Non-quenched and tempered steel, preparation method and application

Abstract

The invention discloses non-quenched and tempered steel, a preparation method and application. The non-quenched and tempered steel comprises, by weight, 0.38%-0.45% of C, 0.15%-0.35% of Si, 0.60%-0.90% of Mn, smaller than or equal to 0.025% of P, smaller than or equal to 0.025% of S, smaller than or equal to 0.20% of Cr, smaller than or equal to 0.05% of V, smaller than or equal to 0.15% of Ni, smaller than or equal to 0.20% of Cu, smaller than or equal to 0.10% of Mo, 0.010%-0.050% of Al, 0.005%-0.020% of N and the balance Fe and other inevitable impurities. The non-quenched and tempered steel is medium-carbon non-quenched and tempered steel with high strength, high toughness and fatigue performance, and can be applied to piston rods of large engineering machinery.

Description

Technical Field

[0001] The present invention relates to the technical field of engineering machinery, and in particular to non-quenched and tempered steel, a preparation method and application thereof. Background Art

[0002] There are two types of piston rods: solid rods and hollow rods. Solid rods have higher strength, simpler processing, and more applications. Hollow piston rods refer to hollow piston rods with a circular cross section. Under the same material and equal cross-sectional area, hollow piston rods have stronger torsion resistance and can withstand larger external torques, saving materials, reducing weight, and reducing the inertia of the piston rod during movement, thereby reducing the friction between the piston rod and the oil seal and extending the service life of the piston rod.

[0003] Solid piston rods are mostly made of high-quality carbon steel cold-drawn material 35 steel, 45 steel, 55 steel to reduce cutting processing. For hydraulic cylinders used in corrosive gases or water media, piston rods are mostly made of stainless steel (35CrMo, Cr18Ni19). The rod body of the hollow piston rod is mostly made of high-quality carbon steel cold-drawn seamless steel pipe.

[0004] With the rapid development of construction machinery, construction machinery has higher and higher requirements on the strength, toughness, and welding performance of hydraulic cylinder piston rod materials. The use of quenched and tempered steel to produce piston rods has low efficiency and high cost of quenching and tempering heat treatment, and is prone to deformation and cracking during heat treatment. Non-quenched and tempered steel is made by adding a small amount of alloying elements (niobium, vanadium, and titanium) to ordinary low-alloy steel. By controlling the precipitation behavior of micro-alloying elements, the grain size is refined, the strength and toughness of the steel is improved, and good formability and weldability are obtained. While ensuring good comprehensive performance, the production cost is further reduced. The use of non-quenched and tempered steel to produce piston rods can omit the quenching and tempering process, reduce deformation and cracking during the quenching and tempering process, and greatly save production cycles and costs. Therefore, non-quenched and tempered steel is increasingly used in piston rods.

[0005] The function of the piston rod is to connect the piston and the crosshead, transmit the force acting on the piston and drive the piston to move. The basic requirements for the piston rod are: sufficient strength, rigidity and stability; good wear resistance and high processing accuracy and surface roughness requirements; minimize the influence of stress concentration in the structure; ensure reliable connection and prevent loosening; the design of the piston rod structure should facilitate the disassembly and assembly of the piston.

[0006] Since the piston rod is usually exposed to rain, salt, ash and other serious pollution for a long time, and the working conditions are bad, the pressure and tension under working state are large, and the movement is relatively frequent, it is easy to be damaged and broken. Therefore, the piston rod must have sufficient wear resistance, corrosion resistance, strength, rigidity and toughness.

[0007] The piston rods of large excavators are generally made of large-sized bars. There are four main ways to connect the piston rod earrings to the rod body: integral forging, threaded connection, gas shielded welding, and friction welding.

[0008] Due to the poor working conditions of gas shielded welding, the welding pool area is only in the surface connection area, and friction welding has the advantages of high welding quality (can achieve the same strength of weld seam as base material), high welding efficiency, stable quality and good consistency. If large-scale engineering machinery uses large-size non-quenched and tempered steel to produce piston rods using friction welding technology, the larger the product specifications, the smaller the round steel rolling compression ratio, and the higher requirements for friction welding technology, the more difficult it is to friction weld large-size piston rods. In order to ensure the performance of the piston rod after friction welding, higher requirements are placed on the uniformity of the round steel composition, core segregation, and performance.

[0009] In view of this, the present invention is proposed. Summary of the invention

[0010] The purpose of the present invention is to provide non-quenched and tempered steel, a preparation method and application thereof, so as to improve the strength, toughness and fatigue resistance of the non-quenched and tempered steel.

[0011] The present invention is achieved in that:

[0012] In a first aspect, the present invention provides a non-quenched and tempered steel, comprising, by weight percentage: C 0.38-0.45%, Si 0.15-0.35%, Mn 0.60-0.90%, P≤0.025%, S≤0.025%, Cr≤0.20%, V≤0.05%, Ni≤0.15%, Cu≤0.20%, Mo≤0.10%, Al 0.010-0.050%, N 0.005-0.020%, and the rest being Fe and other inevitable impurities.

[0013] In an optional embodiment, 0.60≤Ceq≤0.80, wherein Ceq=(W C +0.07W Si +0.16W Mn +0.61W P +0.19W Cu +0.17W Ni +0.20W Cr +0.4W Mo +W V )×100, W is the mass fraction of the corresponding element.

[0014] In an optional embodiment, the alloy comprises, by weight percentage: C 0.38-0.44%, Si 0.15-0.35%, Mn 0.75-0.90%, P≤0.020%, S≤0.020%, Cr≤0.20%, V 0.01-0.05%, Ni≤0.15%, Cu≤0.20%, Mo≤0.10%, Al 0.010-0.050%, and N 0.010-0.020%.

[0015] In an alternative embodiment, 0.60≤Ceq≤0.70.

[0016] In an optional embodiment, the non-quenched and tempered steel has a yield strength ≥400Mpa, a tensile strength ≥600Mpa, an impact KU2 ≥40J, a ferrite grade of 5 to 8, a hot-rolled hardness ≥200HBW, and a core segregation diameter ≤2mm; and the non-metallic inclusions meet the requirements of: Class A ≤1.5, Class B ≤2, Class C ≤0, and Class D ≤1.

[0017] In a second aspect, the present invention provides a method for preparing the non-quenched and tempered steel described in the aforementioned embodiment, comprising hot rolling, wherein the hot rolling step comprises:

[0018] Preheating stage: temperature ≤780℃, heating time ≥120min;

[0019] A heating section, temperature 800-1100℃, heating time ≥60min;

[0020] Second heating stage, temperature 1150℃-1250℃, heating time 50-100min;

[0021] The uniform heating section has a temperature of 1150°C-1250°C and a heating time of 50-100 minutes.

[0022] In a third aspect, the present invention provides a method for preparing non-quenched and tempered steel according to the aforementioned embodiment, comprising converter smelting, LF ladle furnace refining, RH vacuum degassing, continuous casting and hot rolling performed in sequence, wherein the preparation method satisfies at least one of the following features AD:

[0023] A. In the converter smelting step, the converter end carbon is controlled at 0.05% to 0.30%, and the tapping temperature is ≥1600°C;

[0024] B. In the LF ladle furnace refining step, argon is blown and stirred throughout the process, slag-making materials are added to make slag, and the refined slag is kept for 10-15 minutes; preferably, vanadium iron alloy is added during the refining process to adjust the composition;

[0025] C. In the RH vacuum degassing step, the vacuum degree is controlled within 266Pa, the high vacuum time is controlled within 15-35min, and the calcium wire is not fed after the vacuum treatment. Preferably, carbonized rice husk is added for heat preservation after the vacuum treatment, the soft blowing time is controlled within 20-40min, and the argon flow rate is controlled at 1-30Nm3 / h.

[0026] D. In the continuous casting step, the temperature of the molten steel in the middle ladle is controlled at 1505°C-1525°C, and the superheat of the molten steel is controlled at ≤35°C. Preferably, the molten steel is stirred by combining the electromagnetic stirring of the crystallizer and the electromagnetic stirring of the end, wherein the electromagnetic stirring parameters of the crystallizer are controlled at 150-300A and the electromagnetic stirring parameters of the end are controlled at 600-700A.

[0027] In a fourth aspect, the present invention provides a piston rod made of the non-quenched and tempered steel described in any one of the aforementioned embodiments.

[0028] In a fifth aspect, the present invention provides a method for preparing the piston rod described in the aforementioned embodiment, comprising: welding the piston rod to a 45# rod head using a friction welding process, the welding operation environment temperature is ≥15°C, and the welding process parameters include: rotation speed 1000-1800rpm, pressure 2.5-8kN, and welding time 1-10s.

[0029] In a sixth aspect, the present invention provides a method for preparing a piston rod according to the aforementioned embodiment, comprising sequentially performing turning, surface quenching, friction welding, grinding, electroplating and polishing, wherein the method satisfies at least one of the following features ac:

[0030] a. During the turning step, the turning depth is ≤1.5mm;

[0031] b. In the surface quenching step, the surface of the round steel is quenched using medium frequency quenching equipment;

[0032] c. In the electroplating step, the plated layers are Ni layer and Cr layer.

[0033] The present invention has the following beneficial effects:

[0034] The invention provides a piston rod that can be used for large-scale engineering machinery The medium carbon non-quenched and tempered steel has high strength, high toughness and fatigue performance. The non-quenched and tempered steel can reach a yield strength ≥400Mpa, a tensile strength ≥600Mpa, an impact KU2 ≥40J, a ferrite grade of 5 to 8, a hot rolled hardness ≥200HB W, and a core segregation diameter ≤2mm; the non-metallic inclusions meet the following requirements: Class A ≤1.5, Class B ≤2, Class C ≤0, and Class D ≤1. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without creative work.

[0036] Figure 1 This is a metallographic picture of the piston rod prepared in Example 1;

[0037] Figure 2 This is a metallographic picture of the piston rod prepared in Comparative Example 1;

[0038] Figure 3 This is a metallographic picture of the piston rod prepared in Comparative Example 2;

[0039] Figure 4 This is a metallographic picture of the piston rod prepared in Comparative Example 3;

[0040] Figure 5 This is a metallographic picture of the piston rod prepared in Comparative Example 4;

[0041] Figure 6 This is a metallographic picture of the piston rod prepared in Comparative Example 5. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0043] An embodiment of the present invention provides a non-quenched and tempered steel, which includes, by weight percentage, C 0.38-0.45%, Si 0.15-0.35%, Mn 0.60-0.90%, P≤0.025%, S≤0.025%, Cr≤0.20%, V≤0.05%, Ni≤0.15%, Cu≤0.20%, Mo≤0.10%, Al 0.010-0.050%, N 0.005-0.020%, and the rest is Fe and other inevitable impurities.

[0044] The embodiment of the present invention provides a piston rod that can be used for large-scale engineering machinery. Medium carbon non-quenched and tempered steel, medium carbon non-quenched and tempered steel with high strength, high toughness and fatigue performance, the non-quenched and tempered steel can reach yield strength ≥400Mpa, tensile strength ≥600Mpa, impact KU2 ≥40J, ferrite grade 5-8, hot rolled hardness ≥200HBW, core segregation diameter ≤2mm; non-metallic inclusions meet the following requirements: Class A ≤1.5, Class B ≤2, Class C ≤0, Class D ≤1.

[0045] In an optional embodiment, the alloy comprises, by weight percentage: C 0.38-0.44%, Si 0.15-0.35%, Mn 0.75-0.90%, P≤0.020%, S≤0.020%, Cr≤0.20%, V 0.01-0.05%, Ni≤0.15%, Cu≤0.20%, Mo≤0.10%, Al 0.010-0.050%, and N 0.010-0.020%.

[0046] C is a necessary component to ensure the strength of the connecting rod. If the C content is too low, the pearlite content will be reduced, thereby reducing the strength and improving the toughness; if the C content is too high, it will have an adverse effect on the fatigue performance and cutting performance of the steel. Therefore, the present invention determines that the C content is 0.38-0.45%, and more preferably, the C content is 0.38-0.44%.

[0047] Si does not form carbides in non-quenched and tempered steels, but exists in the form of solid solution in ferrite, that is, it has a significant effect of solid solution strengthening ferrite and improving material strength.

[0048] Mn can be dissolved in ferrite and play a role in solid solution strengthening. It is a necessary component to ensure the strength of the connecting rod. At the same time, Mn and S form MnS inclusions to improve the machinability of steel. However, if the Mn content is too high, abnormal structures such as bainite are likely to appear, which affects the service life. Therefore, the present invention determines that the Mn content is 0.60-0.90%, and more preferably, the Mn content is 0.75-0.90%.

[0049] The P element can form micro segregation when the molten steel solidifies, and then segregate at the grain boundary when heated at high temperature, which significantly increases the brittleness of the steel. Too high a P content will significantly deteriorate the hot workability of the steel and increase the difficulty of smelting. Therefore, the present invention determines that P≤0.020%.

[0050] Cr can increase the hardenability of steel and improve the mechanical properties of steel, but too high a Cr content will affect the cutting performance of the steel. Therefore, the present invention determines that the Cr content is ≤0.20%.

[0051] S and Mn can form fine, dispersed MnS to improve the machinability of steel. Too high S content will affect the electroplating performance of the piston rod and cause defects such as surface pinholes, so the S content is controlled to be ≤0.025%. More preferably, the S content is ≤0.020%.

[0052] V is a strong carbide-forming element, which mainly inhibits the deformation and recrystallization of austenite and prevents the growth of its grains during hot working. As the austenitizing temperature increases, the amount of V (C, N) dissolved in austenite increases, and the amount of fine V (C, N) precipitation also increases, but if V is too high, the cost of steel increases. Therefore, the present invention determines that the content of V is ≤0.05%. More preferably, the content of V is 0.01-0.05%.

[0053] The purpose of increasing the N content is to ensure that most of the V in the non-quenched and tempered steel is precipitated in the form of V (C, N), converting the solid-solution vanadium in the steel into precipitated vanadium, giving full play to the precipitation strengthening effect of vanadium, not only significantly improving the strength of the steel, but also significantly refining the ferrite grains of the steel, and improving the comprehensive properties of the steel. Therefore, the present invention determines that the N content is 0.005-0.020%, and more preferably, the N content is 0.010-0.020%.

[0054] In an optional embodiment, 0.60≤Ceq≤0.80, wherein Ceq=(W C +0.07W Si +0.16W Mn +0.61W P +0.19W Cu +0.17W Ni +0.20W Cr +0.4W Mo +W V )×100, W is the mass fraction of the corresponding element.

[0055] In an alternative embodiment, 0.60≤Ceq≤0.70.

[0056] A carbon equivalent within a reasonable range is beneficial to ensuring the strength and welding performance of non-quenched and tempered steel while improving toughness and reducing the occurrence of cracks.

[0057] The embodiment of the present invention further provides a method for preparing the non-quenched and tempered steel described in the above embodiment, comprising heating and rolling, wherein the heating and rolling step comprises:

[0058] Preheating stage: temperature ≤780℃, heating time ≥120min;

[0059] A heating section, temperature 800-1100℃, heating time ≥60min;

[0060] Second heating stage, temperature 1150℃-1250℃, heating time 50-100min;

[0061] The uniform heating section has a temperature of 1150°C-1250°C and a heating time of 50-100 minutes.

[0062] Since this type of steel has a high nitrogen content and cracks are easily formed on the surface, the heating temperature and heating time of each stage are strictly controlled. In particular, the solubility in steel is affected by temperature and the nitrogen content in the steel. The solid solubility product of vanadium carbonitride increases with increasing temperature, which means that at higher temperatures, the solubility of vanadium carbonitride in steel will increase. Therefore, the present invention determines the heating temperature of the uniform heating stage of the ingot to be 1150℃-1250℃.

[0063] In an optional embodiment, the preparation method comprises converter smelting, LF ladle furnace refining, RH vacuum degassing, continuous casting and hot rolling performed in sequence, and the preparation method satisfies at least one of the following features AD:

[0064] A. In the converter smelting step, the converter end carbon is controlled at 0.05% to 0.30%, and the tapping temperature is ≥1600°C;

[0065] B. In the LF ladle furnace refining step, argon is blown and stirred throughout the process, slag-forming materials are added to form slag, and the refined slag is kept for 10-15 minutes; preferably, vanadium iron alloy is added during the refining process to adjust the composition; the slag-forming material can be lime, fluorite or a deoxidizer.

[0066] C. During the RH vacuum degassing step, the vacuum degree is controlled within 266 Pa, the high vacuum time is controlled within 15-35 min, and after the vacuum treatment is completed, the calcium wire is not fed; preferably, carbonized husk is immediately put into the vacuum treatment for insulation, the soft blowing time is controlled within 20-40 min, and the argon flow rate is controlled within 1-30 Nm 3 / h. In order to prevent the formation of large-sized Al and Ca inclusions in the steel, it is not allowed to feed calcium wire after the vacuum treatment.

[0067] D. In the continuous casting step, the temperature of the molten steel in the tundish is controlled at 1505°C-1525°C, and the superheat of the molten steel is controlled at ≤35°C. Preferably, the molten steel is stirred by combining the electromagnetic stirring of the crystallizer and the electromagnetic stirring of the end, wherein the electromagnetic stirring parameters of the crystallizer are controlled at 150-300A and the electromagnetic stirring parameters of the end are controlled at 600-700A. In order to reduce the secondary oxidation of the molten steel, argon blowing is used to protect the casting throughout the continuous casting process.

[0068] After hot rolling, the sample structure and performance can be tested. For example, the impact toughness, ferrite grain size, mechanical properties, Brinell hardness, and non-metallic inclusions of the rolled round steel can be tested according to national standards.

[0069] An embodiment of the present invention further provides a piston rod, the material of which is the non-quenched and tempered steel described in any one of the aforementioned embodiments.

[0070] The piston rod can be large size Piston rod parts, the piston rod meets the following requirements: the structure is pearlite + ferrite, there is no bainite and martensite structure, and the microhardness is ≤350HV1.

[0071] An embodiment of the present invention also provides a method for preparing the piston rod described in the aforementioned embodiment, comprising: welding the piston rod to a 45# rod head using a friction welding process, the welding operation environment temperature is ≥15°C, and the welding process parameters include: rotation speed 1000-1800rpm, pressure 2.5-8kN, and welding time 1-10s.

[0072] Controlling the temperature of the welding operating environment is one of the key factors to ensure welding quality and safety. During the welding process, controlling the ambient temperature can effectively manage the temperature of the welding area, thereby affecting the welding quality, safety, welding machine working efficiency and piston rod service life. Therefore, the present invention determines the welding operating environment temperature: ≥15°C.

[0073] In an optional embodiment, the method comprises turning, surface quenching, friction welding, grinding, electroplating and polishing performed in sequence, and the preparation method satisfies at least one of the following features ac:

[0074] a. In the turning step, the round steel is cut into sections and the surface of the round steel is turned using a CNC lathe, with a turning depth of ≤1.5mm;

[0075] b. In the surface quenching step, the surface of the round steel is quenched using medium frequency quenching equipment;

[0076] c. In the electroplating step, the piston rod is surface plated, and the plated layers are Ni layer and Cr layer.

[0077] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0078] Example 1

[0079] This embodiment provides a piston rod, wherein the preparation method of the non-quenched and tempered steel for the piston rod comprises converter smelting, LF ladle furnace refining, RH vacuum degassing, continuous casting and hot rolling in sequence, wherein:

[0080] In the converter smelting step, the converter end point carbon is controlled at 0.1327% and the tapping temperature is 1605°C;

[0081] In the LF ladle furnace refining step, argon blowing and stirring are performed throughout the process, slag-making materials are added to make slag, the refined slag is kept for 12 minutes, and vanadium iron alloy is added during the refining process to adjust the composition;

[0082] In the RH vacuum degassing step, the vacuum degree was controlled within 266Pa, the high vacuum time was controlled within 13min, and after the vacuum treatment, no calcium wire was fed; after the vacuum treatment, carbonized rice husk was put in for heat preservation, the soft blowing time was controlled within 25min, and the argon flow rate was controlled at 30Nm 3 / h.

[0083] During the continuous casting step, the temperature of the molten steel in the middle ladle is controlled at 1517°C, and the superheat of the molten steel is controlled at ≤27°C. Preferably, the molten steel is stirred by combining the crystallizer electromagnetic stirring and the end electromagnetic stirring, wherein the crystallizer electromagnetic stirring parameters are controlled at 200A / 2.5Hz and the end electromagnetic stirring parameters are controlled at 630A / 3.5Hz.

[0084] The heating and rolling steps include: a preheating section with a temperature of 750°C and a heating time of 135 minutes; a first heating section with a temperature of 964°C and a heating time of 69 minutes; a second heating section with a temperature of 1195°C and a heating time of 60 minutes; and an even heating section with a temperature of 1193°C and a heating time of 62 minutes.

[0085] The preparation method of the piston rod comprises turning, surface quenching, friction welding, grinding, electroplating and polishing in sequence, wherein:

[0086] In the turning step, the turning depth is 1.5 mm;

[0087] In the surface quenching step, the surface of the round steel is quenched by medium frequency quenching equipment;

[0088] In the friction welding step, the piston rod and the 45# rod head are welded by friction welding process. The welding operation environment temperature is 20°C. The welding process parameters include: rotation speed 1200rpm, pressure 3.5kN, welding time 3s

[0089] In the electroplating step, the plated layers are Ni layer and Cr layer.

[0090] Comparative Examples 1-3

[0091] The composition of the non-quenched and tempered steel for the piston rod is shown in Table 1, and the main differences between the production method of the non-quenched and tempered steel and the production method of the piston rod and Example 1 are shown in Table 2.

[0092] Table 1 Chemical composition of non-quenched and tempered steel for piston rod (wt%)

[0093]

[0094] Table 2 Main production process parameters

[0095]

[0096]

[0097] The organization and performance of the round steel and piston rod prepared in each embodiment and comparative example were tested, and the results are as follows: Figure 1-6 And as shown in Table 3.

[0098] Table 3 Organization and performance of round steel and piston rod

[0099]

[0100]

[0101] Compared with Comparative Example 1, Example 1 has too low carbon equivalent, resulting in too low strength and impact work of the round steel; Compared with Comparative Example 2, Example 1 has unreasonable welding step conditions, resulting in bainite appearing in the piston rod, and the microhardness is too high; Compared with Comparative Example 3, Example 1 has slightly higher carbon equivalent, resulting in bainite and martensite appearing in the piston rod, and the microhardness is too high; Compared with Comparative Example 4, Example 1 has too high carbon equivalent, resulting in high content of bainite and martensite appearing in the piston rod, the microhardness is too high, and the risk of piston rod fracture is high; Compared with Comparative Example 5, Example 1 has unreasonable round steel production step conditions, resulting in obvious regional segregation bands in the core of the round steel, resulting in martensitic bands appearing in the piston rod, the microhardness is too high, and the piston rod is at risk of fracture.

[0102] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A non-quenched and tempered steel, characterized in that: The alloy comprises, by weight percentage, C 0.38-0.45%, Si 0.15-0.35%, Mn 0.60-0.90%, P≤0.025%, S≤0.025%, Cr≤0.20%, V≤0.05%, Ni≤0.15%, Cu≤0.20%, Mo≤0.10%, Al 0.010-0.050%, N 0.005-0.020%, and the rest is Fe and unavoidable impurities.

2. The non-quenched and tempered steel according to claim 1, characterized in that: 0.60≤Ceq≤0.80, where Ceq=(W C +0.07W Si +0.16W Mn +0.61W P +0.19W Cu +0.17W Ni +0.20W Cr +0.4W Mo +W V )×100, W is the mass fraction of the corresponding element.

3. The non-quenched and tempered steel according to claim 1, characterized in that: The alloy comprises, by weight percentage, C 0.38-0.44%, Si 0.15-0.35%, Mn 0.75-0.90%, P≤0.020%, S≤0.020%, Cr≤0.20%, V 0.01-0.05%, Ni≤0.15%, Cu≤0.20%, Mo≤0.10%, Al 0.010-0.050%, and N 0.010-0.020%.

4. The non-quenched and tempered steel according to claim 1, characterized in that: 0.60≤Ceq≤0.

70.

5. The non-quenched and tempered steel according to claim 1, characterized in that: The yield strength of the non-quenched and tempered steel is ≥400Mpa, the tensile strength is ≥600Mpa, the impact KU2 is ≥40J, the ferrite grade is 5-8, the hot-rolled hardness is ≥200HBW, and the core segregation diameter is ≤2mm; the non-metallic inclusions meet the following requirements: Class A ≤1.5, Class B ≤2, Class C ≤0, and Class D ≤1.

6. A method for preparing non-quenched and tempered steel according to claim 1, characterized in that: The method comprises heating rolling, wherein the heating rolling step comprises: Preheating stage: temperature ≤780℃, heating time ≥120min; A heating section, temperature 800-1100℃, heating time ≥60min; Second heating stage, temperature 1150℃-1250℃, heating time 50-100min; The uniform heating section has a temperature of 1150°C-1250°C and a heating time of 50-100 minutes.

7. The method for preparing non-quenched and tempered steel according to claim 6, characterized in that: The method comprises converter smelting, LF ladle furnace refining, RH vacuum degassing, continuous casting and hot rolling performed sequentially, and the preparation method satisfies at least one of the following features AD: A. In the converter smelting step, the converter end carbon is controlled at 0.05% to 0.30%, and the tapping temperature is ≥1600°C; B. In the LF ladle furnace refining step, argon is blown and stirred throughout the process, slag-making materials are added to make slag, and the refined slag is kept for 10-15 minutes; preferably, vanadium iron alloy is added during the refining process to adjust the composition; C. In the RH vacuum degassing step, the vacuum degree is controlled within 266Pa, the high vacuum time is controlled within 15-35min, and after the vacuum treatment, the calcium wire is not fed; preferably, after the vacuum treatment, carbonized husk is added for heat preservation, the soft blowing time is controlled within 20-40min, and the argon flow rate is controlled at 1-30Nm3 / h; D. In the continuous casting step, the temperature of the molten steel in the middle ladle is controlled at 1505°C-1525°C, and the superheat of the molten steel is controlled at ≤35°C. Preferably, the molten steel is stirred by combining the electromagnetic stirring of the crystallizer and the electromagnetic stirring of the end, wherein the electromagnetic stirring parameters of the crystallizer are controlled at 150-300A and the electromagnetic stirring parameters of the end are controlled at 600-700A.

8. A piston rod, characterized in that: The material is the non-quenched and tempered steel as described in any one of claims 1 to 5; Preferably, the piston rod meets the following requirements: the structure is pearlite+ferrite, there is no bainite and martensite structure, and the microhardness is ≤350HV1.

9. A method for preparing a piston rod according to claim 8, characterized in that: include: The piston rod and the 45# rod head are welded by friction welding process. The welding operation environment temperature is ≥15℃. The welding process parameters include: rotation speed 1000-1800rpm, pressure 2.5-8kN, and welding time 1-10s.

10. The method for preparing a piston rod according to claim 9, characterized in that: The method comprises turning, surface quenching, friction welding, grinding, electroplating and polishing performed in sequence, and the preparation method satisfies at least one of the following features ac: a. During the turning step, the turning depth is ≤1.5mm; b. In the surface quenching step, the surface of the round steel is quenched using medium frequency quenching equipment; c. In the electroplating step, the plated layers are Ni layer and Cr layer.

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