High-strength and high-toughness easy-cutting medium-carbon low-alloy steel, and manufacturing method therefor

Abstract

Disclosed in the present invention is high-strength and high-toughness easy-cutting medium-carbon low-alloy steel. The steel comprises Fe and unavoidable impurities, and further comprises the following chemical elements in percentages by mass: 0.32-0.37% of C, 0.05-0.44% of Si, 1.16-1.44% of Mn, 0.021-0.054% of S, 0.15-0.44% of Cr, 0.02-0.30% of Ni, 0.06-0.24% of Mo, 0.03-0.29% of Cu, 0.020-0.050% of Al, 0.003-0.030% of Nb, 0.06-0.14% of V, 0.005-0.014% of N, and 0.0005-0.0034% of Ca. Further disclosed in the present invention is a manufacturing method for high-strength and high-toughness easy-cutting medium-carbon low-alloy steel. The method comprises the steps of: smelting; casting; heating: controlling the heating temperature to be 1050-1250°C, and maintaining the temperature for 3-24 h; and forging or rolling: controlling the final rolling temperature or the final forging temperature to be greater than or equal to 900°C, and performing cooling after rolling or after forging.

Description

A high-strength and tough free-machining medium-carbon low-alloy steel and its manufacturing method Technical Field

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

[0002] High-strength medium-carbon steel has a wide range of applications in industrial production. It is often used to manufacture high-safety machinery and structural components, which can enhance strength, reduce weight, improve quality and efficiency, such as automotive parts or key load-bearing components of engineering machinery. High-strength medium-carbon steel not only needs to have high strength, but also high toughness, easy processing, and precisely controllable hardenability to ensure that the mechanical parts made from it have sufficient safety and reliability.

[0003] In the current technology, when preparing high-strength medium-carbon alloy steel, those skilled in the art generally select appropriate chemical composition and then use quenching + tempering heat treatment or controlled rolling + controlled cooling processes for production.

[0004] In the production of high-strength steel using a quenching and tempering process, optimizing the alloy element content can enable the steel to form a martensitic structure during cooling. After heat treatment, a tempered martensitic structure with comprehensive strength and toughness can be obtained. This type of high-strength steel, dominated by (tempered) martensite, has a high dislocation density, which leads to poor impact toughness. Moreover, if micro-defects such as microcracks occur during tensile testing, the steel will rapidly fracture and fail.

[0005] While high-strength steel can be produced without quenching and tempering, thus obtaining non-quenched and tempered steel, the process of controlling the rolling and cooling process is more difficult, which can affect the mechanical properties and overall uniformity of the steel. Summary of the Invention

[0006] One objective of this invention is to provide a high-strength, high-toughness, free-machining medium-carbon low-alloy steel. This steel has a microstructure primarily composed of bainite, exhibiting high strength along with good impact toughness, elongation, and reduction of area. Furthermore, the medium-carbon low-alloy steel described in this invention also possesses excellent machinability and precise control over hardenability, meeting the performance requirements of steel in automotive and construction machinery applications, and thus showing promising prospects for widespread application.

[0007] To achieve the above objectives, this invention proposes a high-strength, high-toughness, free-machining medium-carbon low-alloy steel, wherein the low-alloy steel, in addition to containing Fe and unavoidable impurities, also contains the following chemical elements in weight percentage:

[0008] C: 0.32–0.37%, Si: 0.05–0.44%, preferably 0.16–0.37%, Mn: 1.16–1.44%, S: 0.021–0.054%, preferably 0.031–0.043%, Cr: 0.15–0.44%, Ni: 0.02–0.30%, preferably 0.08–0.27%, Mo: 0.06–0.24%, preferably 0.12–0.19%, Cu: 0.03 ~0.29%, preferably 0.09~0.25%; Al: 0.020~0.050%, preferably 0.028~0.047%; Nb: 0.003~0.030%, preferably 0.014~0.030%; V: 0.06~0.14%; N: 0.005~0.014%, preferably 0.007~0.013%; Ca: 0.0005~0.0034%, preferably 0.0010~0.0024%.

[0009] Preferably, in the high-strength, high-toughness, free-machining medium-carbon low-alloy steel of the present invention, the low-alloy steel has the following chemical elements in weight percentage:

[0010] C: 0.32–0.37%, Si: 0.05–0.44%, preferably 0.16–0.37%, Mn: 1.16–1.44%, S: 0.021–0.054%, preferably 0.031–0.043%, Cr: 0.15–0.44%, Ni: 0.02–0.30%, preferably 0.08–0.27%, Mo: 0.06–0.24%, preferably 0.12–0.19%, Cu: 0.03–0.29%. Preferably, the content is 0.09–0.25%, Al: 0.020–0.050%, preferably 0.028–0.047%, Nb: 0.003–0.030%, preferably 0.014–0.030%, V: 0.06–0.14%, N: 0.005–0.014%, preferably 0.007–0.013%, Ca: 0.0005–0.0034%, preferably 0.0010–0.0024%; the balance is Fe and unavoidable impurities.

[0011] In this invention, the inventors added vanadium, niobium, and aluminum microalloying elements to the steel. This elemental composite microalloying enhances the precipitation strengthening effect of the microalloying elements and refines the grain size of the steel's microstructure. Additionally, a certain amount of sulfur and copper were added to the steel to improve the machinability of the medium-carbon low-alloy steel prepared by this invention. Furthermore, the contents of phosphorus, oxygen, and hydrogen were controlled to reduce the material's hydrogen embrittlement sensitivity and improve the fatigue resistance of the resulting parts.

[0012] Specifically, in the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention, the design principles of each chemical element are as follows:

[0013] C: In the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, carbon (C) can improve the hardenability of the steel, enabling it to form a phase transformation structure with higher hardness during quenching and cooling. When the C content in the steel is too low, the phase transformation structure, such as bainite, will be too low, and the steel will not obtain sufficient tensile strength. At the same time, the C content in the steel should not be too high. While increasing the C content will increase the proportion of hard phases and improve the hardness of the steel, it will also lead to a decrease in toughness. Therefore, considering the influence of C content on the performance of the steel, the mass percentage of C in the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention is controlled between 0.32% and 0.37%.

[0014] Si: In the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, Si is beneficial for improving hardenability and increasing the strength of the steel. Adding an appropriate amount of Si can prevent the formation of coarse carbides. However, it should be noted that the Si content in the steel should not be too high, as excessive Si content will reduce the impact toughness of the steel. Therefore, in the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, the mass percentage of Si is controlled between 0.05% and 0.44%, preferably between 0.16% and 0.37%.

[0015] Mn: In the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, Mn mainly exists in solid solution form. It can effectively improve the hardenability of the steel and form a high-strength low-temperature phase transformation structure during quenching, resulting in good strength and toughness. However, it should be noted that the Mn content in the steel should not be too high. Excessive Mn content leads to the formation of more retained austenite, reducing the yield strength of the steel and easily causing center segregation. Therefore, in the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, the mass percentage of Mn is controlled between 1.16% and 1.44%.

[0016] S: In the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention, sulfur (S) can form sulfides with manganese (Mn), controlling their morphology and effectively improving the steel's machinability. However, it should be noted that excessively high S content in the steel leads to coarse inclusions, which not only hinders hot working but also reduces the steel's impact resistance and fatigue performance. Therefore, in the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention, the mass percentage of S is controlled between 0.021% and 0.054%, preferably between 0.031% and 0.043%.

[0017] Cr: In the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, Cr can significantly improve the hardenability of the steel and can also form a hardened bainitic structure, thereby increasing the strength of the steel. Correspondingly, the Cr content in the steel should not be too high. When the Cr content in the steel is too high, coarse carbides will form, reducing the impact performance of the steel. Therefore, considering the influence of Cr content on the properties of steel, in the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, the mass percentage of Cr is controlled between 0.15% and 0.44%.

[0018] Ni: In the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, Ni exists in solid solution form, which can improve hardenability and effectively enhance the toughness of the steel. However, the Ni content in the steel should not be too high. Excessive Ni content not only increases costs but also leads to an excessively high content of retained austenite, thereby reducing the strength of the steel. Therefore, in the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, the mass percentage of Ni is controlled between 0.02% and 0.30%, preferably between 0.08% and 0.27%.

[0019] Mo: In the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention, Mo can be dissolved in the steel and improve its hardenability, which is beneficial for controlling the microstructure and thus improving its strength and toughness. However, considering that Mo is a precious metal element, the Mo content in the steel should not be too high in order to effectively control the cost of the alloy. Therefore, considering the production cost and the beneficial effects of adding Mo, the mass percentage of Mo in the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention is controlled between 0.06% and 0.24%, preferably between 0.12% and 0.19%.

[0020] Cu: In the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention, adding an appropriate amount of Cu can improve hardenability and strength, and also enhance the steel's weather resistance, corrosion resistance, and machinability. However, it should be noted that the Cu content in the steel should not be too high. If the Cu content is too high, it will accumulate at the grain boundaries during heating, leading to grain boundary weakening and cracking. Therefore, in the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention, the mass percentage of Cu is controlled between 0.03% and 0.29%, preferably between 0.09% and 0.25%.

[0021] Al: In the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention, Al can combine with N to form fine precipitates, thereby pinning grain boundaries and inhibiting austenite grain growth. However, it should be noted that the Al content in the steel should not be too high. Excessive Al content will lead to the formation of larger oxides, and coarse hard inclusions will reduce the impact toughness and fatigue performance of the steel. Therefore, in the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention, the mass percentage of Al is controlled between 0.020% and 0.050%, preferably between 0.028% and 0.047%.

[0022] Nb: In the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention, the addition of Nb to the steel can form fine precipitates, which can inhibit recrystallization of the steel and effectively refine the grains. Grain refinement plays an important role in improving the mechanical properties of steel, especially strength and toughness. Simultaneously, grain refinement also helps reduce the hydrogen embrittlement sensitivity of the steel. However, the Nb content in the steel should not be too high. Besides cost considerations, when the Nb content in the steel is too high, coarse NbC particles will form during the smelting process. These particles not only fail to provide precipitation strengthening but also disrupt the continuity of the material due to the inclusions, leading to a decrease in the strength of the steel. Therefore, to maximize the beneficial effects of Nb, in the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention, the mass percentage of Nb is controlled between 0.003% and 0.030%, preferably between 0.014% and 0.030%.

[0023] V: In the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, vitamin V can improve the hardenability of the steel and is an important alloying element for strengthening non-quenched and tempered steel. V can form precipitates with carbon (C) or nitrogen (N) in the steel, resulting in precipitation strengthening, and can pin grain boundaries, refine grains, and further improve the strength of the steel. Correspondingly, the content of V in the steel should not be too high. If the V content is too high, coarse VC particles will form, reducing the impact toughness of the steel. Therefore, in the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, the mass percentage of V is controlled between 0.06% and 0.14%.

[0024] N: In the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention, N is an interstitial atom that can form nitrides or carbonitrides in the steel, i.e., MX-type precipitates (X refers to C and / or N elements), playing a role in precipitation strengthening and grain refinement strengthening. However, it should be noted that the N content in the steel should not be too high. When the N content in the steel is too high, coarse particles will form, which cannot play a role in grain refinement. This is because N, as an interstitial atom, will accumulate at grain boundaries and defects, leading to a decrease in the impact toughness of the steel. Therefore, to avoid the accumulation of N in the steel, in the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention, the mass percentage content of N is controlled between 0.005% and 0.014%, preferably 0.007% to 0.013%.

[0025] Ca: In the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, Ca can improve the size and morphology of sulfide inclusions in the steel. However, Ca easily forms coarse inclusions, which affects the fatigue performance of the final product. Therefore, in the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, the mass percentage content of Ca is controlled at 0.0005–0.0034%, preferably 0.0010–0.0024%.

[0026] Preferably, the unavoidable impurities in the high-strength and tough free-machining medium-carbon low-alloy steel of the present invention include P, Sn, O and H, wherein P≤0.015%, Sn≤0.020%, O≤0.002%, and H≤0.0002%.

[0027] In this invention, P, O, H, and Sn are all impurity elements in steel. Where technical conditions permit, to obtain steel with better performance and superior quality, the content of impurity elements in the material should be reduced as much as possible. Specifically:

[0028] P: In the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, phosphorus (P) tends to segregate at grain boundaries, reducing grain boundary bonding energy and deteriorating the steel's impact toughness. Therefore, in the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, the mass percentage of phosphorus can be controlled to P ≤ 0.015%.

[0029] Sn: In the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention, the low melting point and soft texture of Sn can effectively improve the machinability of the steel within a certain content range; however, Sn is a harmful element in steel and is not conducive to environmental recycling. Therefore, in the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention, the mass percentage content of Sn can be controlled to Sn≤0.020%.

[0030] O: In the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, oxygen (O) can form oxides and composite oxides with al (Al) in the steel. Therefore, in the economical high-hardenability medium-carbon low-alloy steel described in this invention, to ensure the uniformity of the steel's microstructure, low-temperature impact resistance, and fatigue performance, the mass percentage of oxygen (O) can be controlled to O ≤ 0.002%.

[0031] H: In the economical high hardenability medium-carbon low-alloy steel described in this invention, hydrogen (H) tends to accumulate at defects, making it prone to hydrogen-induced delayed fracture, which is particularly sensitive to high-strength steels. Therefore, in the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention, the mass percentage of H can be controlled to H ≤ 0.0002%.

[0032] Preferably, in the high-strength, high-toughness, free-machining medium-carbon low-alloy steel described in this invention, the machinability coefficient M of the low-alloy steel is... f The range is 0.20 to 0.50, preferably 0.27 to 0.50, where M f = [Cu] + 10 × [Sn] + 5 × [S], where the element symbol is replaced with the value before the percentage sign of the element's mass percentage content.

[0033] In this invention, sulfur (S) can help Mn form fine MnS inclusions, which is beneficial for improving machinability. However, a large amount of S can lead to coarse inclusions and potentially weaken grain boundaries. Cu and Sn also contribute to machinability, but excessive Cu can negatively impact the high-temperature plasticity of steel, while excessive Sn can worsen the plasticity and toughness of steel; therefore, their content needs to be controlled. Thus, the machinability coefficient M... f Set it to 0.20~0.50.

[0034] Preferably, in the high-strength, high-toughness, free-machining medium-carbon low-alloy steel of the present invention, the microalloying element coefficient rM / N of the low-alloy steel ranges from 25 to 58, preferably from 26 to 56, wherein r M / N = (7×[Al]+2×[Nb]+[V] / 2) / [N], where the element symbol is replaced with the value before the percentage sign of the element's mass percentage content.

[0035] In this invention, the microalloying element coefficient r is used. M / N This term describes the fineness and dispersion of MX precipitates. Al, Nb, and V can all form MX microalloyed precipitates, playing a role in refining grains and maintaining grain size stability. If the microalloying element coefficient is too large, coarse precipitates are easily formed during steel preparation, reducing the steel's impact toughness and fatigue life; conversely, if the microalloying element coefficient is too small, an appropriate amount of fine precipitates will not form, failing to refine the grains.

[0036] In addition, the inventors improved the microalloying element coefficient r. M / N The range was optimized. Microalloying coefficient r M / N Related to nanoscale precipitates, when the microalloying coefficient r M / N When the value is higher than 58, it leads to the presence of coarse precipitates in the steel, which cannot play a precipitation strengthening role, resulting in the adverse effects of inclusions and a decrease in fatigue strength; while when the microalloying coefficient r is higher... M / N When the value is below 25, the amount of precipitates is insufficient, failing to achieve the effect of dispersion strengthening. Therefore, in this invention, the inventors have adjusted the microalloying element coefficient r. M / N The range is controlled between 25 and 58.

[0037] Preferably, in the high-strength, high-toughness, free-machining medium-carbon low-alloy steel of the present invention, the critical ideal diameter DI for hardenability of the low-alloy steel is 2.70–4.80 inches, preferably 2.76–4.77 inches, where DI = 0.54 × [C] × (5.10 × [Mn] - 1.12) × (0.70 × [Si] + 1) × (0.363 × [Ni] + 1) ×

[0038] (2.16×[Cr]+1)×(3.00×[Mo]+1)×(0.365×[Cu]+1)×(1.73×[V]+1), where the element symbols are replaced with the values ​​before the percentage sign of the element's mass percentage content.

[0039] In this invention, the critical ideal diameter (DI) for hardenability of high-strength, high-toughness, free-machining medium-carbon low-alloy steel was optimized and controlled within the range of 2.7 to 4.8 inches. This is because: when the DI value is below 2.7 inches, the hardenability of the steel is insufficient; while when the DI value is above 4.8 inches, manufacturing difficulties and higher costs arise.

[0040] Preferably, in the high-strength, high-toughness, free-machining medium-carbon low-alloy steel of the present invention, the bainite transformation temperature coefficient T of the low-alloy steel is... B The temperature range is 531–556℃, preferably 533–551℃, where T B =630-45×[Mn]-40×[V]-35×[Si]-30×[Cr]-25×[Mo]-20×[Ni], where the element symbols are replaced with the values ​​before the percentage sign of the element's mass percentage content.

[0041] In this invention, the steel is cooled to a temperature equal to or less than the bainite transformation temperature T. B This can cause bainite to form within the steel. Therefore, the bainite transformation temperature T... B The value is controlled between 531 and 556℃.

[0042] Preferably, the microstructure of the high-strength and tough free-machining medium-carbon low-alloy steel of the present invention includes bainite, and the area ratio of bainite on any cross section of the low-alloy steel is more than 80%.

[0043] In some embodiments, the microstructure of the high-strength, high-toughness, free-machining medium-carbon low-alloy steel of the present invention further includes one or more of ferrite, pearlite, martensite, and retained austenite.

[0044] Preferably, the austenitic grain size of the high-strength and tough free-machining medium-carbon low-alloy steel of the present invention is grade 6 or above.

[0045] Preferably, the high-strength and tough free-machining medium-carbon low-alloy steel of the present invention meets at least one of the following requirements: tensile strength R m ≥800MPa, yield strength R eL ≥550MPa, elongation A≥15%, reduction of area Z≥35%, Charpy impact energy Akv≥40J.

[0046] Preferably, the end hardenability of the high-strength and tough free-machining medium-carbon low-alloy steel of the present invention satisfies the following Rockwell hardness: 51-54 HRC at J5mm, 35-47 HRC at J13mm, and 26-40 HRC at J25mm.

[0047] Another objective of this invention is to provide a method for manufacturing high-strength, tough, and easy-to-cut medium-carbon low-alloy steel. This method is simple to produce and has a reasonable process design. The high-strength medium-carbon low-alloy steel obtained not only has good impact toughness and plasticity, but is also easy to machine and can meet the requirements for precise control of hardenability. The parts prepared by this method have good fatigue resistance and low hydrogen embrittlement sensitivity, and have good application prospects.

[0048] To achieve the above objectives, the present invention provides a method for manufacturing high-strength, high-toughness, free-machining medium-carbon low-alloy steel, comprising the following steps performed sequentially:

[0049] Smelting;

[0050] Casting;

[0051] Heating: Control the heating temperature to 1050~1250℃, and the holding time to 3~24h;

[0052] Forging or rolling: Control the final rolling temperature or final forging temperature ≥900℃, and cool after rolling or forging.

[0053] In this invention, controlling the heating temperature to be 1050–1250°C and the holding time to be 3–24 h ensures that the non-quenched and tempered steel of this invention is fully austenitized during the heating process.

[0054] In some embodiments, in step (4) of the manufacturing method of the present invention, the material is directly rolled or forged to the finished size.

[0055] In some other embodiments, when step (4) of the manufacturing method of the present invention is rolling, the method further includes the following steps after step (4):

[0056] (5) Secondary heating: The temperature of the secondary heating is controlled at 1050~1250℃, the holding time is 3~12h, and then the secondary rolling is carried out. The final rolling temperature of the secondary rolling is controlled at ≥900℃, and then the rolling is cooled.

[0057] The high-strength, high-toughness, free-machining medium-carbon low-alloy steel and its manufacturing method described in this invention have the following advantages and beneficial effects compared to the prior art:

[0058] The high-strength, tough, and free-machining medium-carbon low-alloy steel described in this invention is a non-quenched and tempered steel, which does not require a quenching and tempering heat treatment process. This non-quenched and tempered steel has a bainite-based microstructure with fine precipitates dispersed in the bainite matrix, giving it good plasticity and toughness, and making it easy to machine.

[0059] The high-strength and tough free-machining medium-carbon low-alloy steel described in this invention, through rational design of chemical composition and combined with optimized process, can effectively overcome the problem of insufficient toughness in existing medium-carbon low-alloy steels, thereby achieving high fatigue life and high safety and reliability.

[0060] In some embodiments, the tensile strength R of the high-strength, high-toughness, free-machining, medium-carbon low-alloy steel described in this invention... m ≥800MPa, yield strength R eL ≥550MPa, elongation A≥15%, reduction of area Z≥35%, Charpy impact energy A kv ≥40J.

[0061] In some embodiments, the end hardenability of the high-strength and tough free-machining medium-carbon low-alloy steel of the present invention satisfies the following Rockwell hardness: 51-54 HRC at J5mm, 35-47 HRC at J13mm, and 26-40 HRC at J25mm.

[0062] The manufacturing method for high-strength and tough free-cutting medium-carbon low-alloy steel described in this invention has a reasonable process design and a wide process window, enabling mass commercial production on bar production lines. The obtained medium-carbon low-alloy steel can meet the high-strength and toughness requirements of automobiles and engineering machinery, and has a very good prospect for promotion and application. Detailed Implementation

[0063] The following will further explain and illustrate the high-strength and tough free-machining medium-carbon low-alloy steel and its manufacturing method according to the present invention with reference to specific embodiments. However, this explanation and description do not constitute an improper limitation on the technical solution of the present invention.

[0064] Examples 1-8 and Comparative Examples 1-5

[0065] The high-strength, high-toughness, free-machining medium-carbon low-alloy steels described in Examples 1-8 of this invention were all prepared using the following steps:

[0066] (1) Smelting: Smelting can be carried out by electric furnace smelting or converter smelting, and then refined and vacuum treated.

[0067] In some implementations, a vacuum induction furnace can be used for smelting.

[0068] In some specific implementations, smelting can be carried out using a 50kg vacuum induction furnace, a 150kg vacuum induction furnace, or a 500kg vacuum induction furnace. Alternatively, smelting can be carried out using an electric furnace smelting + ladle refining + vacuum degassing method. Another method is to use a converter + ladle refining + vacuum treatment method.

[0069] (2) Casting: Casting is carried out by die casting or continuous casting to obtain ingots;

[0070] (3) Heating: The obtained ingot is fed into a heating furnace for heating, and the heating temperature is controlled at 1050-1250℃, and the holding time is 3-24h.

[0071] (4) Forging or rolling: Control the final rolling temperature or final forging temperature ≥900℃, and cool after rolling or forging.

[0072] In some implementations, when a forging process is used, the billet can be forged directly to the final finished size; when a rolling process is used, the billet can be rolled directly to the final finished size.

[0073] Alternatively, in some specific implementations, when using a rolling process, the steel billet can be rolled to a specified intermediate billet size first, and then subjected to secondary heating and secondary rolling to the final finished product size. The intermediate heating temperature of the intermediate billet can be controlled between 1050 and 1250°C, the holding time can be controlled between 3 and 12 hours, the secondary final rolling temperature can be controlled at ≥900°C, and cooling is performed after rolling.

[0074] The specific production process of the high-strength and high-toughness free-machining medium-carbon low-alloy steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4 is as follows:

[0075] Example 1: Smelting was carried out in a 50kg vacuum induction furnace. The molten steel obtained from smelting was cast into ingots, and the ingots were heated and then forged. The heating temperature was controlled at 1050℃, and after holding at that temperature for 3 hours, forging was carried out. The final forging temperature was controlled at 910℃, and the final forging was into a bar with a diameter of Φ60mm, which was then air-cooled.

[0076] Example 2: Smelting was carried out in a 150kg vacuum induction furnace. The molten steel obtained from smelting was cast into ingots, and the ingots were heated and then forged. The heating temperature was controlled at 1100℃, and after holding at that temperature for 24 hours, forging was carried out. The final forging temperature was controlled at 1000℃, and the final forging was into a Φ92mm bar, which was then slowly cooled in a stack.

[0077] Example 3: The steel was smelted in an electric furnace, followed by LF refining and VD vacuum treatment, and then continuously cast into 320mm × 425mm continuous casting billets. After preheating, the billets were gradually heated to 1250℃ and held at that temperature for 4 hours before subsequent rolling. The billets were descaled by high-pressure water after exiting the heating furnace and then rolled. The final rolling temperature was controlled at 1000℃, ultimately rolling them into Φ130mm bars, which were then air-cooled.

[0078] Example 4: The steel was smelted in an electric furnace, followed by LF refining and VD vacuum treatment, and then continuously cast into 280mm × 280mm continuous casting billets. The billets were slowly heated to 1150℃, held at that temperature for 16 hours, and then rolled. After exiting the heating furnace, the billets were descaled by high-pressure water and then rolled, with the final rolling temperature controlled at 970℃, ultimately rolled into Φ80mm bars, which were then air-cooled.

[0079] Example 5: The steel was smelted in an electric furnace, followed by LF refining and VD vacuum treatment, and then cast into a 320mm × 425mm continuous casting billet. The billet was preheated to 1230℃ and held for 5 hours before rolling. After exiting the furnace, the billet was descaled with high-pressure water and rolled into an intermediate billet. The final rolling temperature was controlled at 1050℃, resulting in an intermediate billet size of 220mm × 220mm, which was then air-cooled. The intermediate billet was then slowly heated to 1200℃, held for 6 hours, and then entered the soaking zone. After exiting the furnace, it was descaled with high-pressure water and then rolled, with the final rolling temperature further controlled at 950℃. The finished bar stock had a diameter of Φ60mm and was air-cooled after rolling.

[0080] Example 6: The steel was smelted in an electric furnace, refined, and vacuum-treated, then continuously cast into 220mm × 220mm billets. The billets were slowly heated to 1180℃ and held for 6 hours before rolling. After descaling with high-pressure water, the billets were rolled into intermediate billets, with the final rolling temperature controlled at 1000℃. The resulting intermediate billet dimensions were 140mm × 140mm. The intermediate billets were then heated to 1250℃, held for 3 hours, and then descaled with high-pressure water before rolling. The final rolling temperature of the intermediate billets was controlled at 920℃, resulting in finished bars with a diameter of Φ30mm. The finished bars were then air-cooled.

[0081] Example 7: The billet was smelted in a converter, refined and vacuum treated, and then cast in a die to obtain a billet. The billet was slowly heated to 1200°C and held for 9 hours before rolling. The final rolling temperature was controlled at 970°C, and the billet was finally rolled into Φ90mm bars and air-cooled after rolling.

[0082] Example 8: The material was smelted in a converter, refined, and vacuum-treated before being cast into a billet. The billet was slowly heated to 1180°C and held for 12 hours before being removed from the furnace and descaled by high-pressure water. Rolling then began to produce an intermediate billet, with the final rolling temperature controlled at 950°C. The resulting intermediate billet had dimensions of 260mm × 260mm. The intermediate billet was then preheated to 1050°C and held for 12 hours before being removed from the furnace and descaled by high-pressure water. Rolling continued, with the final rolling temperature of the intermediate billet further controlled at 900°C. The finished bar had a diameter of Φ50mm and was air-cooled after rolling.

[0083] Comparative Example 1: Smelting was carried out in a 50kg vacuum induction furnace. Molten steel was cast into steel ingots, heated and forged into billets. The heating temperature was 1050℃, and after holding at that temperature for 3 hours, forging was carried out. The final forging temperature was controlled at 910℃, and the final forging was into bars with a diameter of Φ60mm. After forging, the bars were air-cooled.

[0084] Comparative Example 2: Smelting was carried out in a 150kg vacuum induction furnace. Molten steel was continuously cast into steel ingots, heated and forged into billets. The heating temperature was 1100℃, and after holding at that temperature for 4 hours, forging was carried out. The final forging temperature was controlled at 1000℃, and the final forging was into Φ92mm bars. After slow cooling, the bars were machined and stripped.

[0085] Comparative Example 3: Selected from commercially available steel, therefore its preparation process cannot be described here.

[0086] Comparative Example 4: The material was smelted in an electric furnace, refined, and vacuum-treated before being cast into a 320mm × 425mm continuous casting billet. The billet was slowly heated to 1230℃ and held for 8 hours before being removed from the furnace and descaled by high-pressure water before rolling to form an intermediate billet. The final rolling temperature was controlled at 1050℃, resulting in an intermediate billet size of 220mm × 220mm. The intermediate billet was then preheated to 1200℃ and held for 6 hours before being removed from the furnace and descaled by high-pressure water before rolling. The final rolling temperature was controlled at 950℃, resulting in a finished bar with a diameter of Φ60mm. After rolling, the bar was air-cooled.

[0087] Comparative Example 5: Selected from commercially available steel, therefore its preparation process cannot be described here.

[0088] Tables 1-1, 1-2, and 1-3 list the mass percentages of each chemical element in the high-strength, high-toughness, free-machining medium-carbon low-alloy steels of Examples 1-8 and the comparative steels of Comparative Examples 1-5.

[0089] Table 1-1. (wt.%, balance Fe and other unavoidable impurities other than P, O, H and Sn)

[0090] Table 1-2. (wt.%, balance Fe and other unavoidable impurities other than P, O, H and Sn)

[0091] Table 1-3. Note: In the table above, DI = 0.54*[C]*(5.10*[Mn]-1.12)*(0.70*[Si]+1)*(0.363*[Ni]+1)*(2.16*[Cr]+1)*(3.00*[Mo]+1)*(0.365*[Cu]+1)*(1.73*[V]+1); r M / N =(7*[Al]+2*[Nb]+[V] / 2) / [N];M f = [Cu] + 10 * [Sn] + 5 * [S]; T B =630-45*[Mn]-40*[V]-35*[Si]-30*[Cr]-25*[Mo]-20*[Ni]; In the above formula, the element symbols are replaced with the values ​​before the percentage sign of the element's mass percentage content.

[0092] Tables 2-1 and 2-2 list the specific process parameters for the high-strength, high-toughness, free-machining medium-carbon low-alloy steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4.

[0093] Table 2-1.

[0094] Table 2-2.

[0095] It should be noted that in the rolling process of Embodiments 5, 6, 8 and Comparative Example 4 of the present invention, the steel billet is first rolled to the specified intermediate billet size, and then subjected to secondary heating and secondary rolling to the final finished product size.

[0096] To verify the performance of the high-strength and high-toughness free-machining medium-carbon low-alloy steel described in this invention, samples were taken from the high-strength and high-toughness free-machining medium-carbon low-alloy steels of Examples 1-8 and the comparative steels of Comparative Examples 1-5. Samples were prepared according to GB / T 2975, and then metallographic examination was performed according to GB / T 13299. The metallographic structure was analyzed, and the area ratio of bainite on any cross-section of the medium-carbon low-alloy steel was calculated using the truncation method. The austenite grain size was determined according to ASTM E112, with an austenitizing temperature of 910℃ and a holding time of 4 hours, followed by water quenching. For the austenite grain size analysis, the steel samples from each example and comparative example were fully austenitized and then water-quenched to prepare metallographic samples. Picric acid-alcohol solution was used to etch and reveal the austenite grain boundaries, and the austenite grain size was analyzed using a metallographic microscope according to ASTM E112.

[0097] Table 3 lists the metallographic structure and austenite grain size analysis results of the high-strength and tough free-machining medium-carbon low-alloy steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4.

[0098] Table 3.

[0099] As can be seen from Table 3 above, the microstructure of the high-strength and tough free-machining medium-carbon low-alloy steels of Examples 1-8 prepared by the manufacturing method described in this invention is mainly bainite, and the area of ​​bainite on any cross-section of the medium-carbon low-alloy steel is greater than or equal to 80%. In addition, besides bainite, the microstructure may contain one or more of ferrite, pearlite, retained austenite, or martensite. Although the microstructure of Comparative Examples 1-5 is also bainite and ferrite + pearlite or martensite and a small amount of retained austenite, the bainite content of Comparative Examples 3-5 is relatively low, especially the microstructure of Comparative Example 5, which is ferrite + pearlite and does not contain bainite.

[0100] Furthermore, as can be seen from Table 3 above, the austenite grain size of the high-strength and tough free-machining medium-carbon low-alloy steels of Examples 1-8 prepared by the manufacturing method described in this invention is all grade 6 or above, while the austenite grain size of Comparative Example 1 is only grade 4.5, which is relatively coarse.

[0101] To further illustrate the performance of the medium-carbon low-alloy steel prepared in this invention, samples of the high-strength and tough free-machining medium-carbon low-alloy steels of Examples 1-8 and the comparative steels of Comparative Examples 1-5 were taken again for performance testing, and the test results are listed in Table 4. Wherein:

[0102] Mechanical property testing: Samples were prepared in accordance with GB / T 2975, and tensile and impact tests were performed in accordance with GB / T 228.1 and GB / T 229 respectively to obtain the mechanical properties of the steels in each embodiment and comparative example.

[0103] Impact energy test: Samples are taken and prepared from hot-rolled round steel according to national standard GB / T 225. End hardenability test (Jominy test) is carried out with reference to GB / T 5216. The normalizing temperature is controlled at 920±10℃ and the quenching temperature is controlled at 870±5℃. Rockwell hardness test is carried out according to GB / T 230.2 to obtain the hardness value (HRC) at a specific location, such as the hardness at 5mm away from the quenched end, i.e., J5mm.

[0104] Machining performance test: The steels of each embodiment and comparative example were machined using a conventional lathe, and the chips were collected to evaluate the machinability of the steels: granular chips that are easy to break were rated as "good", while spiral chips that are continuous and not easy to break were rated as "poor", and chips that are in between and have a "C" shape were rated as "medium".

[0105] Table 4 lists the performance test results of the high-strength and tough free-machining medium-carbon low-alloy steels of Examples 1-8 and Comparative Examples 1-5.

[0106] Table 4. Note: In Table 3 above, there are two sets of data in a single column, representing the results obtained from two tests.

[0107] As can be seen from Table 4 above, the comprehensive performance of the high-strength and high-toughness free-machining medium-carbon low-alloy steels of Examples 1-8 prepared by the manufacturing method described in this invention is significantly better than that of the comparative steels of Examples 1-5. Specifically, the high-strength and high-toughness free-machining medium-carbon low-alloy steels of Examples 1-8 have a yield strength greater than 550 MPa, a tensile strength Rm greater than 800 MPa, an elongation A greater than 15%, a reduction of area Z greater than 35%, and a Charpy impact energy A0. kv2 All values ​​are greater than 40 J. Furthermore, the high-strength, high-toughness, medium-carbon low-alloy steel of this invention not only possesses good impact toughness and plasticity, but also good end-hardenability, with J5mm values ​​between 51-54 HRC, J13mm values ​​between 35-47 HRC, and J25mm values ​​between 26-40 HRC; it is also easy to cut (the chips are all granular), meeting the needs of automotive and construction machinery applications.

[0108] In contrast, Comparative Examples 1-5 all contained parameters in their chemical element composition design that did not meet the design specifications of this invention. Therefore, compared to the high-strength and tough free-machining medium-carbon low-alloy steels of Examples 1-8, the austenite grains in Comparative Example 1 were coarse, and the impact toughness was low (not exceeding 35J); the comparative steel in Comparative Example 2 was mainly bainite with a small amount of pearlite, and the steel strength was relatively low (tensile strength not exceeding 800MPa); Comparative Example 3 had high strength, but poor impact toughness and plasticity, posing a potential safety hazard in service; Comparative Example 4 contained a small amount of martensite, had low toughness, and was not easy to break chips during cutting, resulting in low processing efficiency and difficulty in meeting the application requirements; and the end hardenability of Comparative Example 5 was low (J13mm as low as 34HRC, J25mm as low as 24HRC), which could not meet the requirements for large-diameter bar stock.

[0109] Furthermore, 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 low-alloy steel, characterized in that, In addition to Fe and unavoidable impurities, the low-alloy steel contains the following chemical elements in percentage by mass: C: 0.32–0.37%, Si: 0.05–0.44%, preferably 0.16–0.37%, Mn: 1.16–1.44%, S: 0.021–0.054%, preferably 0.031–0.043%, Cr: 0.15–0.44%, Ni: 0.02–0.30%, preferably 0.08–0.27%, Mo: 0.06–0.24%, preferably 0.12–0.19%, Cu: 0.03 ~0.29%, preferably 0.09~0.25%; Al: 0.020~0.050%, preferably 0.028~0.047%; Nb: 0.003~0.030%, preferably 0.014~0.030%; V: 0.06~0.14%; N: 0.005~0.014%, preferably 0.007~0.013%; Ca: 0.0005~0.0034%, preferably 0.0010~0.0024%.

2. The low-alloy steel as described in claim 1, characterized in that, The low-alloy steel has the following chemical elements in percentage by mass: C: 0.32–0.37%, Si: 0.05–0.44%, preferably 0.16–0.37%, Mn: 1.16–1.44%, S: 0.021–0.054%, preferably 0.031–0.043%, Cr: 0.15–0.44%, Ni: 0.02–0.30%, preferably 0.08–0.27%, Mo: 0.06–0.24%, preferably 0.12–0.19%, Cu: 0.03–0.29%. Preferably, the content is 0.09–0.25%, Al: 0.020–0.050%, preferably 0.028–0.047%, Nb: 0.003–0.030%, preferably 0.014–0.030%, V: 0.06–0.14%, N: 0.005–0.014%, preferably 0.007–0.013%, Ca: 0.0005–0.0034%, preferably 0.0010–0.0024%; the balance is Fe and unavoidable impurities.

3. The low-alloy steel as described in claim 1 or 2, characterized in that, Unavoidable impurities include P, Sn, O, and H, where P ≤ 0.015%, Sn ≤ 0.020%, O ≤ 0.0020%, and H ≤ 0.0002%.

4. The low-alloy steel as described in claim 1 or 2, characterized in that, The machinability coefficient M of the low alloy steel f The range is 0.20 to 0.50, preferably 0.27 to 0.50, where M f = [Cu] + 10 × [Sn] + 5 × [S], where the element symbol is replaced with the value before the percentage sign of the element's mass percentage content.

5. The low-alloy steel as described in claim 1 or 2, characterized in that, The microalloying element coefficient r of the low alloy steel M / N The range is 25–58, preferably 26–56, where r M / N = (7×[Al]+2×[Nb]+[V] / 2) / [N], where the symbol of each element is replaced with the value before the percentage sign of the mass percentage content of that element.

6. The low-alloy steel as described in claim 1 or 2, characterized in that, The critical ideal diameter DI for hardenability of the low alloy steel is 2.70 to 4.80 inches, preferably 2.76 to 4.77 inches, where DI = 0.54 × [C] × (5.10 × [Mn] - 1.12) × (0.70 × [Si] + 1) × (0.363 × [Ni] + 1) × (2.16 × [Cr] + 1) × (3.00 × [Mo] + 1) × (0.365 × [Cu] + 1) × (1.73 × [V] + 1), where the element symbols are replaced with the values ​​preceding the percentage sign of the element's mass percentage content.

7. The low-alloy steel as described in claim 1 or 2, characterized in that, The bainitic transformation temperature coefficient T of the low alloy steel B The temperature range is 531–556℃, preferably 533–551℃, where T B =630-45×[Mn]-40×[V]-35×[Si]-30×[Cr]-25×[Mo]-20×[Ni], where the element symbols are replaced with the values ​​before the percentage sign of the element's mass percentage content.

8. The low-alloy steel as described in claim 1 or 2, characterized in that, The microstructure of the low alloy steel includes bainite, and the area ratio of bainite on any cross section of the low alloy steel is more than 80%.

9. The low-alloy steel as described in claim 1 or 2, characterized in that, The austenitic grain size of the low alloy steel is grade 6 or above.

10. The low-alloy steel as described in claim 1 or 2, characterized in that, The low alloy steel meets at least one of the following requirements: tensile strength R m ≥800MPa, yield strength R eL ≥550MPa, elongation A≥15%, reduction of area Z≥35%, Charpy impact energy A kv ≥40J.

11. The low-alloy steel as described in claim 1 or 2, characterized in that, The end hardenability of the low alloy steel meets the following Rockwell hardness requirements: 51-54 HRC for J5mm, 35-47 HRC for J13mm, and 26-40 HRC for J25mm.

12. A method for manufacturing low-alloy steel as described in any one of claims 1-11, characterized in that, The method includes the following steps performed sequentially: (1) Smelting; (2) Casting; (3) Heating: Control the heating temperature to 1050~1250℃ and the heat preservation time to 3~24h; (4) Forging or rolling: Control the final rolling temperature or final forging temperature ≥900℃, and cool after rolling or forging.

13. The method as described in claim 12, characterized in that, In step (4), the product is directly rolled or forged to the finished size.

14. The method as described in claim 12, characterized in that, When step (4) is rolling, the method further includes the following steps after step (4): (5) Secondary heating: The temperature of the secondary heating is controlled at 1050~1250℃, the holding time is 3~12h, and then the secondary rolling is carried out. The final rolling temperature of the secondary rolling is controlled at ≥900℃, and then the rolling is cooled.

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