A 900mpa grade ultra-fine grain steel material and a method of manufacturing the same

Abstract

The application discloses a 900MPa grade ultra-fine grain steel material, which contains Fe and inevitable impurities, and further contains the following chemical elements with the mass percentage as follows: C: 0.20-0.28%, Si: 0.15-0.35%, Mn: 0.90-1.40%, Cr: 0.80-1.30%, Mo: 0.30-0.65%, Ni: 0.86-1.25%, Nb: 0.01-0.08%, and Al: 0.015-0.05%; and the mass percentage of each chemical element satisfies 6<= (Mn+Ni) / C<=12 and 4<= (Cr+Mo) / C<=8. Correspondingly, the application further provides a manufacturing method of the 900MPa grade ultra-fine grain steel material. The yield strength ratio of the steel material is controllable, and the steel material has good low-temperature toughness and elongation while ensuring high strength.

Description

Technical Field

[0001] This invention relates to a type of steel and a method for manufacturing the same, and more particularly to a high-strength steel and a method for manufacturing the same. Background Technology

[0002] Yield strength is the basis for judging whether a material has undergone plastic deformation and is usually the strength design criterion for steel structural components. In general design and material selection, a higher yield strength is beneficial to improve the safety of the system. However, an excessively high yield strength will increase the yield-to-strength ratio of the material. An excessively high yield-to-strength ratio will lead to an increase in the brittleness of the material, which is not conducive to stress release in certain stress concentration areas and is prone to brittle fracture.

[0003] Therefore, controlling the yield strength ratio of the workpiece during the current workpiece processing is more beneficial to the service performance of the parts. However, the yield strength ratio should not be too low, as an excessively low yield strength ratio can easily cause the workpiece to yield and fail during service. Therefore, in many engineering applications, many companies hope to control the yield strength ratio of the workpiece steel within a specific required range so that the steel can obtain high strength while also possessing excellent impact toughness.

[0004] In the field of materials science, steel strengthening mechanisms known to those skilled in the art can include various methods such as solid solution strengthening, dislocation strengthening, dispersion strengthening, and grain refinement strengthening. However, in actual manufacturing, due to the influence of factors such as the increase in steel strength grade, the adjustment of alloy element ratios, and the diversity of heat treatment processes, the effects of these different strengthening methods are difficult to control effectively, which also leads to a sharp increase in the difficulty of controlling the yield strength ratio. Steel produced using conventional grain refinement methods generally has a grain size of not less than 20 μm, and its effect on improving the low-temperature toughness and strength of the steel is limited. Summary of the Invention

[0005] One of the objectives of this invention is to provide a 900MPa grade ultrafine grain steel, which, through the rational design of chemical composition and combined with optimized process, yields a high-strength steel with fine grain size. The yield strength ratio of this high-strength steel is controllable, and while ensuring high strength, it also has good low-temperature toughness and elongation.

[0006] To achieve the above objectives, this invention proposes a 900MPa grade ultrafine crystalline steel, which contains Fe and unavoidable impurities, and also contains the following chemical elements in the following mass percentages:

[0007] C: 0.20~0.28%, Si: 0.15~0.35%, Mn: 0.90~1.40%, Cr: 0.80~1.30%, Mo: 0.30~0.65%, Ni: 0.86~1.25%, Nb: 0.01~0.08%, Al: 0.015~0.05%;

[0008] Its mass percentage content of each chemical element meets the following requirements:

[0009] 6≤(Mn+Ni) / C≤12;

[0010] 4≤(Cr+Mo) / C≤8.

[0011] Furthermore, the present invention also provides a 900MPa grade ultrafine-grained steel, the mass percentage content of each chemical element being:

[0012] C: 0.20–0.28%, Si: 0.15–0.35%, Mn: 0.90–1.40%, Cr: 0.80–1.30%, Mo: 0.30–0.65%, Ni: 0.86–1.25%, Nb: 0.01–0.08%, Al: 0.015–0.05%; balance Fe and unavoidable impurities.

[0013] Its mass percentage content of each chemical element meets the following requirements:

[0014] 6≤(Mn+Ni) / C≤12;

[0015] 4≤(Cr+Mo) / C≤8.

[0016] The design principles of each chemical element in the 900MPa grade ultrafine grain steel described in this invention are as follows:

[0017] C: In the 900MPa grade ultrafine-grained steel described in this invention, carbon (C) can effectively improve the hardenability of the steel. The C content in the steel directly determines the martensitic phase structure, thus affecting the strength of the steel. Furthermore, C can also form carbides with some alloying elements in the steel, and these carbides can produce a dispersion strengthening effect in the matrix. However, it should be noted that the C content in the steel should not be too high. Excessive C content is very detrimental to the low-temperature impact toughness of the steel, leading to a decrease in its impact toughness. At the same time, the C content should not be too low either. When the C content is too low, the content of low-temperature phase transformation structures such as martensite and lower bainite will be low, making it impossible for the steel to obtain high tensile strength. Based on this, considering the influence of C on the performance of steel, in the 900MPa grade ultrafine-grained steel described in this invention, the mass percentage of C is controlled between 0.20% and 0.28%.

[0018] Si: In the 900MPa grade ultrafine-grained steel described in this invention, Si exists in the steel as a substitutional solid solution. It can increase the lattice distortion energy, enhance the resistance to dislocation movement, and thus improve the steel's strength. Simultaneously, Si can reduce the diffusion ability of C in ferrite. During tempering, an appropriate amount of Si can prevent the formation of coarse carbides precipitating at defects. However, the Si content in the steel should not be too high, as a high Si content will adversely affect the weldability and low-temperature impact toughness of the steel. Therefore, in the 900MPa grade ultrafine-grained steel described in this invention, the mass percentage of Si is controlled between 0.15% and 0.35%.

[0019] Mn: In the 900MPa grade ultrafine-grained steel described in this invention, Mn is a good deoxidizer and desulfurizer. Adding a certain amount of Mn to the steel can produce a good solid solution strengthening effect and improve the strength of the steel. During the quenching process of the steel, Mn will inhibit diffusion-type phase transformation, improve the hardenability of the steel, and form a low-temperature phase transformation structure, which has high strength. However, it should be noted that the Mn content in the steel should not be too high. When the Mn content in the steel is too high, it will increase the yield strength of the steel matrix, thereby increasing the yield strength ratio of the steel. At the same time, the Mn content in the steel should not be too low. When the Mn content in the steel is too low, it will lead to insufficient strength of the steel. Based on this, considering the influence of Mn on the performance of steel, in the 900MPa grade ultrafine-grained steel described in this invention, the mass percentage of Mn is controlled between 0.90% and 1.40%.

[0020] Cr: In the 900MPa grade ultrafine-grained steel described in this invention, Cr reduces the driving force of the γ-α phase transformation and improves hardenability. Simultaneously, Cr is a strong carbide-forming element; adding an appropriate amount of Cr to the steel allows for precipitation strengthening during tempering, resulting in the formation of high-strength martensite during heat treatment, thus obtaining steel with high strength. Furthermore, if Cr carbides are not completely dissolved during heating, they will pin to grain boundaries, effectively refining the grains. However, it is important to note that the Cr content in the steel should not be too high. Excessive Cr content leads to the formation of coarse carbides, deteriorating low-temperature impact performance. Therefore, to ensure the strength and low-temperature impact performance of the steel, the Cr content in the 900MPa grade ultrafine-grained steel described in this invention is controlled between 0.80% and 1.30%.

[0021] Mo: In the 900MPa grade ultrafine-grained steel described in this invention, adding an appropriate amount of Mo can effectively improve the steel's resistance to temper brittleness, inhibit the segregation of harmful elements, and form fine carbides under high-temperature tempering conditions, thereby increasing the steel's strength. Mo can reduce the carbon activity in steel, and the carbides are very stable, do not easily grow, and can refine the grain size. Considering that Mo is a valuable alloying element, adding too much Mo will lead to increased costs. Therefore, in the 900MPa grade ultrafine-grained steel described in this invention, to achieve a balance between strength, toughness, and weldability, the mass percentage of Mo is controlled between 0.30% and 0.65%.

[0022] Ni: In the 900MPa grade ultrafine-grained steel described in this invention, Ni exists in solid solution form. Especially in the composition system designed in this invention, Ni can effectively lower the ductile-brittle transition temperature of the material, improve the matrix toughness, and simultaneously inhibit diffusion-type phase transformation, thus improving the hardenability of the steel. Furthermore, it should be noted that since Ni is an austenitizing element, excessively high Ni content will lead to an excessively high content of retained austenite in the steel, which in turn increases the instability of the steel's microstructure and properties. Therefore, in the 900MPa grade ultrafine-grained steel described in this invention, to ensure the low-temperature impact toughness and strength of the steel, the mass percentage of Ni is controlled between 0.86% and 1.25%.

[0023] Nb: In the 900MPa grade ultrafine-grained steel described in this invention, the addition of Nb to the steel will form high-melting-point carbide and nitride precipitates with C and N in the steel. These precipitates can inhibit recrystallization of the steel. These precipitates tend to agglomerate at dislocation lines, exerting a strong drag effect on dislocation movement. However, it should be noted that the Nb content in the steel should not be too high. When the Nb content in the steel is too high, coarse NbC particles will form under high-temperature tempering conditions, which will deteriorate the low-temperature impact resistance of the steel. Therefore, in the 900MPa grade ultrafine-grained steel described in this invention, the mass percentage of Nb is controlled between 0.01% and 0.08% to ensure the mechanical properties of the steel.

[0024] Al: In the 900MPa grade ultrafine-grained steel described in this invention, Al is a commonly used deoxidizer. Adding an appropriate amount of Al can effectively reduce the oxygen content in the steel. Simultaneously, Al forms fine AlN precipitates during steelmaking, which can effectively pin grain boundaries and inhibit austenite grain growth during rolling and cooling, thereby refining austenite grains and improving the low-temperature toughness of the steel. However, it should be noted that the Al content in the steel should not be too high. When the mass percentage of Al in the steel is too high, large Al oxides will be formed, and coarse Al2O3 hard inclusions will deteriorate the fatigue performance of the steel. Therefore, to improve the toughness of the steel, the mass percentage of Al in the 900MPa grade ultrafine-grained steel described in this invention is controlled between 0.015% and 0.05%.

[0025] Furthermore, this invention also designs the synergistic relationship of elements. By controlling the ratios of (Mn+Ni) / C and (Cr+Mo) / C, and combining these ratios with the corresponding heat treatment processes, carbon atoms can be precipitated from interstitial solid solutions through tempering to form carbides. This transforms solid solution strengthening into precipitation strengthening, achieving multiple couplings of various strengthening mechanisms such as Mn atom solid solution strengthening, C atom solid solution strengthening, carbide precipitation strengthening, and grain refinement strengthening. This results in precise control of the yield strength ratio of steel.

[0026] Furthermore, the 900MPa grade ultrafine crystalline steel of the present invention also contains at least one of the following chemical elements in the following mass percentages:

[0027] 0 < Ti ≤ 0.04%;

[0028] 0 < Ca ≤ 0.004%;

[0029] 0 < Cu ≤ 0.15%.

[0030] In some implementations, to achieve better results, appropriate amounts of at least one of Ti, Cu, and Ca elements can be added to the steel.

[0031] In the 900MPa grade ultrafine grain steel described in this invention, Ti can form compounds with C and N in the steel. The formation temperature of TiN is above 1400℃. It usually precipitates in the liquid phase or δ-ferrite, and its dissolution temperature in austenite exceeds 1200℃. It can effectively prevent austenite grain growth during the heating process. During the rolling process, the precipitated phase of Ti can act as nucleation particles, increase the recrystallization nucleation rate, and prevent grain coarsening. Therefore, an appropriate amount of Ti can be added to the steel.

[0032] However, it is important to note that the Ti content in the steel should not be too high. When the mass percentage of Ti is too high, coarse TiN precipitates will form, which will reduce the impact and fatigue properties of the steel. Furthermore, during tempering, an excessively high Ti content will increase the fluctuation range of the steel's low-temperature impact energy. Therefore, in the 900MPa grade ultrafine-grained steel described in this invention, it is preferable to add an appropriate amount of Ti, and control the mass percentage of Ti to satisfy 0 < Ti < 0.04%.

[0033] In addition, a suitable amount of Cu can be added to the steel. Cu will form fine nano-sized ε-Cu precipitation during tempering, thereby improving the strength of the steel. Simultaneously, adding a certain amount of Cu also helps improve the corrosion resistance of the steel. However, considering the low melting point of Cu, if the mass percentage of Cu added is too high, Cu will accumulate at the grain boundaries during the austenitization process, leading to grain boundary weakening and cracking. Based on the above considerations, in the 900MPa grade ultrafine-grained steel described in this invention, a suitable amount of Cu can be added, and the amount of Cu added can be controlled to be 0 < Cu ≤ 0.15%.

[0034] In addition, an appropriate amount of Ca can be added to the 900MPa grade ultrafine grain steel described in this invention, and the amount of Ca added is controlled to be 0 < Ca ≤ 0.004%. Ca can combine with S in steel to form CaS, which can improve the size and morphology of inclusions and increase the fatigue life of steel.

[0035] Furthermore, in the 900MPa grade ultrafine crystalline steel described in this invention, the mass percentage content of each element also satisfies: 0.2≤(Nb+2Ti) / (C+N)≤0.6.

[0036] In some embodiments of the present invention, while controlling the mass percentage content of a single chemical element, the synergistic relationship between the elements is controlled to satisfy the above formula, which can further improve the performance of the steel.

[0037] Furthermore, in the unavoidable impurities of the 900MPa grade ultrafine-grained steel described in this invention, the content of each impurity element satisfies at least one of the following conditions:

[0038] P≤0.015%;

[0039] S≤0.003%;

[0040] N≤0.015%;

[0041] O ≤ 0.0025%.

[0042] In the 900MPa grade ultrafine-grained steel described in this invention, P, S, N, and O are all impurities in the steel. Where technical conditions permit, their content in the steel should be as low as possible. Specifically:

[0043] P (phosphorus): P in steel tends to segregate at grain boundaries, which reduces the binding energy of the grain boundaries and thus deteriorates the low-temperature impact performance of the steel. Furthermore, the coexistence of P and Mn exacerbates the temper brittleness of the steel. To reduce intergranular fracture caused by P grain boundary segregation, decrease the energy absorbed by the steel upon impact, and ensure the low-temperature impact toughness of round steel, in some implementations, the mass percentage of P can be controlled to meet the following requirement: P ≤ 0.015%.

[0044] S: S has very low solubility in δ-ferrite and austenite. During the solidification of molten steel, S reacts with Fe and Mn in the steel to form numerous sulfide inclusions. FeS becomes liquid at 989℃, leading to hot brittle cracking during rolling. MnS, with its excellent plasticity, forms elongated strips at grain boundaries after rolling, disrupting the continuity of the steel matrix and negatively impacting fatigue life and impact toughness. Therefore, to ensure good low-temperature impact performance and fatigue life, in some embodiments, the mass percentage of S can be controlled to meet the requirement: S ≤ 0.003%.

[0045] Nitrogen (N): Nitrogen can form AlN or TiN in steel and refine austenite grains. However, it should be noted that increasing the mass percentage of N leads to increased enrichment at defects and the formation of coarse nitride precipitates, which in turn affects the low-temperature impact resistance of the steel. Therefore, in some implementations, the mass percentage of N can be controlled to meet the requirement of N ≤ 0.0150%.

[0046] O: O element can combine with Al and Ti elements in steel to form compounds such as Al2O3 and TiO, which are detrimental to the properties of the steel. To ensure uniform steel structure and low-temperature impact toughness, in some implementations, the mass percentage of O can be controlled to meet the requirement of O ≤ 0.0025%.

[0047] Furthermore, in the 900MPa grade ultrafine grain steel described in this invention, its microstructure matrix is ​​tempered martensite + tempered bainite.

[0048] Furthermore, in the 900MPa grade ultrafine grain steel described in this invention, the average grain size of tempered martensite and tempered bainite does not exceed 10μm.

[0049] Furthermore, the 900MPa grade ultrafine crystalline steel of the present invention has a yield strength ≥800MPa, a tensile strength Rm ≥900MPa, a yield-to-tensile ratio of 0.85~0.95, an elongation A ≥15%, a reduction of area ≥50%, and a Charpy impact energy Akv ≥110J at -20℃.

[0050] Accordingly, another objective of the present invention is to provide a method for manufacturing 900MPa grade ultrafine crystalline steel. This manufacturing method has a simple process flow and is easy to control. At the same time, the round steel obtained by combining a reasonable chemical composition ratio ensures high strength while also having good low-temperature toughness and elongation.

[0051] To achieve the above objectives, the present invention also proposes a method for manufacturing 900MPa grade ultrafine-grained steel as described above, comprising the following steps:

[0052] (1) Smelting and casting;

[0053] (2) Heating, forging or rolling: wherein the initial rolling or forging temperature is controlled at 980-1050℃, the final rolling or forging temperature is ≥850℃, and the single-pass deformation is controlled at 30-60%.

[0054] (3) Quenching: The steel is heated to the austenitizing temperature of 860-960℃ and then water quenched;

[0055] (4) Tempering: The tempering temperature is 580-640℃. After tempering, air cooling or water cooling is performed.

[0056] In the manufacturing method described in this invention, by controlling the amount of deformation and rolling temperature in a single rolling pass, the number of carbide precipitate particles is controlled, thereby achieving dynamic recrystallization control of the steel matrix and realizing ultrafine grain control of the steel matrix. This results in a uniform and fine matrix structure, ensuring the size of the martensitic laths after quenching. Furthermore, combined with the tempering process, the diffusion distance of carbon atoms is controllable within a certain range, making the spacing of carbides more uniform and fine, thus improving the carbide's hindering effect on dislocations during material yielding deformation.

[0057] Under the condition of controlling the final rolling or forging temperature to ≥850℃, the steel can undergo recrystallization and strain-induced precipitation, thereby forming a refined bainite and martensite multiphase matrix structure with fine carbonitride precipitation, which can further improve the performance of the steel.

[0058] In the manufacturing method described in this invention, the inventors have also optimized the quenching and tempering heat treatment process. The austenitizing temperature is controlled between 860 and 960°C in the quenching step because: in this quenching process, the carbonitrides of Nb, Ti, Cr and Mo in the steel can be completely or partially dissolved, while the undissolved carbonitrides can effectively pin the austenite grain boundaries, preventing the austenite grains from being too coarse, achieving the purpose of grain refinement after quenching, and improving the strength and toughness of the steel.

[0059] Accordingly, a water quenching process was adopted for cooling after austenitization. During this cooling process, the alloying elements dissolved in the austenite can effectively improve the hardenability of the steel, which can make the final martensite finer, resulting in round steel with high strength and good toughness.

[0060] Furthermore, the reason for controlling the tempering heat treatment of round steel within the tempering temperature range of 580–640℃ after quenching is that: during quenching, the steel matrix forms lower bainite and martensite structures with high defect density, resulting in significant internal distortion energy and stress concentration in some areas, leading to uneven internal stress distribution. During high-temperature tempering, Nb forms fine, dispersed carbonitrides with C and N, while Cr and Mo also precipitate fine carbides, achieving a dispersion strengthening effect. Simultaneously, due to high-density dislocation annihilation and small-angle grain boundary movement, the microstructure of the steel becomes more uniform, thus improving the low elongation after tempering.

[0061] Within the tempering temperature range defined by this invention, the steel can be guaranteed to have good strength, toughness and plasticity, effectively reducing the internal stress of the steel, which is beneficial to the processing and use of the steel, such as the production of high-performance R4 offshore platform mooring chains and steel for engineering structures.

[0062] Additionally, it should be noted that in step (1), steel can be smelted using a converter or electric furnace and cast into a continuous casting billet. Specifically, in-mold casting or continuous casting can be used during the casting process.

[0063] Furthermore, in some embodiments of the manufacturing method described in this invention, in step (2), the billet is heated and then rolled or forged once to directly roll or forge the billet to the size of the final product, wherein the billet heating temperature is 1100-1200°C.

[0064] Furthermore, in some other embodiments of the manufacturing method described in this invention, in step (2), the billet is first heated and rolled into an intermediate billet, and then the intermediate billet is heated and finally rolled or forged to obtain the final finished product size, wherein the billet heating temperature is 1100-1200℃; the intermediate billet heating temperature is 1100-1200℃.

[0065] In some embodiments, the heating temperature is controlled between 1100 and 1200°C because: the heating process at this temperature allows the carbonitrides of Nb, V, and Ti, and the carbides of Cr and Mo, to partially or completely dissolve in austenite; during subsequent rolling or forging and quenching, Nb, V, and Ti form fine carbonitrides, which can pin the austenite grain boundaries, increase the nucleation rate, and facilitate dynamic recrystallization nucleation during rolling, thus refining the rolled steel microstructure. Furthermore, the Cr and Mo dissolved in austenite can also improve the hardenability and quenchability of the steel.

[0066] The 900MPa grade ultrafine crystalline steel and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0067] This invention differs significantly from existing 900MPa grade steel. Current 900MPa grade ultrafine grain steel mainly adopts a low carbon + high manganese technical route, with carbon content generally below 0.20%. However, the technical route adopted in this invention differs significantly from the composition system of existing solutions, and this invention can also achieve precise control of the yield strength ratio within a certain range.

[0068] This invention innovatively employs the control of the ratios of (Mn+Ni) / C and (Mo+Cr) / C, as well as the dynamic recrystallization process during rolling, to achieve the coupling of solid solution strengthening, precipitation strengthening, and fine grain strengthening in steel, thereby achieving precise control of the material's high strength and toughness and yield strength ratio.

[0069] The 900MPa grade ultrafine grain steel described in this invention can form a dual-phase matrix structure of tempered martensite and tempered bainite, and refined carbide precipitation on its matrix, thereby eliminating the internal stress of the steel and exhibiting good structural uniformity.

[0070] In addition, the 900MPa grade ultrafine crystalline steel described in this invention has a wide process window, and can be mass-produced commercially on bar or plate production lines. In some embodiments, it can manufacture round steel with a diameter of 30 to 180 mm.

[0071] In some embodiments, the yield strength Rp of the 900MPa grade ultrafine-grained steel described in this invention 0.2 ≥800MPa, tensile strength R m ≥900MPa, yield strength ratio Rp 0.2 / R m The value is 0.85 to 0.95, the elongation A is ≥15%, the reduction of area Z is ≥50%, and the Charpy impact energy Akv below -20℃ is ≥110J.

[0072] The 900MPa grade ultrafine crystalline steel described in this invention can be effectively applied to applications requiring high strength and toughness materials, such as marine mooring chains, automobiles, mechanical engineering structures, and many other fields. It has wide applicability and good application prospects and value. Attached Figure Description

[0073] Figure 1 The image shows a 500x magnified metallographic image of Example 3 of the present invention under a microscope. Detailed Implementation

[0074] The 900MPa grade ultrafine crystalline steel and its manufacturing method described in this invention will be further explained and described below with reference to the accompanying drawings and specific embodiments. However, this explanation and description do not constitute an undue limitation on the technical solution of this invention.

[0075] The 900MPa grade ultrafine-grained steels of Examples 1-8 and Comparative Examples 1-6 were prepared by the following steps:

[0076] 1) Smelting: The molten steel is smelted in an electric furnace or converter according to the composition ratios shown in Tables 1-1 and 1-2.

[0077] 2) Casting: Molten steel obtained from smelting is cast into billets by die casting or continuous casting.

[0078] 3) Heating; Forging or Rolling: The cast billet is heated and then forged or rolled to obtain a steel billet. The initial rolling or forging temperature is controlled at 980–1050℃, the final rolling or forging temperature is ≥850℃, and the deformation per pass is controlled at 30–60%.

[0079] Examples 5-8 and Comparative Examples 1 and 5 involve only one heating and one rolling or forging process to directly roll or forge the billet to the final dimensions of the finished product.

[0080] In Examples 1-4, Comparative Examples 2-4 and Comparative Example 6, the billets are first rolled to a specified intermediate billet size, and then the intermediate billets are heated and rolled or forged to the final product size. The billet heating temperature is 1100-1200℃, and the intermediate billet heating temperature is 1100-1200℃.

[0081] 4) Quenching: The steel billet is heated to the austenitizing temperature of 860-960℃, and then water quenching is performed after austenitization.

[0082] 5) Tempering: The tempering temperature is 580~640℃, and the tempered product is air-cooled or water-cooled.

[0083] Table 1-1 lists the mass percentage of each chemical element in the 900MPa grade ultrafine crystalline steel of Examples 1-8 and the comparative round steel of Comparative Examples 1-6.

[0084] Table 1-1 (wt.%, balance Fe and other unavoidable impurities other than P, S, N and O)

[0085]

[0086]

[0087] Table 1-2 lists the synergistic relationships of various chemical elements in the 900MPa grade ultrafine crystalline steel of Examples 1-8 and the comparative round steel of Comparative Examples 1-6.

[0088] Table 1-2.

[0089] serial number 6≤(Mn+Ni) / C≤12 4≤(Cr+Mo) / C≤8 0.2≤(Nb+2Ti) / (C+N)≤0.6 Example 1 9.38 5.68 0.30 Example 2 9.31 6.94 0.32 Example 3 9.45 5.75 0.24 Example 4 9.16 7.31 0.21 Example 5 10.24 7.66 0.22 Example 6 7.91 4.18 0.50 Example 7 7.36 6.19 0.44 Example 8 7.71 5.96 0.24 Comparative Example 1 10.20 7.09 0.36 Comparative Example 2 15.28 9.28 0.20 Comparative Example 3 9.26 7.11 0.14 Comparative Example 4 10.20 7.09 0.36 Comparative Example 5 9.22 9.51 0.22 Comparative Example 6 10.20 7.09 0.36

[0090] Note: In Table 1-2, each element in “(Mn+Ni) / C”, “(Cr+Mo) / C”, and “(Nb+2Ti) / (C+N)” should be substituted with the mass percentage of the corresponding chemical element; the results of the above formulas are rounded to two decimal places.

[0091] Table 2 lists the specific process parameters for the 900MPa grade ultrafine crystalline steel of Examples 1-8 and the comparative round steel of Comparative Examples 1-6 in the above process steps.

[0092] Table 2

[0093]

[0094] The present invention also sampled the 900MPa grade ultrafine grain steel of Examples 1-8 and Comparative Examples 1-6 to observe its microstructure using a metallographic microscope, and the observation results are listed in Table 3.

[0095] Table 3.

[0096]

[0097]

[0098] As can be seen from Table 3 above, the microstructure matrix of each embodiment of the present invention is tempered martensite + tempered bainite, and it has ultrafine grains with an average grain size of ≤8.5μm.

[0099] also, Figure 1 The image shows a 500x metallographic image of the 900MPa grade ultrafine-grained steel of Example 3 under an optical microscope.

[0100] from Figure 1 As can be seen from the data, the microstructure of this steel is a multiphase structure consisting of tempered martensite and tempered bainite.

[0101] To verify the implementation effects of the various embodiments of the present invention, samples of 900MPa grade ultrafine-grained steel from Examples 1-8 and Comparative Examples 1-6 were taken for mechanical property testing, and the test results are listed in Table 4. The relevant test procedures are as follows:

[0102] Tensile test: Under room temperature conditions, the yield strength Rp of the round steel samples of each embodiment and comparative example was measured according to GB / T 228-2010 "Metallic materials - Tensile testing - Room temperature test method". 0.2 Tensile strength R m Elongation A and reduction of area Z.

[0103] Impact test: The Charpy impact energy Akv of the round steel samples of each embodiment and comparative example at -20℃ was measured according to GB / T 229-2020 "Metallic Materials Charpy Pendulum Impact Test Method".

[0104] Table 4.

[0105]

[0106]

[0107] Note: Table 4 records the values ​​of the longitudinal impact energy Akv at -20℃ from three parallel tests.

[0108] As shown in Table 4, the steels of Examples 1-8 of this invention all have a yield strength greater than 826 MPa, a tensile strength greater than 911 MPa, a yield-to-tensile ratio of 0.88 to 0.92, a Charpy impact energy (Akv) at -20℃ greater than 120 J, an elongation ≥17%, and a reduction of area ≥67%. This fully demonstrates that the 900 MPa-grade ultrafine-grained steel of this invention has a controllable yield-to-tensile ratio, and while ensuring high strength, it also possesses good low-temperature toughness and elongation.

[0109] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0110] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A 900 MPa grade ultra-fine grained steel material, characterized in that, The mass percentage of each chemical element is: C: 0.20-0.28%, Si: 0.15-0.35%, Mn: 0.90-1.40%, Cr: 0.80-1.30%, Mo: 0.30-0.65%, Ni: 0.86-1.25%, Nb: 0.01-0.08%, Al: 0.015-0.05%; the balance is Fe and other inevitable impurities; The mass percentage of each chemical element satisfies: 6≤(Mn+Ni) / C≤12; 4≤(Cr+Mo) / C≤8; The microstructure matrix of the 900MPa grade ultra-fine grain steel material is tempered martensite + tempered bainite, the average grain size of the tempered martensite and the tempered bainite is not more than 10μm, the yield strength is ≥800MPa, the tensile strength Rm is ≥900MPa, the yield strength ratio is 0.85-0.95, the elongation A is ≥15%, the reduction of area is ≥50%, and the Charpy impact energy Akv at -20℃ is >120J.

2. The 900 MPa grade ultrafine-grained steel material according to claim 1, characterized in that, It also contains at least one of the following chemical elements with the following mass percentage: 0 0 0 3. The 900 MPa grade ultrafine-grained steel material according to claim 1, characterized in that, The mass percentage of each element also satisfies: 0.2≤(Nb+2Ti) / (C+N)≤0.

6.

4. The 900 MPa grade ultrafine-grained steel material according to claim 1, characterized in that, Among other inevitable impurities, the content of each impurity element satisfies at least one of the following: P≤0.015%, S≤0.003%, N≤0.015%, O≤0.0025%.

5. The method of producing a 900 MPa grade ultra-fine grained steel material according to any one of claims 1 to 4, characterized in that, It includes the steps of: (1) smelting and casting; (2) heating, forging or rolling, wherein the open rolling or open forging temperature is controlled to be 980-1050℃, the final rolling or final forging temperature is ≥850℃, and the single pass deformation amount is controlled to be 30-60%; (3) quenching: heating the steel material to an austenitizing temperature of 860-960℃, and then water quenching; (4) tempering: the tempering temperature is 580-640℃.

6. The production method according to claim 5, wherein In step (2), the cast blank is heated, and then once rolled or forged to directly roll or forge the cast blank to the size of the final product, wherein the cast blank heating temperature is 1100-1200℃.

7. The production method according to claim 5, wherein In step (2), the cast blank is first heated and intermediate blank rolled to roll the cast blank into an intermediate blank, and then the intermediate blank is heated and finally rolled or forged to obtain the size of the final product, wherein the cast blank heating temperature is 1100-1200℃; the intermediate blank heating temperature is 1100-1200℃.

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