A method for producing a large-thickness nm600 steel plate

Abstract

A kind of production method of large-thickness NM600 steel plate, the chemical composition of steel is as follows: C=0.43%~0.45%, Si=0.10%~0.20%, Mn=0.95%~1.05%, P≤0.008%, S≤0.0015%, Cr=1.05%~1.15%, Mo=0.14~0.18%, Ti=0.008%~0.015%, V=0.030%~0.040%, Alt=0.050%~0.070%, B=0.0008%~0.0020%, broken space H≤0.00010%, ladle H≤0.00012%, N≤0.0035%, O≤0.0015%, and the rest is Fe and inevitable impurities.The present application is produced by 260mm casting blank, through reasonable casting, rolling and heat treatment process, produces maximum thickness 80mm NM600 high-grade wear-resistant steel, surface Brinell hardness 570~640HB, yield strength ≥1600MPa, tensile strength ≥2000MPa, elongation ≥7%, improves the service performance of wear-resistant steel in heavy load field such as mine machinery, engineering equipment.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology and relates to a method for producing thick NM600 steel plates. Background Technology

[0002] Wear-resistant steel is widely used in metallurgy, mining, building materials, railways, and other fields, playing a crucial role in key wear-prone parts of equipment such as mining dump trucks, coal mine scraper conveyors, bulldozers, excavators, mixers, and loaders. With the rapid development of modern industry, wear failure problems have become increasingly prominent. The service environment for wear-resistant materials is becoming increasingly harsh, thus requiring extremely high strength, hardness, and wear resistance to extend equipment lifespan, shorten component replacement cycles, and significantly improve operating time. Hardness is an important indicator for measuring the wear resistance of metallic materials; the quality of wear-resistant steel is most directly distinguished by its hardness level. In recent years, significant progress has been made in the research and development of low-grade wear-resistant steels such as NM360, NM400, NM450, and NM500, but their alloy content is generally high, which is detrimental to weldability and increases production costs. Currently, although a few manufacturers have developed NM600 steel, its thickness is limited to 40mm and below.

[0003] Chinese patent CN119433371A discloses a low-alloy NM600 grade wear-resistant steel and its preparation method. The thickness range of the NM600 grade wear-resistant steel produced is only 10-30mm. The content of harmful elements such as P, S, N, and O is still too high, and the control of hydrogen content is not sufficient, which is not conducive to controlling hydrogen-induced cracking and reducing the risk of temper brittleness and hot brittleness tendency.

[0004] Chinese patent CN119710447A discloses a high-strength, medium-thin wear-resistant steel plate NM600 and its production method. The thickness of the wear-resistant steel plate produced ranges from 2.0 to 12.0 mm. It has high levels of harmful elements such as phosphorus (P) and sulfur (S), and the steel plate has a yield strength ≥1200 MPa, a tensile strength ≥1400 MPa, and generally good mechanical properties.

[0005] Therefore, developing a high-grade wear-resistant steel of 600HB with low crack risk, high wear resistance, and large thickness is of great significance to the development of the engineering machinery industry. Summary of the Invention

[0006] The purpose of this invention is to provide a method for producing thick NM600 steel plates, wherein the produced NM600 steel plates have a thickness of 50-80 mm, a surface hardness of 570-640 HBW, a yield strength ≥1600 MPa, a tensile strength ≥2000 MPa, and an elongation ≥7%.

[0007] The technical solution of the present invention:

[0008] A method for producing thick NM600 steel plates, with a thickness of 50-80 mm; the chemical composition of the steel (mass percentage) is: C=0.43%-0.45%, Si=0.10%-0.20%, Mn=0.95%-1.05%, P≤0.008%, S≤0.0015%, Cr=1.05%-1.15%, Mo=0.14%-0.18%, Ti=0.008%-0.015%, V=0.030%-0.040%, Alt=0.050%-0.070%, B=0.0008%-0.0020%, void H≤0.00010%, intermediate ladle H≤0.00012%, N≤0.0035%, O≤0.0015%, with the remainder being Fe and unavoidable impurities; key process steps include:

[0009] (1) Smelting: The BOF-LF-RH production process is adopted. The molten iron is treated to remove sulfur and phosphorus. When the LF furnace exits the station, a 150m pure calcium wire is fed in. Then it enters the RH furnace for vacuum treatment. The vacuum degree is ≤67Pa and the holding time is 25 min. Soft blowing is carried out on the platform for 28-34 min. After breaking the vacuum, a 150m pure calcium wire is fed in one go. When the molten steel exits the station, the gas content is measured to be H≤0.00010%, N≤0.0035%, and O≤0.0015%.

[0010] (2) Continuous casting: Full-process protective casting, control of moisture in protective slag and covering agent, tundish baking, and slow cooling and hydrogen expansion measures with the billet covered with heat insulation cover. The tundish H is controlled at ≤0.00012%; the tundish superheat is ≤20℃ for continuous casting furnace and ≤25℃ for open casting furnace. The continuous casting adopts dynamic light reduction technology. The continuous casting billet size is thickness × width × length = 260mm × 2250mm × billet length mm.

[0011] (3) Heating: Preheating section temperature 650~800℃, heating section temperature 1100~1230℃, soaking section temperature 1160~1220℃, furnace time 239~252min;

[0012] (4) Rolling: The finished steel plate thickness is 50-80mm. Two-stage controlled rolling is adopted. The rough rolling start temperature is ≤1040℃, the rough rolling cumulative compression ratio is ≥2, and the intermediate billet thickness is 110-130mm; the finish rolling start temperature is ≤950℃, the finish rolling cumulative compression ratio of 50mm steel plate is 2.2, and the finish rolling cumulative compression ratio of 80mm steel plate is 1.6.

[0013] (5) Heat treatment: Quenching + low-temperature tempering process is adopted;

[0014] (6) Post-furnace cutting: The steel plate after heat treatment is cut with heat or preheating, and then colored or magnetic particle testing is performed to detect whether there are delayed cracks.

[0015] Further, in step (1) smelting: the molten iron is treated by KR desulfurization and smelted in a top and bottom blown converter. The converter slag formation and desulfurization are combined with the deep desulfurization process of slag removal after the furnace.

[0016] Further, in step (2) continuous casting: the billet is covered with an insulation cover and slowly cooled to 200±20℃ before being loaded into the furnace.

[0017] Further, step (5) heat treatment: quenching heating temperature 830~850℃, heating rate 1.6~2.2min / mm, holding time 15~30min, quenching cooling rate 20~40℃ / s, quenching water cooling to room temperature; tempering heating temperature 150~170℃, heating rate 3.5~4.0min / mm, holding time 15~30min; after tempering, stack and slowly cool to room temperature.

[0018] Further, step (6) post-furnace cutting: the temperature of the plate after cutting is ≥50℃.

[0019] Invention principle:

[0020] The wear-resistant steel of this invention possesses characteristics such as high strength, high hardness, high wear resistance, and low risk of cracking. The following explains the role and mechanism of the main alloying elements in the chemical composition of the steel of this invention.

[0021] By controlling the carbon (C) range, the surface hardness of the steel plate is ensured to meet the target requirements; the addition of Mn, B, Si, Cr, and Mo elements improves the hardenability of the steel plate, ensuring a uniform microstructure; the strength properties of the steel plate are improved through a combination of microstructure strengthening and alloy strengthening; the grain refinement of V, Ti, and Al precipitates, combined with a reasonable rolling heat treatment process, aims to improve strength, toughness, and hardness; and harmful elements such as P, S, H, N, and O in the molten steel are strictly controlled. This invention effectively combines scientific alloy element ratios with a reasonable rolling heat treatment process to ensure that the steel plate has excellent mechanical properties. The specific rationale for each major added element is as follows:

[0022] C: Effectively improves the strength and hardness of steel, increases hardenability, and can significantly improve the surface hardness of wear-resistant steel plates; however, excessive carbon has adverse effects on the toughness, cold forming, and weldability of steel. Based on the above comprehensive considerations, the carbon content of this invention is controlled at 0.43% to 0.45%.

[0023] Manganese (Mn) is the most effective alloying element for improving hardenability. When dissolved in ferrite, it strengthens through solid solution and improves the heat treatment properties of steel, refining pearlite grains and increasing strength and hardness. Adding a certain amount of manganese to steel is beneficial, but the content should not be too high. This is because Mn is a segregating element. During solidification, Mn causes the molten steel to accumulate at the solidification end, forming central segregation. This results in coarse cementite in the core of the steel plate, which is detrimental to core performance. Simultaneously, Mn lowers the martensitic transformation temperature in the material. When Mn accumulates to a certain proportion in the segregation zone, it will produce high-hardness microstructures such as martensite and bainite during post-weld cooling, significantly reducing toughness. Based on these comprehensive considerations, the manganese content in this invention is controlled between 0.95% and 1.05%.

[0024] Si (Si): A good reducing agent and deoxidizer in steelmaking, it easily forms a solid solution in ferrite, which improves the strength of steel, especially its yield strength. However, when the Si content is too high, the low-temperature toughness decreases, and the iron oxide scale on the surface of the steel plate becomes difficult to remove. The Si content of the steel in this invention is controlled within the range of 0.10% to 0.20%, which is beneficial to its overall performance.

[0025] Phosphorus (P) has a strong solid solution strengthening effect, significantly increasing the strength and hardness of steel. However, phosphorus is a segregating element, increasing temper brittleness and negatively impacting the low-temperature toughness of steel. Therefore, the phosphorus content in steel should be strictly controlled. In this invention, the P content in the steel is controlled below 0.008%.

[0026] Sulfur (S): As a harmful element in steel, sulfur has a tendency to cause hot cracking, and sulfide inclusions significantly reduce the toughness of steel. Therefore, the sulfur content should be kept as low as possible. In this invention, the sulfur content in the steel is controlled to be below 0.0015%.

[0027] Cr: One of the basic elements in wear-resistant steel, it improves hardenability, strengthens the matrix through solid solution, refines grains, and enhances the strength, hardness, and wear resistance of steel; it can also significantly improve the oxidation resistance of steel and enhance its corrosion resistance. In this invention, the Cr content is controlled at 1.05% to 1.15%.

[0028] Mo: The appropriate addition of molybdenum can improve the hardenability of steel to a limited extent. It mainly exists in the form of carbides in steel, which are dispersed in the matrix to strengthen it. Increasing the Mo content in steel can effectively improve the uniformity of the thickness of the entire cross section of the steel plate and the tempering stability. Considering that Mo is a precious alloy, the Mo content in this invention is controlled at 0.14% to 0.18%.

[0029] V: It has the effect of precipitation strengthening and grain refinement, forming carbides, and improving the strength and toughness of steel by strengthening and refining ferrite grains through ferrite precipitation. In this invention, V is controlled at 0.030% to 0.040%.

[0030] Ti: A strong carbide-forming element, it has extremely strong bonding forces with carbon and nitrogen. During the solidification process of steel, Ti combines with N to produce stable TiN, which can strongly hinder the migration of austenite grain boundaries, thereby refining austenite grains; Ti combines with C to form TiC, which can play a precipitation strengthening role. In this invention, Ti is controlled at 0.008% to 0.015%.

[0031] Al: As a deoxidizer in steel, it can refine grain size. In this invention, the Al content is controlled at 0.050% to 0.070%.

[0032] Boron (B): A key element for improving the hardenability of steel plates. Adding even a small amount of B (0.0008%–0.0030%) to steel can significantly improve its hardenability. Therefore, adding a small amount of B to quenched and tempered steel can effectively replace some expensive hardenability-enhancing alloying elements; however, the B content in wear-resistant steel should not exceed 0.0040%, otherwise, non-solid-solid borides are easily formed, causing boron embrittlement. In this invention, the B content is controlled at 0.0008%–0.0020%.

[0033] O: Oxygen is a harmful element; high content leads to numerous inclusions and negatively impacts the bending performance of steel plates. This invention employs VD vacuum deoxidation, wire feeding precipitation deoxidation, and diffusion deoxidation during LF furnace white slag desulfurization. It also controls the oxygen content of the steel tapped from the converter at the source, and takes measures such as slag blocking, soft blowing, protective casting, and prevention of secondary oxidation, thus controlling O to ≤0.0015%.

[0034] Nitrogen (N) is an unavoidable element in steel. Since dissolved or free nitrogen is highly detrimental to the impact toughness of steel, especially its aging impact toughness, nitrides must be formed to prevent the presence of free nitrogen. In this invention, TiN and BN are mainly formed, which hinder the growth of austenite grains at high temperatures of 1100–1400°C. However, this effect is only noticeable with a very small amount of nitrogen, especially since BN has a relatively small mass, resulting in a higher precipitation volume fraction for the same weight of N. Considering the controllability and economic efficiency of steelmaking, this invention controls N to ≤0.0035%.

[0035] H: Hydrogen is a harmful gaseous element. Hydrogen-induced delayed cracking is one of the main causes of failure in martensitic wear-resistant steel during cutting, bending, and other processes. This invention strictly controls the H content (H in voids ≤ 0.00010%, H in intermediate ladles ≤ 0.00012%), reducing the amount of diffusible hydrogen and its accumulation, effectively preventing the occurrence of hydrogen-induced delayed cracking.

[0036] The main innovations and beneficial effects of this invention are as follows: This invention employs a low-alloy design, rationally designing C, Mn, and Cr alloys to effectively reduce Mo and eliminate the need for expensive alloys such as Ni, thus lowering costs. Combined with a strict controlled rolling process and a quenching + low-temperature tempering heat treatment process, the final mechanical properties of the steel plate are ensured, resulting in a low-cost, high-wear-resistance 600HB high-grade wear-resistant steel. 1) Mechanical properties: Surface hardness 570~640HBW, yield strength ≥1600MPa, tensile strength ≥2000MPa, elongation ≥7%; 2) Thickness ranges from 50~80mm (maximum thickness 60mm according to national standard GB / T 24186-2022), with a maximum thickness of 80mm for NM600 ultra-high-strength wear-resistant steel plates for engineering machinery, further enhancing the service performance of wear-resistant steel in heavy-duty fields such as mining machinery and engineering equipment. Attached Figure Description

[0037] Figure 1 Metallographic image of the steel produced in Example 1.

[0038] Figure 2 Metallographic image of the steel produced in Example 2. Detailed Implementation Example 1

[0039] A method for producing thick NM600 steel plates. One heat of NM600 wear-resistant steel is smelted and rolled into 50mm thick steel plates. The chemical composition (weight percentage) of the steel is shown in Table 1, with the remainder being Fe and essential impurities, CEV=0.86. The method includes the following key process steps:

[0040] (1) Smelting: Top and bottom combined blowing converter is used for smelting. The molten iron is pre-desulfurized and desulfurized by KR. When leaving the LF station, 150m of pure calcium wire is fed in and then enters the RH furnace for vacuum treatment. The vacuum degree is ≤67Pa and the holding time is 25 min. Soft blowing is carried out on the platform for 28 min. After breaking the vacuum, 150m of pure calcium wire is fed in one go.

[0041] (2) Continuous casting: Full protection casting is adopted, the tundish superheat is 19℃, and dynamic light pressure is adopted to improve the internal quality (low magnification results of the furnace: segregation C grade 1.5, porosity grade 1.0, no internal cracks). The billet is covered with a heat insulation cover and slowly cooled to 200±20℃ before being loaded into the furnace for rolling.

[0042] (3) Heating: Preheating section temperature 680~800℃, heating section temperature 1110~1229℃, soaking section temperature 1160~1210℃, furnace time 239min.

[0043] (4) Rolling: The finished steel plate is 50mm thick and is rolled in two stages. The roughing rolling temperature is 1040℃, the intermediate billet thickness is 110mm, and the finishing rolling temperature is 830℃. The cumulative compression ratios of roughing and finishing rolling are 2.4 and 2.2, respectively. The rolling process parameters are detailed in Table 2.

[0044] (5) Heat treatment: Quenching + low temperature tempering process is adopted. The quenching heating temperature is 841℃, the heating rate is 1.6min / mm, the holding time is 23min, the quenching cooling rate is 32℃ / s, and the quenching is water-cooled to room temperature; the tempering heating temperature is 162℃, the heating rate is 3.7min / mm, and the holding time is 21min; after tempering, the stack is slowly cooled to room temperature. The heat treatment process parameters are detailed in Table 3.

[0045] (6) Post-furnace cutting: Steel plates that have been quenched and tempered at low temperature should be cut with heat or preheating. The temperature of the plate after cutting should be 63°C. There should be no sparking on the cut edge, and no obvious sharp edges, burrs, or slag. Right-angled edges should be polished smooth with a hand-held grinder to prevent stress concentration and cracking. Magnetic particle testing should be performed on the four cut edges of each sub-plate, and the results should be normal.

[0046] Example 2

[0047] A method for producing thick NM600 steel plates involves smelting one heat of NM600 wear-resistant steel and rolling it into steel plates with a thickness of 80 mm. The chemical composition of the steel by weight percentage is shown in Table 1, with the remainder being Fe and essential impurities, and a CEV of 0.86. The method includes the following key process steps:

[0048] (1) Smelting: Top and bottom blowing converter is used for smelting. The molten iron is pre-desulfurized and desulfurized by KR. When the LF exits the station, 150m of pure calcium wire is fed in. Then it enters the RH furnace for vacuum treatment. The vacuum degree is ≤67Pa and the holding time is 25 min. Soft blowing is carried out on the platform for 34 min. After breaking the vacuum, 150m of pure calcium wire is fed in one go.

[0049] (2) Continuous casting: Full protection casting is adopted, the tundish superheat is 17℃, and dynamic light pressure is adopted to improve the internal quality (low magnification results of the furnace: segregation C grade 1.0, porosity grade 1.0, no internal cracks). The billet is covered with a heat insulation cover and slowly cooled to 200±20℃ before being loaded into the furnace for rolling.

[0050] (3) Heating: Preheating section temperature 700~800℃, heating section temperature 1120~1230℃, soaking section temperature 1160~1220℃, furnace time 252min.

[0051] (4) Rolling: The finished steel plate is 80mm thick and is rolled in two stages. The roughing rolling temperature is 1040℃, the intermediate billet thickness is 130mm, and the finishing rolling temperature is 820℃. The cumulative compression ratios of roughing and finishing rolling are 2.0 and 1.6, respectively. The rolling process parameters are detailed in Table 2.

[0052] (5) Heat treatment: Quenching + low temperature tempering process is adopted. The quenching heating temperature is 843℃, the heating rate is 1.8min / mm, the holding time is 25min, the quenching cooling rate is 20~40℃ / s, and the quenching is water-cooled to room temperature; the tempering heating temperature is 163℃, the heating rate is 3.8min / mm, and the holding time is 22min; after tempering, the stack is slowly cooled to room temperature. The heat treatment process parameters are detailed in Table 3.

[0053] (6) Post-furnace cutting: Steel plates that have been quenched and tempered at low temperature should be cut with heat or preheating. The temperature of the plate after cutting should be 58°C. There should be no sparking on the cut edge, and no obvious sharp edges, burrs, or slag. Right-angled edges should be polished smooth with a hand-held grinder to prevent stress concentration and cracking. Magnetic particle testing should be performed on the four cut edges of each sub-plate, and the results should be normal.

[0054] The rolling process parameters for Examples 1 and 2 are shown in Table 2, the heat treatment process parameters are shown in Table 3, the mechanical property results are shown in Table 4, and the metallographic structures of the obtained wear-resistant steels are shown in Table 5. Figure 1 and Figure 2 .

[0055] Table 1. Chemical composition by mass percentage (wt.%)

[0056] .

[0057] Table 2 Rolling process parameters

[0058] .

[0059] Table 3 Heat treatment process parameters

[0060] .

[0061] Table 4 Mechanical property test results of the examples

[0062] .

Claims

1. A method of producing a large thickness NM600 steel plate, characterized in that: The produced steel plates have a thickness of 50–80 mm; the chemical composition of the steel (by mass percentage) is: C = 0.43%–0.45%, Si = 0.10%–0.20%, Mn = 0.95%–1.05%, P ≤ 0.008%, S ≤ 0.0015%, Cr = 1.05%–1.15%, Mo = 0.14%–0.18%, Ti = 0.008%–0.015%, V = 0.030%–0.0 40%, Alt=0.050%~0.070%, B=0.0008%~0.0020%, void H≤0.00010%, intermediate H≤0.00012%, N≤0.0035%, O≤0.0015%, the remainder is Fe and unavoidable impurities; the surface hardness of the steel is 570~640HBW, yield strength ≥1600MPa, tensile strength ≥2000MPa, elongation ≥7%; Key process steps include: (1) Smelting: The BOF-LF-RH production process is adopted. The molten iron is treated to remove sulfur and phosphorus. When the LF furnace exits the station, a 150m pure calcium wire is fed in. Then it enters the RH furnace for vacuum treatment. The vacuum degree is ≤67Pa and the holding time is 25min. Soft blowing is carried out on the platform for 28-34min. After breaking the vacuum, a 150m pure calcium wire is fed in one go. When the molten steel exits the station, the gas content is measured to be H≤0.00010%, N≤0.0035%, and O≤0.0015%. (2) Continuous casting: Full-process protective casting, control of moisture in protective slag and covering agent, tundish baking, and slow cooling and hydrogen expansion measures with the billet covered with heat insulation cover. The tundish H is controlled at ≤0.00012%; the tundish superheat is ≤20℃ for continuous casting furnace and ≤25℃ for open casting furnace. The continuous casting adopts dynamic light reduction technology. The thickness of the continuous casting billet is 260mm and the width is 2250mm. (3) Heating: Preheating section temperature 650~800℃, heating section temperature 1100~1230℃, soaking section temperature 1160~1220℃, furnace time 239~252min; (4) Rolling: The finished steel plate thickness is 50-80mm. Two-stage controlled rolling is adopted. The rough rolling start temperature is ≤1040℃, the rough rolling cumulative compression ratio is ≥2, and the intermediate billet thickness is 110-130mm; the finish rolling start temperature is ≤950℃, the finish rolling cumulative compression ratio of 50mm steel plate is 2.2, and the finish rolling cumulative compression ratio of 80mm steel plate is 1.

6. (5) Heat treatment: Quenching + low temperature tempering process is adopted. The quenching heating temperature is 830~850℃, the heating rate is 1.6~2.2min / mm, the holding time is 15~30min, the quenching cooling rate is 20~40℃ / s, and the quenching is water-cooled to room temperature; the tempering heating temperature is 150~170℃, the heating rate is 3.5~4.0min / mm, and the holding time is 15~30min; after tempering, the cells are stacked and slowly cooled to room temperature. (6) Post-furnace cutting: The steel plate after heat treatment is cut with heat or preheating, and then colored or magnetic particle testing is performed to detect whether there are delayed cracks.

2. The method of producing a large thickness NM600 steel plate according to claim 1, characterized in that Step (1) Smelting: The molten iron is treated by KR desulfurization and then smelted in a top and bottom blown converter. The converter slag formation and desulfurization are combined with the deep desulfurization process of slag removal after the furnace.

3. The method of producing a large thickness NM600 steel plate according to claim 1, characterized in that Step (2) Continuous casting: The billet is covered with an insulation cover and slowly cooled to 200±20℃ before being loaded into the furnace.

4. The method for producing a thick NM600 steel plate according to claim 1, characterized in that... Step (6) Post-furnace cutting: The temperature of the plate after cutting is ≥50℃.

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