A low-cost, high-thickness, high-performance NM450 steel plate and its manufacturing method

Abstract

The present invention provides a low-cost high-performance NM450 steel plate with large thickness and a manufacturing method thereof. The composition is as follows: C: 0.17 - 0.25%, Si: 0.1 - 0.3%, Mn: 1.0 - 1.25%, P ≤ 0.015%, S ≤ 0.005%, Alt: 0.015 - 0.04%, Cr: 0.5 - 0.65%, Nb: 0.01 - 0.03%, Ti: 0.01 - 0.02%, B: 0.001 - 0.003%, Re: 0.0015 - 0.003%, N: 0.003 - 0.005%, and the rest is Fe and residual elements. Compared with the prior art, through composition design and in combination with the production process in the present invention, the hardness at 1 / 2 of the product thickness is ≥ 440 HB, and the longitudinal impact energy at -40 °C is ≥ 60 J, which is higher than the standard. The maximum thickness of the steel plate can reach 100 mm, and no cutting delay crack will occur.

Description

Technical Field

[0001] The present invention belongs to the field of alloys, and particularly relates to a low-cost large-thickness high-performance NM450 steel plate and a manufacturing method thereof. Background Art

[0002] NM450 steel plates are mainly applied in fields such as mining machinery, loading and transportation equipment, and large-scale excavation equipment. The application environment is harsh, the usage area is extensive, and processing methods such as flame cutting and welding are required. Therefore, NM450 steel plates must have good wear resistance, low-temperature toughness, and crack sensitivity resistance.

[0003] In view of the above performance requirements, especially for large-thickness NM450 steel plates with a thickness of 80 - 100 mm, due to the thickness effect, domestic manufacturers generally add relatively high alloying elements Cr, Mo, and Ni to improve the hardenability of the steel plate and improve the low-temperature toughness. This not only greatly increases the production cost, but also the segregation of alloying elements easily causes stress concentration after quenching, resulting in cracking of the steel plate after flame cutting and welding.

[0004] A patent with the publication number of CN 107099728 A, published on August 29, 2017, a manufacturing method of a thin-specification high-Ti wear-resistant steel NM450, discloses steps including: blast furnace hot metal, hot metal pretreatment, converter smelting, LF furnace refining, RH furnace refining, traditional slab continuous casting, heating furnace, high-pressure water descaling, hot strip mill, ultra-fast cooling, coiling, leveling, heating, quenching, tempering, and finishing; its composition is: C is 0.16 - 0.20 wt%, Si is 0.2 - 0.4 wt%, Mn is 0.8 - 1.5 wt%, Mo is 0.10 - 0.20 wt%, Cr is 0.30 - 0.50 wt%, Nb is 0.02 - 0.05 wt%, Ti is 0.10 - 0.15 wt%, B is 0.0005 - 0.0010 wt%, P < 0.015 wt%, S < 0.010 wt%, and the rest is Fe and inevitable impurities; this patent selects a low-cost high-Ti microalloying technology, with Ti being 0.10 - 0.15 wt%, and the high Ti content results in a high cost; moreover, this technology cannot solve the problem of segregation in the core of the steel plate of this steel grade. Summary of the Invention

[0005] A low-cost, large-thickness, high-performance NM450 steel plate and its manufacturing method provided by the present invention reduce production costs by designing the composition, adding B and RE, and not adding alloys such as Mo and Ni. Combining with the production process of this application, the obtained product reduces the segregation in the core of the steel plate and optimizes the uniformity of the tissue performance. The produced steel plate meets the requirements that the hardness at half of the thickness is ≥ 440 HB, the longitudinal impact energy at -40 °C is ≥ 60 J, and all mechanical properties meet the national standards with high standards and are higher than the standards. The maximum thickness of the steel plate can reach 100 mm, and no cutting delay cracks will occur, meeting the market demand for large-thickness NM450 steel plates with guaranteed core performance.

[0006] The specific technical solution of the present invention is as follows:

[0007] A low-cost, large-thickness, high-performance NM450 steel plate, including the following components by mass percentage:

[0008] C: 0.17 - 0.25%, Si: 0.1 - 0.3%, Mn: 1.0 - 1.25%, P ≤ 0.015%, S ≤ 0.005%, Alt: 0.015 - 0.04%, Cr: 0.5 - 0.65%, Nb: 0.01 - 0.03%, Ti: 0.01 - 0.02%, B: 0.001 - 0.003%, RE: 0.0015 - 0.003%, N: 0.003 - 0.005%, and the rest are Fe and residual elements.

[0009] In the prior art, by increasing the contents of elements Mn, Cr, Mo, Ti, and Ni, the hardenability and low-temperature toughness of the steel plate are improved to ensure that the core hardness is ≥ 420 HB and the longitudinal impact energy at -40 °C is ≥ 60 J. However, elements Mn and Cr are extremely easy to form composition segregation in the center of the continuous casting billet, resulting in a core segregation zone, making the surface and core structures of thick-specification NM450 uneven. This not only deteriorates the impact toughness of the core but also generates significant thermal stress and tissue stress at the processing site during thermal cutting and welding. When the stress exceeds the tensile strength of the steel plate, microcracks will form at the weak positions of the segregation, and after expansion, delayed cracks will be caused. The prices of elements Mo and Ni alloys are high. At the same time, NiS network structures will be generated in the Ni-containing steel billet during the heating process, increasing the viscosity of the scale on the surface of the steel billet and making it difficult to remove subsequently. Anti-oxidation coatings must be applied, increasing the manufacturing cost of the steel plate. The present invention adds 0.0015 - 0.003% of rare earth RE, and uses the property of rare earth elements to purify molten steel to compound and modify the composite inclusions of (Mn, Ca)S into smaller-sized and more numerous RE2O2S and RE x S yInclusions can provide more nucleation sites for the subsequent grain nucleation during the rolling process, which is beneficial to refine grains during rolling, improve the hardenability of the steel plate, and enhance the low-temperature toughness. In addition, rare earth RE forms fine inclusions with elements such as Al, O, and S, which are enriched at the grain boundaries of the microstructure, reducing the grain boundary interface energy and increasing the "energy barrier" for the segregation of other elements at the grain boundaries. This can, to a certain extent, inhibit the segregation of element B at the grain boundaries and more effectively exert the ability of element B to improve hardenability, thereby achieving the effects of reducing the contents of Mn and Cr, not adding alloys such as Mo and Ni, reducing production costs, reducing the center segregation of the steel plate, and optimizing the uniformity of the microstructure properties.

[0010] The thickness of the low-cost large-thickness high-performance NM450 steel plate is 80 - 100 mm;

[0011] The microstructure of the low-cost large-thickness high-performance NM450 steel plate is martensite; the microstructures at 1 / 4 and the center (1 / 2) are both 100% martensite;

[0012] For the low-cost large-thickness high-performance NM450 steel plate, the yield strength of the steel plate is ≥1150 Mpa, the tensile strength is ≥1400 Mpa, the elongation is ≥20%, the longitudinal impact energy at -40°C is ≥60 J, the surface Brinell hardness is ≥450 HB, and the grain size is 9 - 10 grades; at 1 / 2 of the thickness, the yield strength is ≥1000 Mpa, the tensile strength is ≥1300 Mpa, the elongation is ≥20%, the longitudinal impact energy at -40°C is ≥60 J, the Brinell hardness is ≥440 HB, and the grain size is 9 - 10 grades.

[0013] A manufacturing method of a low-cost large-thickness high-performance NM450 steel plate provided by the present invention includes smelting, heating, rolling, post-rolling cooling, and heat treatment processes.

[0014] The smelting includes: converter smelting - LF furnace refining - RH furnace vacuum degassing;

[0015] The smelting includes feeding rare earth wire after breaking the vacuum in the RH furnace vacuum smelting;

[0016] For the smelting, during LF furnace refining, the electric slag melting time is ≥10 min, the temperature is measured and samples are taken, deoxidation is carried out to make white slag, ensuring that FeO + MnO in the white slag ≤1.0%, the holding time of the white slag is ≥10 min to ensure the desulfurization effect and ensure that the S content in the molten steel is less than 0.005%.

[0017] The RH furnace is used for vacuum degassing. The vacuum treatment time is ≥8 min, the vacuum degree is ≤130 Pa, the holding time of the ultimate vacuum is ≥8 min. After 2 min of breaking the vacuum during RH vacuum treatment, seamless calcium wire is fed at a rate of (0.50 - 0.70) kg / ton of steel, and the wire feeding speed is (1.5 - 1.7) m / s. Then, rare earth alloy wire is fed at a rate of (1.0 - 1.2) kg / ton of steel, and the wire feeding speed is (1.3 - 1.5) m / s. After wire feeding, soft blowing is carried out. The argon bottom blowing pressure in the ladle is (0.20 - 0.4) MPa, and the flow rate is (150 - 300) NM 3 / h, and the time is ≥10 min, and it is ensured that [H] ≤ 3 ppm when leaving the station. Feeding the rare earth alloy wire after breaking the vacuum is mainly to modify the complex inclusions of (Mn, Ca)S remaining in the molten steel after desulfurization. Since rare earth elements are extremely active, adding rare earth alloy before breaking the vacuum will quickly react with oxygen in the steel to form rare earth oxides, thus losing the inclusion modification effect. Ensuring [H] ≤ 3 ppm when leaving the vacuum station is mainly to prevent a large number of hydrogen atoms in the steel from attaching to the intergranular voids or defects of the steel plate, combining into hydrogen molecules, and being released in the form of hydrogen after subsequent flame cutting of the steel plate, causing hydrogen-induced delayed cracks in the steel plate.

[0018] The heating is as follows: The hot charging and hot direct charging process is adopted;

[0019] Specifically, the hot direct charging temperature of the billet is 700 - 800 °C, the hot charging temperature is 500 °C - 680 °C, the heating temperature in the preheating section of the billet is 500 - 900 °C, the temperature in the first heating section is 900 - 1150 °C, the temperature in the second heating section is 1200 °C - 1260 °C, the soaking temperature is 1200 - 1250 °C, and the total heating time is ≥360 minutes.

[0020] In the prior art, generally, the billet needs to be coated with anti-oxidation coating before entering the furnace, and the hot charging and hot direct charging process cannot be adopted. The maximum furnace inlet temperature is only 60 °C, which not only increases the heating energy consumption, but also to achieve temperature uniformity of the billet, it is necessary to extend its soaking time in the soaking section, increasing the risk of grain coarsening and affecting the hardenability of the steel plate. The present invention does not add Ni element, and the hot charging and hot direct charging process can be adopted. The maximum furnace inlet temperature can reach 680 °C. When the billet enters the furnace, the overall temperature is uniform, which can greatly shorten the heating time, not only reducing the heating energy consumption and improving the production efficiency, but also reducing the risk of grain growth, providing a finer grain size for subsequent rolling and heat treatment.

[0021] The rolling is as follows: Two-stage controlled rolling process of recrystallization zone rolling + non-recrystallization zone;

[0022] The rolling is specifically as follows: A two-stage controlled rolling process is adopted. In the first stage, hot rolling in the recrystallization zone is carried out, with the starting rolling temperature ≥ 1050°C, the finishing rolling temperature > 950°C, the cumulative reduction ratio 50% - 60%, and the reduction per pass 20 - 30 mm. In the second stage, rolling in the non-recrystallization zone is carried out, with the starting rolling temperature ≤ 880°C, the finishing rolling temperature 830 - 850°C, the cumulative reduction ratio 30% - 40%, and the reduction per pass ≤ 20 mm. The rolling temperature in the first stage is controlled above 950°C. During the rolling process in this temperature range, the austenite grains of the steel plate will undergo dynamic recrystallization, meta-dynamic recrystallization, and static recrystallization respectively as the temperature of the steel plate drops. The original austenite grains nucleate and grow, and the grains are refined to a certain extent and are not easy to grow. The rolling temperature in the second stage is controlled at 830 - 850°C. In this temperature range, the deformation-induced precipitation effect during the rolling process is utilized to promote the precipitation of the second-phase particles of the micro-alloying element Nb at dislocations and grain boundaries, providing more nucleation sites for subsequent relaxation cooling.

[0023] The post-rolling cooling is: relaxation + rapid cooling;

[0024] The post-rolling cooling is specifically as follows: Before the rapid cooling of the steel plate, it stays on the roller table for 100 - 120 s (relaxation time), the roller table swing is turned on, the relaxation temperature is 810 - 830°C. After the relaxation ends, it enters the ultra-rapid cooling equipment for rapid cooling, the starting cooling temperature ≥ 800°C, the speed of the cooling roller table is 0.3 - 0.5 m / s, the water volume in the high-pressure section is 6000 - 7500 m 3 / h, the water ratio is 1.2 - 1.6, the water volume in the low-pressure section is 3000 - 5000 m 3 / h, the water ratio is 1.2 - 1.6, and the red-return temperature is 300 - 350°C.

[0025] In the prior art, due to the addition of a large amount of alloying elements, in order to avoid large internal stresses generated by rapid post-rolling cooling, resulting in cracks during the sizing process before quenching, the post-rolling air-cooling process is adopted, with a slow cooling rate. During the cooling process, the structure will further grow, affecting the hardenability and structure uniformity of the steel plate. In this application, the addition of alloying elements is reduced. In order to refine the grains of the steel plate, improve the structure uniformity and hardenability, a post-rolling relaxation + rapid cooling process is adopted. The steel plate stays on the roller table for 100 - 120 seconds after rolling, enabling the full precipitation of NbC in the steel, providing more nucleation sites for the rapid-cooling phase transformation, and achieving the effect of refining the grains of the steel plate. The rapid cooling mainly increases the supercooling degree of austenite in the post-rolled steel plate, increases the nucleation rate during the phase transformation process, and at the same time prevents the coarsening of the second-phase particles of Nb precipitated during the two-stage rolling and relaxation processes, providing a finer original grain structure for subsequent offline quenching.

[0026] The heat treatment is: quenching in a trinitrotoluene solution + low-temperature tempering.

[0027] Specifically, the adopted aqueous solution of trinitrate includes the following components by mass percentage: sodium nitrate: 15 - 25%, sodium nitrite: 20 - 30%, potassium nitrate: 20 - 25%, and the balance is water; the quenching and heat preservation temperature is 890 - 910°C, and the furnace holding time is 3×H minutes; H represents the thickness of the steel plate in mm. When calculating, just substitute the value before the unit into the formula; the temperature of the quenched steel plate is 20 - 40°C;

[0028] For the low-temperature tempering: the tempering temperature is 180 - 220°C, and the furnace holding time is 1.5×H + 150 minutes. H represents the thickness of the steel plate in mm. When calculating, just substitute the value before the unit into the formula.

[0029] In the prior art, water is generally used as the quenching cooling medium. The cooling rate above 500°C is relatively low, and a large amount of alloy needs to be added to improve the hardenability, resulting in a high cost. The present invention uses an aqueous solution of trinitrate as the quenching cooling medium. The cooling rate above 500°C is twice that of water. Combined with rare earth RE to refine the grain size of the microstructure, reduce the segregation of the strong hardenability element B, improve the hardenability of the steel plate, further reduce the content of precious alloys, reduce the cost, and reduce the segregation.

[0030] Compared with the prior art, through composition design and in combination with the production process, the present invention obtains a steel plate with a more uniform microstructure. The produced steel plate not only meets the national standards but also meets the requirements that the hardness at 1 / 2 of the thickness is ≥440 HB, the longitudinal impact energy at -40°C is ≥60 J. All mechanical properties meet the national standards with high standards and are higher than the standards. The maximum thickness of the steel plate can reach 100 mm, and no cutting delay cracks will occur, meeting the market demand for producing large-thickness NM450 steel plates with excellent core properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Microstructure at the surface of Example 1;

[0032] Figure 2 Microstructure at 1 / 2 of the thickness of Example 1;

[0033] Figure 3 Photo of the cutting end face after cutting of Example 1.

[0034] Figure 4 Microstructure at the surface of Example 2;

[0035] Figure 5 Microstructure at 1 / 2 of the thickness of Example 2;

[0036] Figure 6 Photo of the cutting end face after cutting of Example 2.

[0037] Figure 7 Microstructure at the surface of Example 3;

[0038] Figure 8The tissue at the 1 / 2 thickness of Example 3;

[0039] Figure 9 The photo of the cut end face after cutting in Example 3.

[0040] Figure 10 The tissue at the surface of Comparative Example 1;

[0041] Figure 11 The tissue at the 1 / 2 thickness of Comparative Example 1;

[0042] Figure 12 The photo of the cut end face after cutting in Comparative Example 1.

[0043] Figure 13 The tissue at the surface of Comparative Example 2;

[0044] Figure 14 The tissue at the 1 / 2 thickness of Comparative Example 2;

[0045] Figure 15 The photo of the cut end face after cutting in Comparative Example 2. Detailed implementation method

[0046] Example 1

[0047] A low-cost large-thickness high-performance NM450 steel plate with a steel plate thickness H of 80 mm. The chemical composition and its mass percentage of the steel plate are as follows: C: 0.20%, Si: 0.26%, Mn: 1.1%, P: 0.009%, S: 0.004%, Alt: 0.03%, Cr: 0.6%, Nb: 0.024%, Ti: 0.018%, B: 0.0023%, RE: 0.0027%, N: 0.004%, and the rest are Fe and residual elements.

[0048] The manufacturing method of the large-thickness rare-earth NM450 steel plate in this example includes smelting, heating, rolling, post-rolling cooling, and heat treatment processes. The specific processes and parameters are as follows:

[0049] Smelting process: 100 tons of molten steel is smelted in a converter, and converter smelting - LF furnace refining - RH furnace vacuum degassing is adopted. The LF furnace is electrified to slag for 15 min, deoxidized to make white slag, and the white slag holding time is 12 min. The FeO + MnO in the slag ≤ 1.0% to ensure that the S content in the molten steel is less than 0.005%; the vacuum treatment time of the RH furnace is 10 min, the vacuum degree is 125 Pa, the ultimate vacuum holding time is 8 min, and after breaking the vacuum for 2 min, 60 Kg of seamless calcium wire is fed at a wire feeding speed of 1.6 m / s, and then 112 kg of rare-earth alloy wire is fed at a wire feeding speed of 1.5 m / s. After wire feeding, soft blowing is carried out, and the argon bottom blowing pressure in the ladle is (0.2 - 0.4) MPa, and the flow rate is (150 - 300) NM 3 / h, Soft blowing time is 15 min, and [H] measured at the outlet is 3 ppm.

[0050] Heating process: The hot charging and hot sending process is adopted. The hot sending temperature of the steel billet is 780 °C, the hot charging temperature is 670 °C, the heating temperature of the preheating section of the steel billet is 500 - 900 °C, the temperature of the first heating section is 900 - 1150 °C, the temperature of the second heating section is 1200 °C - 1260 °C, the soaking section temperature is 1200 - 1250 °C, and the total heating time is 370 minutes.

[0051] Rolling process: In the first stage of rolling, the starting rolling temperature is 1055 °C, the final rolling temperature is 955 °C, the cumulative reduction ratio is 55%, and the reduction per pass is 20 - 30 mm. In the second stage of rolling, the starting rolling temperature is 872 °C, the final rolling temperature is 836 °C, the cumulative reduction ratio is 35%, and the reduction per pass is ≤20 mm.

[0052] Post-rolling cooling process: The steel plate stays on the roller table for 120 s (relaxation time) before entering the ultra-fast cooling device. During the relaxation process, the temperature of the steel plate is controlled at 810 - 830 °C. The roller table swing is started during the stay, with a swing speed of 0.05 m / s. After the swing is completed, rapid cooling is carried out. The starting cooling temperature is 802 °C. The total water volume in the high-pressure section of the ultra-fast cooling is 7000 m 3 / h, water ratio: 1.5, roller speed 0.5 m / s, the total water volume in the low-pressure section is 4500 m 3 / h, water ratio 1.4, roller speed 0.5 m / s, and the return red temperature of the steel plate after cooling is 340 °C.

[0053] Heat treatment process: Adopt quenching with trinitrotoluene aqueous solution + low-temperature tempering. The mass fraction ratio of the trinitrotoluene aqueous solution is: sodium nitrate: 20%, sodium nitrite: 25%, potassium nitrate: 20%, and the rest is water. The quenching holding temperature is 900 °C ± 10 °C, the furnace storage time is 240 minutes, the temperature of the steel plate after quenching is 35 °C, and the low-temperature tempering process: the tempering temperature is 200 °C ± 20 °C, and the furnace storage time is 270 minutes (H represents the thickness of the steel plate, in mm).

[0054] For the microstructure and cutting end face of the steel plate in this embodiment, see Figures 1 - 3 , the microstructure at 1 / 4 and the center (1 / 2) of the steel plate is 100% martensite, the grain size of the steel plate is 9.5 - 10 grades, the structure is fine and uniform, the mechanical properties of the steel plate are shown in Table 1, and the hardness properties are shown in Table 2. Since the maximum thickness of NM450 in the national standard GB / T24186 - 2009 is 80 mm, the performance detection method for the steel plate of the present invention for manufacturing NM450 with a thickness of 80 - 100 mm refers to the NM450 steel plate standard in GB / T24186 - 2009, and the performance test at 1 / 2 of the thickness is carried out with reference to this standard.

[0055]

[0056]

[0057] From the performance test results in Table 1 and Table 2, it can be seen that the mechanical properties at the 1 / 4 thickness and 1 / 2 thickness of the 80mm NM450 steel plate are good, fully meeting the requirements of the corresponding standards, and having good strength and toughness matching. At the same time, the surface and core Brinell hardness also exceed the standard requirements, possessing good wear resistance.

[0058] Example 2

[0059] A low-cost, large-thickness, high-performance NM450 steel plate with a steel plate thickness H of 100mm. The chemical composition and its mass percentage of the steel plate are as follows: C: 0.23%, Si: 0.27%, Mn: 1.15%, P: 0.005%, S: 0.004%, Alt: 0.03%, Cr: 0.63%, Nb: 0.027%, Ti: 0.017%, B: 0.002%, RE: 0.003%, N: 0.004%, and the rest are Fe and residual elements.

[0060] The production method of the low-cost, large-thickness, high-performance NM450 steel plate in this example includes smelting, heating, rolling, post-rolling cooling, and heat treatment processes. The specific processes and parameters are as follows:

[0061] Smelting process: 100 tons of molten steel are smelted in a converter, and converter smelting - LF furnace refining - RH furnace vacuum degassing are adopted. The LF furnace is electrified for slag melting for 16 minutes, deoxidized to make white slag, and the white slag holding time is 15 minutes; ensure that FeO + MnO in the white slag ≤ 1.0%, and ensure that the S content in the molten steel is less than 0.005%; the RH furnace vacuum treatment time is 12 minutes, the vacuum degree is 126 Pa, the ultimate vacuum holding time is 8 minutes, 65 kg of seamless calcium wire is fed 2 minutes after breaking the vacuum, the wire feeding speed is 1.6 m / s, 114 kg of rare earth alloy wire is fed after feeding, the wire feeding speed is 1.5 m / s, soft blowing is carried out after feeding, the bottom blowing argon pressure of the ladle is (0.20 - 0.4) MPa, the flow rate is (150 - 300) NM 3 / h, the soft blowing time is 14 minutes, and [H] is measured to be 2 ppm when leaving the station.

[0062] Heating process: The hot charging temperature of the billet is 750°C, the hot charging temperature is 620°C, the heating temperature of the billet preheating section is 500 - 900°C, the first heating section temperature is 900 - 1150°C, the second heating section temperature is 1200°C - 1260°C, the soaking section temperature is 1200 - 1250°C, and the total heating time is 370 minutes.

[0063] Rolling process: In the first stage of rolling, the starting rolling temperature is 1060°C, the finishing rolling temperature is 952°C, the cumulative reduction ratio is 57%, and the reduction per pass is 20 - 30 mm. In the second stage of rolling, the starting rolling temperature is 868°C, the finishing rolling temperature is 845°C, the cumulative reduction ratio is 33%, and the reduction per pass is ≤20 mm.

[0064] Post-rolling cooling process: The steel plate stays on the roller table for 115 s before entering the ultra-fast cooling device. During the relaxation process, the temperature of the steel plate is controlled at 810 - 830°C. The roller table swing is started during the stay, with a swing speed of 0.05 m / s. After the swing is completed, rapid cooling is carried out. The starting cooling temperature is 815°C. The total water volume in the high-pressure section of the ultra-fast cooling is 7500 m 3 / h, the water ratio is 1.4, the roller speed is 0.5 m / s. The total water volume in the low-pressure section is 5000 m 3 / h, the water ratio is 1.4, the roller speed is 0.5 m / s. The return red temperature of the steel plate after cooling is 321°C.

[0065] Heat treatment process: Quenching with a trinitrotoluene solution + low-temperature tempering is adopted. The mass fraction ratio of the trinitrotoluene solution is: sodium nitrate: 25%, sodium nitrite: 25%, potassium nitrate: 25%, and the rest is water. The quenching and holding temperature is 900°C ± 10°C, and the furnace holding time is 300 minutes. The low-temperature tempering process: The tempering temperature is 200°C ± 20°C, and the furnace holding time is 300 minutes (H represents the thickness of the steel plate, in mm).

[0066] For the steel plate in this embodiment, the microstructures and cutting end faces are shown in Figures 4 - 6 , and the structures at 1 / 4 and the center (1 / 2) are both 100% martensite. The grain size of the steel plate is 9.5 - 10 grades, and the structure is fine and uniform. The mechanical properties of the steel plate are shown in Table 3, and the hardness properties are shown in Table 4.

[0067]

[0068]

[0069] It can be seen from the performance test results in Table 3 and Table 4 that the mechanical properties at 1 / 4 and 1 / 2 of the thickness of the 100 mm NM450 steel plate are good, fully meeting the requirements of the corresponding standards, and having good strength and toughness matching. At the same time, the surface and core Brinell hardness also exceed the standard requirements, and it has good wear resistance.

[0070] Example 3

[0071] A low-cost, high-thickness and high-performance NM450 steel plate with a steel plate thickness H of 90 mm. The chemical composition and its mass percentage of the steel plate are as follows: C: 0.17%, Si: 0.18%, Mn: 1.06%, P: 0.005%, S: 0.004%, Alt: 0.017%, Cr: 0.52%, Nb: 0.020%, Ti: 0.014%, B: 0.0013%, RE: 0.0017%, N: 0.003%, and the rest are Fe and residual elements.

[0072] The production method of the low-cost, high-thickness and high-performance NM450 steel plate in this embodiment includes smelting, heating, rolling, post-rolling cooling and heat treatment processes. The specific processes and parameters are as follows:

[0073] Smelting process: 100 tons of molten steel is smelted in a converter, and converter smelting - LF furnace refining - RH furnace vacuum degassing is adopted. The LF furnace is electrified to slag for 12 minutes, deoxidized to make white slag, and the white slag holding time is 11 minutes to ensure that FeO + MnO in the white slag ≤ 1.0% and the S content in the molten steel is less than 0.005%; the vacuum treatment time of the RH furnace is 8 minutes, the vacuum degree is 130 Pa, the ultimate vacuum holding time is 8 minutes, and 2 minutes after breaking the vacuum, 63 Kg of seamless calcium wire is fed at a wire feeding speed of 1.6 m / s. After feeding, 110 kg of rare earth alloy wire is fed at a wire feeding speed of 1.5 m / s. After wire feeding, soft blowing is carried out. The argon bottom blowing pressure in the ladle is (0.20 - 0.4) MPa, and the flow rate is (150 - 300) NM 3 / h, and the soft blowing time is 10 minutes. Measure [H] at the time of tapping: 3 ppm.

[0074] Heating process: The hot charging temperature of the billet is 710 °C, the hot charging temperature is 520 °C, the heating temperature of the billet preheating section is 500 - 900 °C, the first heating section temperature is 900 - 1150 °C, the second heating section temperature is 1200 °C - 1260 °C, the soaking section temperature is 1200 - 1250 °C, and the total heating time is 385 minutes.

[0075] Rolling process: The starting rolling temperature in the first stage of rolling is 1070 °C, the final rolling temperature is 961 °C, the cumulative reduction rate is 59%, and the single-pass reduction is 20 - 30 mm. The starting rolling temperature in the second stage of rolling is 875 °C, the final rolling temperature is 842 °C, and the cumulative reduction rate is 32%, and the single-pass reduction ≤ 20 mm.

[0076] Post-rolling cooling process: The steel plate stays on the roller table for 113 s before entering the ultra-fast cooling device. During the relaxation process, the temperature of the steel plate is controlled at 810 - 830 °C. The roller table swing is turned on during the stay, and the swing speed is 0.05 m / s. After the swing is completed, rapid cooling is carried out. The starting cooling temperature is 811 °C. The total water volume in the high-pressure section of the ultra-fast cooling is: 6500 m 3 / h, the water ratio: 1.4, the roller speed: 0.5 m / s, and the total water volume in the low-pressure section is 4500 m3 / h, water ratio 1.3, roll speed 0.5 m / s, and the temperature of the steel plate after cooling and turning red is 316 °C.

[0077] Heat treatment process: Quenching with a trinitrotoluene aqueous solution + low-temperature tempering. Mass fraction ratio of the trinitrotoluene aqueous solution: Sodium nitrate: 25%, Sodium nitrite: 25%, Potassium nitrate: 25%, and the rest is water. Quenching holding temperature: 900 °C ± 10 °C, furnace holding time 270 minutes. Low-temperature tempering process: Tempering temperature: 200 °C ± 20 °C, furnace holding time: 285 minutes (H represents the thickness of the steel plate, in mm).

[0078] For the steel plate in this example, the microscopic structure and the cutting end face are shown in Figures 7 - 9 , the structures at 1 / 4 and the core (1 / 2) are both 100% martensite. The grain size of the steel plate is 9.5 - 10 grades, the structure is fine and uniform. The mechanical properties of the steel plate are shown in Table 5, and the hardness properties are shown in Table 6.

[0079]

[0080]

[0081] It can be seen from the performance test results in Table 5 and Table 6 that the mechanical properties at 1 / 4 and 1 / 2 of the thickness of the 90 mm NM450 steel plate are good, fully meeting the requirements of the corresponding standards, and having good strength and toughness matching. At the same time, the Brinell hardness on the surface and at the core also exceeds the standard requirements, with good wear resistance.

[0082] It can be seen from the microscopic structure diagrams of the steel plates and the photos of the cutting ends after flame cutting in Example 1, Example 2, and Example 3 that the structures on the surface and at the core of the steel plates are martensite, the grain size of the structure is relatively fine and uniform, ensuring the uniformity of the tissue properties in the thickness direction of the steel plate. At the same time, no delayed cracks are generated on the cutting end face after cutting.

[0083] Comparative Example 1

[0084] An NM450 steel plate with a thickness of 80 mm. The chemical composition and its mass percentage content of the steel plate are: C: 0.20%, Si: 0.15%, Mn: 1.21%, P: 0.004%, S: 0.005%, Alt: 0.021%, Cr: 0.62%, Nb: 0.025%, Ti: 0.016%, B: 0.0014%, RE: 0.0022%, N: 0.005%, and the rest is Fe and residual elements.

[0085] The production method of this comparative example includes smelting, heating, rolling, post-rolling cooling, and heat treatment processes. The specific processes and parameters are as follows:

[0086] Smelting process: 100 tons of molten steel are smelted in a converter, using converter smelting - LF furnace refining - RH furnace vacuum degassing. The LF furnace is electrified to slag for 13 minutes, deoxidized to make white slag, and the white slag holding time is 14 minutes; the RH furnace vacuum treatment time is 9 minutes, the vacuum degree is 128 Pa, the ultimate vacuum holding time is 10 minutes, and 2 minutes after breaking the vacuum, 59 kg of seamless calcium wire is fed at a wire feeding speed of 1.5 m / s, 113 kg of rare earth alloy wire is fed at a wire feeding speed of 1.5 m / s. After wire feeding, soft blowing is carried out, and the argon bottom blowing pressure in the ladle is (0.20 - 0.4) MPa, and the flow rate is (150 - 300) NM 3 / h, the soft blowing time is 12 minutes, and [H] is measured to be 3 ppm when leaving the station.

[0087] Heating process: The hot charging temperature of the steel billet is 770 °C, the hot charging temperature is 635 °C, the heating temperature in the preheating section of the steel billet is 500 - 900 °C, the temperature in the first heating section is 900 - 1150 °C, the temperature in the second heating section is 1200 °C - 1260 °C, the soaking temperature is 1200 - 1250 °C, and the total heating time is 377 minutes.

[0088] Rolling process: The starting rolling temperature in the first stage of rolling is 1055 °C, the final rolling temperature is 957 °C, the cumulative reduction ratio is 55%, and the reduction per pass is 20 - 30 mm. The starting rolling temperature in the second stage of rolling is 872 °C, the final rolling temperature is 833 °C, the cumulative reduction ratio is 35%, and the reduction per pass is ≤20 mm.

[0089] Post-rolling cooling process: The steel plate directly enters the ultra-fast cooling device for rapid cooling after rolling, The starting cooling temperature is 811 °C, the total water volume in the ultra-fast cooling high-pressure section is 6600 m 3 / h, the water ratio is 1.4, the roll speed is 0.5 m / s, and the total water volume in the low-pressure section is 4400 m 3 / h, the water ratio is 1.4, the roll speed is 0.5 m / s, and the return red temperature of the steel plate after cooling is 306 °C.

[0090] Heat treatment process: Water medium quenching is adopted + low-temperature tempering, the quenching holding temperature: 900 °C ± 10 °C, the furnace storage time is 270 minutes, and the low-temperature tempering process: the tempering temperature: 200 °C ± 20 °C, the furnace storage time: 285 minutes (H represents the thickness of the steel plate, mm).

[0091] The microscopic structure and cutting end face of the steel plate in this comparative example are shown in Figures 10 - 12 , the surface structure of the steel plate is martensite, the grain size is 10 grades, the structure at 1 / 2 of the thickness is martensite + bainite, the grain size is 7.5 - 8 grades, there are differences in both the structure and the grain size, the mechanical properties of the steel plate are shown in Table 7, and the hardness properties are shown in Table 8.

[0092]

[0093]

[0094] From the performance test results in Table 7 and Table 8, it can be seen that there are significant differences in various mechanical properties and Brinell hardness at the 1 / 4 thickness and 1 / 2 thickness of the 90mm NM450 steel plate in Comparative Example 1, which added rare earth elements but did not adopt relaxation cooling and quenching in trinitrotoluene aqueous solution. The properties at the 1 / 2 thickness cannot meet the standard requirements.

[0095] Comparative Example 2

[0096] NM450 steel plate, the thickness of the steel plate is 80mm, and the chemical composition and its mass percentage content of the steel plate are as follows: C: 0.20%, Si: 0.22%, Mn: 1.25%, P: 0.008%, S: 0.005%, Alt: 0.028%, Cr: 0.61%, Nb: 0.023%, Ti: 0.018%, B: 0.0015%, N: 0.005%, and the rest are Fe and residual elements (without adding rare earth) .

[0097] The production method of this comparative example includes smelting, heating, rolling, post-rolling cooling and heat treatment processes. The specific processes and parameters are as follows:

[0098] Smelting process: 100 tons of molten steel is smelted in a converter, and converter smelting - LF furnace refining - RH furnace vacuum degassing is adopted. The LF furnace is electrified for slag melting for 15 minutes, deoxidized to make white slag, and the white slag holding time is 16 minutes; the RH furnace vacuum treatment time is 10 minutes, the vacuum degree is 128 Pa, the ultimate vacuum holding time is 11 minutes, and after 2 minutes after breaking the vacuum, 55 kg of seamless calcium wire is fed at a wire feeding speed of 1.5 m / s. After wire feeding, soft blowing is carried out, and the argon bottom blowing pressure in the ladle is (0.20 - 0.4) MPa, and the flow rate is (150 - 300) NM 3 / h, the soft blowing time is 13 minutes, and [H] is measured to be 3 ppm when leaving the station.

[0099] Heating process: The hot charging temperature of the steel billet is 750 °C, the hot charging temperature is 586 °C, the heating temperature of the preheating section of the steel billet is 500 - 900 °C, the temperature of the first heating section is 900 - 1150 °C, the temperature of the second heating section is 1200 °C - 1260 °C, the soaking section temperature is 1200 - 1250 °C, and the total heating time is 383 minutes.

[0100] Rolling process: The rolling start temperature in the first stage is 1053 °C, the final rolling temperature is 961 °C, the cumulative reduction ratio is 53%, and the single-pass reduction is 20 - 30 mm. The rolling start temperature in the second stage is 873 °C, the final rolling temperature is 835 °C, the cumulative reduction ratio is 35%, and the single-pass reduction is ≤20 mm.

[0101] Post-rolling cooling process: The steel plate stays on the roller table for 105 s before entering the ultra-fast cooling device. During the relaxation process, the temperature of the steel plate is controlled at 810 - 830 °C. The roller table oscillation is started during the stay, with an oscillation speed of 0.05 m / s. After the oscillation is completed, rapid cooling is carried out. The starting cooling temperature is 813 °C. The total water volume in the high-pressure section of the ultra-fast cooling is 6550 m 3 / h, water ratio: 1.4, roller speed 0.5 m / s. The total water volume in the low-pressure section is 4400 m 3 / h, water ratio 1.3, roller speed 0.5 m / s. The return red temperature of the steel plate after cooling is 314 °C.

[0102] Heat treatment process: Quenching with a trinitro aqueous solution + low-temperature tempering is adopted. The mass fraction ratio of the trinitro aqueous solution is: sodium nitrate: 25%, sodium nitrite: 25%, potassium nitrate: 25%, and the rest is water. The quenching holding temperature is: 900 °C ± 10 °C, and the furnace holding time is 240 minutes. The low-temperature tempering process: tempering temperature: 200 °C ± 20 °C, furnace holding time: 270 minutes (H represents the thickness of the steel plate, mm).

[0103] The microstructure and cutting end face of the steel plate in this example are shown in Figures 13 - 15 , The surface structure of the steel plate is martensite, the grain size is 9.5 grades, the structure at 1 / 2 of the thickness is martensite + bainite, and the grain size is 7.5 - 8.5 grades. There are differences in both the structure and the grain size. The mechanical properties of the steel plate are shown in Table 9, and the hardness properties are shown in Table 10.

[0104]

[0105]

[0106] It can be seen from the performance test results in Table 9 and Table 10 that there are significant differences in various mechanical properties and Brinell hardness at 1 / 4 and 1 / 2 of the thickness of the 80 mm NM450 steel plate without rare earth elements, and the performance at 1 / 2 of the thickness cannot meet the standard requirements.

[0107] The underlined data above do not meet the requirements of the present invention. It can be seen from the microstructure diagrams of the steel plates in Comparative Example 1 and Comparative Example 2 and the photos of the cutting ends after flame cutting that the surface structure of the steel plate is martensite, but there is a certain amount of bainite and composition segregation in the core structure. The non-uniform structure in the thickness direction of the steel plate causes performance differences. At the same time, it is easy to generate tissue stress and thermal stress concentration after flame cutting, resulting in a significant increase in the risk of delayed cracking during cutting.

Claims

1. A low-cost high-performance NM450 steel plate with large thickness, characterized in that, The low-cost high-thickness high-performance NM450 steel plate comprises the following components by mass percentage: C: 0.17 - 0.25%, Si: 0.1 - 0.3%, Mn: 1.0 - 1.25%, P ≤ 0.015%, S ≤ 0.005%, Alt: 0.015 - 0.04%, Cr: 0.5 - 0.65%, Nb: 0.01 - 0.03%, Ti: 0.01 - 0.02%, B: 0.001 - 0.003%, RE: 0.0015 - 0.003%, N: 0.003 - 0.005%, and the balance is Fe and residual elements; The thickness of the low-cost high-thickness high-performance NM450 steel plate is 80 - 100 mm; The structure of the low-cost high-thickness high-performance NM450 steel plate is martensite; the core structures at 1 / 4 and 1 / 2 positions are both 100% martensite; For the low-cost high-thickness high-performance NM450 steel plate, the yield strength of the steel plate ≥ 1150 Mpa, the tensile strength ≥ 1400 Mpa, the elongation ≥ 20%, the longitudinal impact energy at -40 °C ≥ 60 J, the surface Brinell hardness ≥ 450 HB, and the grain size is 9 - 10 grades; at 1 / 2 of the thickness, the yield strength ≥ 1000 Mpa, the tensile strength ≥ 1300 Mpa, the elongation ≥ 20%, the longitudinal impact energy at -40 °C ≥ 60 J, the Brinell hardness ≥ 440 HB, and the grain size is 9 - 10 grades.

2. The manufacturing method of the low-cost high-thickness high-performance NM450 steel plate according to claim 1, comprising smelting, heating, rolling, post-rolling cooling, and heat treatment processes.

3. The manufacturing method according to claim 2, characterized in that, The smelting includes LF furnace refining, specifically: the LF furnace refining has an energized slag melting time ≥ 10 min, temperature measurement and sampling, deoxidation to make white slag, ensuring that FeO + Mn0 in the white slag ≤ 1.0%, the white slag holding time ≥ 10 min, and ensuring that the S content in the molten steel is less than 0.005%.

4. The manufacturing method according to claim 2, characterized in that, The smelting includes RH furnace vacuum degassing, specifically: the vacuum treatment time is ≥8 min, the vacuum degree is ≤130 Pa, the ultimate vacuum holding time is ≥8 min. After 2 min of breaking the vacuum during RH vacuum treatment, seamless calcium wire is fed at 0.50 - 0.70 kg per ton of steel, and the wire feeding speed is 1.5 - 1.7 m / s. Then rare earth alloy wire is fed at 1.0 - 1.2 kg per ton of steel, and the wire feeding speed is 1.3 - 1.5 m / s. After wire feeding, soft blowing is carried out, the argon bottom blowing pressure in the ladle is 0.20 - 0.4 MPa, the flow rate is 150 - 300 NM 3 / h, the time is ≥10 min, and it is ensured that [H] ≤ 3 ppm when leaving the station.

5. The manufacturing method according to claim 2, characterized in that, The heating is as follows: adopting a hot charging and hot sending process, the hot sending temperature of the billet: 700 - 800 °C, the hot charging temperature 500 °C - 680 °C, the heating temperature of the billet preheating section: 500 - 900 °C, the temperature of the first heating section: 900 - 1150 °C, the temperature of the second heating section 1200 °C - 1260 °C, the soaking section temperature 1200 - 1250 °C, and the total heating time ≥ 360 minutes.

6. The manufacturing method according to claim 2, characterized in that The rolling includes rolling in the high-temperature recrystallization zone, the rolling start temperature ≥ 1050 °C, the finishing rolling temperature > 950 °C, the cumulative reduction ratio 50% - 60%, and the single-pass reduction 20 - 30 mm; then adopting rolling in the non-recrystallization zone, the rolling start temperature ≤ 880 °C, the finishing rolling temperature 830 - 850 °C, the cumulative reduction ratio 30% - 40%, and the single-pass reduction ≤ 20 mm.

7. The manufacturing method according to claim 2, characterized in that, The post-rolling cooling is: relaxation + rapid cooling; specifically, the post-rolling cooling is as follows: the steel plate stays on the roller table for 100 - 120 s before rapid cooling, the roller table oscillation is started, the relaxation temperature is 810 - 830 °C, after the relaxation is completed, it enters the ultra-rapid cooling equipment for rapid cooling, the starting cooling temperature is ≥ 800 °C, the speed of the cooling roller table is 0.3 - 0.5 m / s, the water volume in the high-pressure section is 6000 - 7500 m 3 / h, the water ratio is 1.2 - 1.6, the water volume in the low-pressure section is 3000 - 5000 m 3 / h, the water ratio is 1.2 - 1.6, and the re-heating temperature is 300 - 350 °C.

8. The manufacturing method according to claim 2, characterized in that The heat treatment is: quenching with a trinitro aqueous solution + low-temperature tempering; the trinitro aqueous solution used includes the following components by mass percentage: sodium nitrate: 15 - 25%, sodium nitrite: 20 - 30%, potassium nitrate: 20 - 25%, and the balance is water; the quenching and holding temperature: 890 - 910 °C, and the furnace storage time is 3 × H minutes; H represents the thickness of the steel plate, in mm; the temperature of the quenched steel plate is 20 - 40 °C; for the low-temperature tempering: tempering temperature: 180 - 220 °C, furnace holding time: 1.5×H + 150 minutes.

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