Methods to reduce delayed cracking during mold steel plate cutting and mold steel plates

Abstract

This invention discloses a method for reducing delayed cracking during die cutting and a die steel plate. The method for reducing delayed cracking during die cutting includes: the rolled die steel plate passes through a cooling bed and enters a first fire-cutting zone for segmented cutting; after segmented cutting, the die steel plate connected to the head and tail is heat-treated and then transferred to a second fire-cutting zone for head and tail cutting; the resulting die steel plates are then stacked and slowly cooled. This method significantly reduces the incidence of fire-cutting cracks in die steel plates, which not only improves the yield of steel plates but also reduces the variable costs of order re-rolling and the production consumption of heat treatment furnaces, ensuring order delivery time.

Description

Technical Field

[0001] This invention relates to the field of steel plate surface quality control technology, and more specifically, to a method for reducing delayed cracking during mold steel plate cutting and to mold steel plates. Background Technology

[0002] Flame-cut cracks in mold plates are a significant factor affecting the quality of Shaogang mold plates. However, during mass production, the probability of cracks appearing at the cut edge after flame cutting of steel plates is very high. Once flame-cut cracks appear, they are usually removed by secondary or tertiary flame cutting. Repeated flame cutting not only increases the cost of flame cutting and reduces the yield and hit rate of steel plates, but also affects the delivery time.

[0003] In view of this, the present invention is proposed. Summary of the Invention

[0004] The purpose of this invention is to provide a method for reducing delayed cracking during the cutting of mold steel plates and a mold steel plate.

[0005] This invention is implemented as follows:

[0006] In a first aspect, the present invention provides a method for reducing delayed cracking during the cutting of mold steel plates, comprising:

[0007] The rolled mold steel plate is cut into sections. After passing through the cooling bed, it enters the first fire cutting zone for section cutting to obtain mold steel plates of specified specifications and mold steel plates connected to the head and tail. The temperature of the section cutting of the mold steel plate is ≥150℃.

[0008] Head and tail cutting: After the segmented cutting is completed, the mold steel plate connected to the head and tail is heat-treated and then transferred to the second fire cutting zone for head and tail cutting to obtain mold steel plate, head steel plate and tail steel plate of specified specifications. The heat treatment temperature is 500℃-700℃, and the head and tail cutting temperature of the mold steel plate is 260℃-290℃.

[0009] Stacking and slow cooling: The mold steel plates of the specified specifications are stacked and slow cooled.

[0010] In an optional embodiment, the method further includes heat treatment and secondary heat cutting of the mold steel plate that has thermal stress cracks after cutting. The heat treatment involves placing the mold steel plate in an environment of 340℃-360℃ and heating it to 190℃-210℃, and then performing secondary heat cutting on the mold steel plate. The temperature of the mold steel plate during the secondary heat cutting is ≥150℃.

[0011] In an optional embodiment, the segmented cutting speed is 0.18 m / min-0.25 m / min; and / or the head and tail cutting speed is 0.18 m / min-0.25 m / min.

[0012] In an optional embodiment, the maximum temperature difference at different positions on the mold steel plate connected to the head and tail during the head and tail cutting is less than 20°C.

[0013] In an optional embodiment, the maximum temperature difference at different locations on the mold steel plate during the segmented cutting is less than 35°C.

[0014] In an optional implementation, the temperature of the mold steel plate of specified specifications in the stacking slow cooling step drops by 25°C-45°C after 8 hours.

[0015] In an optional embodiment, the heat treatment heating coefficient is 2 min / cm-3 min / cm.

[0016] In an optional embodiment, the thickness of the mold steel plate is 30mm-70mm.

[0017] In an optional implementation, ceramic fiber cotton is covered at the cut edge during the stacking slow cooling step.

[0018] Secondly, the present invention provides a mold steel plate obtained by the method described in any of the foregoing embodiments.

[0019] The present invention has the following beneficial effects:

[0020] The method described in this application significantly reduces the incidence of fire-cut cracks in mold steel plates. This not only improves the yield of steel plates but also reduces the variable costs of order re-rolling and the production costs of heat treatment furnaces, thus ensuring the delivery time of orders. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 Macroscopic morphology of fire-cut cracks in steel plates;

[0023] Figure 2 Fine cracks at the edge of fire-cut cracks in steel plates (×500);

[0024] Figure 3 The microstructure of the steel plate matrix (×500);

[0025] Figure 4 The inclusions within the cracks in the steel plate (×100). Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0027] This embodiment provides a method for reducing delayed cracking during mold steel plate cutting, including:

[0028] The rolled mold steel plate is cut into sections. After passing through the cooling bed, it enters the first fire cutting zone for section cutting to obtain mold steel plates of specified specifications and mold steel plates connected to the head and tail. The temperature of the section cutting of the mold steel plate is ≥150℃.

[0029] Head and tail cutting: After the segmented cutting is completed, the mold steel plate connected to the head and tail is heat-treated and then transferred to the second fire cutting zone for head and tail cutting to obtain mold steel plate, head steel plate and tail steel plate of specified specifications. The heat treatment temperature is 500℃-700℃, and the head and tail cutting temperature of the mold steel plate is 260℃-290℃.

[0030] Stacking and slow cooling: The mold steel plates of the specified specifications are stacked and slow cooled.

[0031] Macroscopic observation reveals that fire-cutting cracks occur during the fire-cutting process of steel plates, specifically at the beginning and end. These cracks appear perpendicular to the cutting direction and run from top to bottom on the cut surface, varying in length, size, and depth. Most fire-cutting cracks penetrate the entire thickness of the steel plate, and in severe cases, can lead to the complete breakage of the plate. Figure 1 As shown. A longitudinal sample was taken along the fire-cut crack extension, and then ground and polished to prepare a metallographic sample. The polished and etched samples were then placed under a metallographic microscope sequentially to obtain... Figures 2-4 Observations revealed that the cracks propagated in a mixed manner along and across grains, with numerous fine cracks at the crack edges, reaching a depth of 0.01-0.02 mm. Oxides were present at the crack sites, and the metallographic structure was tempered sorbite. The cracks were not caused by original cracks in the slab that resulted after rolling; the oxides were produced by the high-temperature heating of the steel plate edges during flame cutting, leading to an oxidation reaction. The cracks were zigzag cracks, indicating stress cracking; therefore, this application primarily aims to reduce stress cracking.

[0032] The main grades of mold steel include 1.2311, 1.2312, and 4140. Table 1 shows the chemical composition of several grades of mold steel.

[0033] Table 1 Chemical composition of mold plate

[0034]

[0035] As can be seen from the table, the die steel has a high Ceq (Ceq=C+ Mn / 6+(Cr+V+Mo) / 5+(Cu+Ni) / 15) and a high cold cracking sensitivity index Pcm (Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B (%)). Therefore, compared with ordinary grades of steel, die steel plates are more prone to cold cracking when cooled to a lower temperature after fire cutting.

[0036] Specifically, in this application, after the die steel plate is rolled, it is first cooled and then cut into sections to the specified specifications according to requirements. The die steel plates that meet the requirements are then stacked and slowly cooled. For die steel plates connected to the head and tail sections, head and tail cutting is performed again, and the head and tail-cut die steel plates are also stacked and slowly cooled. In the section cutting step, the cutting temperature is ≥150℃ to reduce the formation of hardened structures; the head and tail cutting temperature is 260℃-290℃. Due to the greater thickness at the head and tail, the cutting temperature is relatively slow, and the temperature difference control is relatively high; the heat treatment temperature is 500℃-700℃, which helps to eliminate internal stress under heat. Through adjustments to the heat treatment and fire-cutting temperatures, the fire-cutting crack rate is significantly reduced. This not only improves the yield of the steel plate but also reduces the variable costs of order re-rolling and the production consumption of the heat treatment furnace, ensuring the delivery time of orders.

[0037] In an optional embodiment, the method further includes heat treatment and secondary heat cutting of the mold steel plate that has thermal stress cracks after cutting. The heat treatment involves placing the mold steel plate in an environment of 340℃-360℃ and heating it to 190℃-210℃, and then performing secondary heat cutting on the mold steel plate. The temperature of the mold steel plate during the secondary heat cutting is ≥150℃.

[0038] For mold steel plates that develop thermal stress cracks after segmented cutting or head-and-tail cutting, heat treatment should be performed first, followed by secondary fire cutting.

[0039] In an optional embodiment, the segmented cutting speed is 0.18 m / min-0.25 m / min; and / or the head and tail cutting speed is 0.18 m / min-0.25 m / min.

[0040] In an optional embodiment, the maximum temperature difference at different positions on the mold steel plate connected to the head and tail during the head and tail cutting is less than 20°C.

[0041] In an optional embodiment, the maximum temperature difference at different locations on the mold steel plate during the segmented cutting is less than 35°C.

[0042] In an optional implementation, the temperature of the mold steel plate of specified specifications in the stacking slow cooling step drops by 25°C-45°C after 8 hours.

[0043] In an optional embodiment, the heat treatment heating coefficient is 2 min / cm-3 min / cm.

[0044] In an optional embodiment, the thickness of the mold steel plate is 30mm-70mm.

[0045] In an optional implementation, ceramic fiber cotton is covered at the cut edge during the stacking slow cooling step.

[0046] In this embodiment, the ceramic fiber cotton can be selected from ceramic fibers that contain metals such as steel wires, thereby improving the service life of the ceramic fiber cotton.

[0047] Another embodiment of this application provides a mold steel plate obtained by the method described in any of the foregoing embodiments.

[0048] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0049] Example 1

[0050] This embodiment studies the flame cutting temperature for segmented cutting, and the results are shown in Table 2.

[0051] Table 2 Comparison of fire-cutting cracks caused by high-temperature and low-temperature fire cutting.

[0052]

[0053] Note: In this embodiment, the fire-cutting plate is a mold steel of grade 1.2311. The temperature of the segmented cutting is shown in Table 2. The segmented cutting speed is 0.2m / min. The maximum temperature difference at different positions on the mold steel plate during segmented cutting is in the range of 30-35℃. In the stacking and slow cooling step, ceramic fiber cotton is used to cover the cut edge, so that the temperature of the mold steel plate drops by 35℃-45℃ after 8 hours.

[0054] Summary: 1 out of 6 high-temperature fire-cut plates developed a fire-cutting crack, with a crack length of 15mm and a crack incidence rate of 16.7%; 5 out of 6 low-temperature fire-cut plates developed fire-cutting cracks, with an average crack length of 36mm and a crack incidence rate of 83.3%. High-temperature fire cutting resulted in a lower probability of fire-cutting cracks and shorter crack lengths compared to low-temperature fire cutting.

[0055] Example 2:

[0056] This embodiment studies the fire cutting speed of segmented cutting, and the results are shown in Table 3.

[0057] Table 3 Comparison of fire-cutting cracks caused by low-speed and high-speed fire-cutting.

[0058]

[0059] Note: In this embodiment, the fire-cutting plate is a mold steel of grade 1.2311. The temperature of the segmented cutting is in the range of 150-170℃. The segmented cutting speed is shown in Table 3. The maximum temperature difference at different positions on the mold steel plate during segmented cutting is in the range of 10-20℃. In the stacking and slow cooling step, ceramic fiber cotton is used to cover the cut edge, so that the temperature of the mold steel plate drops to 35℃-45℃ after 8 hours.

[0060] Summary: 3 out of 6 high-speed fire-cut plates produced fire-cut cracks, with an average crack length of 20mm and a crack incidence rate of 50%; 0 out of 6 low-speed fire-cut plates produced fire-cut cracks, with a crack incidence rate of 0%; low-speed fire cutting has a lower probability of producing fire-cut cracks compared to high-speed fire cutting.

[0061] Example 3:

[0062] This embodiment studies the effects of preheating and heat cutting (using a cutting torch) and heated heat cutting (cooling to a specified temperature after rolling before segmented cutting or heat treatment to a specified temperature before head and tail cutting) on ​​mold steel. The results are shown in Tables 4 and 5.

[0063] Table 4 Comparison of preheated and heated cutting temperatures.

[0064]

[0065] Note: In this embodiment, the fire-cutting plate is die steel with grade 1.2311. The temperature of segmented cutting is shown in Table 2. The cutting speed of segmented cutting and head and tail cutting is 0.2m / min. The maximum temperature difference at different positions on the die steel plate during segmented cutting and head and tail cutting is in the range of 20-32℃. The temperature difference of the preheated cutting test plate is less than 70℃. The heat treatment temperature is above 150℃. In the stacking slow cooling step, ceramic fiber cotton is used to cover the cut edge, so that the temperature of the die steel plate drops by 35℃-45℃ after 8 hours.

[0066] Table 5 Comparison of flame cutting cracks caused by preheated flame cutting and flame cutting at room temperature.

[0067]

[0068] Summary: The average flame cutting temperature of the 6 preheated flame-cut plates was 143.7℃, with a maximum temperature difference of 70℃, resulting in 3 flame-cut cracked plates with an average crack length of 50mm and a crack incidence rate of 66.7%; the average flame cutting temperature of the 6 rolled and destacking flame-cut plates was 156.4℃, with a maximum temperature difference of 32℃, resulting in 2 flame-cut cracked plates with an average crack length of 40mm and a crack incidence rate of 33.3%; the average flame cutting temperature of the 6 heat-treated and tempered flame-cut plates was 265.7℃, with a maximum temperature difference of 20℃, resulting in 0 flame-cut cracked plates.

[0069] After heat treatment and tempering, the high temperature of fire cutting results in a small temperature difference in the steel plate, leading to fewer fire-cut cracks. This is also the cutting method currently used.

[0070] Example 4:

[0071] This embodiment studies the effect of slow cooling of stacking on mold steel plates, and the results are shown in Tables 6 and 7.

[0072] Table 6 shows the cooling temperature of steel plates with and without stacking and slow cooling.

[0073]

[0074] Note: In this embodiment, the flame-cutting plate is die steel with grade 1.2311. The flame-cutting temperature is shown in Table 6. The segmented cutting speed is 0.2 m / min. The maximum temperature difference at different positions on the die steel plate during segmented cutting is less than 35°C. In the stacking and slow cooling step, ceramic fiber cotton is used to cover the cut edge. The temperature change is shown in Table 6.

[0075] Table 7 Comparison of fire-cutting cracks caused by stacking and slow cooling.

[0076]

[0077] Summary: After fire cutting, the cooling rate of the 6 thin steel plates with a thickness ≤40mm was greater than that of the steel plates with a thickness >40mm after slow cooling without stacking. After 8 hours, the temperature drop ΔT of all steel plates was ≥45℃, resulting in 4 fire-cut cracked plates with an average crack length of 50mm and a crack incidence rate of 66.7%. The cooling rate of the 6 thin steel plates with a thickness ≤40mm after slow cooling with stacking was not significantly different from that of the steel plates with a thickness >40mm. After 8 hours, the temperature drop ΔT of all steel plates was <45℃, resulting in 0 fire-cut cracked plates.

[0078] Example 5:

[0079] This embodiment studies the fire-cutting cracking under different heat treatment processes, and the results are shown in Table 8.

[0080] Heat treatment tempering can reduce the internal stress of steel plates. Different heat treatment processes may affect the occurrence of fire-cut cracks. Comparative tests of different heat treatment parameters were carried out in the heat treatment production. The test steel plate was 40*2200mm 1.2311 steel, and the results are shown in the table below.

[0081] Table 8 Comparison of fire-cutting cracks under different heat treatment parameters

[0082]

[0083] Note: In this embodiment, the test steel plate is 40*2200mm 1.2311 steel. The temperature for segmented cutting is 260℃-290℃, and the temperature for head and tail cutting is 150℃-170℃ or higher. The cutting speed for both segmented cutting and head and tail cutting is 0.2m / min. The maximum temperature difference between different positions on the mold steel plate during segmented cutting is 25-35℃, and the maximum temperature difference between different positions on the mold steel plate during head and tail cutting is 10-20℃. The heat treatment temperature is shown in the table above. In the stacking and slow cooling step, ceramic fiber cotton is used to cover the cut edge, so that the temperature of the mold steel plate drops by 35℃-45℃ after 8 hours.

[0084] In summary, as shown in Table 8, the higher the heat treatment temperature, the fewer the number of fire-cut cracks are generated; the magnitude of the heat treatment heating coefficient has little effect on the fire-cut crack incidence rate.

[0085] Example 6:

[0086] This embodiment statistically analyzes the occurrence time of fire-cutting cracks, and the results are shown in Table 9.

[0087] Because fire-cutting cracks are sudden and delayed, they can appear within 3 days to a week or two after the mold plate is fire-cut. The project team has compiled statistics on the time of fire-cutting crack appearance of the mold plate, as shown in Table 9.

[0088] Table 9. Statistics on the time of occurrence of fire-cutting cracks

[0089]

[0090] Note: In this embodiment, the fire-cutting plate is a mold steel of grade 1.2311. The temperature of the segmented cutting is 150-160℃, the speed of the segmented cutting is 0.2m / min, the maximum temperature difference at different positions on the mold steel plate during segmented cutting is 25-35℃, and ceramic fiber cotton is used to cover the cut edge during the stacking slow cooling step, so that the temperature of the mold steel plate drops by 35℃-45℃ after 8 hours.

[0091] Summary: As shown in Table 9, the period within one week after the die plate is the period with the highest incidence of fire-cutting cracks, with ≥75% of fire-cutting cracks occurring. To reduce the probability of fire-cutting cracks occurring within one week, it is necessary to increase the frequency of surface quality inspection of steel plates within one week after fire cutting and to promptly handle fire-cutting cracked plates, which can effectively prevent fire-cutting cracked steel plates from flowing to customers.

[0092] After adopting the method of this application, the incidence of fire-cutting cracks in mold steel plates has decreased significantly, from the original 25% to less than 8%. Since January 2022, the monthly incidence of fire-cutting cracks in mold plates has been less than 5%. The decrease in the incidence of fire-cutting cracks has not only improved the yield of steel plates, but also reduced the variable costs of order re-rolling and the production consumption of heat treatment furnaces, thus ensuring the delivery time of orders.

[0093] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method of reducing delayed cracking of die steel plate cutting, characterized by, Comprise: Segment cutting, the finished die steel plate is through the cold bed, enters the first fire cutting area and carries out segment cutting, obtains the die steel plate of specified size and the die steel plate connected with the head and tail, the temperature of the die steel plate segment cutting is greater than or equal to 150 DEG C; Head and tail cutting, after the die steel plate connected with the head and tail is carried out heat treatment after completing segment cutting, is transferred to the second fire cutting area and carries out head and tail cutting, obtains the die steel plate of specified size, head steel plate and tail steel plate, the temperature of the heat treatment is 500 DEG C-700 DEG C, the temperature of the die steel plate head and tail cutting is 260 DEG C-290 DEG C; Stacking slow cooling, the obtained die steel plate of specified size is carried out stacking slow cooling; The speed of the segment cutting is 0.18 m / min-0.25 m / min;And / or the speed of the head and tail cutting is 0.18 m / min-0.25 m / min; The maximum temperature difference of different positions on the die steel plate connected with the head and tail during the head and tail cutting is less than 20 DEG C; The maximum temperature difference of different positions on the die steel plate during the segment cutting is less than 35 DEG C.

2. The method of reducing cut lag crack of a die steel plate according to claim 1, characterized by, Still comprise that the die steel plate that appears thermal stress crack after cutting carries out supplementary heat treatment and secondary fire cutting, the supplementary heat treatment is that the die steel plate is placed in 340 DEG C-360 DEG C environment and is heated to 190 DEG C-210 DEG C, then the die steel plate is carried out secondary fire cutting, the temperature of the die steel plate during secondary fire cutting is greater than or equal to 150 DEG C.

3. The method of reducing delayed cracking of a die steel plate according to claim 1, wherein, The temperature of the die steel plate of specified size in the stacking slow cooling step is reduced by 25 DEG C-45 DEG C after 8 h.

4. The method of reducing cut lag crack of a die steel plate according to claim 1, characterized by, The heating coefficient of the heat treatment is 2 min / cm-3 min / cm.

5. The method of reducing cut lag crack of a die steel plate according to claim 1, characterized by, The thickness of the die steel plate is 30 mm-70 mm.

6. The method of reducing cut lag crack of a die steel plate according to claim 1, characterized by, The ceramic fiber cotton is covered at the cutting fracture in the stacking slow cooling step. 7.A die steel plate obtained by the method of any one of claims 1-6.

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