S355NL steel and preparation method thereof

Abstract

The invention provides S355NL steel and a preparation method thereof, and relates to the technical field of metallurgical materials, and the S355NL steel comprises the following chemical components in percentage by mass: 0.11-0.15% of C, 0.15-0.25% of Si, 1.27-1.40% of Mn, less than or equal to 0.013% of P, less than or equal to 0.005% of S, 0.03-0.04% of Nb, 0.02-0.04% of V, 0.02-0.05% of Al, less than or equal to 0.009% of Ti, less than or equal to 0.06% of Cr, less than or equal to 0.3% of Ni, 0.01-0.03% of Mo, less than or equal to 0.2% of Cu, less than or equal to 0.015% of N, 0.0011-0.02% of rare earth RE, less than or equal to 10ppm of O and the balance of Fe and inevitable impurities A trace amount of high-purity rare earth is added into traditional S355NL steel, original large-size and irregularly-shaped inclusions are modified into small-size and spherical rare earth oxide and rare earth sulfide inclusions, meanwhile, the lamellar spacing of pearlite is uniformly refined, the deformation stress concentration is reduced through the synergistic effect of the rare earth oxide and the rare earth sulfide, generation of a crack source is restrained, and the performance of the S355NL steel is improved. And the low-temperature impact performance of the S355NL steel is greatly improved.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgical materials, and particularly relates to S355NL steel and a preparation method thereof. Background Art

[0002] European standard S355NL low alloy high strength steel is widely used in the manufacture of wind turbine flanges, bridge construction, ships and other large structural parts. As one of the main raw materials for wind turbine flanges, S355NL steel needs to have high strength and excellent low temperature toughness under complex alternating loads and low temperature and high humidity environments to ensure long-term operational reliability. However, in the prior art, the impact energy of S355NL steel under low temperature conditions often shows obvious volatility, and even the impact energy is unqualified.

[0003] Composite microalloying methods such as Nb, V, and Ti are usually used to improve the strength and toughness of S355NL steel by refining grains and precipitation strengthening, but this method still has problems with inclusions and mixed crystals. Large-sized and irregularly shaped oxide and sulfide inclusions in molten steel often become stress concentration points, leading to the generation of crack sources and reducing low-temperature impact toughness. In addition, the mixed crystal phenomenon that is prone to occur during rolling will lead to uneven stress distribution inside the material, further weakening its low-temperature toughness. Although adjusting the content of conventional elements such as C, Si, and Mn on the basis of microalloying can improve some mechanical properties, the effect on improving low-temperature toughness is limited, and it is difficult to solve the problem of unstable performance of steel at extremely low temperatures. Summary of the invention

[0004] Therefore, the present invention provides a S355NL steel and a preparation method thereof, the main purpose of which is to improve the low-temperature impact toughness of the S355NL steel.

[0005] In order to solve the above problems, the present invention provides an S355NL steel. The chemical composition of the S355NL steel includes, by mass percentage, C: 0.11-0.15%, Si: 0.15-0.25%, Mn: 1.27-1.40%, P≤0.013%, S≤0.005%, Nb: 0.03-0.04%, V: 0.02-0.04%, Al: 0.02-0.05%, Ti≤0.009%, Cr≤0.06%, Ni≤0.3%, Mo: 0.01-0.03%, Cu≤0.2%, N≤0.015%, rare earth RE: 0.0011-0.02%, O≤10ppm, and the balance is Fe and unavoidable impurities.

[0006] Further, the S355NL steel has an equiaxed structure, in which grain boundaries with a mismatch angle of not less than 15 degrees account for more than 86% of the grain boundaries; and / or

[0007] The inclusions in the S355NL steel are spherical or nearly spherical; the average size of the inclusions is less than 3 μm; the inclusions include rare earth oxides and rare earth sulfides; and / or

[0008] The microstructure of the S355NL steel includes ferrite and pearlite; the interlamellar spacing of the pearlite is less than 170 nm.

[0009] Furthermore, the rare earth RE is cerium Ce or yttrium Y; and / or

[0010] The impact energy of the S355NL steel at -50°C is ≥269.67 J; the tensile strength of the S355NL steel is ≥512 MPa, the yield strength is ≥390 MPa, the elongation after fracture is ≥35.5%, and the section shrinkage is ≥80%.

[0011] In another aspect, the present invention provides a method for preparing S355NL steel, comprising the following steps:

[0012] Smelting: preparing materials according to the chemical composition of the S355NL steel, performing smelting treatment, and obtaining a steel ingot after casting;

[0013] Annealing: performing homogenization annealing treatment on the steel ingot;

[0014] Forging: forging the steel ingot after the homogenization annealing treatment to obtain a forged material;

[0015] Heat treatment: The forged material is heat treated to obtain the S355NL steel.

[0016] Furthermore, in the smelting step: a medium frequency vacuum induction furnace is used for smelting treatment;

[0017] Preferably, the vacuum degree of the medium frequency vacuum induction furnace is 1.5-2Pa;

[0018] Preferably, the pressure of the medium frequency vacuum induction furnace is 8-9 MPa;

[0019] Preferably, the smelting treatment includes smelting treatment and refining treatment; the time of the smelting treatment is 15-20 minutes; the temperature of the smelting treatment is 1500-1580°C; the temperature of the refining treatment is 1550-1590°C; and the time of the refining treatment is 5-6 minutes.

[0020] Furthermore, in the smelting step: molten steel is obtained after smelting treatment, and when the temperature of the molten steel is 1550-1570°C, casting is performed to obtain a steel ingot.

[0021] Furthermore, in the annealing step: the temperature of the homogenization annealing treatment is 1200±10°C, and the time is 10-10.5h.

[0022] Furthermore, in the forging step: the temperature of the forging treatment is 1200±10°C.

[0023] Furthermore, an air hammer is used to forge the steel ingot after the homogenization annealing treatment into a square ingot.

[0024] Furthermore, the heat treatment temperature is 910±10°C, and the heat treatment time is 25-30 minutes.

[0025] The S355NL steel and the preparation method thereof provided by the present invention have the following beneficial effects:

[0026] 1. On the one hand, the present invention provides an S355NL steel, whose chemical composition, in terms of mass percentage, includes: C: 0.11-0.15%, Si: 0.15-0.25%, Mn: 1.27-1.40%, P≤0.013%, S≤0.005%, Nb: 0.03-0.04%, V: 0.02-0.04%, Al: 0.02-0.05%, Ti≤0.009%, Cr≤0.06%, Ni≤0.3%, Mo : 0.01-0.03%, Cu≤0.2%, N≤0.015%, rare earth RE: 0.0011-0.02%, O≤10ppm, the balance is Fe and inevitable impurities; the present invention adds rare earths into traditional S355NL steel, and the original large-sized and irregularly shaped inclusions are modified into small-sized and spherical rare earth oxide and rare earth sulfide inclusions. The modified inclusions reduce stress concentration, reduce the generation of crack sources, and improve the low-temperature impact performance of the steel.

[0027] 2. Furthermore, the present invention increases the number of high-angle grain boundaries (grain boundaries with a mismatch angle of not less than 15 degrees) in S355NL steel by adding rare earths. The increase in high-angle grain boundaries makes the movement of dislocations more obviously hindered. When S355NL steel is subjected to external force, the increase in high-angle grain boundaries makes it difficult for dislocations to easily pass through the grain boundaries, and a greater external force is required to move the dislocations, thereby improving the strength of S355NL steel. At the same time, when impacted, the increase in high-angle grain boundaries, when cracks encounter high-angle grain boundaries, the direction of expansion changes, and more energy is required to continue to expand, which enables S355NL to absorb more energy under impact loads and significantly improves impact toughness.

[0028] 3. On the other hand, the present invention provides a preparation method of the above-mentioned S355NL steel, comprising the following steps of smelting: batching according to the chemical composition of the above-mentioned S355NL steel, performing smelting treatment, and obtaining a steel ingot after casting; annealing: performing homogenization annealing treatment on the steel ingot; forging: performing forging treatment on the steel ingot after the homogenization annealing treatment to obtain a forged material; heat treatment: performing heat treatment on the forged material to obtain the above-mentioned S355NL steel. The impact energy of the S355NL steel obtained by the method of the present invention at -50°C is ≥269.67J; the tensile strength of the S355NL steel is ≥512MPa, the yield strength is ≥390MPa, the elongation after fracture is ≥35.5%, and the cross-sectional shrinkage is ≥80%; wherein, the alloy elements can be effectively diffused through homogenization annealing treatment, component segregation can be eliminated, the structural uniformity of the steel can be improved, and the performance difference and potential defects caused by structural inhomogeneity can be reduced; an equiaxed structure can be obtained through heat treatment, and the addition of rare earth increases the proportion of large-angle grain boundaries (grain boundaries with a mismatch angle of not less than 15 degrees) in the equiaxed structure, thereby being beneficial to the comprehensive performance of the S355NL steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the implementation methods of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the implementation methods or the description of the prior art. The drawings described below are only exemplary, and for ordinary technicians in this field, other implementation drawings can be derived from the provided drawings without creative work.

[0030] Figure 1 Middle: ae are the organization diagrams of S355NL steel with rare earth contents of 0, 11ppm, 49ppm, 190ppm, and 860ppm respectively;

[0031] Figure 2 Middle: ae are the morphology and energy spectrum of inclusions in S355NL steel with rare earth contents of 0, 11ppm, 49ppm, 190ppm, and 860ppm respectively;

[0032] Figure 3 It is the low temperature impact toughness of S355NL steel with rare earth contents of 0, 11ppm, 49ppm, 190ppm and 860ppm respectively;

[0033] Figure 4 The yield strength and tensile strength of S355NL steel with rare earth contents of 0, 11ppm, 49ppm, 190ppm, and 860ppm, respectively;

[0034] Figure 5is the proportion of high-angle grain boundaries in equiaxed grains in S355NL steel with rare earth contents of 0, 11ppm, 49ppm, 190ppm, and 860ppm, respectively;

[0035] Figure 6 is the pearlite lamellar spacing diagram of S355NL steel with rare earth contents of 0, 11ppm, 49ppm, 190ppm, and 860ppm respectively;

[0036] Figure 7 It is the statistics of pearlite lamellar spacing of S355NL steel with rare earth contents of 0, 11ppm, 49ppm, 190ppm and 860ppm respectively. DETAILED DESCRIPTION

[0037] In order to more clearly illustrate the implementation methods of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for the implementation methods or the description of the prior art. The drawings described below are only exemplary, and for ordinary technicians in this field, other implementation drawings can be derived from the provided drawings without creative work.

[0038] The invention provides an S355NL steel. The chemical composition of the S355NL steel comprises, by mass percentage, C: 0.11-0.15%, Si: 0.15-0.25%, Mn: 1.27-1.40%, P≤0.013%, S≤0.005%, Nb: 0.03-0.04%, V: 0.02-0.04%, Al: 0.02-0.05%, Ti≤0.009%, Cr≤0.06%, Ni≤0.3%, Mo: 0.01-0.03%, Cu≤0.2%, N≤0.015%, rare earth RE: 0.0011-0.02%, O≤10ppm, and the balance is Fe and unavoidable impurities.

[0039] S355NL steel has an equiaxed structure, in which grain boundaries with a mismatch angle of not less than 15 degrees account for more than 86% of the grain boundaries; the inclusions in S355NL steel are spherical or nearly spherical; the average size of the inclusions is less than 3μm; the inclusions include rare earth oxides and rare earth sulfides; the microstructure of S355NL steel includes ferrite and pearlite; the interlamellar spacing of pearlite is less than 170nm.

[0040] Among them, the rare earth RE is cerium Ce or yttrium Y; the total oxygen content in S355NL steel; the impact energy of S355NL steel at -50°C is ≥269.67J; at room temperature, the tensile strength of S355NL steel is ≥512MPa, the yield strength is ≥390MPa, the elongation after fracture is ≥35.5%, and the cross-sectional shrinkage is ≥80%.

[0041] The mismatch angle refers to the orientation difference between adjacent grains (the orientation difference between grains).

[0042] In another aspect, the present invention provides a method for preparing S355NL steel, comprising the following steps:

[0043] Smelting: according to the chemical composition of S355NL steel, the materials are prepared, and then loaded into a medium frequency vacuum induction furnace, and the medium frequency vacuum induction furnace is evacuated to 1.5-2Pa, and the evacuation time is 35 minutes; after reaching the vacuum requirement, the gas valve is closed, and argon gas is filled to make the pressure reach 8-9MPa; the induction coil is energized for smelting treatment, and the charge is melted after the induction heating time is 15-20 minutes, and the refining treatment is started after the raw materials are melted; among them, the temperature of smelting treatment is 1500-1580℃; the temperature of refining treatment is 1550-1590℃, and the time of refining treatment is 5-6 minutes;

[0044] Among them, the order of loading is C, Si, Mn, Al, Nb, V, Ti, rare earth;

[0045] When the temperature of the molten steel is 1550-1570℃, casting is carried out to obtain a steel ingot, and then the ingot mold is slowly cooled; by slowly cooling the steel ingot in the ingot mold, the rare earth elements are more evenly distributed in the steel, which plays a role in improving the structure and performance, while avoiding excessive thermal stress caused by fast cooling, the formation of coarse grains, and defects such as cracks and cracks;

[0046] Annealing: Heat the steel ingot to 1200±10℃ for 10-10.5h homogenization annealing. Homogenization annealing can effectively diffuse the alloy elements, eliminate component segregation, and improve the uniformity of the steel structure.

[0047] Forging: The steel ingot after homogenization annealing is forged at 1200±10℃ using an air hammer to obtain a forged material; then air cooling is performed; the forged material is a square ingot; air cooling helps the austenite in the steel to have enough time to transform, thereby forming uniform and fine grains, which is beneficial to ensuring the comprehensive performance of S355NL steel;

[0048] Heat treatment: The air-cooled forging material is processed by a wire cutting machine, and then the processed forging material is heat treated at 910±10°C for 25-30 minutes to obtain S355NL steel.

[0049] Based on the above method, the present invention adds rare earth to the traditional S355NL steel, and the rare earth reacts with oxygen in the molten steel first to form stable rare earth oxides. The generation of these rare earth oxides will consume the oxygen in the molten steel. At the same time, according to the principle of thermodynamics, the rare earth oxides formed in the molten steel will further reduce the activity of oxygen, making oxygen more inclined to combine with the rare earth oxides, thereby further reducing the oxygen content in the steel. Low oxygen content can reduce the formation of inclusions in the steel, and at the same time control the inclusion size to be small and evenly distributed, which is conducive to forming a more uniform microstructure and can show higher tensile strength and impact energy.

[0050] In S355NL steel without rare earth, inclusions are generally Al2O3 and MnS inclusions, among which Al2O3 is a hard and brittle inclusion, which has a great negative impact on the impact toughness of the material and easily leads to crack initiation and brittle fracture. MnS has good plasticity, but it can cause anisotropy and lamellar tearing.

[0051] After adding rare earth in the present invention, the element composition of the inclusion formation changes significantly, and the inclusion changes from Al, Mn, O, S elements to Ce, O, S as the main elements, that is, the formed inclusion is rare earth oxide or rare earth sulfide; a small amount of Mn and C elements are enriched. The size and shape of the inclusion change from the original 5μm to less than 3μm, and the shape of the inclusion tends to be spherical and ellipsoidal, and the edge of the inclusion is smoother. The hardness and brittleness of the modified rare earth inclusion are significantly lower than the original inclusion, and the thermal expansion coefficient is higher than the original inclusion. Therefore, the rare earth inclusion has a higher matching degree with the matrix, which reduces the possibility of stress concentration at this location becoming a crack source, which is beneficial to improving the comprehensive mechanical properties of S355NL steel.

[0052] When the rare earth content increases to 860 ppm (ie, 0.086% by mass), the shape of the rare earth inclusions begins to become irregular, and the excess rare earth begins to react with the As element.

[0053] In the present application, the addition of rare earth promotes the nucleation of pearlite. Due to the increase in nucleation sites and the increase in the number of pearlite clusters, the interlamellar spacing decreases at the same transformation volume; on the other hand, the presence of rare earth elements inhibits the diffusion of certain elements (such as C), making the growth rate of pearlite lamellae more balanced in all directions, which helps to refine the interlamellar spacing of pearlite. When the interlamellar spacing of pearlite is smaller, the resistance to dislocation movement is greater. When subjected to external forces, dislocations need to overcome more obstacles to move, thereby improving the yield strength and tensile strength of steel. Smaller interlamellar spacing of pearlite causes more phase interfaces to be encountered during crack propagation, and the crack propagation path becomes tortuous, consuming more energy, thereby improving the toughness of S355NL steel; and the segregation of rare earth elements at grain boundaries can purify grain boundaries and reduce the segregation of impurity elements at grain boundaries (such as elements such as P and S), thereby improving the bonding force of grain boundaries and further enhancing toughness.

[0054] On the other hand, the present invention increases the number of high-angle grain boundaries (grain boundaries with a mismatch angle of not less than 15 degrees) in S355NL steel by adding rare earths, and the increase in high-angle grain boundaries makes the dislocation movement more obviously hindered. When S355NL steel is subjected to external force, the increase in high-angle grain boundaries makes it difficult for dislocations to easily pass through the grain boundaries, and a larger external force is required to move the dislocations, thereby improving the strength of S355NL steel. At the same time, when impacted, the increase in high-angle grain boundaries, when cracks encounter high-angle grain boundaries, the direction of expansion changes, and more energy is required to continue to expand, which enables S355NL to absorb more energy under impact loads, and the impact toughness is significantly improved.

[0055] In summary, the present invention obtains rare earth inclusions with refined size and regular shape and a high proportion of large-angle grain boundaries by adding rare earths, thereby obtaining S355NL steel with excellent comprehensive performance.

[0056] The present invention is further described below with reference to specific embodiments and comparative examples.

[0057] Among them, the performance testing of the S355NL steel obtained in the embodiment and the comparative example includes: processing the obtained rectangular specimen into a V-notch, and finally processing it into a 10mm×10mm×55mm standard Charpy V-type pendulum impact specimen that meets the national standard (GB / T 229-2020); The obtained cylindrical specimen is processed into a M10φ5mm tensile specimen that meets the national standard (GB / T 228.1-2021), and then the processed impact specimen is ground to 1500 mesh with metallographic sandpaper to ensure that the surface roughness is less than 5μm, and then the mechanical properties are tested.

[0058] Example 1

[0059] This embodiment provides a method for preparing S355NL steel, comprising the following steps:

[0060] Smelting: The materials are prepared according to the chemical composition of S355NL steel, and then loaded into a medium-frequency vacuum induction furnace, and the medium-frequency vacuum induction furnace is evacuated to 1.8Pa for 35 minutes; after the vacuum requirement is reached, the gas valve is closed and argon gas is filled to make the pressure reach 8.5MPa; the induction coil is energized for smelting treatment, and the charge is melted after the induction heating time of 18 minutes, and the refining treatment is started after the raw materials are melted; the smelting treatment temperature is 1530℃; the refining treatment temperature is 1560℃, and the refining time is 5 minutes;

[0061] Among them, the chemical composition of S355NL steel includes, by mass percentage: C: 0.15%, Si: 0.15%, Mn: 1.40%, P < 0.005%, S: 0.001%, Nb: 0.034%, V: 0.04%, Al: 0.046%, Ti < 0.001%, Cr < 0.03%, Ni < 0.03%, Mo: 0.02%, Cu < 0.03%, N: 0.002%, O: 9ppm, Ce: 0.0011% (i.e. 11ppm), the balance is Fe and unavoidable impurities;

[0062] When the temperature of the molten steel is 1550-1570°C, casting is performed to obtain a steel ingot, and then the ingot mold is slowly cooled;

[0063] Annealing: heating the steel ingot to 1200°C for 10 hours of homogenization annealing treatment; forging: forging the steel ingot after homogenization annealing treatment at 1200°C with an air hammer to obtain a forged material; and then air cooling; wherein the forged material is a square ingot;

[0064] Heat treatment: The above ingots were processed into 11 mm × 11 mm × 57 mm rectangular specimens and 11 × 62 mm cylindrical specimens by wire cutting machine; then the specimens were heat treated at 910°C for 25 min to obtain S355NL steel.

[0065] Example 2

[0066] This embodiment provides a method for preparing S355NL steel, comprising the following steps:

[0067] Smelting: The materials are prepared according to the chemical composition of S355NL steel, and then loaded into a medium-frequency vacuum induction furnace, and the medium-frequency vacuum induction furnace is evacuated to 1.8Pa for 35 minutes; after the vacuum requirement is reached, the gas valve is closed and argon gas is filled to make the pressure reach 8.5MPa; the induction coil is energized for smelting treatment, and the charge is melted after the induction heating time of 18 minutes, and the refining treatment is started after the raw materials are melted; the smelting treatment temperature is 1550℃; the refining treatment temperature is 1580℃, and the refining time is 5 minutes;

[0068] Among them, the chemical composition of S355NL steel includes, by mass percentage: C: 0.15%, Si: 0.2%, Mn: 1.38%, P < 0.006%, S: 0.0019%, Nb: 0.037%, V: 0.035%, Al: 0.047%, Ti < 0.001%, Cr < 0.03%, Ni < 0.03%, Mo: 0.020%, Cu < 0.03%, N: 0.0015%, O: 8ppm, Ce: 0.0049% (i.e. 49ppm), and the balance is Fe and unavoidable impurities;

[0069] When the temperature of the molten steel is 1550-1570°C, casting is performed to obtain a steel ingot, and then the ingot mold is slowly cooled;

[0070] Annealing: heating the steel ingot to 1200°C for 10 hours of homogenization annealing treatment; forging: forging the steel ingot after homogenization annealing treatment at 1200°C with an air hammer to obtain a forged material; and then air cooling; wherein the forged material is a square ingot;

[0071] Heat treatment: The above ingots were processed into 11 mm × 11 mm × 57 mm rectangular specimens and 11 × 62 mm cylindrical specimens by wire cutting machine; then the specimens were heat treated at 910°C for 25 min to obtain S355NL steel.

[0072] Example 3

[0073] This embodiment provides a method for preparing S355NL steel, comprising the following steps:

[0074] Smelting: The materials are prepared according to the chemical composition of S355NL steel, and then loaded into a medium-frequency vacuum induction furnace, and the medium-frequency vacuum induction furnace is evacuated to 1.8Pa for 35 minutes; after the vacuum requirement is reached, the gas valve is closed and argon gas is filled to make the pressure reach 8.4MPa; the induction coil is energized for smelting treatment, and the charge is melted after the induction heating time of 18 minutes, and the refining treatment is started after the raw materials are melted; among them, the temperature of the smelting treatment is 1530℃; the temperature of the refining treatment is 1560℃, and the refining time is 5min;

[0075] Among them, the chemical composition of S355NL steel includes, by mass percentage: C: 0.15%, Si: 0.15%, Mn: 1.39%, P < 0.005%, S: 0.001%, Nb: 0.035%, V: 0.04%, Al: 0.049%, Ti < 0.001%, Cr < 0.03%, Ni < 0.03%, Mo: 0.019%, Cu < 0.03%, N: 0.0021%, O: 6ppm, Ce: 0.019% (i.e. 190ppm), the balance is Fe and unavoidable impurities;

[0076] When the temperature of the molten steel is 1550-1570°C, casting is performed to obtain a steel ingot, and then the ingot mold is slowly cooled;

[0077] Annealing: heating the steel ingot to 1200°C for 10 hours of homogenization annealing treatment; forging: forging the steel ingot after homogenization annealing treatment at 1200°C with an air hammer to obtain a forged material; and then air cooling; wherein the forged material is a square ingot;

[0078] Heat treatment: The above ingots were processed into 11 mm × 11 mm × 57 mm rectangular specimens and 11 × 62 mm cylindrical specimens by wire cutting machine; then the specimens were heat treated at 910°C for 25 min to obtain S355NL steel.

[0079] Comparative Example 1

[0080] This comparative example provides a method for preparing S355NL steel, comprising the following steps:

[0081] Smelting: The materials are prepared according to the chemical composition of S355NL steel, and then loaded into a medium-frequency vacuum induction furnace, and the medium-frequency vacuum induction furnace is evacuated to 1.8Pa for 35 minutes; after the vacuum requirement is reached, the gas valve is closed and argon gas is filled to make the pressure reach 8.5MPa; the induction coil is energized for smelting treatment, and the charge is melted after the induction heating time of 18 minutes, and the refining treatment is started after the raw materials are melted; the smelting treatment temperature is 1530℃; the refining treatment temperature is 1560℃, and the refining time is 5 minutes;

[0082] Among them, the chemical composition of S355NL steel includes, by mass percentage: C: 0.15%, Si: 0.15%, Mn: 1.39%, P < 0.005%, S: 0.0027%, Nb: 0.034%, V: 0.037%, Al: 0.036%, Ti < 0.001%, Cr < 0.03%, Ni < 0.03%, Mo: 0.019%, Cu < 0.03%, N: 0.002%, and the balance is Fe and unavoidable impurities;

[0083] When the temperature of the molten steel is 1550-1570°C, casting is performed to obtain a steel ingot, and then the ingot mold is slowly cooled;

[0084] Annealing: heating the steel ingot to 1200°C for 10 hours of homogenization annealing treatment; forging: forging the steel ingot after homogenization annealing treatment at 1200°C with an air hammer to obtain a forged material; and then air cooling; wherein the forged material is a square ingot;

[0085] Heat treatment: The above ingots were processed into 11 mm × 11 mm × 57 mm rectangular specimens and 11 × 62 mm cylindrical specimens by wire cutting machine; then the specimens were heat treated at 910°C for 25 min to obtain S355NL steel.

[0086] The mechanical properties and impact energy of the S355NL steel obtained in this comparative example and in Example 1 are compared as shown in Table 1 and Table 2, respectively.

[0087] Table 1

[0088] Tensile strength / MPa Yield strength / MPa Elongation after break / % Sectional shrinkage / % Comparative Example 1 503 365 36.0 80.0 Example 1 512 390 35.5 80.0

[0089] As shown in Table 1, the tensile strength of the S355NL steel of Comparative Example 1 is 503 MPa, and the yield strength is 365 MPa. The tensile strength of the S355NL steel of Example 1 after adding 11 ppm rare earth can reach 512 MPa, and the yield strength can reach 390 MPa. Compared with Comparative Example 1, the tensile strength of the S355NL steel of Example 1 is increased by 1.79%, and the yield strength is increased by 6.84%.

[0090] Table 2

[0091]

[0092] As shown in Table 2, the average impact energy of the S355NL steel of the comparative example at -50°C is 181.05 J, and the average impact energy of the S355NL steel of Example 1 after adding 11 ppm rare earth at -50°C is 269.67 J. Compared with the comparative example 1, the average impact energy of the S355NL steel of Example 1 at -50°C is increased by 48.9%.

[0093] This is because after rare earth is added in Example 1, the original large-sized and irregularly shaped inclusions are modified into small-sized and spherical rare earth oxide and rare earth sulfide inclusions. The modified inclusions reduce stress concentration, reduce the generation of crack sources, and improve the low-temperature impact performance of steel.

[0094] Comparative Example 2

[0095] This comparative example provides a method for preparing S355NL steel, comprising the following steps:

[0096] Smelting: The materials are prepared according to the chemical composition of S355NL steel, and then loaded into a medium-frequency vacuum induction furnace, and the medium-frequency vacuum induction furnace is evacuated to 1.8Pa for 35 minutes; after the vacuum requirement is reached, the gas valve is closed and argon gas is filled to make the pressure reach 8.5MPa; the induction coil is energized for smelting treatment, and the charge is melted after the induction heating time of 18 minutes, and the refining treatment is started after the raw materials are melted; the smelting treatment temperature is 1530℃; the refining treatment temperature is 1560℃, and the refining time is 5 minutes;

[0097] Among them, the chemical composition of S355NL steel includes C: 0.15%, Si: 0.15%, Mn: 1.40%, P < 0.005%, S: 0.001%, Nb: 0.034%, V: 0.039%, Al: 0.049%, Ti < 0.001%, Cr < 0.03%, Ni < 0.03%, Mo: 0.019%, Cu < 0.03%, N: 0.002%, O: 7ppm, Ce: 0.086% (i.e. 860ppm), and the balance is Fe and unavoidable impurities;

[0098] When the temperature of the molten steel is 1550-1570°C, casting is performed to obtain a steel ingot, and then the ingot mold is slowly cooled;

[0099] Annealing: heating the steel ingot to 1200°C for 10 hours of homogenization annealing treatment; forging: forging the steel ingot after homogenization annealing treatment at 1200°C with an air hammer to obtain a forged material; and then air cooling; wherein the forged material is a square ingot;

[0100] Heat treatment: The above ingots were processed into 11 mm × 11 mm × 57 mm rectangular specimens and 11 × 62 mm cylindrical specimens by wire cutting machine; then the specimens were heat treated at 910°C for 25 min to obtain S355NL steel.

[0101] The microstructures of the S355NL steel obtained in Examples 1 to 3 and Comparative Examples 1 and 2 are as follows: Figure 1 As shown in Figure 2, the morphology and energy spectrum of the inclusions are as follows: Figure 2 The performance test results are shown in Figures 3 to 4 As shown in Figure 2, the content of high-angle grain boundaries is as follows: Figure 5 As shown, the pearlite lamellae spacing is Figure 6 and 7 As shown;

[0102] Among them, from Figure 1It can be seen that with the continuous addition of rare earth, the ferrite and pearlite structures in the steel are refined, and the refinement of black pearlite is more obvious. The refined pearlite can improve the strength of S355NL, the characteristics of equiaxed crystals are more obvious, and the density of the structure is improved. After adding rare earth, the inclusions are small in size and evenly distributed.

[0103] from Figure 2 It can be seen that in S355NL steel without rare earth, the inclusions are composed of Al, Mn, O, and S elements; among them, Al2O3 is a hard and brittle inclusion, which has a great negative impact on the impact toughness of the material and easily leads to crack initiation and brittle fracture. MnS has good plasticity, but it will cause anisotropy and lamellar tearing; in steel containing rare earth, the inclusions are mainly composed of Ce, O, and S, with a small amount of Mn and C elements enriched;

[0104] from Figure 3 and 4 It can be seen that after adding rare earth in the embodiment, since rare earth reacts preferentially with oxygen in the molten steel to form stable rare earth oxides, the generation of these rare earth oxides will consume the oxygen in the molten steel. At the same time, according to the principles of thermodynamics, the rare earth oxides formed in the molten steel will further reduce the activity of oxygen, making oxygen more inclined to combine with rare earth oxides, thereby further reducing the oxygen content in the steel. Low oxygen content can reduce the formation of inclusions in the steel, while controlling the inclusion size to be small and evenly distributed, which is conducive to forming a more uniform microstructure and can show higher tensile strength and impact energy. Therefore, the comprehensive performance of the S355NL steel obtained in Examples 1 to 3 is better;

[0105] In S355NL steel without rare earth (such as comparative example 1), inclusions are generally Al2O3 and MnS inclusions, of which Al2O3 is a hard and brittle inclusion, which has a great negative impact on the impact toughness of the material and easily leads to crack initiation and brittle fracture. MnS has good plasticity, but it will cause anisotropy and lamellar tearing, which is not conducive to the comprehensive performance of steel;

[0106] When the rare earth content increases to 860 ppm (as in Comparative Example 2), the shape of the rare earth inclusions begins to become irregular, and the excess rare earth begins to react with the As element, thereby affecting the comprehensive properties of the steel;

[0107] from Figure 5 It can be seen that the addition of rare earth increases the large-angle grain boundaries of S355NL, and the increase in large-angle grain boundaries makes the dislocation movement more obviously hindered. The increase in the proportion of large-angle grain boundaries has an effect on improving the strength and toughness of S355NL. On the one hand, it shows that rare earth refines the grains and increases the number of grain boundaries. On the other hand, the large-angle grain boundaries effectively prevent dislocation slip. The combined effect of the two can improve the performance.

[0108] from Figure 6 and 7 It can be seen that after rare earth is added in the embodiment of the present invention, the interlamellar spacing of pearlite is less than 170nm; wherein, the addition of rare earth promotes the nucleation of pearlite, and due to the increase in nucleation sites and the increase in the number of pearlite clusters, the interlamellar spacing decreases at the same transformation volume; on the other hand, the presence of rare earth elements will inhibit the diffusion of certain elements (such as C), making the growth rate of pearlite lamellae more balanced in all directions, which is helpful to refine the interlamellar spacing of pearlite.

[0109] It is easy for those skilled in the art to understand that, without conflict, the advantageous technical features of the above-mentioned methods can be freely combined and superimposed.

[0110] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention. The above description is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and variations can be made without departing from the technical principles of the present invention. These improvements and variations should also be regarded as the protection scope of the present invention.

Claims

1. A S355NL steel, characterized in that: The chemical composition of the S355NL steel includes, by mass percentage, C: 0.11-0.15%, Si: 0.15-0.25%, Mn: 1.27-1.40%, P≤0.013%, S≤0.005%, Nb: 0.03-0.04%, V: 0.02-0.04%, Al: 0.02-0.05%, Ti≤0.009%, Cr≤0.06%, Ni≤0.3%, Mo: 0.01-0.03%, Cu≤0.2%, N≤0.015%, rare earth RE: 0.0011-0.02%, O≤10ppm, and the balance is Fe and unavoidable impurities.

2. The S355NL steel according to claim 1, characterized in that The S355NL steel has an equiaxed structure, in which grain boundaries with a mismatch angle of not less than 15 degrees account for more than 86% of the grain boundaries; and / or The inclusions in the S355NL steel are spherical or nearly spherical; the average size of the inclusions is less than 3 μm; the inclusions include rare earth oxides and rare earth sulfides; and / or The microstructure of the S355NL steel includes ferrite and pearlite; the interlamellar spacing of the pearlite is less than 170 nm.

3. The S355NL steel according to claim 1 or 2, characterized in that: The rare earth RE is cerium Ce or yttrium Y; and / or The impact energy of the S355NL steel at -50°C is ≥269.67 J; the tensile strength of the S355NL steel is ≥512 MPa, the yield strength is ≥390 MPa, the elongation after fracture is ≥35.5%, and the section shrinkage is ≥80%.

4. A method for preparing the S355NL steel according to any one of claims 1 to 3, characterized in that: The following steps are involved: Smelting: preparing materials according to the chemical composition of the S355NL steel, performing smelting treatment, and obtaining a steel ingot after casting; Annealing: performing homogenization annealing treatment on the steel ingot; Forging: forging the steel ingot after the homogenization annealing treatment to obtain a forged material; Heat treatment: The forged material is heat treated to obtain the S355NL steel.

5. The method for preparing S355NL steel according to claim 4, characterized in that: In the smelting step: a medium frequency vacuum induction furnace is used for smelting treatment; Preferably, the vacuum degree of the medium frequency vacuum induction furnace is 1.5-2Pa; Preferably, the pressure of the medium frequency vacuum induction furnace is 8-9 MPa; Preferably, the smelting treatment includes smelting treatment and refining treatment; the time of the smelting treatment is 15-20 minutes; the temperature of the smelting treatment is 1500-1580°C; the temperature of the refining treatment is 1550-1590°C; and the time of the refining treatment is 5-6 minutes.

6. The method for preparing S355NL steel according to claim 5, characterized in that: In the smelting step: molten steel is obtained after smelting treatment, and when the temperature of the molten steel is 1550-1570°C, casting is performed to obtain a steel ingot.

7. The method for preparing S355NL steel according to claim 4, characterized in that: In the annealing step: the temperature of the homogenization annealing treatment is 1200±10°C and the time is 10-10.5h.

8. The method for preparing S355NL steel according to claim 4, characterized in that: In the forging step: the temperature of the forging process is 1200±10°C. 9 . The method for preparing S355NL steel according to claim 4 , wherein the steel ingot after homogenization annealing is forged into a square ingot using an air hammer.

10. The method for preparing S355NL steel according to claim 4, characterized in that: The temperature of the heat treatment is 910±10° C., and the time of the heat treatment is 25-30 minutes.

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