A NM500 grade wear-resistant steel with good low-temperature toughness and a manufacturing method thereof

Abstract

The invention discloses a NM500 grade wear-resistant steel with good low-temperature toughness and a manufacturing method thereof, including molten steel smelting, slab continuous casting, slab heating, steel plate rolling, online quenching, and tempering heat treatment. The thickness of the steel plate is 20-40 mm. The invention adopts the composition design of medium carbon, low manganese, and vanadium nitrogen microalloying, coupled with clean steel control technology, controlled rolling system and online quenching process technology, and the wear-resistant steel plate manufactured by the invention has a surface Brinell hardness of ≥500 HB, a yield strength of ≥1400 MPa, a tensile strength of ≥1500 MPa, a 180°C bending pass, a 50°C Charpy V-type longitudinal impact energy of ≥45 J, and excellent internal quality and appearance plate shape, and is suitable for service under low temperature environment, strong impact and large displacement deformation conditions. Compared with the traditional manufacturing process, the present method abandons the two high and one low disadvantages of "high alloy, high energy consumption, and low toughness", significantly improves production efficiency, reduces production costs, and more effectively improves and regulates the steel plate organization and performance, and will have good application prospects in the field of wear-resistant steel production.

Description

Technical Field

[0001] The invention belongs to the technical field of wear-resistant steel, and in particular relates to NM500 grade wear-resistant steel with good low-temperature toughness and a manufacturing method thereof. Background Art

[0002] Wear-resistant steel refers to special plate products designed for use under large-area wear conditions, and mainly has the characteristics of high hardness and high wear resistance. With the vigorous development of my country's infrastructure in recent years, wear-resistant steel has been widely used in mining, construction, agriculture, metallurgy and other industries, and is developing in the direction of low cost, light weight and long life. Therefore, the research and development of new wear-resistant materials is of great significance. Compared with traditional steel plates, NM500-grade wear-resistant steel with good low-temperature toughness can meet the service requirements of harsh environments such as low temperature and strong impact, and replace low-grade wear-resistant steel to achieve lightweight assembly of mechanical materials.

[0003] At present, most of the domestic methods for producing NM500 wear-resistant steel adopt offline heat treatment (offline quenching + tempering), which is highly dependent on heat treatment furnaces and quenching machines, and has the disadvantages of long process, high energy consumption, multiple alloys, and high cost; at the same time, due to the high design of carbon and manganese segregation elements, poor control of molten steel inclusions, mismatched rolling system, and coarse offline quenching grains, the steel plate has low strength and poor toughness, and cannot serve in low temperature and strong impact environments. With the development of ultra-fast cooling equipment, online quenching (DQ) technology has become a key technology for high cooling speed and strong cooling of medium and thick plate products. If the online quenching process is used to produce wear-resistant steel, the shortcomings of offline quenching can be solved. At the same time, coupled with reasonable composition design, good molten steel purity, and appropriate controlled rolling technology, NM500 wear-resistant steel with good low-temperature toughness can be obtained.

[0004] Existing document 1, patent "A 500HB grade wear-resistant steel plate and its production method" (publication number: CN109735776A), using ": C: 0.27-0.37%, Si: 0.15-0.35%, Mn: 1.51-1.56%, P ≤ 0.015%, S ≤ 0.015%, Cr: 1.51-1 .60%, Al: 0.015-0.045%, B: 0.0008-0.0035%, Ti: 0.018-0.040%" composition design, through heating temperature 1150-1260℃, finishing rolling temperature 1000-1150℃, online quenching using UFC cooling equipment, quenching and cooling temperature 840-950℃, final cooling temperature ≤200℃, cooling rate ≥25℃ / s, tempering temperature 300-500℃, steel plate thickness 10-50mm, hardness 470-550HB. The steel plate performance made by this patent has no strength and impact energy indicators, and its usability in low temperature and strong impact environment cannot be known. In addition, the design of C and Mn elements is relatively high, and the carbon equivalent is bound to increase, which is not conducive to the subsequent processing, welding and use process. In addition, high C content is sensitive to toughness, and high Mn content is sensitive to segregation and MnS inclusions. How to ensure the matching of strength and plasticity is worth discussing and exploring. In addition, the tempering temperature is high, and the precipitated carbides are bound to aggregate and grow, which has a certain impact on wear resistance.

[0005] Existing document 2, patent "HB500 grade wear-resistant steel plate and its preparation method" (publication number: CN 102517509A), adopts "C: 0.28-0.35%, Si: 0.10-0.60%, Mn: 0.7-1.0%, P≤0.025%, S≤0.010%, Cr: 0.4-0.8%, Mo: 0.10-0.60%, Nb: 0.01-0.05%, V: 0.01-0.08%, Ti: 0.005-0.05%, B: 0.001-0.003%" composition design, through heating temperature 1200-1250℃, rough rolling start temperature ≤1000℃, rough rolling final rolling temperature ≥970℃, pass deformation rate 10-30%, finishing rolling start temperature ≤1000℃, finishing rolling start temperature ≥ ... The temperature is 870-900℃, the final rolling temperature is Ar3+(80-160)℃, the pass deformation rate is 10-30%, and the cooling is carried out by an online water cooling system. The start cooling temperature is Ar3+(70-150)℃, the cooling rate is ≥20℃ / s, the final cooling is ≤100℃, the tempering temperature is 150-250℃, and the tempering is followed by air cooling. The thickness of the steel plate in the embodiment is 20-30mm, the Brinell hardness is greater than 500HB, and the -20℃ impact toughness is ≥24J. The patent component C element is designed to be relatively high, which also has a certain impact on low-temperature toughness and processing and welding. In addition, the embodiment with a thickness of 20-30mm is added with a high content of 0.35% of the precious alloy Mo, which is bound to significantly increase the cost. During the rough rolling process, since the heating temperature of the casting is ≥1200℃, but the rough rolling start temperature is limited to ≤1000℃, this will inevitably increase the waiting time of the steel billet at high temperature, which will not only aggravate the secondary oxidation of the steel billet, affect the quality of the rolled plate surface, but also reduce the rough rolling efficiency. The impact toughness of the embodiment only reaches 24-39J at -20℃, which does not meet the requirements of low temperature high toughness wear-resistant steel.

[0006] Existing document 3, patent "A high strength and high hardness wear-resistant steel" (publication number: CN 102127705A), using "C: 0.2-0.3%, Si: 0.5-1.0%, Mn: 1.4-2.0%, P≤0.01%, S≤0.005%, Nb :0.02-0.04%,Ti:0.02-0.035%,V:0.03-0.06%,B:0.0015-0.002%,Al:≤0.03%,N:0.002%-0.004%” composition design, through heating temperature 1100-1200℃, rough rolling start temperature 1000-1070℃, rolling with cumulative deformation rate ≥80%, finishing rolling start temperature about 850℃, final rolling temperature 800-850℃, rapid cooling to below 200℃ at ≥20℃ near Ar3, example steel plate thickness 15mm, tensile strength 1450-1641MPa, elongation 13-16%, -20℃ impact energy 28-35J, Brinell hardness 440-460HB. The Si content of the patent is designed to be 0.5-1.0%, which is too high. Although it can improve the strength, it will deteriorate the plastic toughness, and even make the steel brittle, reduce weldability and corrosion resistance. The conventional Si design is ≤0.5%. The Mn content of the embodiment is 1.48-1.98%. High Mn will increase the segregation of the steel billet and MnS inclusions, and seriously deteriorate the plastic toughness of the steel plate. It can be seen from the impact energy of only 30J at -20℃, which verifies the above speculation. In addition, under this component design + online quenching process, the hardness is only about 450HB. At the same time, no tempering heat treatment is performed after quenching, and the internal stress cannot be eliminated, which is bound to affect the stability of the steel plate structure and subsequent processing and welding.

[0007] Existing document 4, "A low alloy high performance wear-resistant steel plate and its manufacturing method" (publication number: CN103205627A), adopts "C: 0.21-0.32%, Si: 0.1-0.5%, Mn: 0.60-1.60%, B: 0.0005-0.0040%, Cr≤1.50%, Mo≤0.80%, Ni≤1.50%, Nb≤0.080%, V≤0.080%, Ti≤0.060%, Al:0.010-0.080%, Ca:0.0010-0.0080%, N≤0.0080%, O≤0.0080%, H≤0.0004%, P≤0.015%, S≤0.010%" composition design, through heating temperature 1000-1200℃, rough rolling temperature 900-1150℃, finishing rolling temperature 780-880℃, water cooling to below 400℃, air cooling, tempering heating temperature 100-400℃, insulation 30-120min. The wear-resistant steel has a tensile strength of >1400MPa, elongation >11%, Brinell hardness >450HB, and -40℃ longitudinal impact energy >50J. It can be seen from the chemical composition of Examples 8-9 that the addition of up to 0.60-0.80% of the precious metal Mo will inevitably greatly increase the alloy cost. At the same time, its -40℃ impact energy is only 60-65J, and the low-temperature toughness is not significantly outstanding. The commercial value is low when applied to large-scale industrial production. In addition, the cooling stop temperature of the steel plate with a thickness of 35-39mm in the embodiment is as high as 250-400℃. It is worth discussing whether the cooling effect can be achieved in the longitudinal section and the core of the steel plate and whether the structure is uniform and stable. Although the steel plate has passed the short-term wear resistance test, the wear resistance and service life in the thickness direction, especially the core, may be affected to a certain extent when it is in service for a long time. Summary of the invention

[0008] The purpose of the present invention is to provide a NM500 grade wear-resistant steel with good low-temperature toughness and a manufacturing method thereof, aiming to adopt a reasonable and concise component design, couple the clean steel smelting and continuous casting process technology, and manufacture a medium-thick specification, Brinell hardness meeting the 500HB level, high-grade wear-resistant steel with high strength and high and low-temperature toughness through two-stage controlled rolling and online quenching technology, so as to solve a series of problems such as the long production process of existing wear-resistant steel, high alloy and energy consumption, and insufficient low-temperature toughness.

[0009] The present invention is achieved through the following technical scheme: a NM500 grade wear-resistant steel with good low-temperature toughness and a manufacturing method thereof, characterized in that the NM500 grade wear-resistant steel with good low-temperature toughness comprises the following components in mass percentage: C: 0.19-0.22%, Si: 0.25-0.45%, Mn: 1.20-1.35%, P≤0.010%, S≤0.003%, Al: 0.020-0.040%, Nb: 0.015-0.025%, Ti: 0.01-0.02%, V: 0.10-0.13%, Cr: 0.80-0.90%, Mo: 0.17-0.27%, B: 0.0015-0.0025%, N: 100-140ppm , H≤2ppm, O≤20ppm, the balance is Fe and unavoidable impurities; carbon equivalent CEV(%) range: 0.60-0.70%, CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.

[0010] The present invention also discloses a method for manufacturing the above-mentioned NM500 grade wear-resistant steel with good low-temperature toughness, comprising the steps of molten steel smelting, slab continuous casting, slab heating, steel plate rolling, online quenching, and tempering heat treatment; the method is characterized in that:

[0011] Molten steel smelting steps: the molten iron is first desulfurized and slag-stripped through pretreatment. After slag-stripping, the bright surface of the molten iron is ≥95%, and the S of the molten iron entering the furnace is ≤0.010%. The top and bottom combined blowing converter is used for smelting. During the blowing process, the blowing gun position adopts the low-high-low mode. The bottom blowing of the converter adopts the nitrogen mode for the first 12 minutes of blowing, with a flow rate of 200m 3 / h, blowing for 12 minutes to the end of blowing, the converter bottom blowing adopts argon mode, flow rate 240m 3 / h, the temperature of the molten steel in the furnace is about 1580℃ when the bottom blowing argon mode is switched; the double slag method is adopted for smelting, slag blocking steelmaking, slag thickness ≤40mm, metal manganese, ferrosilicon and ferroaluminum are used for deoxidation and alloying during steelmaking, the converter steelmaking temperature is 1620±20℃, and the final C, P and Al contents are strictly controlled as follows: C≥0.07%, P≤0.009%, 0.04%≤Al≤0.07%; LF+RH double process is adopted, LF refined white ash is added at a consumption of 8kg per ton of steel, white slag is added after the first power supply, the white slag retention time is ≥12min, the final slag sample composition is controlled to be Al2O3>22%, the CaO / Al2O3 content ratio is 1.7-2.0, and the ferroboron is controlled to be added 5min after the titanium iron is added before leaving the station in the later stage; nitrogen is used as the lifting gas in RH vacuum, the pressure is 5KPa, and the flow rate is 60m 3 / h, high vacuum time ≥12min, after breaking the air, ensure that the N content is 100-140ppm. If the content is insufficient, vanadium-nitrogen alloy can be added to make up for it; adopt calcification treatment, feed 200-300 meters of pure calcium wire, wire feeding speed 180-210m / min, soft argon blowing time before hanging bag ≥10min, and molten steel calming time after soft blowing ≥5min;

[0012] Slab continuous casting steps: maintain a constant casting speed of 0.8-0.9 m / min during casting, control the liquid level fluctuation to ±3mm, and the superheat to 20-25℃; realize protective casting throughout the whole process, and put into electromagnetic stirring and dynamic soft pressure, the electric stirring current is set to 350-420A, the frequency is 5-8HZ, and the dynamic soft pressure increases the pressure reduction in the two-phase zone and the solid phase zone, the pressure reduction rate is 1.0-1.3mm / m, the pressure reduction in the two-phase zone is 5-9mm, and the pressure reduction in the solid phase zone is 0.5-1.5mm; the continuous casting slab thickness is 250mm and it is stacked and slowly cooled offline, and the slow cooling time is ≥48h; ensure that the internal quality center segregation of the slab is ≤C class 0.5 level, and no other defects are allowed;

[0013] Slab heating step: Use a walking beam heating furnace to heat the slab in sections to an even temperature of 1200-1250°C. The temperature of the first heating section is 920-950°C, the temperature of the second heating section is 1020-1220°C, the temperature of the third heating section is 1200-1260°C, the temperature of the fourth heating section even temperature is 1200-1250°C, and the heating time of the slab in the furnace is 250-300 minutes;

[0014] Steel plate rolling steps: adopt two-stage segmented rolling, the starting temperature of the rough rolling stage is ≥1060℃, the ending temperature is ≥1000℃, ensure that the single pass reduction rate of the last three passes is ≥15%, and the total rough rolling reduction rate is ≥50%. After the rough rolling is completed, the thickness of the intermediate billet is ≥3.0 times the thickness of the finished steel plate. The water cooling device is turned on during the intermediate billet waiting for temperature to be forced to cool, and the upper spray flow rate is 30-50m 3 / h. The starting temperature of the finishing rolling stage is ≤950℃, and the final rolling temperature is ≥880℃. The reduction rate of each of the last three passes is ≥20%, and the total reduction rate of finishing rolling is ≥66.6%. After rolling, pre-straightening is performed for 1 pass to flatten the plate shape, and the straightening speed is 0.7-1.2m / s;

[0015] Online quenching steps: After pre-straightening, quickly enter the ultra-fast cooling device for cooling, water inlet temperature ≥830℃, red-return temperature ≤150℃, cooling rate 22-35℃ / s. Thickness 20-40mm adopts DQ+ACC cooling mode, water pressure ≥0.5MPa and ≥0.2MPa respectively, among which the DQ frame adopts gap nozzle, with 4 water groups in total, and the upper spray flow rate of each gap group is 240-260m 3 / h, gap water ratio 2.1-2.2; ACC frame uses high-density nozzles, a total of 26 water groups, each high-density group upper and middle cavity flow 180-200m3 / h, high density water ratio 2.45-2.6. Ultra-fast cold roller speed setting 0.25-0.30m / s, acceleration setting 0-0.003m / s 2 ;

[0016] Tempering heat treatment steps: adopt offline low-temperature tempering process, the tempering process includes: tempering temperature 200-220℃, heating coefficient in furnace 3.5-4.0min / mm, air cooling after tempering.

[0017] Further: In the molten steel smelting step, the RH nitrogen blowing flow rate is preferably ≥ 70m 3 / h, to ensure the nitrogen content is increased; preferably control the LF white slag time ≥14min, the final slag sample composition CaO / Al2O3 ratio is 1.7-1.8, and the feeding pure calcium line length is 250-300 meters, which can effectively control the oxide content and sulfide spheroidization effect.

[0018] Furthermore: in the steel plate rolling step, it is preferred to control the single-pass reduction rate of the last three passes of rough rolling to be ≥18%, and the single-pass reduction rate of the last three passes of finishing rolling to be ≥22%, so as to promote dynamic recrystallization and grain refinement; the starting temperature of finishing rolling is ≤960°C. If it is higher than this value, it is easy to cause excessive two-phase rolling to produce coarse mixed crystal structure, reducing the grain refinement effect; the ending temperature of finishing rolling is ≥870°C. If it is lower than this value, it is easy to cause deformation-induced precipitation of coarse and banded ferrite, resulting in insufficient strength and hardness.

[0019] Furthermore: in the online quenching step, the water entry temperature is preferably ≥840°C, and is not required to be lower than 820°C, to reduce or avoid the precipitation of ferrite before entering the water. The quenching cooling rate is preferably ≥25°C / s. A high cooling rate can make the supercooled austenite avoid the ferrite phase region and undergo medium and low temperature transformation, and can also make the remaining untransformed austenite transform into fine martensite to ensure the uniformity of the structure.

[0020] The advantages of the present invention are:

[0021] (1) The present invention adopts a composition design of medium carbon, low manganese, and vanadium-nitrogen microalloying, and intervenes from the design source to reduce the tendency of C and Mn elements to deteriorate low-temperature toughness and internal segregation. At the same time, through the high vanadium design coupled with the converter / RH nitrogen blowing method, VN particles are induced to precipitate during rolling due to deformation. On the one hand, the grain boundaries are pinned to prevent the growth of austenite grains, and on the other hand, the occurrence of austenite recrystallization is prevented or delayed, thereby achieving precipitation strengthening and grain refinement, thereby replacing or reducing the design addition of precious metals, greatly reducing the alloy cost, and ensuring an excellent match of strength and toughness.

[0022] (2) The present invention adopts the clean steel smelting continuous casting process technology. By strictly controlling the key process technology points such as the S content at the end of pretreatment, the C / P / Al content at the end of converter, the LF white slag time, the CaO / Al2O3 ratio of the end slag sample, the RH high vacuum time, the calcification treatment, the soft blowing time, the continuous casting electromagnetic stirring and the light pressure reduction, the content of harmful elements such as P, S, O, and H is controlled in the whole process, and the central segregation of the ingot is ≤C class 0.5 level without other defects, creating the prerequisite for achieving excellent performance of the steel plate.

[0023] (3) The present invention adopts a two-stage controlled rolling system of recrystallization zone + non-recrystallization zone, calculates the Tnr phase transformation point of 920-940°C through an empirical formula, reasonably designs the end temperature of rough rolling and the start temperature of fine rolling, and fully refines the deformed austenite grains by increasing the cooling of the intermediate billet, the large deformation amount of a single pass, and the large deformation rate of the total pass, so that the essential grain size meets the 8.0 level or above.

[0024] (4) The present invention adopts the method of setting the final rolling temperature at above 880°C and the water inlet temperature at above 830°C to suppress the early precipitation of ferrite and ensure hardness; utilizing the residual heat after rolling and cooperating with the ultra-fast cooling process technology to carry out online quenching, which has a simple process and low energy consumption, and inherits the characteristics of fine grains and dislocations after rolling, making the quenched martensite fine and uniform; adopting the low-temperature tempering process to stabilize the structure, eliminate internal stress, and ensure the subsequent processing and use performance.

[0025] (5) The wear-resistant steel described in the present invention has stable internal quality, the total level of non-metallic inclusions is ≤2.0, there are no Class A inclusions, Class B inclusions are ≤1.0, and the metallographic structure is tempered martensite; excellent mechanical properties, surface Brinell hardness ≥500HB, yield strength ≥1400MPa, tensile strength ≥1500Mpa, 180℃ bending qualified, -50℃ Charpy V-type impact energy ≥45J; the steel plate is flat, and the straightness meets ≤10mm / 2m. Compared with normal steel grades of the same level, it has excellent low-temperature impact toughness and is suitable for low temperature, strong impact and large displacement deformation service requirements. At the same time, the production process is simple and efficient, the manufacturing cost is low, and it has industrial mass production conditions. It has broad application prospects in the field of medium and high-level wear-resistant steel and has certain market competitiveness. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Austenite grain microstructure of 30 mm steel plate in Example

[0027] Figure 2 The microstructure of the longitudinal section of the 30 mm steel plate near the surface of the embodiment

[0028] Figure 3 The microstructure of the longitudinal section 1 / 2 of the 30 mm steel plate in the embodiment DETAILED DESCRIPTION

[0029] The present invention is further described in detail below in conjunction with specific examples. It should be clear that these examples are only used to describe the specific implementation methods of the present invention and are not intended to constitute any limitation on the protection scope of the present invention.

[0030] The mass percentages of the chemical components of the NM500 grade wear-resistant steels of the following Examples 1-3 and the steel plate of Comparative Example 1 are shown in Table 1, wherein the composition design of Comparative Example 1 does not contain V element and the converter / RH does not have nitrogen blowing operation, and the rolling and quenching setting processes are consistent with the examples:

[0031]

[0032] The molten steel smelting process, Examples 1-3 are as follows: the bright surface of the molten iron after pretreatment and slagging is 98 / 99 / 98%, and the molten iron S in the furnace is 0.005 / 0.004 / 0.006%; the blowing gun position during the converter blowing process adopts a low-high-low mode, and the converter bottom blowing adopts a nitrogen mode for 12 minutes before blowing, with a flow rate of 200 / 210 / 225m 3 / h, blowing for 12 minutes to the end of blowing, the converter bottom blowing adopts argon mode, flow rate 240 / 240 / 230m 3 / h, the temperature of molten steel in the furnace was 1585 / 1583 / 1588℃ when the bottom blowing argon mode was switched; double slag method was adopted for slag blocking and steel tapping, slag thickness was 35 / 40 / 38mm, and the tapping temperature of the converter was 1624 / 1636 / 1621℃. The final C, P and Al contents were: C: 0.075 / 0.081 / 0.084%, P: 0.008 / 0.009 / 0.009%, Al: 0.064 / 0.069 / 0.062%; LF refined lime was added 908 / 906 / 910kg, the white slag was kept for 15 / 16 / 15min, and the final slag sample had Al2O3:31.1 / 33.7 / 32.6%, CaO / Al2O3:1.73 / 1.75 / 1.74, ferroboron is added 5 minutes after ferrotitanium is added; nitrogen is used as the lifting gas in RH vacuum, the pressure is 5KPa, and the flow rate is 72 / 75 / 80m 3 / h, high vacuum time 14 / 15 / 15min, N after breaking air: 114 / 127 / 135ppm; feeding pure calcium wire 280 / 300 / 300 meters, wire feeding speed 190 / 200 / 210m / min, soft argon blowing time before hanging bag 12 / 13 / 14min, molten steel calming time after soft blowing 6 / 6 / 7min.

[0033] The slab continuous casting process, Examples 1-3 are: constant pulling speed 0.85 / 0.85 / 0.85m / min, liquid level fluctuation ±2mm, superheat 22 / 24 / 25℃; protective pouring is achieved throughout the process, electromagnetic stirring and dynamic soft pressure are put into use, the electric stirring current is set to 360 / 360 / 370A, the frequency is 5 / 5 / 6HZ, the dynamic soft pressure increases the reduction in the two-phase zone and the solid phase zone, the reduction rate is 1.15 / 1.20 / 1.15mm / m, the reduction in the two-phase zone is 8 / 7 / 9mm, and the reduction in the solid phase zone is 1.3 / 1.2 / 1.4mm; the continuous casting slab thickness is 250mm and it is stacked and slowly cooled offline, and the slow cooling time is 48h; the low-center segregation of the slab is all Class C 0.5, and there are no other defects.

[0034] The heating process of the slab, the heating parameters of Examples 1-3 are: the temperature of the first heating section is 930-950°C, the temperature of the second heating section is 1080-1200°C, the temperature of the third heating section is 1200-1240°C, the temperature of the fourth heating section and the soaking section is 1200-1230°C, and the slab heating time in the furnace is 280 / 295 / 298min.

[0035] The steel plate rolling process, the rolling process parameters of Examples 1-3 are shown in Table 2, and the pass reduction and reduction rate are shown in Table 3.

[0036]

[0037]

[0038] The online quenching process of the steel plate and the water cooling process parameters of Examples 1-3 are shown in Table 4.

[0039]

[0040] During the tempering heat treatment process, the tempering parameters of Examples 1-3 are: tempering temperature 200 / 200 / 220°C, heating coefficient in the furnace 3.5 / 3.8 / 4.0 min / mm, total heating time 70 / 114 / 160 min, and air cooling after tempering.

[0041] The mechanical properties of Examples 1-3 and Comparative Example 1 are shown in Table 5, and the internal quality and appearance of the board are shown in Table 6.

[0042]

[0043] As can be seen from Tables 5 and 6, Examples 1-3 use vanadium nitrogen microalloying, with a yield strength of 1440-1500MPa, a tensile strength of 1580-1630MPa, a 180° cold bending pass, a -50°C longitudinal impact energy of 50-60J, and an austenite grain size of 8.0-8.4. The above description shows that the NM500 grade wear-resistant steel involved in the present invention not only has the characteristics of high strength and high hardness, but also has excellent low-temperature toughness and bending forming performance. In addition, since Comparative Example 1 does not use vanadium nitrogen microalloying, the actual strength is about 56-72MPa lower, the hardness is about 30HB lower, and the -50°C impact energy is less than 45J. This performance is basically consistent with the result that the austenite grain size in Table 6 is only 7.4.

[0044] By the attached Figure 1 It can be seen that for the wear-resistant plate with a thickness of 30 mm in Example 2, the deformed austenite grains are uniform and fine, and the grain size reaches 8.2. The rolling plastic deformation compresses the grains, and elongates and flattens them along the rolling direction, providing a good organizational foundation for subsequent phase transformation. Figure 2 It can be seen that after online quenching and offline tempering, the near-surface structure of the steel plate is mainly tempered martensite. The martensite laths are arranged in parallel and are narrow and slender. At the same time, a small amount of carbides can be seen between the laths, which can provide a certain hardening effect. Figure 3 It can be seen that the 1 / 2 position of the longitudinal section is tempered martensite + a small amount of residual austenite, and the center hardness has reached 509HB after testing. This comprehensively shows that the online quenching cooling intensity has penetrated into the core of the steel plate, with good hardenability and uniform structure, which not only ensures the wear resistance and service life of the steel plate, but also avoids the mixed structure caused by insufficient cooling rate near the center of the thickness, thereby deteriorating the strength and toughness.

[0045] From the above, it can be seen that the 20-40mm thick NM500 grade wear-resistant steel produced according to the present invention has excellent comprehensive performance, which is mainly due to the use of medium carbon low manganese and vanadium nitrogen microalloying composition design to achieve the effect of VN particles in improving strength and maintaining toughness; secondly, it is matched with clean steel smelting and continuous casting process technology to improve the purity of molten steel and the quality of ingots, and the austenite morphology before quenching is controlled by adjusting the rolling system to provide a tissue basis for subsequent phase transformation; finally, the online quenching method is adopted to retain the high-density dislocation defects generated by the controlled rolling plastic deformation in the finishing stage, which are then entangled with the precipitated carbides during tempering to play a dislocation pinning role, and comprehensively achieve the effect of hardening and improving toughness. So far, the present invention has greatly improved production efficiency and saved production costs, and more importantly, improved and regulated the organization and performance of wear-resistant steel, with significant economic benefits and application value.

Claims

1. A method for manufacturing NM500 grade wear-resistant steel with good low-temperature toughness, comprising the following steps: molten steel smelting, slab continuous casting, slab heating, steel plate rolling, online quenching, and tempering heat treatment; characterized in that: The NM500 grade wear-resistant steel with good low-temperature toughness comprises the following components in percentage by mass: C: 0.19-0.22%, Si: 0.25-0.45%, Mn: 1.20-1.35%, P≤0.010%, S≤0.003%, Al: 0.020-0.040%, Nb: 0.015-0.025%, Ti: 0.01-0.02%, V: 0.10-0.13%, Cr: 0.80-0.90%, Mo: 0.17-0.27%, B: 0.0015-0.0025%, N: 100-140ppm, H≤2ppm, O≤20ppm, and the balance is Fe and unavoidable impurities; carbon equivalent CEV range: 0.60-0.70%, CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15; In the method for manufacturing the NM500 grade wear-resistant steel with good low-temperature toughness: (1) Steelmaking steps: The molten iron is first desulfurized and slag-stripped through pretreatment. After slag-stripping, the bright surface of the molten iron is ≥95%, and the S of the molten iron entering the furnace is ≤0.010%. The top and bottom combined blowing converter is used for smelting. During the blowing process, the blowing gun position adopts the low-high-low mode. The bottom blowing of the converter adopts the nitrogen mode for the first 12 minutes of blowing, and the flow rate is ≥200m 3 / h, blowing for 12 minutes to the end of blowing, the converter bottom blowing adopts argon mode, flow rate ≥ 240m 3 / h, the temperature of molten steel in the furnace is 1580℃±10℃ when the bottom blowing argon mode is switched; double slag method is adopted for smelting, slag blocking for steel tapping, slag thickness ≤40mm, metal manganese, ferrosilicon and ferroaluminum are used for deoxidation and alloying during steel tapping, the tapping temperature of the converter is 1620±20℃, and the final C, P and Al contents are strictly controlled as follows: C≥0.07%, P≤0.009%, 0.04%≤Al≤0.07%; LF+RH double process is adopted, LF refined white ash is added at a consumption of 8kg per ton of steel, white slag is made after the first power supply, the white slag retention time is ≥12min, the final slag sample composition is controlled to be Al2O3>22%, the CaO / Al2O3 content ratio is 1.7-2.0, and the ferroboron is controlled to be added 5min after the titanium iron is added before leaving the station in the later stage; nitrogen is used as the lifting gas in RH vacuum, the pressure is 5KPa, and the flow rate is ≥60m 3 / h, high vacuum time ≥12min, ensure N content 100-140ppm after breaking the air; adopt calcification treatment, feed 200-300 meters of pure calcium wire, wire feeding speed 180-210m / min, soft argon blowing time before hanging bag ≥10min, molten steel calming time after soft blowing ≥5min; (2) Slab continuous casting steps: During the casting process, a constant casting speed of 0.8-0.9 m / min is maintained, and the liquid level fluctuation is controlled to be ±3mm, and the superheat is 20-25°C; protective casting is implemented throughout the process, and electromagnetic stirring and dynamic soft pressure are applied. The electromagnetic stirring current is set to 350-420A, and the frequency is 5-8Hz. Dynamic soft pressure increases the pressure reduction in the two-phase region and the solid phase region, and the pressure reduction rate is 1.0-1.3mm / m, the pressure reduction in the two-phase region is 5-9mm, and the pressure reduction in the solid phase region is 0.5-1.5mm; the continuous casting slab is 250mm thick and stacked offline for slow cooling, and the slow cooling time is ≥48h; the internal mass center segregation of the slab is ensured to be ≤C class 0.5 level; (3) Slab heating step: the slab is heated in sections to a uniform temperature of 1200-1250°C by using a walking beam heating furnace; wherein the temperature of the first heating section is 920-950°C, and the slab is heated in the furnace for 250-300 min; the temperature of the second heating section is 1020-1220°C, and the slab is heated in the furnace for 250-300 min; the temperature of the third heating section is 1200-1260°C, and the slab is heated in the furnace for 250-300 min; (4) Steel plate rolling steps: adopt two-stage segmented rolling, the starting temperature of the rough rolling stage is ≥1060℃, and the ending temperature is ≥1000℃, ensuring that the single-pass reduction rate of the last three passes is ≥15%, and the total rough rolling reduction rate is ≥50%; after the rough rolling is completed, the thickness of the intermediate billet is ≥3.0 times the thickness of the finished steel plate. During the intermediate billet waiting for temperature, the water cooling device is turned on for forced cooling, and the upper spray flow rate is 30-50m 3 / h; the starting temperature of the finishing rolling stage is ≤950℃, the final rolling temperature of the last pass is ≥880℃, the single pass reduction rate of the last three passes is guaranteed to be ≥20%, and the total finishing rolling reduction rate is ≥66.6%; after rolling, pre-straightening is performed for 1 pass to flatten the plate shape, and the straightening speed is 0.7-1.2m / s; (5) Online quenching steps: After pre-straightening, the steel plate is quickly cooled in an ultra-fast cooling device, with a water inlet temperature of ≥830°C, a red-return temperature of ≤150°C, and a cooling rate of 22-35°C / s; the steel plate thickness is 20-40mm and the DQ+ACC cooling mode is adopted, with water pressures of ≥0.5MPa and ≥0.2MPa respectively. The DQ frame adopts a slit nozzle, with a total of 4 water groups, and the spray flow rate of each slit group is 240-260m 3 / h, gap water ratio 2.1-2.2; ACC frame uses high-density nozzles, a total of 26 water groups, each high-density group has an upper and middle cavity flow rate of 180-200m 3 / h, high density water ratio 2.45-2.6; ultra-fast cold roller speed setting 0.25-0.30m / s, acceleration setting 0-0.003m / s 2 ; (6) Tempering heat treatment step: an offline low-temperature tempering process is adopted, and the tempering process includes: tempering temperature 200-220°C, heating coefficient in the furnace 3.5-4.0min / mm, and air cooling after tempering.

2. The method for manufacturing a NM500 grade wear-resistant steel with good low-temperature toughness as claimed in claim 1, characterized in that: The NM500 grade wear-resistant steel has good low-temperature toughness, the steel plate austenite grain size is ≥8.0, the total level of non-metallic inclusions is ≤2.0, there are no Class A inclusions, the metallographic structure is tempered martensite, its surface Brinell hardness is ≥500HB, the yield strength is ≥1400MPa, the tensile strength is ≥1500Mpa, the 180°C bending is qualified, the -50°C Charpy V-type longitudinal impact energy is ≥45J, and the steel plate straightness meets the requirement of ≤10mm / 2m.

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