High-strength and high-toughness non-quenched and tempered steel and manufacturing method therefor

Abstract

The present disclosure relates to non-quenched and tempered steel, which contains, in addition to Fe and inevitable impurities, the following chemical elements in percentage by mass: 0.36-0.42% of C, 0.70-0.85% of Si, 1.30-1.50% of Mn, 0.040-0.055% of S, 0.10-0.20% of Cr, 0.15-0.25% of Cu, 0.08-0.12% of V, and 0.006-0.014% of N. In addition, the present disclosure relates to a method for manufacturing the non-quenched and tempered steel of the present disclosure, comprising the following steps carried out in sequence: (1) smelting and casting to obtain a casting billet; (2) heating and rolling the casting billet; (3) cooling; and (4) cold drawing. The non-quenched and tempered steel of the present disclosure has the characteristics of high strength, excellent impact toughness and fatigue performance, and low costs, and is especially applicable to motor shafts.

Description

High-toughness non-quenched and tempered steel and manufacturing method thereof TECHNICAL FIELD

[0001] The present disclosure relates to a steel material and a manufacturing method thereof, and in particular to a high-toughness non-quenched and tempered steel and a manufacturing method thereof. BACKGROUND

[0002] The driving motor shaft is one of the core components in the motor system, and its main function is to transmit power. Since the driving motor shaft needs to bear a large load and torque when working, its failure mode is mainly fatigue fracture. The fatigue resistance of the motor shaft is closely related to its strength and toughness and the uniformity of the structure. At present, the motor shaft of new energy vehicles mainly adopts two modes: medium-carbon steel / medium-carbon alloy steel quenching and tempering + surface induction hardening and low-carbon alloy steel + carburizing. Some directly adopt the medium-carbon steel normalizing + surface induction hardening mode, and some adopt the 50MnSiV, 46MnVS6, etc. non-quenched and tempered steel + surface induction hardening mode to produce the motor shaft. At present, green and low-carbon development is popular, in order to reduce carbon by eliminating the heat treatment process, the use of non-quenched and tempered steel to manufacture automobile parts has become a major development direction.

[0003] The existing technology of motor shaft steel mainly has three directions: one is low-carbon MnCr and CrMo, followed by carburizing heat treatment; one is medium-low-carbon CrNiMoV for large specifications, such as 30Cr2Ni4MoV, 25CrNi1MoV, etc., followed by quenching and tempering heat treatment; and the other is non-quenched and tempered steel, which is strengthened and toughened by Nb, V, and Ti micro-alloying on medium-carbon steel, followed by controlled rolling and controlled cooling, and then surface induction heat treatment for wear resistance requirements.

[0004] The prior art related to non-quenched and tempered steel is as follows: a Chinese patent document with publication number CN114875317A and publication date August 9, 2022, entitled "Production method of non-quenched and tempered steel for motor shaft" discloses a production method of non-quenched and tempered steel for motor shaft. In the technical solution disclosed in the patent document, the weight percentage of each chemical component of the non-quenched and tempered steel is as follows: C = 0.45%-0.51%, Si = 0.30%-0.60%, Mn = 0.70%-1.00%, P ≤ 0.025%, S = 0.025%-0.035%, Nb = 0.018%-0.023%, Ti = 0.010%-0.040%, Cr = 0.10%-0.30%, V = 0.13%-0.15%, N = 0.0100%-0.0200%, Cu ≤ 0.35%, Mo ≤ 0.25%, and the rest is Fe and unavoidable impurities. The process route adopted by the non-quenched and tempered steel is converter + vacuum degassing + continuous casting + rolling + heat preservation and slow cooling, wherein the rolling is as follows: hot rolling with large reduction is adopted for rough rolling, the opening rolling temperature is ≥980℃, and the reduction of the first two passes is 25%-30%; finish rolling is adopted in the dual-phase zone, the KOCKS unit is used to produce finished products, the intermediate water tank is opened, and the finish rolling temperature is 760-800℃; controlled cooling: slow cooling is used for the rolling after the cooling bed using the heat preservation cover, and slow cooling is used for the heat preservation pit after the cooling bed.

[0005] For another example: a Chinese patent document with publication number CN109439836A and publication date March 8, 2019, entitled "Non-quenched and tempered steel for motor shaft of new energy electric vehicle and production method thereof" discloses a non-quenched and tempered steel for motor shaft of new energy electric vehicle and a production method thereof. In the technical solution disclosed in the patent document, the weight percentage of each chemical component of the non-quenched and tempered steel is as follows: C: 0.45-0.51, Si: 0.30-0.55, Mn: 1.0-1.25, P: ≤0.020, S: 0.010-0.025, Cr: 0.10-0.20, Ni: 0.20-0.30, Al: 0.015-0.035, V: 0.08-0.12, Nb: ≤0.035, Ti: 0.010-0.025, N: 130-170ppm, and the balance is Fe. The production method includes the following steps: molten iron desulfurization → LF → RH → continuous casting → cutting → cooling → inspection → heating furnace heating → controlled rolling → controlled cooling → finishing.

[0006] However, the strength of the non-quenched and tempered steel of the above-mentioned prior art is still not high enough, and the strength and toughness are not excellent enough.

[0007] Therefore, there is still a need in the art to develop non-quenched and tempered steel with high strength and toughness to meet the needs of users for fatigue-resistant non-quenched and tempered steel for motor shaft. SUMMARY

[0008] In view of the above-mentioned defects and shortcomings of the prior art, the present inventors have designed a chemical composition in accordance with the working conditions of motor shafts by precisely identifying the effects of various alloying elements on the strength and toughness of non-quenched and tempered steel, and have obtained non-quenched and tempered steel (in particular, non-quenched and tempered round steel, in particular, non-quenched and tempered round steel for motor shafts) having high strength and toughness. In addition, the present inventors have further improved the strength and toughness of the steel by optimizing the cooling process after rolling and the amount of cold drawing deformation.

[0009] Therefore, in a first aspect, the present disclosure provides a non-quenched and tempered steel containing, in addition to Fe and unavoidable impurities, the following chemical elements in mass percent: C: 0.36-0.42%, Si: 0.70-0.85%, Mn: 1.30-1.50%, S: 0.040-0.055%, Cr: 0.10-0.20%, Cu: 0.15-0.25%, V: 0.08-0.12%, N: 0.006-0.014%.

[0010] In a preferred embodiment, the non-quenched and tempered steel of the present disclosure contains the following chemical elements in mass percent: C: 0.36-0.42%, Si: 0.70-0.85%, Mn: 1.30-1.50%, S: 0.040-0.055%, Cr: 0.10-0.20%, Cu: 0.15-0.25%, V: 0.08-0.12%, N: 0.006-0.014%, the balance being Fe and unavoidable impurities.

[0011] In a preferred embodiment, the content of V and N elements in the above-mentioned non-quenched and tempered steel satisfies the following relationship: [V] x [N] ≥ 7 x 10 -4 , and / or [V] / [N] ≥ 7.5, where [V] and [N] are the values before the mass percent of the corresponding element content. For example, when the content of V in the steel is 0.08%, 0.08 is substituted for calculation.

[0012] In addition, unless otherwise specified, the content of an element in the steel refers to its mass percent in the steel.

[0013] In a preferred embodiment, the unavoidable impurities include P, O, H and B. In a preferred embodiment, P ≤ 0.015%, O ≤ 0.0015%, H ≤ 0.0002% and / or B ≤ 0.0008%.

[0014] In a preferred embodiment, the microstructure of the non-quenched steel of the present disclosure is ferrite + pearlite. In a more preferred embodiment, there is no observable or detectable bainite and martensite in the microstructure of the non-quenched steel of the present disclosure. In a preferred embodiment, the area percentage of the pearlite is 55%-65%. In a preferred embodiment, the interlamellar spacing of the pearlite is 0.3-1.2 μm.

[0015] In a preferred embodiment, the yield strength of the non-quenched steel of the present disclosure is ≥ 1100 MPa, the tensile strength is ≥ 1150 MPa, the yield strength ratio is ≥ 0.96, the elongation is ≥ 12%, the reduction of area is ≥ 50%, the room temperature longitudinal impact energy Akv2 is ≥ 15 J, and the -20°C longitudinal impact energy Akv2 is ≥ 8 J. Preferably, for the comprehensive consideration of manufacturing cost and performance, the yield strength of the non-quenched steel of the present disclosure is usually below 1200 MPa, the tensile strength is below 1250 MPa, the reduction of area is below 60%, the room temperature longitudinal impact energy Akv2 is below 20 J, and the -20°C longitudinal impact energy Akv2 is below 10 J.

[0016] In a preferred embodiment, the fatigue life of the non-quenched steel of the present disclosure is ≥ 15 million times, preferably ≥ 20 million times.

[0017] The high-strength and high-toughness non-quenched steel of the present disclosure has the characteristics of high strength, excellent impact toughness and fatigue performance, and low cost. The high-strength and high-toughness non-quenched steel of the present disclosure can meet the needs of users for non-quenched fatigue-resistant motor shaft steel without any heat treatment.

[0018] In another aspect, the present disclosure provides a method for manufacturing the above-mentioned non-quenched steel, which comprises the following steps performed in sequence:

[0019] (1) smelting and casting to obtain a cast blank;

[0020] (2) heating and rolling the cast blank;

[0021] (3) cooling;

[0022] (4) cold drawing.

[0023] In a preferred embodiment, in step (1), the smelting is performed by an electric furnace or a converter.

[0024] In a preferred embodiment, in step (1), the casting is mold casting or continuous casting.

[0025] In a preferred embodiment, during the heating in step (2), the casting billet has an entry temperature ≤ 400 °C, and the lower limit of the entry temperature is not particularly limited, and is generally above room temperature, the heating temperature is 1110-1230 °C, and the heating time is ≥ 180 min, preferably ≤ 240 min. In a preferred embodiment, during the rolling in step (2), the finish rolling temperature is ≥ 920 °C, preferably ≤ 960 °C.

[0026] In a preferred embodiment, in step (3), the cooling is carried out in a holding pit, the entry temperature is ≥ 840 °C, the cooling rate is 0.2-0.8 °C / s, and the exit temperature is ≤ 300 °C, and the air cooling after exiting the pit is to room temperature.

[0027] In a preferred embodiment, in step (4), the cold drawing reduction is 18%-22%.

[0028] In a preferred embodiment, the method further comprises the steps of shot blasting and / or phosphor saponification between steps (3) and (4).

[0029] A high-strength and tough non-quenched and tempered steel with high strength, excellent impact toughness and fatigue performance, and low cost can be obtained by the manufacturing method of the present disclosure, and no heat treatment is required. BRIEF DESCRIPTION OF DRAWINGS

[0030] FIG. 1 is a metallographic structure diagram (200 times magnification) of the non-quenched and tempered steel of Example 1. DETAILED DESCRIPTION

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.

[0032] As used herein, the term “and / or” refers to any and all possible combinations of one or more of the listed items.

[0033] In this context, the yield strength (Rel) is the yield limit at which the metal material exhibits a yield phenomenon, that is, the stress resisting a slight plastic deformation.

[0034] In this context, the tensile strength (Rm) is the critical value at which the metal transitions from uniform plastic deformation to local concentrated plastic deformation, and is also the maximum load-carrying capacity of the metal under static tension. The tensile strength represents the maximum uniform plastic deformation resistance of the material.

[0035] In this context, the yield strength to tensile strength ratio is the ratio of the yield strength to the tensile strength of the material, and is an important indicator for measuring the strength difference between the elastic deformation stage and the plastic deformation stage of the material.

[0036] In the present text, the elongation, i.e. the percentage of the total deformation ΔL of the gauge length section after tensile fracture of the material to the original gauge length L, δ = ΔL / L x 100%, is an indicator for the plastic properties of the material.

[0037] In the present text, the reduction of area is the percentage of the maximum reduction of the cross-sectional area at the reduced diameter of the sample after tensile fracture to the original cross-sectional area.

[0038] In the present text, the room temperature longitudinal impact energy Akv2 refers to the energy absorbed by the sample until fracture when the impact test is carried out at room temperature along the longitudinal direction (parallel to the rolling direction). This value is usually used to measure the toughness of the material, i.e. the ability of the material to resist fracture under impact load.

[0039] In the present text, the -20℃ longitudinal impact energy Akv2 refers to the energy absorbed by the sample until fracture when the impact test is carried out at -20℃ along the longitudinal direction (parallel to the rolling direction). This value is used to measure the toughness and impact resistance of the material at low temperature.

[0040] In the present text, the microstructure of the non-quenched and tempered steel, including the proportion (area percentage) of pearlite and the pearlite interlamellar spacing, is determined according to GB / T 13298-2015 "Metallic Materials - Methods of Test for Determining Microstructure".

[0041] In the present text, the yield strength, tensile strength, yield ratio, elongation, reduction of area, room temperature longitudinal impact energy Akv2 and -20℃ longitudinal impact energy Akv2 of the non-quenched and tempered steel are determined according to GB / T 228.1-2021 "Metallic Materials - Tensile Test - Part 1: Method of Test at Room Temperature" and GB / T 229-2020 "Metallic Materials Charpy Impact Test Method". The fatigue life is a statistical result of user testing, which is carried out according to GB / T 3075-2021 "Metallic Materials - Fatigue Test - Axial Force Control Method".

[0042] In the present text, the cold drawing reduction is the proportion of the reduction of the cross-sectional area of the material during cold drawing.

[0043] In the non-quenched and tempered steel of the present disclosure, the design principles of each chemical element are as follows:

[0044] C: In the non-quenched and tempered steel of the present disclosure, C is a necessary strengthening element to ensure the strength of the steel. Increasing the C content in the steel will increase the non-equilibrium structure transformation ability of the steel, thereby significantly improving the strength of the steel. However, the C content in the steel should not be too high, and too high C content will adversely affect the plasticity and toughness of the steel, especially too high carbon content will cause the toughness of the steel to decrease sharply during subsequent cold drawing. Based on this, in the non-quenched and tempered steel of the present disclosure, the mass percentage of C element is controlled between 0.36-0.42%.

[0045] Si: In the non-quenched and tempered steel of the present disclosure, Si is an element for strengthening ferrite, replaces Fe atoms in the steel in a substitutional manner, hinders dislocation movement, and is beneficial to the improvement of steel strength, especially the improvement of yield strength of the steel and the improvement of the yield strength ratio. However, excessive Si can easily cause the generation of martensite in the steel, reduce the plasticity and toughness of the steel, and is not conducive to subsequent cold drawing. Therefore, in the non-quenched and tempered steel of the present disclosure, the mass percentage of Si is controlled to be between 0.70-0.85%.

[0046] Mn: In the non-quenched and tempered steel of the present disclosure, Mn dissolves into ferrite to form a substitutional solid solution, producing the effect of solid solution strengthening. At the same time, Mn and S in the steel form MnS which is easy to cut, which can improve the chip breaking effect and improve the cutting performance in the subsequent motor shaft machining process. Mn can also promote the precipitation strengthening effect of Cu in the steel. However, excessive Mn content can cause serious segregation of the steel, which is not conducive to the subsequent cold drawing process. Therefore, in the non-quenched and tempered steel of the present disclosure, the mass percentage of Mn is controlled to be between 1.30-1.50%.

[0047] S: In the non-quenched and tempered steel of the present disclosure, S is an easy-to-cut machining element, and the MnS formed with Mn can improve the cutting machining performance of the motor shaft. The cutting machining performance is better with the increase of S content. However, S will segregate during the solidification process of the molten steel, and high S content will form more coarse sulfide inclusions, which will harm the fatigue resistance of the steel. Therefore, in the non-quenched and tempered steel of the present disclosure, the mass percentage of S is controlled to be between 0.040-0.055%.

[0048] Cr: In the non-quenched and tempered steel of the present disclosure, appropriate amount of Cr element can improve the strength of the steel. However, the content of Cr element should not be too high, because when the content of Cr element is too high, coarse carbides will be formed, which will deteriorate the impact performance of the steel. Therefore, in the non-quenched and tempered steel of the present disclosure, the mass percentage of Cr is controlled to be between 0.10-0.20%.

[0049] Cu: In the non-quenched and tempered steel of the present disclosure, Cu can improve the fluidity of the molten steel and improve the toughness of the steel. Cu plays a role of replacing Mo, Ni and Al in strengthening, and has a lower cost than Mo, Ni and Al. Cu can be second phase precipitated in the continuous cooling process, resulting in precipitation strengthening. The second phase precipitates are Cu-rich transition phases, and the precipitation behavior is related to the cooling rate and the Cu content. When the cooling rate is 0.1-1 ℃ / s, the second phase precipitates become finer and denser as the cooling rate increases. The precipitates have the maximum density when the cooling rate is 1 ℃ / s. When the cooling rate exceeds 1 ℃ / s, the precipitates gradually decrease as the cooling rate increases. However, too high Cu content can easily cause copper brittleness, which can deteriorate the plasticity and toughness of the steel. Therefore, in the non-quenched and tempered steel of the present disclosure, the mass percentage of Cu is controlled to be 0.15-0.25%.

[0050] V: In the non-quenched and tempered steel of the present disclosure, V can form precipitates by combining with C or N in the steel to improve the strength of the steel. However, if the V content is too high, coarse VC particles can be formed, which can deteriorate the plasticity and toughness of the steel. Therefore, in the non-quenched and tempered steel of the present disclosure, the mass percentage of V is controlled to be 0.08-0.12%.

[0051] N: In the non-quenched and tempered steel of the present disclosure, N is a gap atom and also an MX-type precipitate forming element. However, too high N content can cause the enrichment of N at defects to increase, and coarse nitride precipitate particles can be formed, especially TiN particles with sharp angles, which can affect the impact toughness and fatigue performance of the steel. Therefore, in the non-quenched and tempered steel of the present disclosure, the mass percentage of N is controlled to be 0.006-0.014%.

[0052] Preferably, in the non-quenched and tempered steel of the present disclosure, the mass percentages of V and N are controlled to satisfy: [V] x [N] ≥ 7 x 10 -4 , [V] / [N] ≥ 7.5, where [V] and [N] are the numerical values before the mass percentages of the corresponding elements. By controlling the contents of V and N to satisfy the above relationship, the amount of VN can be sufficient to fully play the role of fine-grain strengthening due to the precipitation of VN at the grain boundaries to improve the strength and toughness of the steel; and the contents and sizes of other nitrides can also be controlled to prevent the formation of TiN particles with sharp angles due to too high N content, which can adversely affect the impact toughness and fatigue performance of the steel.

[0053] In one embodiment, in the non-quenched and tempered steel of the present disclosure, unavoidable impurities include P, O, H and B, wherein P ≤ 0.015%, O ≤ 0.0015%, H ≤ 0.0002% and / or B ≤ 0.0008%.

[0054] Considering that P is segregated at the grain boundary, it will reduce the binding energy of the grain boundary and deteriorate the impact performance of the steel. P segregated at the grain boundary will cause intergranular fracture of the steel under impact load, form a larger cleavage plane, and reduce the energy absorbed by the steel under impact. Based on this, in the non-quenched and tempered steel of the present disclosure, the mass percentage of P element is controlled to be P≤0.015%.

[0055] In addition, considering that O can form oxides and complex inclusions with deoxidizing elements in steel, which is not conducive to the fatigue resistance of the steel. Based on this, in the non-quenched and tempered steel of the present disclosure, the mass percentage of O element is controlled to be O≤0.0015%.

[0056] In addition, considering that H is affected by the hydrostatic pressure field of the edge dislocation in the steel, it will gather at defects and form hydrogen embrittlement. In steel with high tensile strength, the density of dislocations, subgrain boundaries, etc. is high, and if the content of H is too high, it will cause more H atoms to be enriched at defects. The aggregation of H atoms will form H molecules, causing the steel to occur delayed fracture. Based on this, in the non-quenched and tempered steel of the present disclosure, the mass percentage of H element is controlled to be H≤0.0002%.

[0057] B belongs to residual elements in steel. B has strong affinity with N and O, and is extremely easy to react with metals such as Ti to form extremely hard boride TiB2, and a small amount of B can also significantly improve the hardenability of the steel. However, high B content will deteriorate the plasticity and toughness. Based on this, in the non-quenched and tempered steel of the present disclosure, the mass percentage of B element is controlled to be B≤0.0008%.

[0058] In a preferred embodiment, in the method for manufacturing non-quenched and tempered steel of the present disclosure, during the heating process in step (2), the billet entry temperature is ≤400℃, the heating temperature is 1110-1230℃, and the heating time is ≥180min. In a preferred embodiment, during the rolling process in step (2), the finish rolling temperature is ≥920℃.

[0059] By making the billet entry temperature ≤400℃, the high-temperature mechanical properties caused by the billet being charged in the two-phase region (proeutectoid ferrite film and the segregation of AlN on it, coarse grains and mixed grains) can be avoided, and the intergranular micro-cracks caused by the large tensile stress in the early stage of heating (Ac1-Ac3) can also be avoided, which can cause the intermediate billet or round steel to have a network of cracks after rolling.

[0060] In addition, the heating temperature is an important factor affecting the size of the original austenite grains during the slab heating. In the method for manufacturing the non-quenched and tempered steel according to the present disclosure, by controlling the heating temperature to be 1110-1230°C, the final performance of the obtained non-quenched and tempered steel can be made more optimal. During the complete austenitization heating at 1110-1230°C, the carbonitrides of Cr and V and part of Cu are dissolved in the austenite. During the subsequent rolling and cooling, Cu is secondarily precipitated to produce precipitation strengthening, and Cr and V form fine carbonitride precipitates to refine the as-rolled structure. The Mn and Cr partially dissolved in the austenite increase the strength of the steel.

[0061] In a preferred embodiment, in the manufacturing method according to the present disclosure, during the rolling in step (2), the ingot is rolled to the finished size by using a one-fire material forming process or a two-fire material forming process.

[0062] The one-fire material forming process refers to directly rolling the billet to the final specification by one heating. The two-fire material forming process includes two times of heating and rolling, i.e., first rolling the billet to a specified intermediate billet size, and then hot rolling to the final finished size. The two-fire material forming process helps to refine the grains inside the material and make the structure more uniform, which helps to improve the mechanical properties of the steel material, such as strength, toughness, etc. The above two processes can be selected as needed by those skilled in the art, and the two-fire material forming process is more preferred.

[0063] In a preferred embodiment, in the manufacturing method according to the present disclosure, in step (3), the cooling is performed in a soaking pit, the entry temperature is ≥840°C, the cooling rate is 0.2-0.8°C / s, and the exit temperature is ≤300°C. In an embodiment, after exiting the pit, the steel is air-cooled to room temperature.

[0064] The post-rolling cooling is a phase transformation process of the deformed austenite. In the manufacturing method according to the present disclosure, by controlling the cooling rate to be 0.2-0.8°C / s and the exit temperature (stop cooling temperature) to be ≤300°C, the ferrite nucleation is facilitated to obtain a refined structure. Under the conditions of entering the soaking pit at a temperature ≥840°C and cooling at a rate of 0.2-0.8°C / s, the steel material is slowly cooled in the soaking pit, the second phase precipitates of Cu are fine and dense, the precipitation strengthening effect is optimal, and the austenite gradually transforms into ferrite and / or pearlite. The pearlite lamellae formed in the range of Al (eutectoid temperature) to 650°C are relatively thick, and the interlamellar spacing is greater than 0.5μm; in the range of 650-600°C, the formed lamellae are finer, and the interlamellar spacing is about 0.3-0.4μm; and the lower the temperature, the finer the interlamellar spacing. The long-time soaking at a temperature above 300°C is also beneficial to the hydrogen expansion of the high-strength steel. After the transformation is completed, the steel is exited from the pit at a temperature ≤300°C and air-cooled. At this time, the steel material (round bar) has been prepared for the subsequent cold drawing in terms of the structure.

[0065] In a preferred embodiment, in the manufacturing method of the present disclosure, the cold-drawing reduction ratio in step (4) is 18%-22%.

[0066] The cold-drawing process improves the strength and toughness of the steel material by causing plastic deformation of the steel material. The inventors have found through repeated experiments that the strength and toughness of the steel material are optimally matched when the cold-drawing reduction ratio is controlled in the range of 18%-22%. At this time, the pearlite proportion (area percentage) is 55%-65%, and the pearlite lamellar spacing is 0.3-1.2 μm.

[0067] In a preferred embodiment, in the manufacturing method of the present disclosure, a step of shot blasting and / or phosphating is further included between steps (3) and (4).

[0068] Shot blasting and phosphating are surface treatment processes. Shot blasting removes impurities such as surface oxide scales, strengthens, polishes, deburrs, and enables the steel material to achieve the required brightness, cleanliness, roughness, and to strengthen the surface of the workpiece, thereby improving the service life and aesthetics of the steel material. Steel material subjected to phosphating can adsorb a large amount of lubricating oil to reduce friction, thereby improving the corrosion resistance, corrosion resistance, and wear resistance of the steel material surface. Those skilled in the art can select to use the above two processes as needed.

[0069] The non-quenched and tempered steel and the manufacturing method thereof of the present disclosure have the following beneficial effects:

[0070] 1) The non-quenched and tempered steel of the present disclosure adopts a reasonable alloy composition design, meeting the user's requirements for high strength and toughness of the non-quenched and tempered steel for motor shafts.

[0071] 2) The non-quenched and tempered steel of the present disclosure uses the precipitation strengthening effect of Cu under different cooling speeds to replace the commonly used strengthening of Ni, Mo, and Al. Cu is lower in cost than Ni, Mo, and Al. And since Cu has a small deteriorating effect on plasticity and toughness, the Si content can be appropriately increased to increase the yield strength ratio and the yield strength.

[0072] 3) The non-quenched and tempered steel of the present disclosure controls the content of the elements V and N, which are prone to fluctuations in production, within a suitable range (i.e., [V]×[N]≥7×10 -4 and / or [V] / [N]≥7.5), greatly reducing the impact of the variation in the content of the two elements in different heats on the strength and toughness of the steel material.

[0073] 4) The manufacturing method of the present disclosure controls the cooling process after rolling and the amount of cold-drawing deformation, so that the strength and toughness of the steel material are fully realized, and the produced non-quenched and tempered round steel achieves the strength and toughness ratio that ordinary steel species achieve only through quenching and tempering heat treatment without any heat treatment, is low in carbon and environmentally friendly, and can meet the user's demand for low-cost high-strength and tough non-quenched and tempered steel for motor shafts.

[0074] The present disclosure will be further described in detail with reference to the accompanying drawings and examples. The following examples are only used to illustrate the present disclosure and not to limit the scope of the present disclosure.

[0075] Examples

[0076] Examples 1-6 and Comparative Examples 1-4

[0077] The non-quenched and tempered steels of Examples 1-6 were prepared by the following steps:

[0078] (1) Smelting and casting according to the chemical composition shown in Table 1: smelting by electric furnace or converter, and casting into continuous casting billets or ingots; during casting, using mold casting or continuous casting.

[0079] (2) Heating and rolling: controlling the billet entry temperature ≤ 400 ℃, the high-temperature section temperature (heating temperature) is 1110-1230 ℃, the heating time is ≥ 180 min, and the finish rolling temperature is ≥ 920 ℃. The steel billet is directly rolled to the final size, or the steel billet is rolled to the specified intermediate billet size, and then heated and rolled to the final product size.

[0080] (3) Cooling: after rolling, enter the holding pit for cooling, the entry temperature is ≥ 840 ℃, the cooling speed is 0.2-0.8 ℃ / s, and the temperature is ≤ 300 ℃, then air cooling is performed after exiting the pit.

[0081] (4) Optionally, shot blasting or phosphor saponification.

[0082] (5) Cold drawing: the cold drawing reduction is 18%-22%.

[0083] The non-quenched and tempered steels of Comparative Examples 1-2 were also prepared by the above steps. The non-quenched and tempered steels of Comparative Examples 3-4 were also prepared by the above steps, but the difference lies in the cooling process and the cold drawing process parameters. The chemical composition and specific process parameters of Examples 1-6 and Comparative Examples 1-4 are shown in Table 1 and Table 2. Table 1 lists the mass percentage of each chemical element of the non-quenched and tempered steels of Examples 1-6 and Comparative Examples 1-4. Table 2 lists the specific process parameters of the non-quenched and tempered steels of Examples 1-6 and Comparative Examples 1-4.

[0084] Then, the obtained rod of Examples 1-6 and Comparative Examples 1-4 were sampled and tested for microstructure and various related properties. The test results are listed in Table 3-1 and Table 3-2, respectively.

[0085] The test methods of the rod of each example and comparative example are as follows:

[0086] The microstructure of the rod was detected by GB / T 13298-2015 “Metal Microstructure Test Method”;

[0087] The mechanical properties of the rod were tested according to GB / T 228.1-2021 “Metallic Materials-Tensile Testing-Part 1: Method of Test at Room Temperature” and GB / T 229-2020 “Metallic Materials Charpy Impact Test Method”.

[0088] The fatigue life of the motor shaft was obtained from the statistical results of user tests. The tests were performed according to GB / T 3075-2021 “Metallic Materials Fatigue Test Axial Force Control Method”.

[0089] Table 3-1 lists the microstructure of the non-quenched and tempered steels of Examples 1-6 and Comparative Examples 1-4.

[0090] Table 3-1

[0091] Table 3-2 lists the mechanical properties of the non-quenched and tempered steels of Examples 1-6 and Comparative Examples 1-4.

[0092] Table 3-2

[0093] As can be seen from Table 3-1, the microstructure of the non-quenched and tempered steels of Examples 1-6 is ferrite + pearlite, without bainite and martensite, wherein the proportion of pearlite is 55%-65%, the pearlite interlamellar spacing is 0.3-1.2 pm, and both the proportion of pearlite and the interlamellar spacing are normal. Figure 1 shows the metallographic structure of the non-quenched and tempered steel of Example 1, and it can be seen that the metallographic structure thereof is ferrite + pearlite. In contrast, the proportion of pearlite of Comparative Example 2 is insufficient; bainite appears in the microstructure of Comparative Example 3; and the pearlite interlamellar spacing of Comparative Example 4 is too large.

[0094] As can be seen from Table 3-2, the mechanical properties of the non-quenched and tempered steels of Examples 1-6 all satisfy the requirements of yield strength ≥ 1100 MPa, tensile strength ≥ 1150 MPa, yield ratio ≥ 0.96, elongation ≥ 12%, reduction of area ≥ 50%, room temperature impact energy Akv2 ≥ 15 J, -20 °C impact energy Akv2 ≥ 8 J, and fatigue life ≥ 20 million times. In contrast, the plasticity and toughness and fatigue performance of Comparative Example 1 are poor, and the elongation, reduction of area, longitudinal impact energy at room temperature Akv2, longitudinal impact energy at -20 °C Akv2, and fatigue life are lower than those of the examples. The strength of Comparative Example 2 is not high, and the yield strength, tensile strength, and fatigue life are lower than those of the examples. The plasticity and toughness of Comparative Example 3 are insufficient, and the elongation, reduction of area, longitudinal impact energy Akv2, longitudinal impact energy at -20 °C Akv2, and fatigue life are lower than those of the examples. The strength of Comparative Example 4 is slightly low, and the yield strength, tensile strength, and fatigue life are lower than those of the examples.

[0095] All publications, patent applications, patents, and other references mentioned in this disclosure are incorporated by reference in their entirety.

[0096] While the disclosure has been illustrated and described with reference to certain preferred embodiments thereof, it is understood that the foregoing is intended to be illustrative only and not limiting of the scope of the disclosure. Changes can be made to the disclosure in form and detail without departing from the spirit thereof.

Claims

1. A non-quenched and tempered steel, characterized in that, The non-quenched and tempered steel contains, in addition to Fe and inevitable impurities, the following chemical elements in mass percentage: C: 0.36-0.42%, Si: 0.70-0.85%, Mn: 1.30-1.50%, S: 0.040-0.055%, Cr: 0.10-0.20%, Cu: 0.15-0.25%, V: 0.08-0.12%, N: 0.006-0.014%.

2. The non-quenched and tempered steel according to claim 1, characterized in that, The non-quenched and tempered steel contains, in addition to Fe and inevitable impurities, the following chemical elements in mass percentage: C: 0.36-0.42%, Si: 0.70-0.85%, Mn: 1.30-1.50%, S: 0.040-0.055%, Cr: 0.10-0.20%, Cu: 0.15-0.25%, V: 0.08-0.12%, N: 0.006-0.014%, and the balance being Fe and inevitable impurities.

3. The non-quenched and tempered steel according to claim 1 or 2, characterized in that, The content of V and N elements in the non-quenched and tempered steel satisfies the following relation: [V] x [N] ≥ 7 x 10 -4 -4, and / or [V] / [N] ≥ 7.5, where [V] and [N] are the values before the mass percentage of the corresponding element content.

4. The non-quenched and tempered steel according to claim 1 or 2, characterized in that, The inevitable impurities include P, O, H and B, wherein P≤0.015%, O≤0.0015%, H≤0.0002% and / or B≤0.0008%.

5. The non-quenched tempered steel according to claim 1 or 2, characterized in that, The microstructure of the non-quenched and tempered steel is ferrite + pearlite.

6. The non-quenched and tempered steel according to claim 5, characterized in that, The area percentage of the pearlite in the non-quenched and tempered steel is 55%-65%.

7. The non-quenched and tempered steel according to claim 5, characterized in that, The interlamellar spacing of the pearlite in the non-quenched and tempered steel is 0.3-1.2 μm.

8. The non-quenched and tempered steel according to claim 1 or 2, characterized in that, The non-quenched and tempered steel has a yield strength ≥1100 MPa, a tensile strength ≥1150 MPa, a yield strength / tensile strength ratio ≥0.96, an elongation ≥12%, a reduction of area ≥50%, a room temperature longitudinal impact energy Akv2 ≥15 J, and a -20℃ longitudinal impact energy Akv2 ≥8 J.

9. The non-quenched tempered steel according to claim 1 or 2, characterized in that, The non-quenched and tempered steel has a fatigue life of 15 million times or more, preferably 20 million times or more.

10. A method of producing the non-quenched and tempered steel according to any one of claims 1 to 9, characterized by, The method comprises the following steps performed in sequence: (1) smelting and casting to obtain a cast blank; (2) heating and rolling the cast blank; (3) cooling; (4) cold drawing.

11. The method of claim 10, wherein, In step (1), the smelting is performed by an electric furnace or a converter, and / or the casting is mold casting or continuous casting.

12. The method of claim 10 or 11, wherein, In the heating process in step (2), the cast blank has an entry temperature ≤400℃, a heating temperature of 1110-1230℃, and a heating time ≥180 min; and / or in the rolling process in step (2), the final rolling temperature is ≥920℃.

13. The method of claim 10 or 11, wherein, In step (3), the cooling is performed in a holding pit, with an entry temperature ≥840℃, a cooling speed of 0.2-0.8℃ / s, and an exit temperature ≤300℃, followed by air cooling to room temperature.

14. The method of claim 10 or 11, wherein, In step (4), the cold drawing has a reduction of area of 18%-22%.

15. The method of claim 10 or 11, wherein, The method further comprises the steps of shot blasting and / or phosphor saponification between steps (3) and (4).

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