A method for producing a 200-380 mm thick free-cutting 1.2312 die steel

Abstract

The application discloses a production method of 200-380mm thick easy-to-cut 1.2312 die steel plate, and adopts the following chemical components (unit, wt%): C: 0.42-0.44, Si: 0.20-0.40, Mn: 1.40-1.50, Cr: 1.70-1.80, Mo: 0.18-0.25, P≤0.015, S: 0.050-0.090, Ti: 0.020-0.040, B: 0.001-0.002, and the rest is Fe and inevitable impurities; the purity of molten steel is improved by optimizing the adding process of sulfur alloy, and the hardenability of the steel plate is improved by adding B elements; the controlled cooling process of ACC water cooling and cold bed online air cooling is used after rolling; and then the hardness of the steel plate is directly tempered and adjusted, so that the 200mm-380mm thick easy-to-cut 1.2312 die steel plate is obtained, the whole plate hardness is 32-34HRC, the section hardness difference is within 2HRC, the detection flaw meets GB / T 6402-2008 level three, and the requirements of the easy-to-cut 1.2312 die steel plate are fully met.

Description

Technical Field

[0001] This invention belongs to the field of medium and heavy plate production, specifically involving a production method for 200-380mm thick free-cutting 1.231 mold steel plates. Background Technology

[0002] With the rapid development of the plastic mold steel processing industry, the demand for plastic mold steel plates both domestically and internationally is becoming increasingly diversified and widespread, with a growing urgency for high performance and low cost. Especially for products with thicknesses exceeding 120mm, the inherent defects of conventional ingots are difficult to eliminate, often compromising internal quality. While some companies have adopted electroslag remelting ingot casting to ensure internal quality, these products suffer from drawbacks such as high energy consumption, high cost, low yield, and low production efficiency, making it difficult to meet the demands of the increasingly competitive mold steel processing industry.

[0003] Currently, the majority of free-cutting steels produced in large quantities are sulfur-containing free-cutting steels. Poor control of the sulfur content can easily lead to sulfide inclusions in the molten steel. Furthermore, the high alloy content of this steel poses a risk of cracking during heat treatment. For example, Chinese patent 2008101414973 discloses a smelting method for sulfur-added free-cutting plastic mold steel. This method involves adding iron-sulfur wire after vacuum treatment to adjust the sulfur content to 0.09–0.125%, followed by the addition of calcium-silicon blocks at a weight of 0.1–0.2% of the molten steel, thus completing the smelting process. However, the vacuum holding time after adding the iron-sulfur wire is short, resulting in uneven sulfur content in the molten steel, which easily leads to sulfur segregation and sulfide inclusions.

[0004] For example, Chinese patent 2013105690687 discloses a method for producing free-cutting plastic mold steel plates. The produced steel plates are ≤120mm thick. During the soft blowing stage at the end of LF smelting, 900-1000m of sulfur wire is fed in, and the sulfur content in the molten steel is controlled at 0.08%-0.10%. However, this method does not control the amount of slag in the molten steel. Therefore, during the vacuum treatment RH process, sulfur is easily absorbed by the alkaline slag, and the sulfur recovery rate is unstable, making it difficult to control the sulfur content. The heat treatment adopts normalizing + tempering, but the microstructure of the steel plate after normalizing is unstable, which easily leads to uneven properties and uneven hardness in the thickness direction of the steel plate.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a method for producing 200-380mm thick free-machining 1.231 mold steel plates. The free-machining mold steel plates obtained by this method have an overall plate hardness of 32-34 HRC, a cross-sectional hardness difference of less than 2 HRC, and a flaw detection quality that meets the GB / T 6402-2008 Level 3 standard, fully satisfying the requirements for free-machining 1.2312 mold steel plates.

[0007] The technical solution adopted in this invention is as follows: A method for producing a 200-380mm thick free-machining 1.2312 mold steel plate, using the following chemical composition (unit, wt%) by mass fraction: C: 0.42-0.44, Si: 0.20-0.40, Mn: 1.40-1.50, Cr: 1.70-1.80, Mo: 0.18-0.25, P≤0.015, S: 0.050-0.090, Ti: 0.020-0.040, B: 0.001-0.002, with the remainder being Fe and unavoidable impurities;

[0008] The steel production process includes smelting, casting, heating, rolling, controlled cooling, and tempering, as detailed below:

[0009] (1) Smelting: During the LF refining process, maintain the white slag with good fluidity and foamy properties for 45 min to 60 min. The basicity of the white slag R > 3.0. After refining, remove the slag to ensure that the slag layer thickness is ≤ 40 mm. Before VD pressure holding, add 150 to 200 kg of ferrosulfide. The vacuum degree of VD pressure holding is ≤ 65 Pa. The pressure holding time is 18 to 20 min. During the pressure holding process, control the argon pressure to 0.5-1.0 kPa and the argon flow rate to 150-220 L / min. After VD venting, add 10 to 20 kg of ferroborone alloy and gently blow argon for 10 to 15 min. During the gentle blowing process, the steel liquid surface should only fluctuate slightly to obtain high purity molten steel. Control the gas content in the molten steel to [H] ≤ 1.2 ppm, [O] ≤ 30 ppm, and [N] ≤ 50 ppm.

[0010] (2) Casting: Water-cooled ingot mold with a casting thickness greater than 600mm;

[0011] (3) Heating: During the steel ingot simmering stage, the furnace temperature is controlled at 400-500℃ and the simmering time is 5-10 min / mm. During the heating stage, the heating rate is controlled at ≤50℃ / h. During the heat preservation stage, the furnace temperature is controlled at 1290±20℃ and the heat preservation time is 5-10 min / mm. The heating rate is controlled at ≤50℃ / h and the heat preservation temperature is 1290±20℃. The heat preservation time is 5-10 min / mm.

[0012] (4) Rolling: When rolling begins, the surface temperature of the steel ingot is >980℃, and the reduction per pass is 50-60mm. High-pressure water is used to remove phosphorus once after each of the first three passes. In order to control the plate shape, water is not used in subsequent passes. Rolling is stopped when the thickness reaches 300-450mm and the steel is cooled. When the temperature drops to 800-850℃, the second rolling is started. First, the vertical roller is used for edge extrusion. The single pass of vertical roller edge extrusion is controlled within 10mm. Multiple edge extrusions are used to achieve flush edges. Then, precision rolling is performed to control the thickness tolerance of the steel plate. After rolling is completed, the plate enters ACC cooling.

[0013] (5) Cooling control: Control the roller speed to 0.6-1.0m / s and the cooling rate to 15-20℃ / min. After each ACC water cooling, the plate will be reddened for 1 minute. The reddening temperature of the whole plate will be controlled between 400-500℃. The plate will then be placed on the cooling bed. Four high-pressure fans will be installed at the tail end of the steel plate on the cooling bed. The diameter of the fan outlet is 1.2m and the distance between each fan is 0.5m. Fans with different wind directions and angles will be staggered. The front end of the fan will be 1-2m away from the tail end of the steel plate. Two fans will have their outlets facing upward at 45° and flush with the lower surface of the steel plate. Two fans will have their outlets facing downward at 45° and flush with the upper surface of the steel plate. After the steel plate arrives at the cooling bed, the fans will be turned on immediately and the cooling rate will be controlled to 2-5℃ / min. When the temperature drops below 250℃, the fans will be turned off and the plate will be loaded into the furnace for tempering in time.

[0014] (6) Tempering: The tempering temperature is controlled at 600-610℃, and the holding time is 4min / mm. After the steel plates are taken out of the furnace, they are stacked and air-cooled to room temperature.

[0015] Compared with the prior art, the beneficial effects of the present invention include:

[0016] 1. In terms of composition design, adding trace amounts of boron to steel can improve the density and rolling performance of steel, increase strength and hardenability, and then the hardness value and hardness uniformity requirements of mold steel can be met by controlling the cooling of the steel plate.

[0017] 2. A reasonable process for adding ferrosulfide alloy can reduce steel slag, which is beneficial for controlling the yield of ferrosulfide and the precise control of S content. On the other hand, after adding ferrosulfide, it is stirred with argon gas to make the sulfur content in the molten steel uniform. Furthermore, due to the vacuum degree of the VD furnace, the generation of sulfide inclusions can be effectively reduced.

[0018] 3. Water-cooled die casting with a compression ratio greater than 2 is used to ensure the quality of internal flaw detection of steel ingots. Reasonable controlled rolling, controlled cooling, and stacked tempering are used to control the cooling to ensure the hardness and uniformity of the steel plate. Finally, free-machining 1.231 die steel plates with a thickness of 200-380mm, a plate hardness of 32-34HRC, and a cross-sectional hardness difference within 2HRC are obtained. Detailed Implementation

[0019] The present invention will be further described below with reference to the embodiments.

[0020] The production of a 380mm thick free-machining 1.2312 die steel uses a chemical composition (wt%) containing the following mass fractions: C: 0.43, Si: 0.35, Mn: 1.45, Cr: 1.75, Mo: 0.20, P: 0.14, S: 0.075, Ti: 0.035, B: 0.0015, with the remainder being Fe and unavoidable impurities.

[0021] Its production process is as follows:

[0022] 1. Smelting: During the LF refining process, the white slag with good fluidity and foamy properties was maintained for 45 minutes, and the basicity of the white slag R = 3.5. After the steel refining was completed, the slag was removed, and the thickness of the remaining slag layer in the molten steel was 30 mm. Before the VD pressure holding, 200 kg of ferrosulfite was added. The vacuum degree of VD pressure holding was 45 Pa, the vacuum holding time was 20 minutes, the pressure during the pressure holding process was 0.8 kPa, and the argon flow rate was 220 L / min. After the VD venting, 18 kg of ferroborone was added, and argon was soft-blown for 15 minutes. During the soft-blowing process, the steel liquid surface fluctuated slightly. The gas content in the molten steel was detected to be [H] = 0.8 ppm, [O] = 25 ppm, and [N] = 35 ppm.

[0023] 2. Water-cooled mold casting: Water-cooled ingot mold with a thickness of 840mm is used for casting to ensure a compression ratio >2, a casting temperature of 1540℃, a water flow rate of 420L / min, a steel output rate of 6t / min during casting, a uniform rise in the molten steel level, and demolding 8 hours after casting is completed.

[0024] 3. Steel ingot heating: During the steel ingot simmering stage, the furnace temperature is 450±10℃, the simmering time is 600min, and the heating rate is 50℃ / h. During the heat preservation stage, the furnace temperature is 1290±20℃, and the heat preservation time is 800min.

[0025] 4. Rolling: The surface temperature of the steel ingot is 1115℃ when rolling begins, and the reduction per pass is 50-55mm. The first three passes are descaled once after each rolling pass. To control the plate shape, water is not sprayed in the subsequent passes. The steel is cooled after rolling to 450mm. When the temperature drops to 850℃, rolling begins again. First, the edges are extruded by vertical rollers. The edge extrusion of vertical rollers is controlled at 10mm per pass. Multiple extrusions are performed to make the edges of the steel plate flush. Then, the steel plate is finished rolled to a thickness of 382mm. After rolling, the steel plate enters the ACC cooling equipment.

[0026] 5. Cooling Control: After the steel plate is rolled, it enters the ACC cooling equipment with a roller speed of 1.0 m / s. Each time it passes through the ACC water cooling equipment, it will be reddened for 1 minute. The final reddening temperature of the whole plate is controlled at 500℃, and the cooling rate of the steel plate is 15℃ / min. After entering the cooling bed, four fans are turned on immediately. When the steel plate drops to 250℃, the fans are turned off, and the cooling rate of the steel plate is controlled at 5℃ / min. Then it is loaded into the furnace for tempering.

[0027] 6. Tempering: The tempering temperature is controlled at 600℃, and the holding time is 4min / mm. After the holding time is completed, the steel plates are stacked and air-cooled to room temperature.

[0028] 7. After the steel plate temperature drops to room temperature, perform external inspection, flaw detection, and hardness testing after sawing.

[0029] The 380mm thick free-machining 1.2312 die steel obtained in the example underwent external inspection, flaw detection, and sawing. The hardness of the steel plate was tested according to GB / T230~2018 standard, in HRC units. The specific physical quality is as follows:

[0030] Table 1 Surfaceness testing unit HR

[0031]

[0032] Table 2 Cross-sectional hardness test (unit: HRC)

[0033] Location 100mm 500mm 1000mm 1500mm 2000mm 2500mm 3000mm upper surface 32.2 32.2 33.8 33.7 33.7 33.8 33.2 1 / 4 33.1 32.2 33.3 32.4 32.2 32.8 33.5 1 / 2 32.3 32.3 32.4 32.2 32.8 32.2 32.3 1 / 4 33.5 32.3 32.5 32.7 32.8 33.2 33.5 lower surface 33 32.8 33.8 33.3 33.3 33.8 33.5

[0034] Inclusions were graded according to GB / T 10561 "Determination of Non-metallic Inclusion Content in Steel - Standard Rating Chart Microscopic Examination Method", and the test results are shown in Table 3 below:

[0035] Table 3. Detection of Impurities

[0036]

[0037] Non-destructive testing was conducted in accordance with GB / T 6402~2008 "Ultrasonic Testing of Steel Forgings" standard, and the result was classified as Level III.

[0038] The above description is only a preferred embodiment of the present invention. The above specific embodiments are not intended to limit the present invention. Any modifications, alterations or equivalent substitutions made by those skilled in the art based on the above description shall fall within the protection scope of the present invention.

Claims

1. A method for producing a 200 to 380 mm thick free-cutting 1.2312 die steel sheet, characterized in that, The chemical composition is as follows (by mass percentage): C: 0.42–0.44%, Si: 0.20–0.40%, Mn: 1.40–1.50%, Cr: 1.70–1.80%, Mo: 0.18–0.25%, P≤0.015%, S: 0.050–0.090%, Ti: 0.020–0.040%, B: 0.001–0.002%, with the remainder being Fe and unavoidable impurities. The steel plate is produced through a process including smelting, casting, heating, rolling, controlled cooling, and tempering, as detailed below: 1) Smelting: During the LF refining process, maintain the white slag with good fluidity and foamy properties for 45-60 minutes. The basicity of the white slag R > 3.

0. After refining, remove the slag to ensure that the slag layer thickness is ≤ 40 mm. Before VD pressure holding, add 150-200 kg of ferrosulfide. The vacuum degree of VD pressure holding is ≤ 65 Pa. The pressure holding time is 18-20 minutes. During the pressure holding process, control the argon pressure at 0.5-1.0 kPa and the argon flow rate at 150-220 L / min. After VD venting, add 10-20 kg of ferroboron alloy and gently blow argon for 10-15 minutes. During the gentle blowing process, the steel liquid surface should only fluctuate slightly to obtain high-purity molten steel. Control the gas content in the molten steel to [H] ≤ 1.2 ppm, [O] ≤ 30 ppm, and [N] ≤ 50 ppm. 2) Casting: Water-cooled ingot molds with a casting thickness greater than 600mm; 3) Heating: During the steel ingot simmering stage, the furnace temperature is controlled at 400-500℃ and the simmering time is 5-10 min / mm. During the heating stage, the heating rate is controlled at ≤50℃ / h. During the holding stage, the furnace temperature is controlled at 1290±20℃ and the holding time is 5-10 min / mm. 4) Rolling: When starting rolling, the surface temperature of the steel ingot is >980℃, and the reduction per pass is 50-60mm. High-pressure water is used to remove phosphorus after each of the first three passes. To control the plate shape, water is not used in subsequent passes. Rolling is stopped when the thickness reaches 300-450mm and the steel is cooled. When the temperature drops to 800-850℃, the second rolling begins. First, vertical roll edge extrusion is performed. The single pass of vertical roll edge extrusion is controlled within 10mm. Multiple edge extrusions are performed to achieve flush edges. Then, finish rolling is performed to control the thickness tolerance of the steel plate. After rolling, the plate enters ACC cooling. 5) Cooling Control: Control the roller speed to 0.6-1.0 m / s and the cooling rate to 15-20℃ / min. After each ACC water cooling cycle, allow 1 minute for the steel plate to regain its red color. Control the overall red color temperature of the plate to between 400-500℃ before it is removed from the line and placed on the cooling bed. Install 4 high-pressure fans at the tail end of the steel plate on the cooling bed. The fan outlet diameter is 1.2m, and the fans are spaced 1m apart. Fans with different wind directions and angles are staggered. The front end of the fans is 1-2m away from the tail end of the steel plate. Two fans have their outlets facing upward at 45° and flush with the lower surface of the steel plate, and two fans have their outlets facing downward at 45° and flush with the upper surface of the steel plate. After the steel plate arrives on the cooling bed, immediately turn on the fans and control the cooling rate to 2-5℃ / min. Turn off the fans when the temperature drops below 250℃ and load the plate into the furnace for tempering in a timely manner. 6) Tempering: The tempering temperature is controlled at 600~610℃, and the holding time is 4min / mm. After the steel plates are taken out of the furnace, they are stacked and air-cooled to room temperature.

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