rockwell hardness between the surface layer and the core of 1.6 HRC or less.

10. A heating process in which the steel billet is placed in a heating furnace and heated in three stages, with a preheating stage temperature of 850-950°C, a residence time in the preheating stage of 60 min or more, a heating stage temperature of 1100-1220°C, and a soaking stage temperature of 1210-1250°C, and the residence time in the furnace of 240 min or more; a rolling step of rolling the steel billet discharged from the furnace in the heating step into a steel plate at a rolling start temperature of 1060 to 1140°C and a final rolling temperature of 980 to 1050°C; a post-rolling cooling step in which the steel sheet obtained by the final rolling is transferred to a cooling bed and air-cooled to 200°C or less; The steel sheet cooled in the cooling step after rolling is subjected to normalizing at a temperature T Ｎ Ac ３ +60°C≦T Ｎ ≦Ac ３ A normalizing process for performing a normalizing treatment to a temperature of +90°C; The steel plate that has been subjected to the normalizing process is transferred to a cooling bed, and B ｆ -50℃≦T Ｆ ≦B ｆ Temperature T that meets -20°C Ｆ A cooling process after normalizing in which the material is air-cooled; The steel plate is cross-stacked with a ferrite-pearlite steel plate having a temperature of 450 to 550°C, and the steel plate is self-tempered during the stacking period. When the temperature of the steel plate reaches B again, ｆ -50℃≦T Ｍ ≦B ｆ Temperature T that meets -20°C Ｍ and a cross-stacking self-tempering process in which the temperature is lowered to 100° C., the steel is de-stacking, and the steel is naturally cooled to room temperature. The cross-stacking refers to a method for manufacturing steel plates for plastic molds, in which the bottom and top layers are both ferritic-pearlitic steel plates, and the steel plates and the ferritic-pearlitic steel plates are alternately stacked one layer at a time, the stacking period refers to a period during which the cross-stacked state is maintained, and the de-stacking refers to a method for separating the stacked steel plates and the ferritic-pearlitic steel plates one by one.

11. The length L of the steel plate ２ , width W ２ , thickness H ２ and the length L of the ferrite-pearlite steel plate １ , width W １ , thickness H １ L １ ≧L ２ +500mm, W １ ≧W ２ +300mm, H １ ≧H ２ The method for producing a plastic mold steel plate according to claim 10, characterized in that:

12. Either or both of the cooling step after rolling and the cooling step after normalizing are First, the steel plate is transferred to the cooling bed, and the temperature of the top surface of the steel plate is B ｓ +15°C≦T Ａ ≦B ｓ Temperature T that meets +35°C Ａ Cool naturally until the temperature drops to . After that, the temperature of the top surface of the steel plate becomes T Ｆ 11. The method for manufacturing a plastic mold steel plate according to claim 10, further comprising: turning on the fan to disturb the air below the steel plate with the fan until the temperature drops to 0.2°C, and controlling the temperature difference between the upper surface and the lower surface of the steel plate to be 5°C or less; and the direction of the air blown by the fan is parallel to the lower surface of the steel plate or obliquely downward away from the lower surface of the steel plate.

13. The method for producing a plastic mold steel sheet according to claim 10, characterized in that the chemical components of the steel billet used are, in mass percent, C 0.33-0.38%, Si 0.11-0.19%, Mn 0.70-0.90%, P≦0.014%, S≦0.004%, Cr 1.40-1.80%, Ni 0.70-0.90%, Mo 0.16-0.24%, and the ratio of Cr / Mn is 2±0.05, the ratio of Cr / (Mn+Ni) is 1±0.05, Mn+Cr+Ni+Mo is 3.0%-3.8%, and the balance is Fe and unavoidable impurities.

14. The method for manufacturing a plastic mold steel plate according to claim 13, characterized in that the obtained steel plate has a yield strength of 700 MPa or more, a tensile strength of 1050 MPa or more, a V-type Charpy impact energy of 15 J or more, a Rockwell hardness of 31-34 HRC, and the difference in Rockwell hardness between the surface layer and the core is 1.6 HRC or less.

15. A plastic mold steel plate manufactured by the manufacturing method according to claim 1, The chemical composition, in mass percent, is C 0.33-0.38%, Si 0.11-0.19%, Mn 0.70-0.90%, P≦0.014%, S≦0.004%, Cr 1.40-1.80%, Ni 0.70-0.90%, Mo 0.16-0.24%, and the ratio of Cr / Mn is 2±0.05, the ratio of Cr / (Mn+Ni) is 1±0.05, Mn+Cr+Ni+Mo is 3.0%-3.8%, and the balance is Fe and unavoidable impurities. A plastic mold steel plate having a yield strength of 700 MPa or more, a tensile strength of 1050 MPa or more, a V-type Charpy impact energy of 15 J or more, a Rockwell hardness of 31 to 34 HRC, and a difference in Rockwell hardness between a surface layer and a core of 1.6 HRC or less.

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