Pre-hardened free-cutting corrosion-resistant die steel plate and method for manufacturing the same

Abstract

A pre-hardened easy-to-cut corrosion-resistant die steel thick plate and a preparation method thereof, the composition contains, by weight percentage, C: 0.30-0.35; Si: 0.30-0.50; Mn: 0.80-1.00; P≤0.020; S: 0.065-0.085; Cr: 15.60-16.50; Ni: 0.30-0.40; and the balance is Fe and inevitable impurities. The content of S in the steel is precisely controlled by adding pure sulfur wire at the end of refining and adjusting the basicity of the slag; the center segregation and porosity of the continuous casting billet are controlled through cooling water and electromagnetic stirring during the continuous casting process; the pre-hardening heat treatment is carried out through surface grinding before rolling of the continuous casting billet, high-temperature rapid rolling, single-pendulum air cooling quenching and tempering after rolling; and the high-quality pre-hardened easy-to-cut corrosion-resistant die steel thick plate is successfully prepared through series process control.

Description

Technical Field

[0001] This invention relates to a method for the composition design, smelting, rolling and heat treatment of pre-hardened free-cutting corrosion-resistant die steel thick plates, belonging to the field of metallurgy and heat treatment. Background Technology

[0002] Corrosion-resistant mold steel is a high-grade variety of plastic mold steel, mainly composed of high-carbon martensitic material. It is primarily used to manufacture plastic molds resistant to F- and Cl- ion corrosion or for use in water vapor environments (rust prevention), and is widely used in the automotive, home appliance, medical, and food packaging mold industries. With the significant increase in labor costs in recent years, the processing efficiency and cost of corrosion-resistant molds have gradually become key concerns in the mold industry.

[0003] Adding sulfur to steel can form a large number of plastic inclusions (MnS), which can effectively improve the machinability of the steel, thereby increasing the processing efficiency of molds and significantly reducing labor costs. However, adding sulfur to corrosion-resistant mold steel (high-carbon martensitic stainless steel) presents many problems. For example, sulfur is severely lost during smelting, and the S content is difficult to control accurately; low-melting-point FeS is prone to appear in sulfur-containing steel, resulting in "hot brittleness," and improper process control during rolling can easily lead to rolling cracks; moreover, sulfur-containing steel is more prone to elemental segregation, causing defects such as non-compliance in steel plate inspection. Therefore, the preparation of free-machining corrosion-resistant mold steel has always been a challenge in the industry.

[0004] This invention precisely controls the sulfur content in steel by adding pure sulfur line at the end of refining and adjusting the basicity of slag; it employs high-temperature fast rolling during the rolling process and pre-hardening heat treatment by air-cooling quenching followed by tempering, and successfully prepares high-quality pre-hardened free-machining corrosion-resistant die steel thick plates through a series of process controls.

[0005] The purpose of this invention is to provide a pre-hardened free-machining corrosion-resistant die steel thick plate and its preparation method. By controlling the process parameters of smelting, rolling and heat treatment, high-quality production of sulfur-containing free-machining corrosion-resistant die steel thick plate can be achieved. Summary of the Invention

[0006] The purpose of this invention is to address the above-mentioned problems by providing a pre-hardened, free-machining, corrosion-resistant thick plate of mold steel and its preparation method.

[0007] The objective of this invention is achieved as follows: a pre-hardened, free-machining, corrosion-resistant mold steel thick plate, with the following chemical composition and weight percentage: C: 0.30-0.35%, Si: 0.30-0.50%, Mn: 0.80-1.00%, P≤0.020%, S≤0.065-0.085%, Cr: 15.60-16.50%, Ni: 0.30-0.40%, with the balance being Fe and unavoidable impurities.

[0008] A method for preparing pre-hardened free-machining corrosion-resistant die steel thick plates includes the following steps: Step 1: Smelting: The smelting process consists of a converter, VOD, and LF in sequence. The converter outlet temperature is ≥1620℃. The VOD treatment is conducted with Ar supplied throughout, under high vacuum ≤2mbar, with strong stirring for dehydrogenation, and a treatment time of 16-20 minutes. In LF smelting, the slag volume is controlled at 100-150mm, and high-manganese and high-chromium alloys are used for carbonization. After feeding silicon-calcium wire, the stirring time is 15-20 minutes. At the end of LF refining, 80-150Kg of quartz sand is added, and S wire with a purity of 99.9% is fed in at a length of 8-9 meters per ton of molten steel. The temperature is adjusted to 1... LF (Leaf Casting) is produced at 550-1560℃, ensuring the S content in the steel is controlled between 0.08-0.09%. The composition of the LF leaving the station is controlled according to the target composition of the steel. Step 2: Continuous Casting: The continuous casting cross-section is 200*1200-1275mm, the superheat is stably controlled at 20-40℃, the tundish steel temperature is controlled at 1498-1518℃, the cooling water flow rate of the crystallizer is 2000-2500L / min for the wide side and 300-600L / min for the narrow side; the process casting speed is constantly controlled at 0.6-0.8m / min, the electromagnetic stirring current is 450-550A, the frequency is 3-5Hz, and the reversing time is 10-2... 0s, after flame cutting, the continuously cast billet is annealed in an annealing furnace. The furnace temperature is required to be 300-400℃, and the billet temperature upon entering the furnace should not be lower than 400℃. It is held at 700-800℃ for 12-15 hours, then cooled to 350-400℃ before being removed from the furnace to ensure sufficient annealing and prevent stress cracking. Step 3: Surface grinding of the continuously cast billet: After annealing, the surface is ground, with a grinding amount of 1-2mm on each side. Step 4: Rolling: The continuously cast billet is heated to 1230-1270℃. Before rolling, it is descaled using high-pressure water. High-temperature rolling is used, requiring a final rolling temperature of 1000-1100℃. The rolling process employs a small reduction-large reduction-small reduction rolling method. The rolling process involves several steps: First, a small reduction is used to maintain the plate shape, with a single pass reduction of 8-13%. Second, a large reduction is used to ensure the core quality, requiring at least one pass reduction of 20-30%. Rolling is performed in 6-9 passes depending on the final plate specifications. The rolled steel plate undergoes two straightening processes. Step five: Heat treatment. After rolling, the steel plate is air-cooled using a single pendulum rapid cooling method, with a cooling rate between water cooling and stack cooling. After cooling to room temperature, the steel plate is loaded into the furnace for tempering. During tempering, shims are used to separate the steel plates. The tempering temperature is set at 560-600℃ and held for 20-25 hours. After the first tempering, a second tempering is performed using the same process to ensure the steel plate hardness is controlled between 280-320 HBW.

[0009] In step one, the argon flow rate for strong stirring is 150-200 NL / min, and the argon flow rate for weak stirring is 20-50 NL / min.

[0010] The beneficial effects of this invention are as follows: This invention is applicable to the production of pre-hardened free-machining corrosion-resistant die steel thick plates. By adjusting the basicity at the end of refining, adding pure sulfur line, and adjusting the temperature, the sulfur content of the steel grade S can be precisely controlled. The segregation of sulfur-containing steel is improved by methods such as continuous casting horizontal stretching speed and strong electromagnetic stirring. By using continuous casting billet surface grinding, high-pressure water descaling, and controlling high rolling temperature, phenomena such as easy cracking and scabbing on the surface of sulfur-containing steel are avoided, thus improving the surface quality of sulfur-containing hot-rolled plates. The reasonable matching of large and small reductions during the rolling process improves the core quality and plate shape unevenness of the steel plate. By using single pendulum air cooling and shim-separated furnace loading, and tempering twice at 580℃ for 20 hours, the hardness is stably controlled at 280-320 HBW, ensuring the requirements for plate shape flatness and hardness uniformity. Finally, it achieves the production of free-machining corrosion-resistant die steel hot-rolled plates with high surface quality and high hardness uniformity. Detailed Implementation

[0011] 1. Production process design: converter smelting + VOD + LF + continuous casting — annealing — surface grinding — continuous casting billet heating — high pressure water descaling — rolling into finished products — single sheet air cooling — low temperature furnace tempering twice.

[0012] Technical solution of the invention: Composition design: Based on the characteristics of sulfur in steel and the difficulty of adding sulfur to martensitic stainless steel, this invention designs a free-machining corrosion-resistant mold steel with the following chemical composition by mass percentage: C: 0.30-0.35; Si: 0.30-0.50; Mn: 0.80-1.00; P≤0.020; S≤0.065-0.085; Cr: 15.60-16.50; Ni: 0.30-0.40; balance is Fe and unavoidable impurities.

[0013] Smelting: The smelting process consists of converter, VOD, and LF in sequence to ensure the control level of gas content and inclusions in the steel. At the end of LF refining, 100 kg of quartz sand is added to reduce the alkalinity of the slag, and S line (purity 99.9%) is fed in. The feeding line length is 8-9 meters / ton of molten steel, and the temperature is adjusted to 1550-1560℃ to exit LF, ensuring that the S content in the steel is accurately controlled between 0.08-0.09%.

[0014] Continuous casting: The continuous casting cross section is 200*1260mm, the superheat is stably controlled at 20-40℃, the molten steel temperature in the tundish is controlled at 1498-1518℃, the cooling water of the crystallizer is 2300L / min on the wide side and 500L / min on the narrow side; the process casting speed is constantly controlled at 0.7m / min. In order to ensure low segregation and core density of the continuous casting billet, the electromagnetic stirring current is 500A, the frequency is 3.5Hz, and the reversal time is 15s. After flame cutting, the continuous casting billet is put into the annealing furnace for red annealing.

[0015] Continuous casting billet grinding: Sulfur-containing steel continuous casting billets are prone to sulfur deposition at the cutting position after flame cutting. After annealing, the continuous casting billet must be ground, with a single-sided grinding amount of 2mm, to ensure the surface quality of the steel plate after rolling.

[0016] Rolling: The continuously cast billet is heated to 1250±10℃ and then rapidly rolled after exiting the furnace. The deformation in at least one pass during rolling is ≥20%. High-pressure water descaling is used before rolling, and high-temperature rolling is employed, requiring a final rolling temperature ≥1050℃ to reduce the risk of surface cracking during the rolling of sulfur-containing steel. The rolling process uses a low-reduction (including widening) – high-reduction – low-reduction rolling method. High-reduction rolling (at least one pass with a deformation ≥20%) ensures the core quality of the steel plate, while low-reduction ensures the flatness of the steel plate. After rolling, the steel plate undergoes two straightening processes to further ensure its flatness.

[0017] Heat treatment: After rolling, the steel plate is air-cooled and quenched using a single pendulum rapid cooling method. The cooling rate is between that of water cooling and stack cooling to avoid the formation of network carbides during cooling, which would affect the quality of the steel plate. This also avoids the problems of water-cooled steel plates deforming and being unable to straighten, and the severe precipitation of network carbides in steel due to the slower cooling rate of stack cooling. The steel plate is cooled to room temperature and promptly loaded into the furnace for tempering to ensure that all the microstructure of the steel undergoes martensitic transformation while avoiding excessive stress that could cause cracking. During the tempering process, shims are used to separate the steel plates to ensure uniform temperature distribution around the steel plates and uniform surface hardness after tempering. The furnace loading amount depends on the size and specifications of the steel plates. Double-row or double-column loading can be adopted according to the width and length. After loading, the temperature is set to 580℃ and held for 20-25 hours to ensure that most of the martensite in the steel is transformed into tempered martensite and a small portion of the retained austenite is transformed into martensite. After the first tempering, the same process is used for a second tempering to ensure that the microstructure is in a stable state and that the hardness is stably controlled between 280-320 HBW. Example 1

[0018] Following the above process control points, experiments were conducted, with the specific details as follows: 1. The sulfur-containing free-machining corrosion-resistant mold steel 3Cr16S was produced using the following process route: 80t converter → 80t VOD refining → 80t LF refining. After the LF alloy composition was adjusted appropriately, 100kg of silica sand was added, followed by feeding 700m of sulfur wire. The electrode energizing temperature was adjusted to 1555℃ before the steel was tapped and sent to the continuous casting platform. The chemical composition control is shown in the table below.

[0019]

[0020] 3. The continuously cast billet is sent for annealing at a hot temperature of 400℃. The temperature is increased at a rate of ≤100℃ / h, held at 750℃ for 12 hours, and then cooled in the furnace to 400℃ before being air-cooled.

[0021] 4. Grind the surface of the continuously cast billet, with a single-sided grinding amount of 2mm.

[0022] 5. The rolling heating temperature is 1230℃. After exiting the heating furnace, the material is descaled by high-pressure water and then rapidly rolled to a diameter of 98mm. The rolling process consists of 6 passes with reduction rates of 9.7%, 8.8%, 23.8%, 8.3%, and 8.7% respectively. After exiting the mill, the material undergoes two straightening processes: pre-straightening and straightening.

[0023] 6. Lay out single steel plates on the cooling bed for cooling. After cooling to 30°C, load them into the tempering furnace, adding pads one by one. Set the furnace temperature in two steps. The first step is 520°C (this temperature is not the final tempering temperature, but the process temperature), and hold for 3 hours to ensure uniform temperature inside and outside the steel plate. The second step is 580°C, and hold for 20 hours. After reaching the temperature, remove the steel plate from the furnace and air cool. After cooling, repeat the tempering process once, using the same process as the first tempering.

[0024] 7. Nine-point hardness test and cross-sectional hardness test were performed on the board surface. The test results are as follows.

[0025]

[0026] In the cross-sectional direction, the surface hardness is 318 HBW and the core hardness is 308 HBW.

[0027] 8. The pass rate for steel plate flaw detection (SEP 1921-1984 E / e grade) is 100%. Example 2

[0028] According to the above process control points, experiments were conducted, and the specific details are as follows: 1. The sulfur-containing free-machining corrosion-resistant mold steel was produced using the following process route: 80t converter → 80t VOD refining → 80t LF refining. After the LF alloy composition was adjusted appropriately, 120Kg of quartz sand was added, followed by feeding 750m of sulfur wire. The electrode energizing temperature was adjusted to 1557℃ before the steel was tapped and sent to the continuous casting platform. The chemical composition control is shown in the table below.

[0029]

[0030] 3. The continuously cast billet is sent for annealing in a hot furnace. The furnace temperature is 430℃, and the temperature is increased at a rate of 80℃ / h. It is held at 750℃ for 14 hours, and then cooled in the furnace to 385℃ before being air-cooled.

[0031] 4. Grind both sides of the continuously cast billet, with a grinding amount of 1.5mm on each side.

[0032] 5. The rolling heating temperature is 1250℃. After exiting the heating furnace, the material is descaled by high-pressure water and then rapidly rolled to a diameter of 68mm. The rolling process is divided into 6 passes with reduction rates of 12.1%, 11.8%, 14.8%, 25.2%, and 24.8%, respectively. After exiting the rolling mill, the material undergoes two straightening processes: pre-straightening and straightening.

[0033] 6. Use single steel plates to be laid out and cooled. After cooling to room temperature, load them into the tempering furnace, adding pads one by one. Set the furnace temperature to 590℃ and hold for 25 hours. After reaching the temperature, remove them from the furnace and air cool. After cooling, repeat the tempering process once, using the same process as the first tempering.

[0034] 7. Perform nine-point hardness tests on the board surface and cross-sectional hardness tests. The test results are shown in the table below.

[0035]

[0036] In the cross-sectional direction, the surface hardness is 303 HBW and the core hardness is 312 HBW.

[0037] 8. The pass rate for steel plate flaw detection (SEP 1921-1984 E / e grade) is 100%.

[0038] The above description is only a specific embodiment of the present invention, but the structural features protected by the present invention are not limited thereto. Any changes or modifications made by those skilled in the art within the scope of the present invention are covered by the patent scope of the present invention.

Claims

1. A pre-hardened, free-machining, corrosion-resistant die steel thick plate, characterized in that: The chemical composition and weight percentage are as follows: C: 0.30-0.35%, Si: 0.30-0.50%, Mn: 0.80-1.00%, P≤0.020%, S≤0.065-0.085%, Cr: 15.60-16.50%, Ni: 0.30-0.40%, with the balance being Fe and unavoidable impurities; A method for preparing a pre-hardened, free-machining, corrosion-resistant die steel thick plate includes the following steps: Step 1: Smelting: The smelting process is as follows: converter, VOD, LF. The converter outlet temperature is ≥1620℃. The VOD treatment is supplied with Ar throughout the process, under high vacuum ≤2mbar, with strong stirring for dehydrogenation, and the treatment time is 16-20 minutes. The LF smelting controls the slag amount to 100-150mm, uses high manganese and high chromium alloys for carbonization, and after feeding silicon-calcium wire, the stirring time is 15-20 minutes. At the end of the LF refining, 80-150Kg of quartz sand is added, and S wire with a purity of 99.9% is fed in. The feeding wire length is 8-9 meters / ton of molten steel. The temperature is adjusted to 1550-1560℃ to exit the LF, ensuring that the S content in the steel is controlled between 0.08-0.09%. The composition of the LF exit station is controlled according to the target composition of the steel. Step 2: Continuous casting: The continuous casting cross-section is 200*1200-1275mm, the superheat is stably controlled at 20-40℃, the molten steel temperature in the tundish is controlled at 1498-1518℃, the cooling water flow rate of the crystallizer is 2000-2500L / min for the wide side and 300-600L / min for the narrow side; the process casting speed is constantly controlled at 0.6-0.8m / min, the electromagnetic stirring current is 450-550A, the frequency is 3-5Hz, and the reversal time is 10-20s. After flame cutting, the continuously cast billet is placed in an annealing furnace for annealing. The annealing requires a furnace temperature of 300-400℃, the billet temperature entering the furnace is not lower than 400℃, and it is held at 700-800℃ for 12-15h. It is then furnace cooled to 350-400℃ before being taken out of the furnace to ensure sufficient annealing and prevent stress cracking. Step 3: Continuous casting billet grinding: After annealing, the surface is ground, with a grinding amount of 1-2mm on one side; Step 4: Rolling: The continuous casting billet is heated to 1230-1270℃. Before rolling, it is descaled with high-pressure water. High-temperature rolling is used, requiring a final rolling temperature of 1000-1100℃. The rolling process adopts a small reduction-large reduction-small reduction rolling method. The small reduction maintains the plate shape, with a single pass reduction of 8-13%. The large reduction maintains the core quality, requiring at least one pass reduction of 20-30%. Rolling is carried out in 6-9 passes depending on the specifications of the rolled steel plate. The rolled steel plate is straightened twice. Step 5: Heat treatment: After rolling, the steel plate is air-cooled and quenched using a single pendulum rapid cooling method. The cooling rate is between that of water cooling and stack cooling. After the steel plate is cooled to room temperature, it is loaded into the furnace for tempering. During the tempering process, the steel plates are separated by shims. After loading into the furnace, the tempering temperature is set to 560-600℃ and held for 20-25 hours. After the first tempering, the same process is used for a second tempering to ensure that the hardness of the steel plate is controlled between 280-320 HBW.

2. The pre-hardened free-machining corrosion-resistant die steel thick plate according to claim 1, characterized in that: In step one, the argon flow rate for strong stirring is 150-200 NL / min, and the argon flow rate for weak stirring is 20-50 NL / min.

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