A 350mpa grade low yield ratio impact-resistant steel for parts, a manufacturing method and a part carburizing treatment method

Abstract

The present application belongs to the field of wear-resistant steel production, and particularly relates to a 350MPa-grade low-yield-ratio impact-resistant steel for parts, a manufacturing method and a part carburizing treatment method. The chemical components of the steel are as follows in terms of percentage by weight: 0.15%<=C<=0.28%, 0.05%<=Si<=0.3%, 0.3%<=Mn<=0.8%, 0.7%<=Cr<=1.5%, 0.1%<=Mo<=0.6%, 0.0010%<=RE<=0.0050%, 0.0010%<=N<=0.010%, P<=0.030% and S<=0.020%, and the balance is Fe and inevitable impurities. The finished part of the present application has excellent performance at 300 DEG C high temperature, the yield strength is above 450MPa, the tensile strength is above 800MPa, the elongation is >=18%, and the impact energy Akv is >=50J.

Description

Technical Field

[0001] This invention belongs to the field of wear-resistant steel production, specifically relating to a 350MPa grade low yield strength ratio impact-resistant steel for parts, its manufacturing method, and a method for carburizing the parts. Background Technology

[0002] Wear-resistant steel is generally heat-treated martensitic steel with poor formability, making it unsuitable for machining complex-shaped parts such as automotive and agricultural machinery clutches. Furthermore, these parts operate in high-temperature environments of 200–300°C, requiring materials with excellent resistance to high-temperature abrasive wear to extend their service life. The combined effect of high temperature and wear leads to rapid material failure. For example, ordinary wear-resistant steel plate NM400, widely used in metallurgy, power, mining, and building materials industries with a hardness of HB400, only reaches a hardness of 290 HB when heated to 500°C. This decrease in hardness under high-temperature conditions significantly reduces its wear resistance and service life, affecting the lifespan of the parts.

[0003] Patent application document 202210690770.8 discloses the following technical content: C: 0.15-0.19%, Si: 0.22-0.35%, Mn: 11-1.7%, P: 0.007-0.015%, S: 0.001-0.015%, Cr: 0.15-0.35%, Mo: 0.11-0.17%, Ni: 0.015-0.045%, N≤0.01%, Al: 0.025-0.035%, Ca: 0.01-0.03%, Ti: 0.01-0.03%, V≤0.02%, B: 0.001-0.003%, Nd≤0.001%, with the balance being Fe and other unavoidable impurities. The performance of wear-resistant steel plates is improved by adding V and Nd to traditional steel plate raw materials. V is an excellent deoxidizer for steel; adding V refines the grain structure, improves strength and toughness, and the carbides formed by V and C can also improve the steel's resistance to hydrogen corrosion under high temperature and pressure. Calcium in the raw materials has a significant effect on the modification of inclusions in cast steel. Adding an appropriate amount of calcium to cast steel can transform long strip-shaped sulfide inclusions into spherical CaS or (Ca,Mn)S inclusions. The oxides and sulfide inclusions formed by calcium have low density and are easy to float and remove. Calcium also significantly reduces the segregation of sulfur at grain boundaries. All of these factors are beneficial to improving the quality of cast steel and thus its performance. Nd in the raw materials effectively improves the weather resistance of the steel plate. The quenching and tempering process is prone to cracking when water-cooled, has poor weldability, and its formability and high-temperature resistance are not mentioned.

[0004] Patent application document discloses a wear-resistant and heat-resistant steel and its preparation method, application number 201810636530.3. The disclosed technical content is as follows: C: 0.24-0.4%, Mn: 9.5-10.5%, Si: 1.5-1.9%, Nb: 0.7-1.20%, V: 0.5-0.8%, Cr: 19-21%, Ni: 5-6%, P: 0-0.045%, S: 0.2-0.3%, with the balance being Fe. It exhibits good heat and wear resistance, with a Brinell hardness between 230 and 285 HB, tensile strength ≥710 MPa, and elongation ≥16%. It possesses high hardness, high strength, and high toughness, making it suitable for various mining and construction machinery such as automobiles, bulldozers, loaders, excavators, and dump trucks. The high Mn and Cr / Ni content results in high costs. The added sulfur ore has a burn-off rate of 30%–35%, and the sulfur addition process employs three different time periods, making it complex and difficult to control. Molding performance and high-temperature performance are not mentioned.

[0005] Patent application document discloses a high-strength, high-wear-resistant NM600 steel plate and its manufacturing method, application number 201811516116.5. The disclosed technical content is as follows: the steel plate thickness is 12mm or more, C≤0.8%~1.2%, Si≤0.70%, Mn: 7.0%~8.5%, P≤0.025%, S≤0.010%, Cr: 2.0%~5.0%, Ni≤1.5%, Mo: 0.2%~2.5%, B≤0.004%, and the rest are Fe and residual elements. This invention relates to a high-carbon deformation-induced hardening wear-resistant steel. After quenching and tempering, this steel contains austenite in a metastable state. When the material surface is subjected to external pressure, impact, or friction, the surface austenite immediately transforms into deformation martensite, achieving rapid surface hardening and a dramatic improvement in wear resistance. Wear tests show that the average hardness at 50 μm from the wear surface (subsurface layer) is 570–650 HB. However, the high Cr content results in high cost, and the martensite transformation from metastable austenite in high-carbon steel is unstable and prone to defects such as cracks. Formability and high-temperature performance are not mentioned.

[0006] Patent application document for martensitic high-temperature wear-resistant steel and its manufacturing method, application number 202010949264.7, discloses the following technical content: C: 0.18%~0.24%, Si: 0.1%~0.35%, Mn: 0.5%~11%, P<0.013%, S<0.004%, Als: 0.03%~0.06%, Nb: 0.025%~0.055%, V: 0.02%~0.04%, Ti: 0.01%~0.025%, Cr: 0.1%~0.35%, Mo: 0.25%~0.45%, B: 0.0005%~0.0018%, N: 0.003%~0.0045%, H<1.7ppm, Ca: 0.002%~0.004%, with the remainder being Fe and unavoidable impurities. The steel plate thickness ranges from 8mm to 30mm, with a room temperature Brinell hardness of 400HBW to 430HBW. The microstructure at 100℃ to 300℃ is well-defined lath martensite with clear grain boundaries. Mechanical properties at 100℃ to 300℃ include a tensile strength of 1250MPa to 1352MPa and a Brinell hardness of 388HBW to 420HBW. Its high-temperature, repeated low-stress impact wear resistance at 100℃ to 300℃ is 2.38 to 6.99 times better than conventional wear-resistant steel, meeting the service requirements of complex working conditions involving repeated impacts and high-temperature wear at 100℃ to 300℃. The addition of nitrogen (B) employs a temperature-controlled and rate-controlled quenching process and a sub-temperature quenching heating process. The quenching and cooling process uses intermittent cooling, making the heat treatment process complex and difficult to control. Adding Nb, Ti, and V after heat treatment easily precipitates large particles, affecting impact resistance. Formability is not mentioned, the maximum temperature resistance is only 300℃, and impact resistance is not specified.

[0007] Patent application document describes a heat-resistant and wear-resistant steel plate, application number 201710068871.0. The disclosed technical content is as follows: C: 0.33%~0.40%, Si: 0.2%~0.4%, Mn: 0.80%~1.00%, Cr: 0.5~0.7%, Mo: 0.2~0.4%, Nb: 0.015%~0.03%, B: 0.0005~0.0022%, Ti: 0.10%~0.20%, Als: 0.015%~0.045%. It is characterized by medium carbon, Ti, and B microalloying, with the addition of Cr, Mo, Nb, B, and Ti, etc. The process involves continuous casting, slow cooling of slab stacks, and hot cleaning at 100~200℃. This process is complex and increases costs. The online quenching microstructure is martensitic, which is prone to cracking without tempering. Furthermore, martensitic wear-resistant steel has poor weldability and poor formability. After tempering at temperatures below 500℃, the surface hardness is greater than HB400, but impact resistance is not mentioned.

[0008] Patent application document discloses a heat-resistant and wear-resistant steel plate, application number CN201310105154.2, with the following technical content: C: 0.10~0.30%, Si: 0.10~1.50%, Mn: 0.50~2.00%, Cr: 0.30~2.00%, Mo: 0.10~1.50%, Ni≤1.00%, V≤0.10%, RE≤0.10%, W≤1.00%, Al: 0.010~0.080%, B: 0.0005~0.0040%, Ti≤0.80%, Ca≤0.0080%, N≤0.0080%, O≤0.0080%, H≤0.0004%. At least one rare earth element such as La, Ce, and Nd is added. The plate undergoes quenching followed by high-temperature tempering at 400℃ or higher, resulting in a Brinell hardness greater than 360HB. Adding appropriate amounts of elements such as Cr, Mo, V, RE, and W improves the high-temperature resistance of the steel plate, ensuring that it retains good hardness at relatively high temperatures (300–600℃), and thus excellent wear resistance at higher temperatures. The tensile strength at 500℃ is greater than 1000 MPa. Specific high-temperature hardness and impact performance indicators are not mentioned. Alloys are often expensive. Martensitic wear-resistant steel has poor weldability and formability, and its hardness decreases after high-temperature tempering, resulting in low heat and wear resistance.

[0009] The wear-resistant steels mentioned in the prior art all require quenching heat treatment to obtain a martensitic structure. However, martensitic steels have high strength and hardness, are difficult to process and form, have poor thermal conductivity and weldability, high residual stress after welding, and a high tendency to harden. The heat-affected zone is prone to developing a hard and brittle martensitic structure, easily leading to problems such as cold cracking, embrittlement of weld joints, and softening in the HAZ (heat-affected zone). None of the aforementioned steel grades and manufacturing methods mention controlling the yield strength ratio to improve formability and high-temperature impact resistance.

[0010] It is evident that existing materials can no longer meet the requirements, and it is necessary to develop heat-resistant and wear-resistant steels that are cost-controllable, have excellent formability, and possess superior comprehensive properties such as hardness and impact resistance at high temperatures. Summary of the Invention

[0011] To address the problems of mismatched strength and toughness, poor formability, and mismatched wear and impact resistance at high temperatures in existing wear-resistant steels, resulting in rapid material failure and short service life due to the combined effects of impact and wear at high temperatures, this invention provides 350MPa grade low yield strength ratio impact-resistant steel for parts, a manufacturing method, and a carburizing treatment method for the parts. The hot-rolled plate has good formability, a yield strength of 350MPa~420MPa, a tensile strength of 550~650MPa, a yield strength ratio ≤0.65, and an elongation ≥20%. The finished parts exhibit excellent performance at 300℃, with a yield strength above 450MPa, a tensile strength above 800MPa, an elongation ≥18%, an impact energy Akv ≥50J, a surface hardness above 45HRC, and a surface hardness difference within ±1HRC.

[0012] To achieve the above objectives, the present invention employs the following technical solution:

[0013] A 350MPa grade low yield strength ratio impact-resistant steel for parts, the chemical composition of the steel by weight percentage is: 0.15%≤C≤0.28%, 0.05%≤Si≤0.3%, 0.3%≤Mn≤0.8%, 0.7%≤Cr≤1.5%, 0.1%≤Mo≤0.6%, 0.0010%≤RE≤0.0050%, 0.0010%≤N≤0.010%, P≤0.030% and S≤0.020%, with the balance being Fe and unavoidable impurities.

[0014] The thickness of the steel plate in this invention is 2 to 20 mm.

[0015] The following details the mechanism of action of each alloy component in the structural steel of this invention, where the percentage symbol % represents a weight percentage:

[0016] C: 0.15%~0.28%;

[0017] Carbon (C) is a major solid solution strengthening element in steel. In this invention, C reacts with Cr and Mo to form carbides, improving strength and hardness, especially high-temperature resistance, ensuring that hardness and wear resistance do not decrease at high temperatures. If the C content is below 0.15%, it is difficult to guarantee the strength and hardness of the steel plate. On the other hand, if the C content is above 0.28%, the strength is too high, affecting the yield strength ratio, deteriorating the toughness and plasticity of the steel, and making it prone to fracture. Therefore, the C content should be controlled between 0.15% and 0.28%.

[0018] Mn: 0.3%~0.8%;

[0019] Manganese (Mn) is relatively inexpensive and is an excellent deoxidizer and desulfurizer, essential for ensuring the strength and toughness of steel. Manganese and iron can form a solid solution indefinitely, increasing hardness and strength. Mn combines with sulfur (S) to form MnS, preventing hot cracking at grain boundaries that could affect the steel's hot formability. Mn is also a good deoxidizer and increases hardenability. Low Mn content in steel cannot meet the requirements for high strength and hardness, while excessive Mn content leads to severe segregation, affecting weldability and formability, and increasing production costs. Therefore, considering both cost and performance requirements, the Mn content should be controlled between 0.3% and 0.8%.

[0020] Si: 0.05%~0.3%;

[0021] Si is a common element in steel, used as a reducing agent and deoxidizer in the steelmaking process. Si is a ferrite-forming element, not a carbide-forming element. Si dissolved in ferrite improves hardenability and tempering resistance, increases strength and hardness, enhances wear resistance, significantly improves elastic limit, yield strength, and yield ratio, and increases fatigue strength, thus extending the service life of steel. However, a Si content exceeding 0.3% leads to surface decarburization and oxidation in steel, severely impacting its high-temperature wear resistance.

[0022] Cr: 0.7% ≤ Cr ≤ 1.5%;

[0023] Cr is a medium-strong carbide-forming element. In this invention, the addition of Cr forms composite carbides with C and Mo, improving the stability of high-temperature hardness and wear resistance. Chromium also increases the temperatures of A3 and A1, shifts the GS line to the upper left, slows austenite decomposition, and improves hardenability; it also enhances the oxidation resistance and corrosion resistance of steel.

[0024] Mo: 0.1% ≤ Mo ≤ 0.6%;

[0025] Mo can increase the stability of supercooled austenite and improve hardenability. In this invention, Mo forms carbides with Cr and other materials, which inhibit grain growth at high temperatures, hinder dislocation movement, improve oxidation and creep resistance, and enhance high-temperature strength and hardness.

[0026] RE: 0.0010%~0.0050%;

[0027] Resin (RE) is a strong oxidizing element that effectively removes inclusions and refines grains in steel. When RE is controlled within the range of 0.0010% to 0.0050%, it forms complex carbides with Cr and Mo, promoting carbide nucleation, spheroidization, and refinement, thus improving high-temperature wear resistance. Simultaneously, RE improves the morphology of inclusions in steel, refines grains, suppresses microsegregation, homogenizes the microstructure, reduces banding, and enhances impact resistance and impact toughness. Below 0.001%, these effects are not significant, while above 0.0050%, coarse inclusions are easily formed, affecting impact resistance.

[0028] P: P≤0.030%, S≤0.020%;

[0029] Both phosphorus (P) and sulfur (S) are unavoidable harmful impurities in steel, and their presence severely degrades the steel's toughness. Therefore, measures must be taken to minimize the P and S content in steel. This invention limits the P content to ≤0.030%. The maximum S content is limited to 0.020% to reduce the formation of MnS.

[0030] N: 0.001% ≤ N ≤ 0.010%;

[0031] Nitrogen (N) is generally considered a harmful element, and the lower the N content in steel, the better. This invention employs a nitrogen-containing composition design, introducing more than 0.001% N during refining to form fine particles ≤100nm with RE, C, Cr, Mo, etc., thereby improving high-temperature wear resistance and impact resistance.

[0032] A method for manufacturing 350MPa grade low yield strength ratio impact-resistant steel for parts and a method for carburizing the parts, specifically including the following steps:

[0033] 1) The steel plate of the present invention is rolled from a continuously cast billet smelted in a converter, refined in an LF electric furnace, and cast by a vertical bending arc continuous casting machine. The arc radius of the vertical bending continuous casting machine is not less than 5 meters; the billet thickness is 170-250 mm; the superheat of the tundish casting is 20-30°C; the continuous casting adopts light reduction and electromagnetic stirring, the light reduction is 5-10 mm, the current is 400-1000 A, the frequency is 2.2-2.9 Hz, the interval time is 3-8 min; the continuous casting speed is 0.9-1.8 m / min, the secondary cooling water ratio is 0.15-0.35 L / kg, the equiaxed crystal ratio is above 50%, and the micro-segregation of the alloy liquid at the end of the columnar crystals is controlled.

[0034] 2) The hot billet is fed into a walking beam furnace for heating. The furnace uses a reducing atmosphere. The preheating section temperature is 500-600℃, and the holding time is more than 30 minutes. The first heating section temperature is 900-1100℃, the second heating section temperature is 1180-1250℃, and the soaking section temperature is 1200-1250℃. The combined holding time of the second heating section and the soaking section is more than 50 minutes.

[0035] 3) High-pressure water descaling is used before roughing and finishing rolling, with a pressure of not less than 20MPa to ensure the surface quality of the steel plate; the total reduction in roughing rolling is 150-180mm, the initial rolling temperature is 1100-1200℃, and the temperature difference along the entire length of the steel strip is ≤20℃; the total reduction in finishing rolling is 40-60mm, the final rolling temperature is 950-1000℃, and the temperature difference along the entire length of the steel strip is ≤10℃.

[0036] 4) After the steel plate exits the finishing mill, it enters laminar flow cooling. The front section is concentrated cooling at a cooling rate of 30-100℃ / s. The difference in cooling rate along the width of the same position is ≤10℃ / s. It is cooled to 500-600℃ and then coiled. It is then slowly cooled to below 300℃ at a cooling rate of 25-35℃ / h and air-cooled to obtain a mixed structure of ferrite and fine spheroidized sorbite.

[0037] 6) Carburizing treatment: The blank of the steel plate is stamped and formed into a part. It is induction heated to 850-930℃, and a mixture of propane and nitrogen with a volume ratio of more than 40% is introduced. The surface is carburized for 10-20 minutes, and then water mist is treated for 10-20 seconds and rapidly cooled to below 300℃. The upper and lower surfaces of the steel plate are fine sorbite with a depth of not less than 1mm, and the core is sorbite + ferrite.

[0038] This invention employs converter smelting and LF electric furnace refining, eliminating the need for RH vacuum treatment after adding RE. LF electric furnace refining involves adding other alloys after the sulfur content is ≤0.020%, while simultaneously purging nitrogen to adjust the N content in the molten steel. Finally, RE is added to alter the composition, quantity, and morphology of non-metallic inclusions, accelerating steel flow, promoting full inclusion flotation, and improving steel purity. The finished steel contains no more than grade 1.5 non-metallic inclusions, and the invention also improves surface finish and eliminates anisotropy in the microstructure. Continuous casting utilizes light reduction and intermittent electromagnetic stirring technology. The light reduction is 5–10 mm, the electromagnetic stirring current is 400–1000 A, the frequency is 2.2–2.9 Hz, and the interval is 3–8 min. The tundish pouring superheat is 20–30 °C to control inclusions and segregation, resulting in an equiaxed crystal ratio of over 50% in the billet, and controlling micro-segregation of the alloy liquid at the columnar crystal ends.

[0039] Meanwhile, the continuous casting speed is 0.9~1.8m / min, the secondary cooling water ratio is 0.15~0.35L / kg, ensuring that the surface temperature deviation of the billet is not greater than 10℃, the billet cooling rate is uniform, the microstructure is uniform, and the strength difference between the rolled strip steel and the coil is ≤20MPa.

[0040] Hot charging and hot delivery of the billet reduces the surface temperature difference and mitigates microsegregation. The heating furnace uses a reducing atmosphere, with a preheating zone temperature of 500–600℃ and a holding time of at least 30 minutes. The first heating zone temperature is 900–1100℃, the second heating zone temperature is 1180–1250℃, and the soaking zone temperature is 1200–1250℃. The combined holding time for the second heating zone and the soaking zone is at least 50 minutes to ensure the surface quality of the billet, prevent oxidation and decarburization, and simultaneously ensure the rolling temperature to form Cr-Mo carbides.

[0041] High-pressure water descaling is performed before both rough and finish rolling, with a pressure of not less than 20 MPa, to ensure the surface quality of the steel plate.

[0042] The roughing rolling total reduction is 150–180 mm, with an initial rolling temperature of 1100℃–1200℃, providing nucleation impetus for the precipitation of various carbides and nitrides. The strip rolling temperature difference is ≤20℃, resulting in uniform microstructure and properties. The finishing rolling total reduction is 40–60 mm, with a final rolling temperature of 950–1000℃, facilitating the formation of a mixed sorbite and ferrite microstructure after subsequent cooling, with banding not exceeding grade 2.0. The strip rolling temperature difference is ≤10℃, resulting in uniform microstructure and properties. Strip thickness fluctuation is within ±0.20 mm, crown is ≤40 μm, and the tensile strength difference within the same coil is ≤20 MPa.

[0043] After exiting the finishing mill, the steel plate undergoes laminar flow cooling, with concentrated cooling in the front section at a rate of 30–100℃ / s. The difference in cooling rate along the width of the plate at the same location is ≤10℃ / s, ensuring uniform microstructure and properties within the same plate, with a strength difference ≤20MPa. The plate is then cooled to 500–600℃ and coiled, followed by further cooling at a rate of 25–35℃ / h to below 300℃. This promotes carbide nucleation, resulting in a uniform, fine-grained mixed microstructure of spheroidized sorbite and ferrite, with banding not exceeding grade 2.0. Cooling rates exceeding 35℃ / h tend to form lamellar pearlite, while cooling rates below 25℃ / h promote the growth of spheroidized sorbite, leading to poor toughness and plasticity. The steel plate exhibits a yield strength of 350–420MPa, a tensile strength of 550–650MPa, a yield-to-tensile ratio ≤0.65, an elongation greater than 20%, and is free of surface oxidation and intergranular oxide layers.

[0044] The blank for stamping is induction heated to 850–930℃, then a mixture of propane and nitrogen (40% or more propane) is introduced. The surface is carburized for 10–20 minutes, followed by water mist treatment for 10–20 seconds and rapid cooling to below 300℃. The core structure is sorbite + ferrite, while the upper and lower surfaces of the steel plate have a fine sorbite structure with a depth of at least 1 mm. This improves surface hardness and impact resistance, resulting in uniform surface hardness and excellent performance. At 300℃, the yield strength is above 450 MPa, the tensile strength is above 800 MPa, the elongation is ≥18%, the impact energy Akv ≥50 J, the surface hardness is above 45 HRC, and the surface hardness difference is within ±1 HRC.

[0045] Compared with the prior art, the beneficial effects of the present invention are:

[0046] Steel plates produced according to the above chemical composition and process are alloyed with elements such as Cr and Mo. Simultaneously, process parameters such as smelting, continuous casting, billet heating, rolling temperature, reduction rate, coiling temperature, and cooling rate are controlled to obtain a uniform, fine mixed sorbite and ferrite microstructure, with banding not exceeding grade 2. The yield strength is 350–420 MPa, tensile strength is 550–650 MPa, yield ratio is ≤0.65, elongation is greater than 20%, and there is no surface oxidation or intergranular oxide layer. Furthermore, through controlled rolling and cooling processes, the strip thickness fluctuation is controlled within ±0.20 mm, the crown is ≤40 μm, and the tensile strength difference within the same coil is ≤20 MPa. After carburizing treatment, the surface of the steel plate contains a fine sorbite microstructure within 1 mm. At 300℃, the yield strength is above 450 MPa, the tensile strength is above 800 MPa, the elongation is ≥10%, the impact energy Akv ≥50 J, the surface hardness is above 45 HRC, and the surface hardness difference is within ±1 HRC. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0048] Table 1 lists the chemical composition of the steel in the examples, Table 2 lists the smelting and hot rolling production process parameters of the steel in the examples, Table 3 lists the carburizing treatment process of the steel in the examples, and Table 4 lists the hot-rolled plates of the examples and their properties after carburizing treatment.

[0049] Table 1 Chemical composition of the examples, %

[0050]

[0051] Table 2. Smelting and hot rolling production processes in the examples (I)

[0052]

[0053] Table 2. Smelting and hot rolling production process (II) of Examples

[0054]

[0055] Table 3 Carburizing treatment process in the examples

[0056]

[0057] Table 4 Performance of Hot-Rolled Plates and Components in Examples

[0058]

[0059] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A 350MPa grade steel for impact-resistant components with low yield strength ratio, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: 0.15%≤C≤0.28%, 0.05%≤Si≤0.3%, 0.3%≤Mn≤0.8%, 0.7%≤Cr≤1.5%, 0.1%≤Mo≤0.6%, 0.0010%≤RE≤0.0050%, 0.0010%≤N≤0.010%, P≤0.030% and S≤0.020%, with the balance being Fe and unavoidable impurities; After rolling and air cooling, a mixed structure of ferrite and fine spheroidized sorbite is obtained. The yield strength of the steel plate is 350-420 MPa, the tensile strength is 550-650 MPa, the yield ratio is ≤0.65, the elongation is greater than 20%, and the difference in tensile strength between the same coil is ≤20 MPa. After carburizing, the upper and lower surfaces of the part have a sorbite structure with a depth of not less than 1 mm, the core has a sorbite + ferrite structure, the yield strength at 300℃ is above 450 MPa, the tensile strength is above 800 MPa, the elongation is ≥18%, the impact energy Akv is ≥50 J, the surface hardness is above 45 HRC, and the surface hardness difference is within ±1 HRC.

2. The 350MPa grade low yield strength ratio impact-resistant steel for components according to claim 1, characterized in that, The thickness of the steel plate is 2 to 20 mm.

3. A method for manufacturing 350MPa grade low yield strength ratio impact-resistant steel for components as described in claim 1 or 2, characterized in that, Specifically, the methods and steps are as follows: 1) The superheat of the tundish casting is 20-30℃; the continuous casting speed is 0.9-1.8m / min; and the secondary cooling water volume is 0.15-0.35L / kg. 2) The billet is hot-charged into the heating furnace for heating; the preheating section temperature is 500-600℃, the holding time is more than 30 minutes, the second heating section temperature is 1180-1250℃, the soaking section temperature is 1200-1250℃, and the sum of the holding time of the second heating section and the soaking section is more than 50 minutes. 3) The total reduction in rough rolling is 150-180 mm, the initial rolling temperature is 1100-1200℃, and the temperature difference in the rolling of the steel strip along its entire length is ≤20℃; the total reduction in finish rolling is 40-60 mm, the final rolling temperature is 950-1000℃, and the temperature difference in the rolling of the steel strip along its entire length is ≤10℃. 4) After the steel plate exits the finishing mill, it enters laminar flow cooling. The front section is centrally cooled at a cooling rate of 30-100℃ / s. The difference in cooling rate along the width of the same position is ≤10℃ / s. The plate is cooled to 500-600℃ and then coiled. It is then slowly cooled to below 300℃ at a cooling rate of 25-35℃ / h and air-cooled.

4. The method for manufacturing 350MPa grade low yield strength ratio impact-resistant steel for parts according to claim 3, characterized in that, The steel plate is rolled from a continuously cast billet produced by converter smelting, LF electric furnace refining, and casting on a vertical curved arc continuous casting machine. The arc radius of the vertical curved arc continuous casting machine is not less than 5 meters.

5. The method for manufacturing 350MPa grade low yield strength ratio impact-resistant steel for parts according to claim 3, characterized in that, The continuous casting process employs light reduction and intermittent electromagnetic stirring. The light reduction is 5–10 mm, the electromagnetic stirring interval is 3–8 min, and the current is 400–1000 A.

6. The method for manufacturing 350MPa grade low yield strength ratio impact-resistant steel for components according to claim 3, characterized in that, High-pressure water descaling is performed before both roughing and finishing rolling, with a pressure of not less than 20 MPa.

7. The method for manufacturing 350MPa grade low yield strength ratio impact-resistant steel for parts according to claim 3, characterized in that, After air cooling in step 4), a mixed structure of ferrite and fine spheroidized sorbite is obtained. The yield strength of the steel plate is 350-420 MPa, the tensile strength is 550-650 MPa, the yield ratio is ≤0.65, the elongation is greater than 20%, and the difference in tensile strength between the same rolls is ≤20 MPa.

8. A method for carburizing treatment of a component, characterized in that, The component is a blank formed by stamping steel of the 350MPa grade low yield strength ratio impact-resistant component as described in claim 1. The carburizing treatment method includes: induction heating to 850-930°C, surface carburizing treatment for 10-20 minutes, water mist treatment for 10-20 seconds, and cooling to below 300°C.

9. A method for carburizing a component according to claim 8, characterized in that, After carburizing, the upper and lower surfaces of the part have a sorbite structure with a depth of not less than 1 mm, the core has a sorbite + ferrite structure, the yield strength at 300℃ is above 450 MPa, the tensile strength is above 800 MPa, the elongation is ≥18%, the impact energy Akv is ≥50 J, the surface hardness is above 45 HRC, and the surface hardness difference is within ±1 HRC.

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