NANO-PRECIPITATE REINFORCED ULTRA-HIGH-STRENGTH STEEL AND MANUFACTURING METHOD THEREFOR

TECHNICAL FIELD

[0001]The present invention relates to the technical field of ultra-high-strength steel, in particular to a nano-precipitate reinforced ultra-high-strength steel and a manufacturing method therefor.

BACKGROUND ART

[0002]Hot-rolled ultra-high-strength steel for engineering machinery with a yield strength of 960 MPa grade is primarily applied in the manufacture of components such as crane booms and placing booms for pump trucks, which demands high requirements for the steel's strength, plasticity, low-temperature toughness, and fatigue performance. Conventional 960 MPa-grade ultra-high-strength steel is generally manufactured by offline quenching and tempering heat treatment or online quenching+tempering process, resulting in a tempered martensite structure. However, tempered martensite exhibits low plasticity, leading to cracking risks during subsequent processing such as bending and hole expansion. For example, the cold bending performance of the 960 MPa-grade ultra-high-strength steel manufactured by the conventional process can only meet D=5~6a at 90 degrees, and an elongation of ≤16%. High-temperature tempering can enhance the plasticity of ultra-high-strength steel, but the strength will be reduced.

[0003]Chinese patent CN103014538B discloses an ultra-high-strength steel with a yield strength of 960 MPa, which is manufactured by online quenching+510~550° C. tempering process, with a structure of tempered martensite.

[0004]Chinese patent CN102134680A discloses a manufacturing method for a high-strength steel with a yield strength of 960 MPa grade, which adopts a low carbon content design and has high Cr content, wherein C: 0.07~0.09%, Cr: 1.05~1.15%. The disclosure does not contain microalloying elements such as Nb, Ti, or V but has a relatively high Cr content.

[0005]Chinese patent CN102560274A develops an ultra-high-strength steel with a yield strength of 1000 MPa grade through offline heat treatment, and the microstructure is tempered martensitic structure. The main components are Cr: 0.30~0.50%; Mo: 0.30~0.50%; Ni: 0.20~0.50%; and V: 0.030~0.050%.

[0006]Chinese patent CN102505096A obtains a tempered martensitic ultra-high-strength steel by online quenching+460~520° C. tempering.

SUMMARY

[0007]In order to solve the above technical problems existed in the prior art, the present invention provides a nano-precipitate reinforced ultra-high-strength steel and a manufacturing method thereof. Preferably, the ultra-high-strength steel has a microstructure of tempered sorbite+a large number of nano-precipitates. More preferably, the steel has a yield strength of ≥1000 MPa, a tensile strength of ≥1050 MPa, an elongation A₅₀ of ≥19%, and an impact energy at −60° C. of ≥90 J.

[0008]The first aspect of the present invention provides a steel, comprising the following components in percentage by weight: C: 0.15~0.21%, Si: ≤0.50%, Mn: 0.60~1.60%, Ti: 0.051~0.15%, V: 0.040~0.12%, Cr: 0.20~1.20%, B: 0.0005~0.0030%, Al: 0.02~0.06%, Ca: 0.0005~0.004%, N: ≤0.005%, P: ≤0.020%, S: ≤0.0050%, O: ≤0.0040%, with the balance comprising Fe and inevitable impurities; and further satisfies:

[0009]a nano-precipitate control index NPI of 5~26, calculated by the following formula: NPI=(Mo+W+2.3*Cr)/(Ti+V), wherein each chemical element in the formula represent the numerical value before the percentage sign of the percentage by mass of the corresponding chemical element; and Ti+V≥0.11%.

[0010]Preferably, the balance of the above steel chemical composition is Fe and inevitable impurities.

[0011]Preferably, the above steel further comprises one or more element selected from Nb, Mg, Ni, Cu, Mo, and W, wherein Nb: 0~0.060%, Mg: 0~0.003%, Ni: 0~0.30%, Cu: 0~0.40%, Mo: 0~0.40%, W: 0~0.30%.

[0012]Preferably, the steel according to the present invention has a microstructure of tempered sorbite and nano-precipitates; wherein the tempered sorbite has a prior austenite grain size (i.e., the initial austenite grain size) of 5~10 μm, the nano-precipitates comprise TiC and VC precipitates, and the nano-precipitates have a size of 2~5 nm.

[0013]Preferably, the steel according to the present invention has a yield strength of ≥1000 MPa, a tensile strength of ≥1050 MPa, an elongation A₅₀ of ≥19%, and an impact energy at −60° C. of ≥90 J. More preferably, the performance of the steel satisfies at least one of the following: a yield strength of 1000~1115 MPa, a tensile strength of 1050~1172 MPa, an elongation A₅₀ of 19~25%, and an impact energy at −60° C. of 90~129 J.

[0014]Unless otherwise specified, the content of each element in the steel composition of the present invention is expressed in weight percentage. In the compositional design of the steel according to the present invention, the effects of each element are as follows:

[0015]Carbon: C exerts solid solution strengthening effect and can adjust the strength, plasticity and toughness of the martensitic structure. Experimental results show that the relationship between the tensile strength of low-carbon martensite after reheating and quenching and the carbon content is as follows: Rm=2510×C+790 (Empirical formula, the numerical value before the percentage symbol of C's weight percentage is substituted into the steel for calculation, unit: MPa). After quenching, tempering is applied to further adjust strength, plasticity, and toughness. Excessive C content increases the overall C equivalent, making the steel prone to cracking during welding. The C content in the present invention is 0.15~0.21%.

[0016]Silicon: A certain amount of Si can play a better role in deoxidizing, and at the same time can inhibit the precipitation of carbide and improve the toughness of steel in the tempering process. Excessive Si may cause the formation of red iron scale. Therefore, the Si content in the present invention is ≤0.50%.

[0017]Manganese: A Mn content of 0.6% or more can enhance the hardenability of steel. However, the content above 1.6% may lead to segregation and formation of inclusions such as MnS, which deteriorates the toughness of martensitic high-strength steel. Therefore, the Mn content in the steel of the present invention is 0.60~1.60%, preferably 1.20~1.60%.

[0018]Titanium: As a microalloying element, Ti can form a large number of nano-scale precipitates with elements such as C and N through controlled rolling and cooling processes. On the one hand, it can strongly inhibit the growth of austenite grains during heat treatment, and on the other hand, it can retain a large number of nano-precipitates after heat treatment and play a role in precipitation strengthening. The Ti content in the steel of the present invention is 0.051~0.15%, preferably 0.051~0.09%, so as to ensure the strength of the steel while reducing the manufacturing cost.

[0019]Vanadium: As a microalloying element, V can form nano-scale precipitates with C. A large number of nano-scale VC precipitates are produced during heat treatment tempering. The V content in the steel of the present invention is 0.040~0.12%, preferably 0.04~0.08%, so as to ensure the strength of the steel while reducing the manufacturing cost.

[0020]Niobium: As a microalloying element, Nb can form nano-scale precipitates with C, effectively inhibiting the growth of austenite grain during hot rolling and consequently refining the structure after phase transformation. The Nb content in the steel of the present invention is 0~0.060%, preferably 0.03% or less.

[0021]Magnesium: By controlling steelmaking and continuous casting process, trace amount of Mg will form fine MgO precipitate particles during steelmaking, so that TiN adheres to MgO to form composite precipitates MgO—TiN, which will modify the shape of cubic TiN, and the composite precipitates will be close to spherical shape. At the same time, the growth of harmful TiN can be controlled, so that the number of large particles of TiN can be reduced. The maximum size of TiN can be reduced from the conventional 8~10 μm to 5 μm or less, so as to improve the toughness and plasticity of the steel. The Mg content in the steel of the present invention is 0~0.003%.

[0022]Chromium: A Cr content of 0.2% or more can enhance the hardenability of steel, which is conducive to the formation of a full martensitic structure during quenching. Cr will form a carbide of Cr during tempering, which has the effect of resisting tempering softening. A Cr content above 1.20% may cause excessive sparking during welding, adversely affecting the quality of welding. Therefore, the Cr content in the steel of the present invention is 0.20~1.20%, preferably 0.20~0.80%.

[0023]Molybdenum: A certain amount of Mo can enhance the hardenability of steel, which is conducive to the formation of a full martensitic structure during quenching. Mo reacts with C to form carbide particles during high-temperature tempering, which has the effect of resisting temper softening and weld joint softening. Excessive Mo content increases carbon equivalent and deteriorates the welding performance. Meanwhile, as a noble metal, high Mo usage raises costs. Therefore, the Mo content in the steel of the present invention is 0~0.40%, preferably 0.25% or less.

[0024]Tungsten: W can enhance the hardenability of steel, form carbide particles during tempering, and have significant resistance to temper softening, as well as resistance to temper embrittlement. The W content in the steel of the present invention is 0~0.30%, preferably 0.22% or less.

[0025]Nickel: A certain amount of Ni has the role of refining the martensite structure and improving the toughness of the steel. Excessive Ni increases carbon equivalent and deteriorates the welding performance. At the same time, as a noble metal, high Ni usage raises costs. Therefore, the Ni content in the steel of the present invention is 0~0.30%, preferably 0.20% or less.

[0026]Copper: Cu contributes to precipitation strengthening during tempering. Additionally, a certain amount of Cu can enhance the corrosion resistance in ultra-high-strength steel for engineering machinery. The Cu content of the present invention is 0~0.40%, preferably 0.25% or less.

[0027]Boron: Trace amount of B can enhance the hardenability and the strength of steel. However, a B content above 0.0030% is prone to produce segregation and form carbon-boron compounds, which seriously deteriorate the toughness of the steel. Therefore, the B content in the steel of the present invention is 0.0005~0.0030%, preferably 0.0005~0.0020%.

[0028]Aluminum: An Al content of 0.2% or more serves as a deoxidizer on one hand, and on the other hand, the resulted trace amount of Al₂O₃ can refine grains during slab heating, and further enhancing microstructure refinement in rolled steel plates. However, an Al content above 0.06% is prone to produce Al oxide inclusion defects. The Al content in the steel of the present invention is 0.02~0.06%, preferably 0.02~0.04%.

[0029]Calcium: Trace amount of Ca can purify molten steel during smelting and modify the shape and size of inclusions such as MnS, thereby improving the toughness of the steel. A Ca content above 0.004% is prone to form Ca compounds with larger sizes, which in turn deteriorates the toughness. Therefore, the Ca content in the steel of the present invention is 0.0005~0.004%, preferably 0.0015~0.0035%.

[0030]Nitrogen: The steel of the present invention contains relatively high Ti content, which tends to form large cubic TiN particles with N that deteriorate the plate's plasticity and toughness. In the present invention, on the one hand, the content of N in the steel is strictly controlled to be 0.0050% or less, preferably 0.0030% or less through refining process, and on the other hand, by adding trace amount of rare earth elements combined with certain superheat in steelmaking and molten steel solidification process, the size and quantity of formed TiN can be effectively reduced.

[0031]Phosphorus, Sulfur and Oxygen: P, S and O are impurity elements in steel, which affect the plasticity and toughness of the steel. The content of the above elements in the steel of the present invention is strictly controlled to P≤0.020%, S≤0.0050%, O≤0.0040%; preferably satisfying at least one of the following: P≤0.012%, S≤0.0035%, O≤0.0035%.

[0049]In the manufacturing method for the ultra-high-strength steel of the present invention:

[0050]When introducing Mg during steelmaking, the superheat of the molten steel in the converter furnace is controlled within 15° C., while the secondary cooling water flow in continuous casting is moderately increased to improve the solidification speed of the molten steel, so as to form fine MgO precipitates. TiN can adhere to MgO to form composite precipitates MgO—TiN, so that the shape of cubic TiN is modified, and the composite precipitates will be close to spherical shape. Meanwhile, the composite precipitates are relatively dispersed, which helps control the growth of harmful TiN and reduces the quantity of large TiN particles. The maximum size of TiN can be reduced from the conventional 8~10 μm to 5 μm or less, so as to improve the toughness and plasticity of the steel.

[0051]In the slab heating step, controlling the heating temperature to 1220~1300° C. with a core holding time >30 minutes ensures complete dissolution of TiC precipitates formed during continuous casting. When the heating temperature exceeds 1300° C., it causes excessive growth of austenite grains, resulting in weakened grain boundary cohesion, which is prone to cracking during rolling.

[0052]In the manufacturing method for the present invention, the finish rolling temperature is 820~920° C., and the austenite grain can be refined by austenite recrystallization. After hot rolling, the steel plate is cooled to 560~680° C. at a cooling rate of 10~30° C./s to obtain ferrite+pearlite+nano-precipitates. Here, the size of precipitated TiC is 6~15 nm.

[0053]In the heat treatment step, the steel plate is heated to Ac₃+ (20~50° C.), and the holding time is controlled at 5~10 minutes. In the present invention, by the addition of a certain amount of Mo, W and/or Cr in combination to make 5≤NPI≤26, the dissolution rate of TiC can be controlled in a suitable range during the heat treatment process, so that the size of TiC is controlled at a micro-nano scale size of 2~5 nm. During this period, the nano-scale TiC can strongly inhibit the growth of austenite and refine the austenite and the structure after quenching.

[0054]During tempering (holding at 500~600° C. for 10~30 minutes), the steel plate develops a tempered sorbite structure. VC and TiC undergo secondary precipitation during the tempering process, with their precipitate sizes controlled within the micro-nano scale range of approximately 2~5 nm. The combined nano-precipitation of TiC and VC can generate a precipitation strengthening effect of 180~280 MPa.

[0055]The steel of the present invention obtains a tempered sorbite+nano-precipitate microstructure through a quenching and high-temperature tempering process. The large amount of nano-precipitates ensures that the strength of 1000 MPa or more can still be achieved after high-temperature tempering. By employing rare earth elements to purify the molten steel and control the size and shape of inclusions, the cracks caused by inclusions during deformation are reduced, and the plasticity of the steel plate is improved.

[0056]The beneficial effects of the present invention are as follows:

[0057]The present invention obtains a steel with a microstructure of tempered sorbite+nano-precipitates through controlled rolling and cooling and heat treatment process. The steel of the present invention has a large number of nano-precipitates of TiC and VC, which ensures that the steel plate has a strength of 1000 MPa or more after high-temperature tempering. The tempered sorbite obtained from high-temperature tempering improves the plasticity of the steel plate. By employing rare earth elements to purify the molten steel and control the size and shape of inclusions, the cracks caused by inclusions during deformation are reduced, and the plasticity of the steel plate is improved.

[0058]Compared with the prior art, the present invention obtains a steel with a microstructure of tempered sorbite+nano-precipitates, and by combining rare earth purification of the molten steel and controlling the size and shape of the inclusions, an ultra-high-strength steel with higher plasticity and toughness is obtained.

BRIEF DESCRIPTION OF DRAWINGS

[0059]FIG. 1 is the metallographic structure photograph of the ultra-high-strength steel after rapid heat treatment in Example 3 of the present invention, taken by an optical microscope.

[0060]FIG. 2 is the metallographic structure photograph of the ultra-high-strength steel after rapid heat treatment in Example 3 of the present invention, taken by a scanning electron microscope.

DETAILED DESCRIPTION

[0061]The present invention will be further explained in conjunction with the examples and accompanying drawings.

[0062]Table 1 shows the compositions and corresponding NPIs of the Examples and Comparative Examples of the present invention, with the balance being Fe and unavoidable impurities. The steels of the Examples and Comparative Examples were manufactured using the method of the present invention described above, with the process parameters shown in Table 2. The corresponding performances of the steels of the Examples and Comparative Examples are shown in Table 3.

[0063]Comparative Examples 1~4 were manufactured using essentially the same method as Example 3 of the present invention. The differences are that the NPIs of Comparative Examples 1 and 2 are not within the range defined by the present invention, the Ti+V of Comparative Example 3 is not within the range defined by the present invention, and the Cr content in Comparative Example 4 is not within the range defined by the present invention.

[0064]The yield strength, tensile strength, and elongation of the steel in Table 3 were tested in accordance with the standard GB/T 228.1-2021 “Metallic materials—Tensile test—Part 1: Test method at room temperature”. The impact energy at −60° C. was determined in accordance with GB/T 229-2020 “Metallic materials—Charpy pendulum impact test method”.

[0065]The average austenite grain size was measured in accordance with the standard GB/T 6394-2017 “Determination method of the average grain size of metal”. The polished samples were etched with picric acid, and 10 or more of the microstructural photographs were taken by an optical microscope to statistically determine the average austenite grain size.

[0066]The size of the nano-precipitates was measured using transmission electron microscopy (TEM) by observing thin-film samples to statistically determine the precipitate size.

[0067]FIGS. 1 and 2 present the optical microscope and scanning electron microscope photographs of Example 3 of the present invention, respectively. As shown in FIG. 1, the steel of Example 3 exhibits a tempered sorbite structure after heat treatment. As shown in FIG. 2, the steel of Example 3 has a large number of micro-nano precipitates in its structure after heat treatment.

[0068]As can be seen from the metallographic photographs in FIGS. 1 and 2, the metallographic structure of the finished steel plate is a uniform equiaxed tempered sorbite with a fine and dense structure, and the average grain size of the initial austenite of the tempered sorbite is about 6 μm. In FIG. 2, it can be seen that there are a large number of granular carbide precipitates in the steel of the present invention, and 90% or more of the TiC and TiV precipitates are in the size of 2~5 nm.

[0069]By adding Mo, V and/or Cr to a steel containing a specific amount of Ti and V and make them satisfy the above NPI, the steels of Examples 1-8 of the present invention can be obtained with a microstructure of tempered sorbite+nano-precipitates having a size of 2~5 nm, resulting in a comprehensive improvement in strength, plasticity and toughness of the steel.

[0070]From Table 3, it can be seen that compared with Comparative Examples 1-4, the present invention can obtain steel with significantly improved strength, plasticity and toughness by controlling the composition of the steel elements.

[0071]In summary, the present invention adopts controlled rolling and cooling combined with offline heat treatment process, and by controlling the chemical composition design, base metal structure, heating rate, holding time, and cooling rate, etc., the steel achieves ultra-high strength while maintaining good elongation and low-temperature impact toughness and so on.