High-strength ultralow-temperature toughness steel NM450 and production method thereof

Abstract

The invention discloses high-strength ultralow-temperature toughness steel NM450 with the thickness of 10-100 mm. The high-strength ultralow-temperature toughness steel NM450 is prepared from, by mass, 0.18-0.24 wt% of C, 0.20-0.50 wt% of Si, 0.9-1.5 wt% of Mn, smaller than or equal to 0.010 wt% of P, smaller than or equal to 0.001 wt% of S, larger than or equal to 0.025 wt% of Als, 0.3-1.0 wt% of Cr, 0.2-0.8 wt% of Ni, 0.2-0.5 wt% of Mo, 0.02-0.05 wt% of Nb, 0.01-0.05 wt% of Ti, 0.0015-0.0025 wt% of B and the balance Fe and residual elements. The carbon equivalent CEV is less than or equal to 0.76%, and the crack sensitivity coefficient Pcm is less than or equal to 0.40%; through continuous casting, heating, rolling and heat treatment, the strength of the obtained steel plate is matched with the low-temperature impact toughness, the main structure of the steel plate is tempered martensite and a small amount of bainite, the tensile strength of the steel plate is larger than or equal to 1300MPa, the yield strength is larger than or equal to 1000MPa, the elongation A is larger than or equal to 9%, the ballistic work at the temperature of minus 40 DEG C is larger than or equal to 45J, the surface hardness is 425-475HBW, and the core hardness is larger than or equal to 400HBW. And the use requirements of high-strength and ultralow-temperature toughness wear-resistant steel are completely met.

Description

Technical Field

[0001] The invention is applicable to the field of medium and thick plate production, and specifically relates to high-strength ultra-low temperature toughness steel NM450 and a production method thereof. Background Art

[0002] Wear-resistant steel plates have good wear resistance and impact resistance, and can be cut, bent, welded, etc. They are widely used in metallurgy, coal, cement, electricity, glass, mining, building materials, bricks and tiles, etc. The hardness of traditional low-alloy high-strength wear-resistant steel is 300-500HB. However, in extremely cold weather conditions, especially when in contact with high-hardness rocks and minerals, low-grade wear-resistant steel plates can no longer meet the use requirements. Wear-resistant steel plates with higher hardness, better low-temperature impact toughness, and higher resistance to welding crack sensitivity are required to make the steel plates more suitable for use in cold areas.

[0003] The patent application number 201310181136.2 is titled "A patented technology of HB500 grade hot-rolled high-strength wear-resistant steel and its production method", which adopts a lower alloy design but does not contain elements such as Ni, so that the low-temperature impact absorption energy of the steel plate is lower. On the other hand, the patent is based on hot rolling and is not suitable for wide and thick plate lines.

[0004] Chinese patent CN103194684B discloses a wear-resistant steel plate and a manufacturing method thereof, wherein the chemical composition by weight percentage is: C 0.15-0.35%, Si 0.10-0.60%, Mn 0.30-1.60%, P≤0.015%, S≤0.010%, Ti0.10-1.00%, Cr 0.20-1.50%, Mo≤0.80%, Ni≤1.50%, V≤0.10%, B 0.0005-0.0040%, Al 0.010-0.080%, Ca 0.0010-0.0080%, N≤0.0080%, O≤0.0080%, H≤0.0004%, and the rest is Fe and unavoidable impurities. Chinese patent CN102002645B discloses a high-strength wear-resistant steel plate and a preparation method thereof, wherein the chemical composition mass percentage of the steel is: C is 0.12%-0.22%, Si is 0.25%-0.50%, Mn is 1.10%-1.80%, Als is 0.025%-0.055%, Cr is 0.30%-1.00%, Ni is 0.20%-0.60%, Mo is 0.10%-0.50%, Ti is 0.010%-0.050%, B is 0.0010%-0.0050%, P≤0.020%, S≤0.015%, and the remaining content is Fe and inevitable inclusions. However, the hardness of the above patented wear-resistant steel plate is relatively low. Summary of the invention

[0005] In order to solve the above technical problems, the purpose of the present invention is to provide a high-strength ultra-low temperature toughness steel NM450, which fully meets the use requirements of high-strength ultra-low temperature toughness wear-resistant steel.

[0006] Another object of the present invention is to provide a method for producing high-strength and ultra-low temperature toughness NM450 steel.

[0007] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a high-strength ultra-low temperature toughness NM450 steel, the thickness of the steel is 10-100 mm, and the chemical components contained in the following mass percentages are: C: 0.18-0.24, Si: 0.20-0.50, Mn: 0.9-1.5, P≤0.010, S≤0.001, Als≥0.025, Cr: 0.3-1.0, Ni: 0.2-0.8, Mo: 0.2-0.5, Nb: 0.02-0.05, Ti: 0.01-0.05, B: 0.0015-0.0025, and the others are Fe and residual elements; its carbon equivalent CEV≤0.76%, and the crack sensitivity coefficient Pcm≤0.40%.

[0008] The principles of composition design are as follows:

[0009] C (Carbon): Carbon is a strengthening element that can improve the strength and hardness of steel by forming solid solution structure and carbide structure; it can slow down the decomposition rate of austenite, thereby increasing the stability of supercooled austenite; C can significantly improve the hardenability of steel, so that the steel can obtain a deeper hardened layer after quenching; carbon can form carbide structures such as cementite (Fe3C) with iron, and these carbide structures can significantly improve the hardness and wear resistance of steel.

[0010] Mn (Manganese): Mn has sufficiently high plasticity and high wear resistance. Therefore, Mn can increase the stability of austenite, expand the austenite in the γ phase, reduce the critical cooling rate during quenching, and reduce the critical point of steel (A1 and A3), so it can improve the hardenability of steel. The presence of Mn can also promote carburization, which can greatly improve the surface hardness and wear resistance of steel.

[0011] Cr (chromium): Cr is a medium carbide-forming element. Among all kinds of carbides, chromium carbide is the smallest one. It can be evenly distributed in the volume of steel, so it has high strength, hardness, yield point, high oxidation resistance, corrosion resistance and high wear resistance. Because it can make the organization refined and evenly distributed, the plasticity and toughness are also good. The beneficial effects of nickel are high strength, high toughness, good hardenability, high resistance and high corrosion resistance.

[0012] Ni (nickel): Ni not only strongly improves the strength of steel, but also keeps the toughness of iron at a very high level, and its brittle temperature is extremely low. (When nickel is less than 0.3%, its brittle temperature is below -100°C. When the amount of Ni increases, about 4% to 5%, its brittle temperature can drop to -180°C.) Therefore, Ni can improve the strength and plasticity of quenched structural steel at the same time. The lattice constant of Ni is similar to that of γ-iron, so it can form a continuous solid solution. This is conducive to improving the hardenability of steel. Ni can reduce the critical point and increase the stability of austenite, so its quenching temperature can be reduced and the hardenability is good.

[0013] Mo (Molybdenum): Mo can refine grains and reduce the overheating tendency of steel. Combined with Cr and Ni, it can greatly improve hardenability, strength, hardness and thermal stability. It can inhibit and reduce temper brittleness and improve impact toughness.

[0014] Nb (niobium): Nb can refine the grains of steel, reduce the overheating sensitivity and temper brittleness of steel, and under certain conditions, it can also improve the strength, toughness and creep resistance of steel.

[0015] Ti (titanium): Ti has a strong affinity with C and N. The formed carbides, nitrides or carbonitrides have a very high dissolution temperature. During the heating process, these undissolved small carbonitride particles increase the nucleation center of austenite and hinder the movement or merging of austenite grain boundaries at high temperatures; during the quenching process, these compounds will produce a strong precipitation strengthening effect, which can refine the grains and improve the strength.

[0016] B (boron): Boron dissolves in solid solution, and its carbides are also very stable, which can prevent other carbides from precipitating and recrystallizing and diffusing. When a trace amount of boron (0.0015-0.0025%) is added to steel, the hardenability of the steel can be significantly improved, and the strength of the steel plate can be increased. After quenching + low-temperature tempering, a good impact value can be obtained, and high temperature is difficult to transfer to the solid solution.

[0017] The production method of the above-mentioned high-strength ultra-low temperature toughness NM450 steel includes continuous casting, heating, rolling and heat treatment, as follows:

[0018] ①Continuous casting: Use billets with a thickness of ≥400mm, control the superheat of the molten steel in the tundish to 5-22℃, use argon protection for the entire casting process, and turn on electromagnetic stirring;

[0019] ② Heating: preheating section temperature 900 ~ 1000 ℃, heating section temperature 1220 ~ 1260 ℃, insulation section temperature 1200 ~ 1240 ℃, heating time 10 ~ 13min / cm;

[0020] ③ Rolling: Two-stage rolling is adopted, the rough rolling temperature is 1050℃~1150℃, the pass reduction is controlled at 40~50mm, the grains are fully broken, the deformation is ensured to penetrate into the core of the steel plate, and the problem of intermediate segregation of the continuous casting billet is compensated and improved. The final rolling temperature is controlled at 950℃~1000℃, and the thickness of the steel is 1.8~2.5 times the thickness of the finished steel plate; the second stage rolling temperature is controlled at 880~920℃ (temperature below the unrecrystallized zone of austenite), and small reduction rolling is adopted to increase the slip band and dislocation density in the austenite grains through thermal deformation, and at the same time increase the effective grain boundary area, creating conditions for the deformation nucleus of the ferrite phase. The final rolling temperature is controlled at 780℃~810℃. After rolling, the steel plate enters ACC cooling to solidify the refined grains after cooling to prevent them from growing again, and the red-returning temperature is controlled at 640~660℃;

[0021] ④ Heat treatment: Two quenching + one tempering, the steel plate is heated to 895±10℃ for the first quenching, the holding time is 1.8~2.2min / mm, and the quenching cooling rate is controlled to be ≥2℃ / S. The temperature of the steel plate for the second quenching treatment is 865±10℃, the holding time is 1.8~2.2min / mm, the quenching cooling rate is controlled to be ≥2℃ / S, the tempering heating temperature is 200±20℃, and the holding time is 4±0.5min / mm. The two quenchings further refine the grain structure, and the low-temperature tempering improves the low-temperature impact toughness of the steel plate.

[0022] It should be noted that in order to obtain the best strength and toughness match of the steel plate after tempering, the ideal quenched structure is not a single martensitic structure, but a mixed structure of martensite + bainite. If a single martensitic structure is obtained after quenching the steel plate, the original austenite grains will form relatively coarse martensite laths, which is not conducive to improving the toughness of the steel plate. If the steel plate is cooled at an appropriate quenching rate, the first formed F in the supercooled austenite will first divide the austenite grains into many small pieces, and then supercool to below the Bs / Ms point, forming various oriented lath bainite / martensite in the divided small pieces, which can refine the bainite / martensite lath bundles and obtain a mixed structure of lath bainite / martensite plus ferrite, which is conducive to improving toughness. During the heat treatment process, through quenching in the two-phase zone, part of the undissolved ferrite is retained to prevent the transformed austenite from growing. Since there is a transmission time of 0 to 3 minutes from the steel plate being taken out of the furnace to entering the water, before the bainite / martensite phase transformation begins, intragranular ferrite with different orientations will be formed in the crystal, which divides a certain amount of austenite grains into multiple regions. After cooling to the starting temperature of bainite / martensite transformation, the precipitates on the subgrain boundaries with different orientations in the divided regions promote the nucleation of bainite / martensite at each subgrain boundary. The orientation difference of the subgrains causes the directions of the bainite / martensite bundles of each subgrain to be different, and the growth process of each bundle of bainite / martensite is restricted by the subgrain boundaries.

[0023] The steel plate obtained by this scheme has a strength matching low-temperature impact toughness, and its main structure is tempered martensite and a small amount of bainite, with a tensile strength ≥1300MPa, a yield strength ≥1000MPa, an elongation A ≥9%, an impact energy of -40℃ ≥45J, a surface hardness of 425~475HBW, and a core hardness ≥400HBW. It fully meets the use requirements of high-strength, ultra-low-temperature toughness wear-resistant steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0025] Figure 1 This is the metallographic structure diagram of the surface of a 60 mm thick steel plate of the present invention (the magnification increases gradually from left to right).

[0026] Figure 2 This is the metallographic structure diagram of the 60 mm thick steel plate at 1 / 4 of the thickness of the present invention (the magnification increases gradually from left to right).

[0027] Figure 3 This is the metallographic structure diagram of the 60 mm thick steel plate at 1 / 2 thickness of the present invention (the magnification increases gradually from left to right). DETAILED DESCRIPTION

[0028] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0029] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0030] Three types of NM450 steel with high strength, high hardness and ultra-low temperature toughness with a thickness of 10 to 100 mm are produced, using the chemical composition in mass percentage as shown in Table 1 below:

[0031] Table 1: Chemical composition of several 10-100 mm thick NM450 steels (Wt, %)

[0032]

[0033] The production process of the steel mainly includes converter smelting, LF refining, VD vacuum refining, continuous casting, billet heating, rolling, stack cooling and heat treatment, as follows:

[0034] (1) Converter smelting: the molten iron S≤0.008%, P≤0.060%, the molten iron temperature≥1300℃, dry high-quality scraps and nickel-containing special scrap steel, the converter loading amount is controlled according to the remaining 4-7 tons of steel casting, argon blowing is not allowed during the steel tapping process, and no deoxidizer and alloy are allowed to be added to the molten steel. At the end of steel tapping, the slag is blocked by a slag cone. If the slag blocking fails, the furnace must be lifted in advance to ensure that the thickness of the converter slag is controlled below 20mm to avoid slag rephosphorization. After the molten steel arrives at the argon station, add 200kg of lime to the molten steel in advance, and blow argon appropriately for stirring.

[0035] (2) LF refining: Slag making is carried out in accordance with the large slag volume process standard (the amount of lime used in a single furnace must be controlled at more than 800 kg) to prevent the occurrence of slag dilution; 2.0 m / t steel-aluminum wire is added, and aluminum particles and calcium carbide are used as deoxidizers in the process. The amount of aluminum particles is controlled at 30-60 kg, and the white slag retention time is ≥ 15 min; after the LF first heating is completed, precious alloys are added during the second heating process under good deoxidation conditions to ensure that the composition reaches the internal control limit.

[0036] (3) VD vacuum refining: The vacuum degree can reach ≤67Pa, and the pressure holding time is ≥20min. After breaking the air, adjust the [Al] content, and add titanium iron and boron iron to adjust the Ti and B components to the target. Carry out calcium treatment and soft blowing for 10min. Avoid exposing the molten steel during soft blowing. After vacuuming, perform H setting operation, and control the H content within 1.8PPm. After H setting, add covering agent to ensure the leaving station temperature is 1550±15℃.

[0037] (4) Continuous casting: the pouring temperature was controlled at 1550-1560°C, 400 mm thick section ingot was used, the pulling speed was 0.68 m / min, the specific water volume was 0.80 L / kg, and electromagnetic stirring was turned on during the whole pouring process, with the electromagnetic stirring at 900 A, 6 Hz, and 30 s.

[0038] (5) Ingot heating: preheating section temperature 900-1000°C, heating section temperature 1220-1260°C, holding section temperature 1200-1240°C, heating time 10-13 min / cm.

[0039] (6) Rolling: Two-stage rolling is adopted. In the first stage, large reduction is adopted, and the reduction per pass is controlled at 40-50 mm. The final rolling temperature is controlled at 950℃~1000℃, and the thickness of the steel plate is 1.8-2.5 times the thickness of the finished steel plate. In the second stage, the starting rolling temperature is controlled at 880℃~920℃, and small reduction rolling is adopted. The final rolling temperature is controlled at 780℃~810℃. After rolling, ACC is used for controlled cooling, and the red-return temperature of the steel plate is controlled at 640-660℃.

[0040] (7) Stack cooling: After rapid straightening, the steel plate is placed in a slow cooling pit for stacking and slow cooling. The stack cooling temperature is ≥350°C and the stack cooling time is ≥48 hours. Its function is not only to effectively eliminate the mechanical and structural stress generated during the rolling of the steel plate, but also to reduce the hydrogen content in the steel to prevent the occurrence of white spots in the steel.

[0041] (8) Heat treatment: Two quenchings + one tempering. The temperature of the first quenching treatment of the steel plate is 895±10℃, the holding time is 2min / mm, and the quenching cooling rate is controlled to be ≥2℃ / S. The temperature of the second quenching treatment of the steel plate is 865±10℃, the holding time is 2min / mm, and the quenching cooling rate is controlled to be ≥2℃ / S. The temperature of the tempering treatment is 200±20℃, and the holding time is 4±0.5min / mm.

[0042] The steel plates obtained in the example were tested for performance. The sampling location and sample preparation of the mechanical properties test pieces of the steel plates were carried out in accordance with the provisions of the standard GB / T 2975. The low-temperature impact toughness test was carried out in accordance with the standard GB / T 229, the tensile properties test was carried out in accordance with the standard GB / T228, and the hardness test was carried out in accordance with the standard GB / T 231.1. The surface hardness test was carried out after the surface was cut at least 0.7 mm.

[0043]

[0044] The specimen is subjected to a 45° cold bending test, the bending diameter is 4 times the thickness of the steel plate, and the specimen matrix has no cracks; the hardness in the thickness direction meets B≥A-2.5C, and the core hardness is ≥400HB, where B is the hardness of the wear-resistant plate specimen at the detection point in the thickness direction (unit HB), A is the surface hardness of the wear-resistant plate specimen (unit HB), and C is the shortest distance from the detection point to the original surface of the wear-resistant plate (unit mm).

[0045] From the above data, we can see that the steel plate has high strength and high hardness as well as good low-temperature impact toughness. The hardness of the steel plate surface, 1 / 4 cross section and 1 / 2 cross section is uniform, which fully meets the relevant technical requirements.

Claims

1. A high-strength ultra-low temperature toughness steel NM450, characterized in that: The thickness of the steel is 10 to 100 mm, and includes the following chemical compositions by mass content (unit, wt%): C: 0.18 to 0.24, Si: 0.20 to 0.50, Mn: 0.9 to 1.5, P≤0.010, S≤0.001, Als≥0.025, Cr: 0.3 to 1.0, Ni: 0.2 to 0.8, Mo: 0.2 to 0.5, Nb: 0.02 to 0.05, Ti: 0.01 to 0.05, B: 0.0015 to 0.0025, and the rest are Fe and residual elements; Its carbon equivalent CEV≤0.76%, crack sensitivity coefficient Pcm≤0.40%; the main structure of the steel plate is martensite and bainite, its tensile strength is ≥1300MPa, yield strength is ≥1000MPa, elongation A≥9%, impact energy at -40℃ is ≥45J, surface hardness is 425~475HBW, and core hardness is ≥400HBW.

2. The production method of the high-strength ultra-low temperature toughness steel NM450 according to claim 1 is characterized in that Including continuous casting, heating, rolling and heat treatment, as follows: ①Continuous casting: Use billets with a thickness of ≥400mm, control the superheat of the molten steel in the tundish to 5-22℃, use argon protection for the entire casting process, and turn on electromagnetic stirring; ② Heating: preheating section temperature 900 ~ 1000 ℃, heating section temperature 1220 ~ 1260 ℃, insulation section temperature 1200 ~ 1240 ℃, heating time 10 ~ 13min / cm; ③Rolling: Two-stage rolling is adopted, the rough rolling start temperature is 1050℃~1150℃, the pass reduction is controlled at 40~50mm, the final rolling temperature is controlled at 950℃~1000℃, and the air-dried steel thickness is 1.8~2.5 times the thickness of the finished steel plate; the second-stage rolling start temperature is controlled at 880~920℃, small reduction rolling is adopted, the final rolling temperature is controlled at 780℃~810℃, and after rolling, the steel plate enters ACC cooling, and the red-return temperature is controlled at 640~660℃; ④ Heat treatment: two quenching + one tempering, the first quenching heating temperature of the steel plate is 895±10℃, the holding time is 1.8~2.2min / mm, the quenching cooling rate is controlled to be ≥2℃ / S, the second quenching temperature of the steel plate is 865±10℃, the holding time is 1.8~2.2min / mm, the quenching cooling rate is controlled to be ≥2℃ / S, the tempering heating temperature is 200±20℃, and the holding time is 4±0.5min / mm.

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