Steel plate for low-welding-sensitivity mirror mold and manufacturing method of steel plate

Abstract

The invention relates to a steel plate for a low-welding-sensitivity mirror mold and a manufacturing method of the steel plate. The die steel plate comprises the following chemical components in percentage by mass: 0.15 to 0.30 percent of C, 0.15 to 0.35 percent of Si, 1.20 to 1.50 percent of Mn, less than or equal to 0.010 percent of P, less than or equal to 0.001 percent of S, 1.20 to 1.80 percent of Cr, 0.25 to 0.65 percent of Mo, 0.9 to 1.4 percent of Ni, 0.02 to 0.10 percent of Al, 0.1 to 0.25 percent of V, less than or equal to 0.007 percent of N and the balance of iron and inevitable impurity elements. The manufacturing method comprises the working procedures of converter or electric furnace smelting, LF refining, vacuum refining, continuous casting, heating, rolling, normalizing accelerated cooling, two times of tempering treatment and the like which are carried out in sequence. The thickness of the continuous casting steel plate is 10-230 mm, and ultrasonic flaw detection meets NB / T 47013.3 T1 level; and the hardness is 35-42 HRC, the hardness fluctuation is smaller than or equal to 3 HRC, the welding crack sensitivity index is smaller than or equal to 0.41%, and good hardness uniformity, surface polishing performance and weldability are achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of special steel smelting, and particularly relates to a steel plate for a low-welding-sensitive mirror mold and a manufacturing method thereof. Background Art

[0002] Currently, plastic molds account for over half of all molds and are widely used in fields such as home appliances, automobiles, and communications. During use, plastic molds inevitably suffer localized damage and minor defects requiring welding repair. However, the carbon content of existing plastic mold steel generally ranges from 0.35% to 0.45%, resulting in a high weld crack sensitivity index (Pcm) (calculated using the formula Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B), typically exceeding 0.50. This results in poor weldability and makes mold repair difficult. Furthermore, with the increasing demand for larger plastic molds, green economies, and high-quality polishing, requirements for mold polishability, hardness uniformity, and cost-effectiveness are becoming increasingly stringent.

[0003] Patent CN108467989A, "Easily Welded Pre-hardened Plastic Mold Steel Plate and Its Production Method," discloses a steel plate with a hardness deviation of ≤3 HRC and a welding sensitivity index (Pcm) ≤0.40, but the hardness is low, at 28-36 HRC, and the mirror finish is poor. Patents CN110295332B, "High-Toughness, High-Mirror Finish Pre-hardened Mold Steel and Its Manufacturing Process," and CN110923574A, "Low-Carbon, Easy-Weld, Corrosion-Resistant Plastic Mold Steel and Its Manufacturing Method," primarily improve hardenability by reducing carbon content and significantly increasing elements like Cr, Mn, and B. The welding sensitivity index (Pcm) is ≥0.5, and the addition of elements like B, in particular, increases welding sensitivity and the risk of cracking. Both inventions utilize electroslag ingot forging, which is inefficient and energy-intensive, making the steel plate production cost-effective. Patent CN116083801A A high-uniformity and high-mirror-polishing performance mold steel and its preparation method discloses a mold steel plate that improves the hardenability and heat treatment hardness of the steel plate by adding a large amount of Cr, Mo, V and N elements, and improves the mirror-polishing performance of the mold steel, but ignores weldability, with a welding sensitivity index Pcm≥0.95, and cannot be welded. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a steel plate for a mirror mold with low welding sensitivity in view of the above-mentioned prior art, and also to provide an efficient and stable continuous casting-rolling-heat treatment production method for the steel plate.

[0005] The technical solution adopted by the present invention to solve the above problems is: a steel plate for low welding sensitivity mirror mold, whose chemical composition and weight percentage are: C: 0.15-0.30%, Si: 0.15-0.35%, Mn: 1.20-1.50%, P: ≤0.010%, S: ≤0.001%, Cr: 1.20-1.80%, Mo: 0.25-0.65%, Ni: 0.9-1.4%, Al: 0.02-0.10%, V: 0.1-0.25%, N: ≤0.007%, B: ≤0.002%, Cu: ≤0.15%, and the balance is iron and unavoidable impurity elements.

[0006] The present invention reduces the welding sensitive elements C and Cr content in the standard die steel composition, and at the same time fully considers the hardenability of the steel plate, adds appropriate amounts of Mo and V, and the steel plate welding sensitivity index Pcm≤0.41, and the hardness after pre-hardening heat treatment is 35-42HRC.

[0007] The following is a detailed description of the effects and dosage of the components in the present invention: C: It is an essential element for the strength and hardness of steel plates, but too high a C content is detrimental to the ductility, toughness, and weldability of the steel. The present invention controls its content to 0.16-0.30%.

[0008] Si: A deoxidizing element in steel, it increases its strength and hardness through solid solution strengthening. A Si content below 0.10% results in poor deoxidation, while higher Si contents increase the brittleness of the steel plate. In the present invention, the Si content is controlled to 0.10-0.30%.

[0009] Mn: An element that improves the hardenability of steel, compensating for the loss of strength and hardness caused by a reduction in carbon content. Therefore, the Mn content in steel must be no less than 0.8%, otherwise it will not fully function. However, excessive Mn content can reduce weldability. Therefore, in the present invention, the Mn content is controlled to 1.0-1.50%.

[0010] Cr: An element that helps improve the strength and hardness of steel plates. However, excessive addition reduces the toughness and weldability of the steel. The present invention controls its content within a range of 1.20-1.80%.

[0011] Nickel: This element enhances the hardenability of steel sheets and significantly improves their toughness. However, nickel is a precious metal, and excessive nickel content increases costs. Therefore, the present invention controls its content to 0.9-1.4%, which helps achieve the best cost-effectiveness.

[0012] Mo: Mo significantly improves steel's hardenability and tempering stability, while reducing temper brittleness. However, Mo is a precious metal, and excessive Mo content increases costs and reduces the material's weldability. In this invention, the Mo content is controlled within a range of 0.25-0.65%.

[0013] V: It can significantly increase the strength and hardness of steel by dispersing and precipitating. Adding a small amount of vanadium can compensate for the reduction in hardness caused by the removal of C and Cr. The present invention controls its content to 0.1-0.25%.

[0014] Al: Al in molten steel easily combines with N to form AlN, which can effectively refine grains. However, too high an Al content will impair the toughness of the steel. In the present invention, the Al content is controlled at 0.02-0.10%.

[0015] S, P, and B are harmful impurity elements in steel, easily causing defects such as segregation and inclusions. As impurity elements, they can adversely affect the toughness of the steel plate and the toughness near the weld, so their content should be minimized. In this invention, P is controlled to ≤ 0.010%, S ≤ 0.001%, and B ≤ 0.002%.

[0016] In order to improve the quality stability of steel plates, reduce production energy consumption and costs, and improve production efficiency, the present invention also discloses a method for manufacturing steel plates for low-welding-sensitivity mirror molds: Smelting: High-quality molten iron and scrap steel are selected to prepare smelting raw materials according to the chemical composition. These raw materials undergo molten iron pretreatment, converter smelting, LF refining, and vacuum refining to produce pure molten steel. The sulfur content of the molten steel after LF refining is preferably no more than 0.001%. The high vacuum treatment time is greater than 25 minutes, ensuring that H ≤ 0.0001% and N ≤ 0.0050%. This reduces inclusions and gas content in the steel plate, reduces processing cracking, and improves the mirror polish of the steel plate. Slab continuous casting utilizes non-oxidizing protection throughout the casting process, with an overheat temperature no greater than 30°C. Dynamic soft reduction technology is used to reduce centerline segregation and porosity in the slab, with a target centerline segregation of Class C and a porosity of Class 0.5. The slab is then slowly cooled for at least 72 hours after exiting the continuous casting line.

[0017] Rolling: After slow cooling, the continuous casting slab is heated in a furnace to 1220-1270°C at a heating rate of 10-12.5 min / cm. After exiting the furnace, it is descaled with high-pressure water, and the slab temperature after descaling is controlled at 980-1080°C. A two-stage rolling process is used. The starting temperature of the rough rolling process is ≥1000°C, with a maximum pass reduction of no less than 50mm and a pass reduction ratio of 15-25% to increase the density of the steel plate and improve flaw detection quality. The finishing rolling process uses a multi-pass low-reduction rolling process with a pass reduction ratio of 8%-12%, and a cumulative rolling process of 5-10 passes to improve the steel plate shape. The rolled steel plate is then placed on a hot steel plate pile for slow cooling for at least 36 hours, and then slowly cooled to room temperature.

[0018] Heat treatment: The steel plate after slow cooling is subjected to a "normalizing accelerated cooling + two tempering" heat treatment. The normalizing temperature is 850-890℃, and the holding time is 1.8-2.2min / mm. The steel plate out of the furnace is accelerated to cool to below 150℃ by air cooling or mist cooling, which speeds up the cooling rate of the steel plate and improves the hardness of the core of the steel plate. The first tempering heating temperature is 600-660℃, and the tempering holding time is 3.5-4.0min / mm. After being taken out of the furnace, it is cooled in the air. The tempered steel plate needs to be heated and tempered again in the furnace for homogenization treatment. The heating temperature is 520-620℃, and the holding time is 3.5-4.0min / mm. After being taken out of the furnace, it is air-cooled to room temperature. The steel plate can be shipped after passing the performance test.

[0019] The present invention has the following characteristics: In response to the current demand for plastic mold steel plates that take into account mirror polishing performance, weldability and high cost-effectiveness, the composition design is optimized to produce high-purity and low-segregation slabs. A targeted "normalizing accelerated cooling + double tempering" heat treatment scheme is adopted to realize the continuous casting and rolling process to produce high-density 10-230mm thick low-crack-sensitive mirror plastic mold steel plates with a hardness of up to 40HRC, a hardness fluctuation of ≤3HRC, a welding crack sensitivity index of ≤0.41%, and good polishability and weldability.

[0020] Compared with the prior art, the advantages of the present invention are: (1) The converter-refining-continuous casting process is used to produce high-purity slabs, and the content of harmful elements and inclusions reaches the level of electroslag production of steel ingots, thereby improving production efficiency and reducing energy consumption and production costs.

[0021] (2) The present invention adopts a two-stage rolling process for slabs. In the rough rolling stage, strong deformation penetration rolling is achieved to fill the shrinkage holes of the slab and improve the density of the steel plate. In the finishing rolling stage, a multi-pass small reduction rolling process is used to improve the shape of the steel plate, improve the dimensional accuracy of the steel plate, reduce the processing volume, and reduce the mold processing cost.

[0022] (3) The present invention adopts a relatively cost-effective component design, which increases the hardenability depth of the steel plate by normalizing accelerated cooling, while avoiding cracking caused by direct water quenching of the steel plate. Normalizing accelerated cooling increases the cooling rate, avoiding the upper bainite transformation zone and entering the lower bainite / martensite transformation zone during the structural transformation. The hardness is higher than that of the brittle upper bainite zone, while the internal stress is lower than that of the martensite zone transformation. The first tempering homogenizes the steel plate structure, reduces the stress of the steel plate, and makes the steel plate hardness reach about 40HRC. The second tempering temperature is lowered to further improve the distribution of carbides in the steel and improve the uniformity of hardness. The two temperings improve the uniformity of the structure, reduce the stress while maintaining the hardness level of the first tempering, which is more conducive to welding stability.

[0023] This invention achieves efficient and low-cost production of low-crack-susceptibility mirror-finish mold steel by optimizing composition design and innovating production processes to ensure steel plate purity, hardenability, and structural uniformity. This plastic mold steel plate, characterized by high hardness and uniformity, excellent polishability (polishing to 5000 mesh and above), and weldability, provides cost-effective plastic mold steel for the Chinese mold industry, enhancing the country's mold manufacturing capabilities and market competitiveness. This patent application, through collaborative innovation in composition and process, resolves the long-standing "high hardness, low weldability, and high polishability" dilemma in the mold steel industry, demonstrating outstanding substantive features and significant advancements. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the metallographic structure 500X of the invention example steel plate after heat treatment.

[0025] Figure 2 It is the grain size of steel plates produced by conventional process (grade 5-6).

[0026] Figure 3 It is the grain size of the steel plate produced by the process of the present invention (grade 9-10). DETAILED DESCRIPTION

[0027] The technical solution of the present invention will be described in more detail with reference to the preferred embodiments of the present invention. However, these embodiments are merely descriptions of preferred implementations of the present invention and do not limit the scope of the present invention in any way. Example 1

[0028] The low crack-sensitive mirror mold steel plate involved in this embodiment has a thickness of 230 mm, and contains the following components and their mass percentages: C: 0.18%, Si: 0.20%, Mn: 1.40%, P: 0.010%, S: 0.001%, Cr: 1.25%, Mo: 0.55%, Ni: 0.95%, Al: 0.023%, V: 0.15%, N: 0.0040%, and the remainder is iron and unavoidable impurity elements.

[0029] The production process of the mold steel plate is as follows: The smelting raw materials are configured according to the chemical composition of the above steel plates and are sequentially subjected to molten iron pretreatment - converter smelting - LF refining - RH refining - thick slab continuous casting - slow cooling - slab heating - rolling - steel plate slow cooling - heat treatment.

[0030] The key processes described above are as follows: Blast furnace hot metal is pretreated to a sulfur content of no more than 0.003%. High-quality hot metal and selected scrap steel are then smelted in a converter, subjected to LF refining, and vacuum refining to produce pure molten steel. The molten steel preferably has a sulfur content of 0.0008% after LF refining. The high vacuum treatment lasts 27 minutes, resulting in a final molten steel with an H content of 0.00005% and an N content of 0.0040%. The pure molten steel is continuously cast into slabs through a ladle, tundish, and crystallizer, all without oxidation protection. The superheat is 10-30°C, and dynamic soft reduction technology is used. The slabs have a low central segregation of C0.5 and a central porosity of 0.5. The slabs are then slowly cooled in the hood for 96 hours.

[0031] After slow cooling, the continuous casting slabs are placed in a walking beam furnace and heated to 1220-1270°C for a total of 10 hours. After exiting the furnace, they are descaled with high-pressure water. The temperature of the descaled slabs is controlled at 1080°C. The descaled slabs undergo rough rolling with reductions of 40mm, 50mm, and 55mm. The finishing rolling process uses multiple passes with small reductions to form 230mm thick steel plates. After finishing rolling, the plates undergo hot straightening to improve their shape. The rolled plates are then placed on a hot steel plate pile and slowly cooled to room temperature for 48 hours.

[0032] After slow cooling, the steel plates undergo a heat treatment consisting of "normalizing, accelerated cooling, and double tempering." The normalizing temperature is 870±10°C, and the holding time is 7.5 hours. The steel plates are then cooled with atomized water to accelerate cooling to below 150°C, increasing the core hardness of the steel plates. The first tempering heating temperature is 630±10°C, and the tempering holding time is 14 hours. After leaving the furnace, the plates are air-cooled on the cooling bed. The tempered steel plates are then heated and tempered again in the furnace for homogenization treatment at 530±10°C, with a holding time of 13.5 hours. The plates are then air-cooled to room temperature.

[0033] The 230mm thick steel plate manufactured using this process meets NB / T 47013.3 T1 ultrasonic flaw detection standards, with a plate roughness of ≤5mm / m, a hardness of 39HRC, and a weld sensitivity coefficient of 0.40. The final mold can be polished to a mesh of 5000 or better. Example 2

[0034] The low crack-sensitive mirror mold steel plate involved in this embodiment has a thickness of 50 mm, and contains the following components and their mass percentages: C: 0.17%, Si: 0.25%, Mn: 1.5%, P: 0.010%, S: 0.0009%, Cr: 1.3%, Mo: 0.55%, Ni: 1.0%, Al: 0.023%, V: 0.11%, N0.004%, and the remainder is iron and unavoidable impurity elements.

[0035] The production process of the mold steel plate is as follows: The smelting raw materials are configured according to the chemical composition of the above steel plates and are sequentially subjected to molten iron pretreatment - converter smelting - LF refining - RH refining - thick slab continuous casting - slow cooling - slab heating - rolling - steel plate slow cooling - heat treatment.

[0036] The key processes described above are as follows: Blast furnace hot metal is pretreated to a sulfur content of no more than 0.003%. High-quality hot metal and selected scrap steel are then smelted in a converter, subjected to LF refining, and vacuum refining to produce pure molten steel. The molten steel preferably has a sulfur content of 0.0008% after LF refining. The high vacuum treatment lasts for 25 minutes, resulting in a final molten steel with an H content of 0.00006% and an N content of 0.0040%. The pure molten steel is continuously cast into slabs through a ladle, tundish, and crystallizer, all without oxidation protection. The superheat is 15-30°C, and dynamic soft reduction technology is used. The slabs have a low central segregation of C0.5 and a central porosity of 0.5. The slabs are then slowly cooled for 72 hours under the hood.

[0037] After slow cooling, the continuous casting slabs are placed in a walking beam furnace and heated to 1220-1270°C for a total of 7 hours. After exiting the furnace, they are descaled with high-pressure water. The temperature of the descaled slabs is controlled at 1080°C. The descaled slabs undergo rough rolling with reductions of 40mm, 50mm, 55mm, and 60mm. The finishing rolling process uses multiple passes with small reductions to form 50mm thick steel plates. After finishing rolling, the steel plates undergo hot straightening to improve their shape. The rolled steel plates are then placed on a hot steel plate pile and slowly cooled to room temperature for 36 hours.

[0038] After slow cooling, the steel plates are subjected to a heat treatment consisting of "normalizing, accelerated cooling, and two tempering treatments." The normalizing temperature is 870±10°C, and the holding time is 1.6 hours. High-power fans are added on both sides of the steel plates to accelerate cooling to below 150°C and increase the core hardness of the steel plates. The first tempering heating temperature is 640±10°C, and the tempering holding time is 3 hours. After leaving the furnace, the steel plates are air-cooled on the cooling bed. The tempered steel plates are then heated and tempered again in the furnace for homogenization treatment. The heating temperature is 530±10°C, the holding time is 3.5 hours, and the plates are air-cooled to room temperature.

[0039] 50mm thick steel plates manufactured using this process meet NB / T 47013.3 T1 ultrasonic flaw detection standards, with a flatness of ≤5mm / m, a hardness of 40HRC, a hardness fluctuation of no more than 2HRC, and a weld sensitivity coefficient of 0.39. The final mold can be polished to a grit of 5000 or better. The steel plates exhibit uniform hardness, mirror finish, and excellent weldability.

[0040] Table 1 Ultrasonic flaw detection description Ultrasonic flaw detection Level Description Process of the present invention NB / T 47013.3T1 The maximum allowable single defect indication area S is ≤ 50 mm² in the middle of the steel plate, and the number of defects is ≤ 10. The maximum allowable single defect indication length is ≤ 20 mm in the edge of the steel plate, and the number of defects is ≤ 2. Traditional crafts NB / T 47013.31 The maximum allowable single defect indication area S is ≤ 100mm² in the middle of the steel plate, and the number of defects is ≤ 10; the maximum allowable single defect indication length is ≤ 30mm, and the number of defects is ≤ 3 in the edge of the steel plate. Although the preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are intended to be included within the scope of protection of the present invention.

Claims

1. A steel plate for a low-welding-sensitivity mirror mold, characterized by: The chemical composition and mass percentage of the steel plate are: C: 0.15-0.30%, Si: 0.15-0.35%, Mn: 1.20-1.50%, P: ≤0.010%, S: ≤0.001%, Cr: 1.20-1.80%, Mo: 0.25-0.65%, Ni: 0.9-1.4%, Al: 0.02-0.10%, V: 0.1-0.25%, N: ≤0.007%, B: ≤0.002%, Cu: ≤0.15%, and the balance is iron and unavoidable impurity elements.

2. The low-welding-sensitivity mirror-finished mold steel plate according to claim 1, characterized in that: The steel plate has a thickness of 230 mm, ultrasonic flaw detection meets NB / T 47013.3 T1 level, hardness is 35-42 HRC, hardness fluctuation is ≤3 HRC, and welding crack sensitivity index is ≤0.41%.

3. A method for manufacturing a steel plate for a low-welding-sensitivity mirror mold according to claim 1, characterized in that: The method mainly includes: 1) Smelting: Select high-quality molten iron and scrap steel to prepare smelting raw materials according to the chemical composition. The smelting raw materials are sequentially subjected to molten iron pretreatment, converter smelting, LF refining, and vacuum refining to produce pure molten steel; 2) Rolling: After the continuous casting slab is slowly cooled, it is heated in a furnace to 1220-1270°C at a heating rate of 10-12.5 min / cm. After being discharged from the furnace, it is descaled with high-pressure water. The temperature of the slab after descaling is controlled at 980-1080°C. A two-stage rolling process is adopted, and the final rolling temperature is not higher than 900°C. The rolled steel plate is covered with a hot steel plate pile for slow cooling for more than 36 hours, and then slowly cooled to room temperature; 3) Heat treatment: The steel plate after slow cooling is subjected to "normalizing accelerated cooling + double tempering" heat treatment, and then air-cooled to room temperature after being taken out of the furnace.

4. The method for manufacturing a steel plate for a low-welding-sensitivity mirror mold according to claim 3, characterized in that: Step 1) After molten steel LF is refined, the S content is not greater than 0.001%, the high vacuum treatment time is greater than 25 minutes, and the H content of the molten steel is ensured to be ≤ 0.0001%, and the N content is ≤ 0.0050%.

5. The method for manufacturing a steel plate for a low-welding-sensitivity mirror mold according to claim 3, characterized in that: Step 1) The continuous casting of the medium slab adopts non-oxidation protection casting throughout the whole process, the overheating degree is not greater than 30°C, and the dynamic soft reduction technology of the slab is adopted to improve the center segregation and center porosity of the slab. The center segregation is Class C and the center porosity is Class 0.

5. The continuous casting slab is slowly cooled under the line cover for not less than 72 hours.

6. The method for manufacturing a steel plate for a low-welding-sensitivity mirror mold according to claim 3, characterized in that: Step 2) The starting rolling temperature of the medium and rough rolling is ≥1000℃, the maximum pass reduction is not less than 50mm, the pass reduction rate is 15-25%, and the finishing rolling stage adopts a multi-pass small reduction rolling process with a pass reduction rate of 8%-12%, and a cumulative rolling of 5-10 passes.

7. The method for manufacturing a steel plate for a low-welding-sensitivity mirror mold according to claim 3, characterized in that: In step 3), the normalizing temperature is 850-890°C, the holding time is 1.8-2.2 min / mm, and the steel plate is cooled to below 150°C by air cooling or mist cooling to speed up the cooling of the steel plate and improve the hardness of the core of the steel plate.

8. The method for manufacturing a steel plate for a low-welding-sensitivity mirror mold according to claim 3, characterized in that: In step 3), the first tempering heating temperature is 600-660°C, and the tempering holding time is 3.5-4.0 min / mm. After being taken out of the furnace, the steel plate is cooled in the air. The tempered steel plate is then put into the furnace again for heating and tempering homogenization treatment at a heating temperature of 520-620°C and a holding time of 3.5-4.0 min / mm.

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