Steel plate with high toughness, high hardness and corrosion resistance for die and manufacturing method

Abstract

The invention relates to a high-toughness, high-hardness and corrosion-resistant steel plate for a mold and a manufacturing method. The steel plate comprises the following chemical components in percentage by mass: 0.15-0.30% of C, 0.20-0.60% of Si and the balance of Fe and inevitable impurities, p < = 0.005%; s < = 0.002%; 0.50 to 1.20% of Mn; 0.02 to 0.04 percent of Al; nb is less than or equal to 0.01%; v is less than or equal to 0.01%; 6.5 to 11.5 percent of Cr + Ni; 0.35 to 0.65 percent of Mo + Cu; ceq < = 2.5; and the balance of Fe and inevitable impurities. The manufacturing process sequentially comprises the procedures of KR molten iron pretreatment, BOF smelting, LF and RH furnace refining, continuous casting, slab heating, TMCP rolling, high-temperature DQ cooling, straightening, cooling bed cooling, tempering, steel plate shearing and the like. According to the design, the steel plate with the thickness of 6-50 mm is produced by adopting a process of composite addition of medium C, Cr, Ni, Cu, Mo and microalloy, TMCP, high-temperature DQ and tempering, and a unique martensite microstructure is produced through specific components and process design; the die steel plate has the 0 DEG C impact property of more than or equal to 200J, the Rockwell hardness of more than or equal to 45HRC and the corrosion weight loss ratio of less than or equal to 0.01 g / m < 2 > * h in a simulated seawater / salt spray corrosion environment, and meets the use requirements of die steel under the service condition of an acid corrosion environment.

Description

Technical Field

[0002] The invention belongs to the technical field of special steel smelting, and specifically relates to a steel plate for a corrosion-resistant mold with high toughness, high hardness and a manufacturing method thereof. Background Art

[0004] In recent years, with the transfer of the world's manufacturing center to China and Southeast Asia, the mold steel industry in the region has been greatly developed. Mold steel is the most basic technology and material source in the mold industry, and mold steel is mainly used in mass-produced parts and manufactured parts. The current development and progress of mold steel can promote the rapid and stable development of the mold industry, and it is also the industrial foundation of the entire manufacturing industry. It is an important indicator of the level of product manufacturing in a country. With the improvement of my country's industrialization level, the import volume of high-end mold steel has also increased year by year, and the domestic requirements for molds have become more stringent. Among them, tire and plastic molds are the most important components of mold steel, accounting for about 60% of the entire mold steel industry. Tires and plastic products will produce corrosive gases such as hydrogen chloride, hydrogen fluoride and sulfur dioxide when the injection mold is working, and the working temperature reaches 200-350℃, which is easy to cause corrosion damage to the cavity of tire / plastic mold steel, affecting the surface finish and service life of the mold. Traditional tire / plastic mold steel can no longer meet the needs of the rapid development of the tire / plastic industry. Therefore, customers of mold forming parts tend to choose mold steel with corrosion resistance, high toughness and high hardness. Research on high-toughness corrosion-resistant die steel at home and abroad is still in the exploratory stage.

[0005] Based on the characteristics of high-toughness, high-hardness, corrosion-resistant mold steel and market demand, this paper proposes a high-toughness, high-hardness, corrosion-resistant mold steel plate and a manufacturing method.

[0006] This patent proposes for the first time a steel plate with high toughness, high hardness, and a manufacturing method for corrosion-resistant molds, and conducts systematic research on the chemical composition, production process, high toughness, high hardness, corrosion resistance, and organizational properties of corrosion-resistant mold steel with high toughness and high hardness; this patent adopts medium C, Cr, Ni, Cu, Mo and micro-alloy composite addition + TMCP + high-temperature DQ + tempering process to produce 6-50mm thick steel plates, and produces a unique martensitic microstructure through specific composition and process design, so that the mold steel plate has 0℃ impact performance ≥200J, Rockwell hardness ≥38HRC, and corrosion weight loss rate ≤0.01g / m in simulated seawater / salt spray corrosion environment. 2 *h, meet the use requirements of mold steel under acid-corrosive environment service conditions.

[0007] Patent announcement number CN114645194A proposes a preparation method of high corrosion-resistant ferritic stainless steel. Compared with this patent, on the one hand, the composition and process are different: the CN114645194A alloy is only designed with high Cr composition, and the process adopts cold rolling annealing process, while this patent adopts Cr, Ni, Cu, Mo and micro-alloy composite addition, and the process adopts slab continuous casting + TMCP + high temperature DQ + tempering hot rolling process; on the other hand, the application is different. CN114645194A is applied to corrosion-resistant stainless steel, and this patent is suitable for high-end mold industry in corrosive environments such as S and high-temperature H2O steam, hydrogen chloride, and hydrogen fluoride. Another patent announcement number CN116194600A proposes high corrosion-resistant stainless steel parts and their manufacturing methods, heat treatment methods for stainless steel parts, and rolling bearings and their manufacturing methods. On the one hand, the chemical composition is different. CN116194600A uses high Cr+Mo+ (W+B+N), while this patent uses Cr, Ni, Cu, Mo and micro-alloy composite additions; on the other hand, the product performance and use are different. This patent is suitable for high-end mold industry in corrosive environments such as S and high-temperature H2O steam, hydrogen chloride, hydrogen fluoride, etc., and has high toughness, while CN116194600A does not have high toughness and mold steel usage conditions. Summary of the invention

[0009] The technical problem to be solved by the present invention is to provide a high toughness, high hardness corrosion-resistant mold steel plate and a manufacturing method for the above-mentioned prior art, and to produce a unique martensite microstructure through specific composition and process design, so that the mold steel plate has a 0℃ impact performance ≥200J, a Rockwell hardness ≥45HRC and a corrosion weight loss rate ≤0.01g / m in a simulated seawater / salt spray corrosion environment 2 *h, meet the use requirements of mold steel under acid-corrosive environment service conditions.

[0010] The technical solution adopted by the present invention to solve the above problems is: a steel plate for corrosion-resistant molds with high toughness and hardness, whose chemical composition mass percentage is C 0.15-0.30%, Si 0.20-0.60%; P≤0.005%; S≤0.002%; Mn 0.50-1.20%; Al 0.02-0.04%; Nb ≤0.01%; V≤0.01%; Cr+Ni 6.5-11.5%; Mo+Cu 0.35-0.65%; Ceq≤2.5; the balance is Fe and unavoidable impurities.

[0011] The functions of all ingredients contained in the present invention and the reasons for selecting their contents are specifically described as follows:

[0012] C: C in steel can increase the strength of the steel, but too high a C content will form carbides with chromium, resulting in a decrease in the corrosion resistance of the steel and an adverse effect on the toughness of the steel. The C content of the present invention is selected in the range of 0.15-0.30%.

[0013] Si: Si improves strength and hardenability, but Si increases overheating sensitivity and cracking tendency in steel. The Si content in the present invention is determined to be within the range of 0.20-0.60%.

[0014] Mn: Mn is a good deoxidizer and desulfurizer, which can improve the toughness of steel and reduce the ductile-brittle transition temperature; however, Mn segregation has an adverse effect on the acid resistance and toughness of steel. Taking comprehensive considerations, the Mn content in the present invention is designed to be in the range of 0.50-1.20%.

[0015] P, S: P is prone to cause segregation when solidifying in steel and increases cold brittleness. The range of P content in the present invention is determined to be ≤0.005%; S is prone to cause hot brittleness and reduce the ductility and toughness of steel. The range of S content in the present invention is determined to be ≤0.002%.

[0016] Al: Al can form AlN to refine the grains, but a large Al content can easily form brittle inclusions during molten steel smelting, reducing the purity of the molten steel. The Al content of the present invention is selected in the range of 0.02-0.04%.

[0017] Cr, Ni: Cr can improve the corrosion resistance and tempering stability of steel, but reduce impact toughness; Ni can improve strength while ensuring good toughness, and Ni increases the electrode potential of the matrix, thereby improving corrosion resistance, but Ni resources are scarce and expensive; taking comprehensive considerations, the Cr+Ni content in the present invention is designed to be in the range of 6.5-11.5%.

[0018] Cu, Mo: Cu can improve the corrosion resistance and cutting performance of the material, but if too much Cu is added, the steel will tend to be hot brittle; Mo can form a more stable passivation film, while refining the grains of the steel and improving the hardenability, but too much will lead to a decrease in toughness; considering comprehensively, the present invention adopts Cu+Mo composite addition, and the total content range is determined to be 0.35-0.65%.

[0019] In terms of composition design, medium C, Cr, Ni, Cu, Mo and micro-alloy composite addition + TMCP + high temperature DQ + tempering process are used to produce 6-50mm thick steel plates. Through specific composition and process design, a unique martensitic microstructure is produced, which makes the mold steel plate have 0℃ impact performance ≥200J, Rockwell hardness ≥45HRC and corrosion weight loss rate ≤0.01g / m in simulated seawater / salt spray corrosion environment. 2 *h, meet the use requirements of mold steel under acid-corrosive environment service conditions.

[0020] In addition, the patent of the present invention also provides a method for manufacturing the above-mentioned high-toughness, high-hardness, corrosion-resistant mold steel. The specific production steps are: the manufacturing process is KR molten iron pretreatment - BOF smelting - LF and RH furnace refining - continuous casting - slab heating - TMCP rolling - high-temperature DQ cooling - straightening - cooling on cooling bed - tempering - steel plate shearing - inspection and warehousing.

[0021] The manufacturing method of the above-mentioned high toughness, high hardness corrosion-resistant mold steel specifically comprises the following steps:

[0022] Step 1: Steelmaking process: KR molten iron pretreatment, BOF converter smelting, LF refining, RH vacuum degassing treatment are used to produce high-purity molten steel, and then a 360mm thick continuous casting machine is used to produce continuous casting slabs. After continuous casting, the continuous casting slabs are subjected to pile slow cooling, and the slow cooling time is ≥72 hours.

[0023] Step 2: heating the continuous casting billet to 1200-1260°C, and descaling the continuous casting billet using high-pressure water after it is taken out of the furnace;

[0024] Step 3: The continuous casting billet is rolled in two stages: the first stage is the rough rolling stage: the starting rolling temperature is 1160-1200℃, where the offline temperature is 1160℃ = finishing rolling 1090℃ + 7 rough rolling passes * 10℃ temperature drop / pass, and the online temperature is 1200℃ according to the temperature drop of 1260℃ after the initial descaling by high-pressure water ≥60℃; the thickness of the intermediate billet in this stage is 1.5h-4.5h (h is the thickness of the finished steel plate), and the thickness in this range can ensure the addition The heating effect of the hot furnace ensures that there are 5 to 7 rolling passes in the finishing rolling; at the same time, the cumulative comprehensive reduction rate in this stage is ≥75%, which can ensure that the austenite grains are completely broken and provide ultra-fine grains for finishing rolling; the second stage is the finishing rolling stage: to ensure that the final rolling temperature is ≥1050℃, according to the on-site temperature drop of the passes, the finishing rolling start is set at 1090-1150℃, and the cumulative reduction rate of the finishing rolling passes is ≥50%, ensuring that the deformation can penetrate into the center of the steel plate and obtain finer grains.

[0025] Step 4: After rolling, the steel plate quickly enters DQ (steel plate speed ≥ 3.5m / s), ensuring that the temperature of the steel plate entering DQ is ≥ 1050℃, and then DQ is quickly cooled (online rapid cooling), the cooling rate is controlled at 15-30℃ / s, the water outlet temperature is 250-350℃, and air cooling is performed after exiting the water.

[0026] Step 5: Temper the steel plate. The tempering process uses a temperature of 450-650℃, a holding time of 4.5-6.5Min / mm, and air cooling. The determination of the tempering temperature mainly takes into account the impact and corrosion resistance of the steel plate. The specific process parameters (tempering temperature, corrosion resistance) and the corresponding impact performance are derived from the regression formula. The regression formula is derived based on field experimental data and experimental test results:

[0027] Z=-708.88+3.7279XY-3.47*10 -3 X 2 , where: X is the tempering temperature °C, Y is the impact energy / J, Z is the corrosion resistance weight loss rate / g·(m 2 h) -1 .

[0028] From the 3D graph, it can be concluded that if the tempering temperature is too high or too low, it will affect the corrosion resistance. The low point in the 3D graph is where the corrosion resistance is best, so using the tempering control temperature here can obtain the best corrosion resistance.

[0029] Compared with the prior art, the advantages of the present invention are:

[0030] (1) The present invention adopts complex pure steel smelting technology to provide excellent billets. For the first time, medium C is used, and Cr, Ni, Cu, Mo and micro-alloy composite additions + TMCP + high-temperature DQ + tempering process is used to produce steel plates with a thickness of 6 to 50 mm. A unique martensitic microstructure is produced through specific composition and process design. The use of medium C improves the fluidity of molten steel, the structure is more uniform, the molten steel has high purity and excellent mechanical properties; the use of TMCP + high-temperature DQ is easy to form a martensitic structure, and tempering produces a small amount of troostite, which is a balanced structure with high hardness and good mechanical properties; (martensite + tempered troostite + needle-shaped carbide) accounts for 75-85% (total), of which martensite accounts for 85-95% (mixed structure), and tempered troostite and needle-shaped carbide account for 5-15% (mixed structure).

[0031] (2) The steel plate provided by the present invention is the first steel plate produced in China and abroad by combining TMCP and high-temperature DQ processes. The steel plate has a unique microstructure, which is mainly composed of a mixed structure of martensite + tempered troostite + needle-shaped carbides. The steel plate has an impact resistance of ≥200J at 0°C, a Rockwell hardness of ≥45HRC, and can meet the requirements for use as mold steel under service conditions in acidic and corrosive environments. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a microstructure photograph of the 15 mm specification of the present invention at 1 / 4 along the plate thickness direction.

[0034] Figure 2 This is a microstructure photograph of the 45mm specification of the present invention at 1 / 4 along the plate thickness direction.

[0035] Figure 3 It is a 3D statistical diagram of tempering temperature, corrosion rate and impact work in the present invention. DETAILED DESCRIPTION

[0037] The technical solution of the present invention is described in more detail in conjunction with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred implementation methods of the present invention and cannot impose any limitation on the scope of the present invention.

[0038] Embodiment 1-2:

[0039] According to the chemical composition range and manufacturing method of the present invention, a new type of high-toughness, high-hardness, and corrosion-resistant mold steel plate with a unique microstructure is manufactured through KR molten iron pretreatment - BOF smelting - LF and RH furnace refining - continuous casting - slab heating - TMCP rolling - high-temperature DQ cooling - straightening - cooling on a cooling bed - tempering - steel plate shearing - inspection and warehousing.

[0040] The specific process of the above-mentioned heating, rapid TMCP rolling, and high-temperature DQ cooling is as follows: heating the 360m thick continuous casting billet to 1250°C, keeping the temperature in the soaking section for 30min (Example 1) or heating the 360m thick continuous casting billet to 1230°C, keeping the temperature in the soaking section for 25min (Example 2), and descaling the continuous casting billet with high-pressure water after it is taken out of the furnace; then two-stage rolling is carried out, the first stage rolling temperature is 1180-1190°C, the cumulative comprehensive reduction rate is ≥80%, and the intermediate billet thickness is 70mm (Example 1) or the first stage rolling temperature is 1170-1180°C, the cumulative comprehensive reduction rate is ≥75%, and the intermediate billet thickness is 140mm (Example 2); the second stage rolling temperature is 1120°C, the cumulative pass reduction rate is 65 %, (Example 1) or the second stage rolling temperature is 1100℃, the cumulative pass reduction rate is 60%, (Example 2), and the final steel plate thickness is 20mm (Example 1) and 45mm (Example 2); after rolling, high-temperature DQ rapid cooling is performed, the DQ entry temperature is 1060℃, the cooling rate is 26℃ / s, the water outlet temperature is 280℃ (Example 1) and the DQ entry temperature is 1080℃, the cooling rate is 28℃ / s, and the water outlet temperature is 260℃ (Example 2); then hot straightening; after hot straightening, the steel plate is air-cooled on a cooling bed; tempering treatment: tempering temperature 520℃, holding time 4.5Min / mm, air cooling (Example 1) and tempering temperature 550℃, holding time 5.0Min / mm, air cooling (Example 2).

[0041] The chemical composition of the test steel plate is shown in Table 1, and the hardness, impact resistance and corrosion resistance are shown in Table 2; the microstructure of the steel plate is shown in Figure 1 and Figure 2 shown.

[0042] Table 1 Chemical composition of steel plates in Examples 1 and 2 (wt.%)

[0043] Note: Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15 Table 2 Hardness, impact resistance and corrosion resistance of the steel plates in Examples 1 and 2

[0044] Note: Cycle 1 is 300 hours; Cycle 2 is 600 hours; Cycle 3 is 1000 hours.

[0045] Although the preferred embodiments of the present invention are described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A steel plate for a corrosion-resistant mold with high toughness and high hardness, characterized in that: The chemical composition of the steel plate is as follows by mass percentage: C 0.15~0.30%, Si 0.20~0.60%; P≤0.005%; S≤0.002%; Mn 0.50~1.20%; Al0.02~0.04%; Nb≤0.01%; V≤0.01%; Cr+Ni 6.5~11.5%; Mo+Cu 0.35~0.65%; Ceq≤2.5; the balance is Fe and unavoidable impurities.

2. The high toughness, high hardness, corrosion-resistant mold steel plate according to claim 1, characterized in that: The steel plate has a 0°C impact performance of ≥200J, a Rockwell hardness of ≥45HRC, and a corrosion weight loss rate of ≤0.01g / m in a simulated seawater / salt spray corrosion environment 2 *h performance.

3. The corrosion-resistant mold steel plate with high toughness and hardness according to claim 1, characterized in that: The metallographic structure of the steel plate is a mixed structure of martensite+tempered troostite+acicular carbide.

4. The corrosion-resistant mold steel plate with high toughness and hardness according to claim 3, characterized in that: The proportion of martensite + tempered troostite + needle-shaped carbide in the organization is 75-85% of the overall organization, the proportion of martensite is 85-95% of the mixed organization, and the proportion of tempered troostite and needle-shaped carbide is 5-15% of the mixed organization.

5. A method for manufacturing a corrosion-resistant mold steel plate with high toughness and hardness as claimed in claim 1, characterized in that: The method comprises the following steps: Step 1: Steelmaking process: KR hot metal pretreatment, BOF converter smelting, LF refining, RH vacuum degassing treatment are used to produce high-purity molten steel, and then a continuous casting machine is used to produce continuous casting slabs. After continuous casting, the continuous casting slabs are subjected to slow cooling in piles, and the slow cooling time is ≥ 72 hours; Step 2: heating the continuous casting billet to 1200-1260°C, and descaling the continuous casting billet using high-pressure water after it is taken out of the furnace; Step 3, the continuous casting billet is rolled in two stages: the first stage is the rough rolling stage: the starting rolling temperature is 1160-1200℃, and the cumulative comprehensive reduction rate in this stage is ≥75%, which can ensure that the austenite grains are completely broken and provide ultra-fine grains for finishing rolling; the second stage is the finishing rolling stage: to ensure that the final rolling temperature is ≥1050℃, according to the on-site pass temperature drop, the finishing rolling start is set to 1090-1150℃, and the cumulative pass reduction rate of finishing rolling is ≥50%, ensuring that the deformation can penetrate into the center of the steel plate and obtain more refined grains; Step 4: The rolled steel plate enters DQ quickly, ensuring that the temperature of the steel plate entering DQ is ≥1050℃, and then the DQ is quickly cooled, the outlet water temperature is 250-350℃, and air-cooled after the water exits; Step 5: Temper the steel plate at a temperature of 450-650°C, a holding time of 4.5-6.5 Min / mm, and air cooling.

6. The method for manufacturing a corrosion-resistant mold steel plate with high toughness and high hardness according to claim 5, characterized in that: In step 3, the offline temperature of the rough rolling stage is 1160℃ = finishing rolling 1090℃ + 7 rough rolling passes * 10℃ temperature drop / pass, and the online temperature is 1200℃ determined based on the temperature drop of 1260℃ out of the furnace after the initial descaling with high-pressure water ≥60℃; the thickness of the intermediate billet in this stage is 1.5h~4.5h, h is the thickness of the finished steel plate, and the thickness in this range can ensure the heating effect of the heating furnace and ensure that the finishing rolling has 5~7 rolling passes.

7. The method for manufacturing a corrosion-resistant mold steel plate with high toughness and high hardness according to claim 5, characterized in that: In step 4, the speed at which the steel plate enters the DQ is ≥3.5 m / s, and the cooling rate is controlled at 15-30 °C / s.

8. The method for manufacturing a corrosion-resistant mold steel plate with high toughness and hardness according to claim 5, characterized in that: The tempering temperature in step 5 is determined according to the following regression formula: Z=-708.88+3.7279XY-3.47*10 -3 X 2 , where: X is the tempering temperature ℃, Y is the impact energy / J, Z is the corrosion resistance weight loss rate / g·(m 2 h) -1 .

Images