Pre-hardened free-cutting corrosion-resistant die steel thick plate and preparation method thereof

Abstract

The invention discloses a pre-hardened free-cutting corrosion-resistant die steel thick plate and a preparation method thereof. The pre-hardened free-cutting corrosion-resistant die steel thick plate comprises the following components in percentage by weight: 0.30-0.35 0.30 to 0.50% of Si; 0.80 to 1.00 of Mn; p < = 0.020; s is from 0.065 to 0.085 percent; the content of Cr is 15.60 to 16.50 percent; 0.30 to 0.40% of Ni; and the balance of Fe and inevitable impurities. According to the method, the content of S in the steel is accurately controlled by adding a pure sulfur line at the final stage of refining, adjusting the alkalinity of slag and the like; controlling segregation and looseness of the core of a continuous casting blank through cooling water and electromagnetic stirring in the continuous casting process; the pre-hardening heat treatment is carried out in the modes of surface coping before rolling of a continuous casting billet, high-temperature rapid rolling, simple pendulum air cooling quenching after rolling and tempering, and the high-quality pre-hardening state free-cutting corrosion-resistant die steel thick plate is successfully prepared through a series of process control.

Description

Technical Field

[0001] The invention relates to a pre-hardened easy-cutting corrosion-resistant die steel thick plate composition design and a smelting, rolling and heat treatment method, belonging to the field of metallurgy and heat processing. Background Art

[0002] Corrosion-resistant mold steel is a relatively high-grade variety of plastic mold steel, mainly composed of high-carbon martensite, and is mainly used to manufacture plastic molds that are resistant to F- and Cl- ion corrosion or used in water vapor environments (rust prevention). It is widely used in mold industries such as automobiles, home appliances, medical treatment, and food packaging. With the substantial increase in labor costs in recent years, the processing efficiency and processing costs of corrosion-resistant molds have gradually become the focus of the mold industry.

[0003] Adding sulfur to steel can form a large amount of plastic inclusions MnS in the steel, which can effectively improve the cutting performance of the steel, thereby improving the processing efficiency of the mold and greatly reducing labor costs. However, there are many problems in adding sulfur to corrosion-resistant mold steel (high-carbon martensitic stainless steel). For example, sulfur is severely burned during the smelting process, and the S content is not easy to accurately control; low-melting-point FeS is prone to appear in S-containing steel, resulting in "hot brittleness" phenomenon, and rolling cracks are prone to occur if the rolling process control is not appropriate; and sulfur-containing steel is more likely to produce element segregation, resulting in defects such as steel plate non-compliance with flaw detection. Therefore, the preparation of easy-to-cut corrosion-resistant mold steel has always been a difficulty in the industry.

[0004] The present invention accurately controls the S content in the steel by adding a pure sulfur line at the end of refining and adjusting the basicity of the slag. High-temperature rapid rolling is adopted during the rolling process, and pre-hardening heat treatment is performed by air-cooling quenching followed by tempering. Through a series of process controls, high-quality pre-hardened easy-to-cut corrosion-resistant die steel thick plates are successfully prepared.

[0005] The purpose of the present invention is to provide a pre-hardened easy-cutting corrosion-resistant die steel thick plate and a preparation method thereof, and to achieve high-quality production of sulfur-containing easy-cutting corrosion-resistant die steel thick plate by controlling process parameters during smelting, rolling and heat treatment. Summary of the invention

[0006] The purpose of the present invention is to provide a pre-hardened easy-to-cut corrosion-resistant die steel thick plate and a preparation method thereof in view of the above problems.

[0007] The object of the present invention is achieved as follows: a pre-hardened free-cutting corrosion-resistant die steel thick plate, the chemical composition and weight percentage are: 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 unavoidable impurities.

[0008] A preparation method of a pre-hardened free-cutting corrosion-resistant die steel thick plate comprises the following steps: step 1: smelting: the smelting process is converter, VOD, LF in sequence, the converter outlet temperature is ≥1620°C, Ar is supplied throughout the VOD treatment, high vacuum degree is ≤2mbar, strong stirring dehydrogenation is performed, and the treatment time is 16-20 minutes; LF smelting controls the slag volume to 100-150mm, uses high manganese and high chromium alloy for carbon addition, feeds silicon calcium wire and then weakly stirs for 15-20 minutes, adds 80-150Kg of quartz sand at the end of LF refining, and feeds S wire with a purity of 99.9%, feeds wire length of 8-9 meters / ton of molten steel, and adjusts the temperature to 1 LF is discharged at 550-1560℃, ensuring that the S content in the steel is controlled between 0.08-0.09%, and the LF discharge composition is controlled according to the target composition of the steel; Step 2: Continuous casting: The continuous casting 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 of the crystallizer is 2000-2500L / min on the wide side and 300-600L / min on the narrow side; the process pulling 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, the continuous casting billet is flame cut and then put into the annealing furnace for annealing. The annealing requires a furnace temperature of 300-400℃, and the billet entering the furnace is not less than 400℃. It is kept at 700-800℃ for 12-15h, and the furnace is cooled to 350-400℃ before being taken out of the furnace to ensure that the annealing is sufficient and stress cracking does not occur; Step 3: Continuous casting billet grinding: surface grinding is performed after annealing, and the single-side grinding amount is 1-2mm; Step 4: Rolling: The continuous casting billet is heated to a temperature of 1230-1270℃, high-pressure water is used for dephosphorization before rolling, and high-temperature rolling is used. The final rolling temperature is required to be 1000-1100℃, and the rolling process adopts small reduction-large reduction-small reduction rolling. Method: small reduction to maintain plate shape, single-pass reduction of 8-13%; large reduction to ensure the quality of the core part requires at least a 20-30% reduction in one pass, and 6-9 passes are rolled according to 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-quenched by single-pendulum rapid cooling, and the cooling rate is between water cooling and pile 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 at 560-600℃ and kept warm for 20-25h. After the first tempering, the same process is used for secondary tempering to ensure that the hardness of the steel plate is controlled between 280-320HBW.

[0009] In step 1, 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 the present invention are as follows: the present invention is suitable for the production of a pre-hardened easy-to-cut corrosion-resistant die steel thick plate, and the sulfur content of the steel grade S can be precisely controlled by adjusting the basicity at the end of refining, adding a pure sulfur line, and adjusting the temperature, and the condition that the sulfur-containing steel is easy to segregate is improved by means of continuous casting transverse pulling speed, strong electromagnetic stirring, and the like; the surface grinding of the continuous casting billet, high-pressure water dephosphorization, and control of high rolling temperature, etc. are adopted to avoid the phenomenon of easy cracking and scarring on the surface of the sulfur-containing steel, and improve the surface quality of the sulfur-containing hot-rolled plate; the core quality of the steel plate and the plate shape unevenness are improved by reasonable matching of large pressure reduction and small pressure reduction in the rolling process; the single-swing air cooling and pad iron separation furnace loading method are adopted, and the hardness is stably controlled at 280-320HBW for 20 hours. The requirements for the flatness of the steel plate shape and the uniformity of the hardness are guaranteed, and finally the production of the easy-to-cut corrosion-resistant die steel hot-rolled plate with high surface quality and high hardness uniformity is realized. DETAILED DESCRIPTION

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

[0012] Technical solution of the invention: Composition design: According to the characteristics of S in steel and the difficulty of adding S to martensitic stainless steel, the present invention designs a free-cutting corrosion-resistant die steel, whose chemical composition is as follows 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; the remainder is Fe and unavoidable impurities.

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

[0014] Continuous casting: The continuous casting 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 pulling 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 reversing time is 15s. After flame cutting, the continuous casting billet is put into the annealing furnace for red annealing.

[0015] Grinding of continuous casting billets: Sulfur deposition is easy to occur at the cutting position of sulfur-containing steel continuous casting billets after flame cutting. The surface of the continuous casting billets must be ground after annealing. The single-side grinding amount is 2mm to ensure the surface quality of the steel plate after the continuous casting billets are rolled.

[0016] Rolling: The continuous casting billet is heated to 1250±10℃, and is rolled quickly after being taken out of the furnace. The deformation of at least one pass in the rolling process is ≥20%. High-pressure water is used to remove phosphorus before rolling, and high-temperature rolling is used. The final rolling temperature is required to be ≥1050℃ to reduce the risk of surface cracking during the rolling of sulfur-containing steel. The rolling process adopts a rolling method of small reduction (including widening)-large reduction-small reduction. Large reduction rolling (at least one pass deformation ≥20%) ensures the core quality of the steel plate, and small reduction ensures the flatness of the steel plate. The steel plate is straightened twice after rolling to further ensure the flatness of the steel plate.

[0017] Heat treatment: After rolling, the steel plate is air-cooled and quenched by single pendulum rapid cooling, with a cooling rate between water cooling and pile cooling, to avoid the formation of network carbides during the cooling process, which affects the quality of the steel plate, and avoids the problem of deformation of water-cooled steel plates that cannot be straightened, and the serious precipitation of network carbides in the steel due to slow pile cooling. The steel plate is cooled to room temperature and promptly loaded into the furnace for tempering to ensure that all the structures in the steel undergo martensitic phase transformation while avoiding cracking caused by excessive stress. During the tempering process, the steel plates are separated by shims to ensure uniform temperature distribution around the steel plates and uniform surface hardness of the steel plates after tempering. The loading capacity depends on the size of the steel plates. The furnace can be loaded in double rows or double columns according to the width and length. After loading, the temperature is set at 580℃ and kept warm for 20-25h to ensure that most of the martensite in the steel is transformed into tempered martensite and a small amount of residual austenite is transformed into martensite. After the first tempering, the same process is used for secondary tempering to ensure that the organization is a steady-state organization and the hardness is stably controlled between 280-320HBW. Example 1

[0018] According to the above process control points, the test was carried out, and the specific situation is as follows: 1. Sulfur-containing free-cutting corrosion-resistant die steel 3Cr16S is produced using the following process route: 80t converter → 80t VOD refining → 80t LF refining. After the LF alloy composition is adjusted appropriately, 100Kg of quartz sand is added, and then 700m of sulfur wire is fed. The electrode power-on temperature is adjusted to 1555℃ and the steel is sent to the continuous casting platform. The chemical composition control is shown in the following table.

[0019]

[0020] 3. Annealing of continuous casting billet, charging furnace temperature is 400℃, heating at a rate of ≤100℃ / h, keeping at 750℃ for 12h, cooling the furnace to 400℃ and air cooling.

[0021] 4. Surface grinding of continuous casting billet, single-side grinding amount is 2mm.

[0022] 5. The rolling heating temperature is 1230℃. After leaving the heating furnace, it is dephosphorized by high-pressure water, and then rolled quickly. The rolling specification is 98mm. The rolling is divided into 6 passes, and the reduction rates are 9.7%, 8.8%, 23.8%, 8.3%, and 8.7% respectively. After leaving the rolling mill, it goes through two straightening processes in sequence: pre-straightening and straightening.

[0023] 6. Spread out a single steel plate on the cooling bed for cooling. After cooling to 30°C, put it into the tempering furnace and add pads one by one. The furnace temperature is set to two steps. The first step is 520°C (this temperature is not the final tempering temperature, but the process temperature) and is kept warm for 3 hours to ensure the temperature uniformity inside and outside the steel plate. The second step is 580°C and is kept warm for 20 hours. After reaching the temperature, take it out of the furnace and air cool it. After cooling, repeat the tempering once. The process is the same as the first tempering.

[0024] 7. Conduct nine-point hardness test on the board surface and cross-section hardness test. 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 qualified rate of steel plate flaw detection (SEP 1921-1984 E / e grade) is 100%. Example 2

[0028] According to the above process control points, the test was carried out, and the specific situation is as follows: 1. Sulfur-containing free-cutting corrosion-resistant die steel is produced using the following process route: 80t converter → 80t VOD refining → 80t LF refining. After the LF alloy composition is adjusted appropriately, 120Kg of quartz sand is added, and then 750m of sulfur wire is fed. The electrode power-on temperature is adjusted to 1557℃ and the steel is sent to the continuous casting platform. The chemical composition control is shown in the following table.

[0029]

[0030] 3. The continuous casting billet is annealed in hot air, with the charging temperature at 430℃, heating at a rate of 80℃ / h, keeping at 750℃ for 14h, cooling the furnace to 385℃ and air cooling out of the furnace.

[0031] 4. Double-sided grinding of continuous casting billet, single-sided grinding amount is 1.5mm.

[0032] 5. The rolling heating temperature is 1250℃. After leaving the heating furnace, it is dephosphorized by high-pressure water, and then rolled quickly. The rolling specification is 68mm. The rolling is divided into 6 passes, and the reduction rates are 12.1%, 11.8%, 14.8%, 25.2%, and 24.8% respectively. After leaving the rolling mill, it goes through two straightening processes in sequence: pre-straightening and straightening.

[0033] 6. Spread out a single steel plate for cooling, and after cooling to room temperature, put it into the tempering furnace, add pads piece by piece, set the furnace temperature to 590℃ and keep it warm for 25 hours, take it out of the furnace and air cool it after reaching temperature, and repeat the tempering after cooling, the process is the same as the first tempering.

[0034] 7. Carry out nine-point hardness test on the board surface and cross-section hardness test. The test results are shown in the following table.

[0035]

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

[0037] 8. The qualified rate of 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 of the protection scope of the present invention are not limited thereto. Any changes or modifications made by any technician in the field of the present invention are all covered by the patent scope of the present invention.

Claims

1. A pre-hardened free-cutting corrosion-resistant die steel thick plate, characterized in that: The chemical composition and weight percentage are: 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 unavoidable impurities.

2. A method for preparing a pre-hardened free-cutting corrosion-resistant die steel thick plate, characterized in that: The following steps are involved: Step 1: Smelting: The smelting process is converter, VOD, LF in sequence. The converter outlet temperature is ≥1620℃. Ar is supplied throughout the VOD process. Under high vacuum degree ≤2mbar, strong stirring dehydrogenation is performed for 16-20 minutes. In LF smelting, the slag volume is controlled to be 100-150mm. High manganese and high chromium alloys are used for carbon addition. After feeding the silicon-calcium wire, the weak 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. The feeding wire length is 8-9 meters / ton of molten steel. The temperature is adjusted to 1550-1560℃ to discharge LF, ensuring that the S content in the steel is controlled between 0.08-0.09%. The LF outlet composition is controlled according to the target composition of the steel. Step 2: Continuous casting: The continuous casting 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 of the crystallizer is 2000-2500L / min on the wide side and 300-600L / min on the narrow side; the process pulling speed is constantly controlled at 0.6-0.8m / min, the electromagnetic stirring current is 450-550A, the frequency is 3-5Hz, the reversing time is 10-20s, and the continuous casting billet is flame cut and then put into the annealing furnace for annealing. The annealing requires a furnace temperature of 300-400℃, and the billet entering the furnace is not less than 400℃. It is kept at 700-800℃ for 12-15h, and the furnace is cooled to 350-400℃ before being discharged from the furnace to ensure that the annealing is sufficient and stress cracking does not occur; Step 3: Grinding of continuous casting billet: Surface grinding is performed after annealing, with a single-side grinding amount of 1-2 mm; Step 4: rolling: the continuous casting billet is heated to 1230-1270℃, high-pressure water is used for dephosphorization before rolling, high-temperature rolling is adopted, and the final rolling temperature is required to be 1000-1100℃. The rolling process adopts a rolling method of small reduction-large reduction-small reduction. Small reduction is used to maintain the plate shape, and a single pass reduction is 8-13%; large reduction is used to maintain the quality of the core part, and the reduction amount of at least one pass is 20-30%. According to the specifications of the steel plate after rolling, 6-9 passes are rolled; the steel plate after rolling is straightened twice; Step 5: Heat treatment: After rolling, the steel plate is air-cooled and quenched by single pendulum rapid cooling. The cooling rate is between water cooling and pile 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 pads. After loading into the furnace, the tempering temperature is set at 560-600℃ and kept warm for 20-25h. After the first tempering, the same process is used for secondary tempering to ensure that the hardness of the steel plate is controlled between 280-320HBW.

3. The method for preparing a pre-hardened free-cutting corrosion-resistant die steel thick plate according to claim 2, characterized in that: In step 1, 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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