High-toughness ferrite-pearlite type non-quenched and tempered steel and method for manufacturing the same

Abstract

The application provides a high-toughness ferrite-pearlite type non-quenched and tempered steel and a manufacturing method thereof, and the composition of the steel is as follows: C 0.35-0.42%, Si 0.35-0.45%, Mn 1.25-1.65%, P ≤0.010%, S 0.025-0.050%, Cr 0.10-0.30%, Nb 0.02-0.05%, Mo 0.010-0.050%, Ni 0.10-0.25%, Al 0.025-0.045%, V 0.10-0.30%, Cu ≤0.10%, Ti 0.010-0.040%, [N] 120-160ppm, [H] ≤1.0ppm, T.O ≤10ppm; the rest is Fe and inevitable impurity elements; the steel is combined with a controlled rolling and controlled cooling process, the ferrite content is increased, the strength and toughness are improved, the strength and toughness are matched, and the cutting performance is improved.

Description

Technical Field

[0001] This invention belongs to the field of alloy structural steel and relates to a high-strength and high-toughness ferritic pearlitic non-quenched and tempered steel and its manufacturing method. It belongs to the Mn-Cr series high-strength, medium-low carbon non-quenched and tempered steel used for forging crankshafts and is suitable for the automotive manufacturing and engineering machinery industries with high requirements for material strength and toughness matching and machinability. Background Technology

[0002] Microalloyed non-quenched and tempered steel (hereinafter referred to as non-quenched and tempered steel) is a type of material controlled by microalloying and rolling / forging. Because its application eliminates the need for quenching and high-temperature tempering, it offers advantages such as simplified production processes, energy and resource savings, and improved product quality. Replacing quenched and tempered steel with non-quenched and tempered steel in the production of various parts (forgings) has become an important development direction in the steel industry, receiving significant attention and application in industries such as construction machinery, automobile manufacturing, and petrochemicals. Currently, non-quenched and tempered steel accounts for a considerable proportion of automotive forgings in countries like Japan and Germany. In recent years, lightweighting of automobile bodies has become increasingly popular, leading to a corresponding development of non-quenched and tempered steel towards higher strength and performance.

[0003] Non-quenched and tempered steels are mainly classified into three types according to their microstructure: ferrite + pearlite (F+P) type, martensitic (M) type, and bainitic (B) type. Among these, the industrial production of martensitic and bainitic non-quenched and tempered steels is complex and difficult; therefore, F+P type non-quenched and tempered steels are currently the dominant type. Their comprehensive properties are achieved through microalloying and process control to achieve a balance between strength and toughness in the microstructure. Existing non-quenched and tempered steels are mainly of the F+P type, belonging to medium-low carbon alloy steels. Their microstructure consists of proeutectoid ferrite + pearlite + a small amount of bainite (core), and they are widely used in engine crankshafts and other components, exhibiting high strength and certain plasticity and toughness. Its main chemical elements (wt.%) are distributed as follows: C: 0.32-0.40%, Si: 0.50-0.70%, Mn: 1.30-1.45%, Cr: 0.10-1.45%, Mo≤0.050%, Ni≤0.30%, P≤0.025%, S: 0.020-0.035%, with the remainder being Fe and unavoidable impurities. The most significant problem with this type of non-quenched and tempered steel currently is its high strength but poor toughness, which severely restricts its serviceability under strong impact loads. Therefore, further improvements in the strength-toughness balance of the steel are needed.

[0004] As is well known, the main control methods for improving the strength and toughness of non-quenched and tempered steel are chemical composition regulation and optimization (microalloying) and controlled rolling and cooling. Among these, the rolling and cooling processes of non-quenched and tempered steel are complex, difficult to control, and greatly affect the overall uniformity of the material's mechanical properties. Compared with process control, leveraging the regulation of microstructure by microalloying elements to improve the strength-toughness balance of the material remains an economical and feasible method.

[0005] Non-quenched and tempered steels mainly adopt national standards such as GB / T 15712, ISO 11692, and EN10267, and are primarily composed of low- to medium-carbon manganese with added microalloying elements. Common F+P type non-quenched and tempered steels include C38N2, 49MnVS3, and 42MnSiVS33. Existing patents for enhancing the strength-toughness balance of non-quenched and tempered steels mainly focus on compositional fine-tuning and process optimization.

[0006] Chinese patent, publication number CN 114892079 A, ​​published on August 12, 2022, discloses a production process for low-carbon bainitic non-quenched and tempered hot-rolled round steel with a carbon content of 0.20-0.30%. By controlling the production process (EAF-LF-RH-CCM-heating-rolling-cooling), bainitic structure is obtained, which improves the material strength, high hardness, and machinability. However, the plasticity is low, and no temperature control measures are mentioned during the rolling process.

[0007] Chinese patent CN 107587073A, published on January 16, 2018, discloses a titanium- and nitrogen-containing non-quenched and tempered steel for automotive engine crankshafts and its preparation method. It adds 0.44%-0.48% C, 0.02%-0.03% Ti, and 0.01%-0.02% N. By adding and optimizing the titanium element in the steel, a pearlite + intergranular / intragranular ferrite structure is obtained, improving the balance between strength and toughness. This patent improves the material's strength and hardness to some extent. However, its tensile strength is only 800 MPa, its elongation is only 13-15%, its plasticity is low, its impact energy is low, and its toughness is poor.

[0008] For example, Chinese patent CN 103614629A, published on March 5, 2014, discloses a 900MPa grade hot-rolled non-quenched and tempered thin steel plate and its preparation method. Through multi-element V, Ti, and Mo microalloying treatment and online hot rolling control, ultrafine-grained ferrite and its grain boundary dispersed precipitates are obtained, greatly improving the strength of the non-quenched and tempered steel. However, its toughness is low, resulting in a mismatch between strength and toughness.

[0009] The international patent, publication number WO 2022152158A1, published on July 21, 2022, provides a high-strength, tough, and easy-to-cut non-quenched and tempered round steel and its manufacturing method. Through micro-alloying (Al, Nb, V), the strength of the material is effectively improved, but the impact toughness and elongation are low, and the strength and toughness are not well matched.

[0010] Currently, the degree of microalloying in non-quenched and tempered steel used in automobile manufacturing is relatively low, and its strength and toughness are mainly achieved through controlled rolling and cooling, forging processes, etc. However, due to the complexity of the production control process and the narrow process window, the main method for improving its strength and toughness still relies on microalloying design. Summary of the Invention

[0011] The purpose of this invention is to provide a high-strength and high-toughness ferritic pearlitic non-quenched and tempered steel and its manufacturing method. In view of the common problem of insufficient strength and toughness matching in non-quenched and tempered steel, the invention adopts microalloying technology and provides a production control process with matching formula. It adopts a low-temperature controlled rolling and cooling production process to increase the ferrite content in the steel, improve its strength and toughness, achieve a balance between strength and toughness, and improve its machinability.

[0012] The specific technical solution of this invention is as follows:

[0013] A high-strength and high-toughness ferritic pearlitic non-quenched and tempered steel, comprising the following components by weight percentage:

[0014] C: 0.35-0.42%, Si: 0.35-0.45%, Mn: 1.25-1.65%, P≤0.010%, S: 0.025-0.050%, Cr: 0.10-0.30%, Nb: 0.02-0.05%, Mo: 0.010-0.050%, Ni: 0.10-0.25%, Al: 0.025-0.045%, V: 0.10-0.30%, Cu≤0.10%, Ti: 0.010-0.040%, [N]: 120-160ppm, [H]≤1.0ppm, TO≤10ppm, 0.13%≤Nb+V+Ti≤0.39%; the remainder is Fe and unavoidable impurity elements.

[0015] The composite microalloying of Nb, V, and Ti exhibits varying degrees of enhancement in toughness and strength: below 0.13% of the total content, the strengthening effect is negligible; above 0.39%, the strengthening effect saturates and no longer significantly increases, while also significantly increasing manufacturing costs. Therefore, this invention proposes a microalloying control range of 0.13%-0.39% for the core alloying elements. This invention uses V as the primary alloying element, supplemented by Nb and Ti, with V being an order of magnitude higher than the latter two. V has high solid solubility during forging heating, resulting in significant precipitation strengthening during deformation (forging / rolling) and controlled cooling. Furthermore, the precipitation strengthening effect is significant and only slightly diminishes over a wide temperature range (1200-1250℃). However, the strengthening effect of V is not linearly related to its content, exhibiting a peak strengthening effect. Based on extensive experimental research, this invention limits the main microalloying strengthening element system to 0.13% ≤ Nb + V + Ti ≤ 0.39%, achieving a better strengthening effect.

[0016] The composition of the high-strength and tough ferritic pearlitic non-quenched and tempered steel also satisfies: 2.98≤Mn / C≤4.71, preferably 3.50≤Mn / C≤3.82; when Mn is added within the C content range of 0.35~0.42%, and the above dosage ratio is controlled, the content of proeutectoid ferrite precipitation increases. Adding Mn within this range can significantly increase the toughness and strength of ferrite.

[0017] The composition of the high-strength and tough ferritic pearlitic non-quenched and tempered steel also satisfies: 2.0≤Al / [N]≤2.8. Within this range, the grain refinement effect is obvious and it is not easy to cause mixed crystals.

[0018] The microstructure of the high-strength and tough ferritic pearlitic non-quenched and tempered steel consists of ferrite, pearlite, and a small amount of carbides. The ferrite volume fraction is 10%-13%, the grain size is grade 8.0 or higher, and the carbide volume content is ≤4%. Ferrite is mainly distributed at the grain boundaries, and the microalloying effect of alloying elements promotes the formation of a large number of nucleation sites within the grains, which further grow and increase the content of ferrite within the grains, making the content of ferrite within the grains account for more than 4% of the total. The aspect ratio of sulfides is ≤9.0.

[0019] After heat treatment, the mechanical properties of the high-strength and high-toughness ferritic pearlitic non-quenched and tempered steel reach: Rm≥920MPa, R p0.2 ≥550MPa, A≥15%, Z≥40%; impact energy measured at room temperature (KU2)≥30J.

[0020] The manufacturing method of high-strength and high-toughness ferritic pearlitic non-quenched and tempered steel provided by the present invention has the following full-process production steps: batching - electric arc furnace smelting (EAF) - LF refining - RH degassing - round billet continuous casting (CCM) - low temperature controlled rolling - slow cooling - flaw detection / grinding - finished round steel.

[0021] The low-temperature controlled rolling specifically refers to:

[0022] Control the temperature of the heating furnace soaking section: 1180-1220℃, the total time for preheating, heating and soaking is ≥10h, the initial rolling temperature is 950-1000℃, and the final rolling temperature is 690-740℃.

[0023] The slow cooling process involves cooling the rolled material to 550-600°C on a cooling bed before placing it in a slow cooling pit. An insulation cover is added around the slow cooling pit to achieve slow and uniform cooling of the rolled material. The cooling time shall not be less than 4 hours, cooling the material to 150-200°C; preferably 4-5 hours.

[0024] After the steel is removed from the pit, the surface and ends are ground to ensure good surface quality, while reducing the sensitivity to surface decarburization and ensuring that the surface is free of decarburization and has zero defects.

[0025] This invention promotes grain refinement in steel and changes the morphology and distribution of sulfides through a low-temperature controlled rolling and cooling process. The F+P non-quenched and tempered steel produced according to the composition design scheme and production method of this invention has fine and uniform grain structure, which significantly improves the strength and toughness of the steel.

[0026] The mechanisms and degrees of influence of various elements in non-quenched and tempered steel on improving the strength-toughness balance are not entirely the same. Through composition adjustment, microalloying, and other treatments, the strength and toughness of the steel can be improved to achieve a better balance. The roles and effects of each element in non-quenched and tempered steel are as follows:

[0027] Carbon (C) is the most important and fundamental element affecting the strength and toughness of non-quenched and tempered steel, and is the fundamental source of material strength and hardness. On the one hand, C is the most basic and effective strengthening element in steel, affecting material strength, hardenability, and wear resistance, and its cost is relatively low. On the other hand, excessive C content will affect the phase transformation temperature, reduce the density of movable dislocations in the steel to a certain extent, decrease the yield strength ratio, increase decarburization sensitivity, and thus worsen the fatigue performance and processing performance of the steel. For non-quenched and tempered steel, an increase in C content will correspondingly increase the volume fraction of pearlite in the steel, leading to a linear increase in material strength and hardness, while the material's plasticity and toughness will decrease accordingly. In order to improve the plasticity and toughness of non-quenched and tempered steel, this invention adopts a medium carbon content design, appropriately reducing the C content of the steel grade, ultimately reducing the pearlite content in the steel, increasing the volume fraction of ferrite, thereby improving the plasticity and toughness of the steel to a certain extent; however, the premise of improving toughness is to ensure the strength of the steel, so the C element content is 0.35-0.42%.

[0028] The main functions of silicon (Si) in steel are deoxidation and solid solution strengthening. The latter can improve the strength and hardness of the material, but also affect its hardenability. Furthermore, Si can significantly increase the deformation resistance of steel, which is detrimental to cold heading and cold extrusion processes. If the Si content is too high, on the one hand, it will reduce the plasticity and toughness of the material, increase the activity of carbon (C), and promote decarburization and graphitization during rolling and forging; on the other hand, it will increase the difficulty of steel smelting, easily forming inclusions and banded structures, thus worsening the fatigue resistance of the steel. Si can inhibit the continuous network precipitation of proeutectoid cementite along the austenite grain boundaries, producing "grain boundary ferrite," transforming the grain boundary phase from highly brittle cementite to a strong and tough ferrite phase. Generally, the Si content in steel is controlled at around 0.30%. However, in this invention, to leverage the effect of Si in increasing grain boundary ferrite and thus increasing the ferrite volume fraction in the steel, the Si content is appropriately increased to 0.35-0.45%.

[0029] The main roles of manganese (Mn) in non-quenched and tempered steel are: 1) Solid solution strengthening. When the Mn content is below 0.8%, the solid solution strengthening effect is not obvious; however, when the Mn content exceeds 1.5%-1.6%, it promotes the formation of bainite in the steel. Therefore, for pearlite-ferrite type non-quenched and tempered steel, the Mn content is generally controlled within the range of 1.5%. 2) Grain refinement strengthening. Mn is an alloying element that expands the austenite region in steel. Increasing the Mn content can, on the one hand, reduce the carbon content at the eutectoid point of the steel, increase the pearlite content, and thus improve the strength of the steel; on the other hand, it can lower the phase transformation temperature of the steel, improve hardenability, and refine ferrite grains, pearlite clusters, and pearlite lamellar spacing, thereby improving the strength of the steel. For non-quenched and tempered steel, the strengthening effect of Mn is mainly through refining ferrite grains, solid solution strengthening, and a certain degree of grain refinement strengthening. Furthermore, Mn in steel can increase the solid solubility of V (C, N) in austenite, refine the size of precipitates, and increase the content of precipitates, thus aiding in enhancing the precipitation strengthening effect of V. Mn can also hinder and delay the transformation of ferrite to pearlite, reducing pearlite content and increasing the volume fraction of ferrite, lowering the bainite transformation temperature, thereby exerting a grain-refining strengthening effect and contributing to improved material plasticity and toughness. With Mn ≤ 1.5%, as the Mn content in steel increases, the material strength significantly improves, which can appropriately compensate for the strength decrease caused by the reduction in C content, without sacrificing the plasticity and toughness of non-quenched and tempered steel. This invention, based on the design principle of slightly reducing C content, employs a reasonable C-Mn combination to leverage the role of Mn in refining ferrite grains and increasing the volume fraction of the ferrite phase. The invention defines the Mn content range as 1.25-1.65% to achieve the optimal ratio with C, 2.98≤Mn / C≤4.71, with the optimal ratio being Mn / C=3.65, thereby improving the material's overall performance, such as strength, toughness, and ductility.

[0030] In steel, Cr readily combines with C to form insoluble carbides, which can effectively increase the crystal nucleation rate and inhibit grain growth under short-term heating conditions. Furthermore, Cr delays the bainitic transformation, significantly enhancing the strength of bainite and ferrite through solid solution strengthening; simultaneously, it reduces the reactivity of C, thereby reducing surface decarburization during heating, rolling, and cold working, which is beneficial for achieving higher fatigue performance. Similar to C and Mn, appropriately increasing the Cr content increases the volume fraction of ferrite; conversely, increasing the Mn and Cr content reduces the pearlite lamellar spacing in steel. For F+P type non-quenched and tempered steel with a pearlite volume fraction of over 80%, the pearlite lamellar spacing has a significant effect on the strength of the non-quenched and tempered steel. Studies have shown that when the pearlite lamellar spacing decreases by 10 nm, the yield strength of the material increases by approximately 20 MPa, and its hardness also increases accordingly. In this invention, the Cr content should not be too high, and should be controlled within the range of 0.10-0.30%.

[0031] Mo and Ni are relatively expensive elements, and their addition should be minimized during design and application. Ni stabilizes austenite, enhances the hardenability of steel, improves low-temperature toughness, and reduces the notch sensitivity of non-quenched and tempered steel. Furthermore, Ni improves the surface rust layer structure, increases density and surface adhesion, enhances corrosion resistance, and inhibits hydrogen adsorption, thus improving resistance to delayed fracture. Mo significantly improves hardenability, prevents temper brittleness and overheating tendency; it also refines grains, but its beneficial effects are limited if its content is too low. Excessive Mo content promotes the formation of grain boundary ferrite films, which is detrimental to the hot plasticity of steel and increases the tendency for reheat cracking. In this invention, considering the high cost of Mo and Ni, their content should not be too high; therefore, the design is Mo: 0.010-0.050%, Ni: 0.10-0.25%.

[0032] Al is an effective deoxidizer, enhancing the oxidation resistance of steel and forming AlN pinned austenite grain boundaries to inhibit its growth. This refines the grain size and improves the material's resistance to delayed fracture. The combined addition of Al and Ti allows TiN's nitrogen-fixing effect to dissolve Al into ferrite, resulting in a dispersed distribution and inhibiting C diffusion. When the Al content is below 0.020%, the effect is not significant; above 0.040%, it easily forms coarse oxide inclusions, worsening the steel's impact toughness and fatigue performance. This invention fully utilizes the effect of Al in forming Al-N pinned austenite grain boundaries in steel to prevent its growth, thereby refining the grain size. A suitable ratio of 2.0 ≤ Al / [N] ≤ 2.8 is used, meaning that in the non-quenched and tempered steel involved in this invention, the Al content is controlled within the range of 0.025-0.045%.

[0033] The role of sulfur (S) in steel is twofold: Firstly, S readily combines with manganese (Mn) in steel to form MnS inclusions, causing hot brittleness. Secondly, treatment with Ca or Te can alter the shape and distribution of sulfide inclusions, making precipitates nucleation sites and promoting rapid precipitation of intragranular ferrite, thus increasing the proportion of ferrite in the overall microstructure and improving the toughness of non-quenched and tempered steel. Furthermore, a certain amount of sulfides can improve the machinability of steel and reduce the difficulty of processing. Studies have shown that as the S content increases from 0.025% to 0.065%, the amount of MnS in the steel increases, and significant elongation occurs along the rolling direction, while the grains are refined to a certain extent. This invention utilizes the advantages of sulfur (S) by controlling its content within the range of 0.025-0.050%, aiming to promote grain refinement in steel. Simultaneously, this invention provides a low-temperature controlled rolling and cooling manufacturing method that crushes long, ribbon-like aggregated sulfides in steel, transforming them into short rods, near-ellipsoidal shapes, and dispersed distributions. This significantly reduces the aspect ratio of sulfides in the steel, thereby improving the machinability of the steel.

[0034] Phosphorus (P) is a segregating element that tends to accumulate at grain boundaries in steel, significantly increasing the steel's susceptibility to cold brittleness and delayed fracture, thus affecting its plasticity and negatively impacting the material's microstructure and properties. This invention addresses this harmful element by controlling its content to below 0.010%.

[0035] Nitrogen (Nb), Ti, and V are the most commonly used microalloying elements in steel, typically with their content controlled below 0.1%. They precipitate a second phase through their affinity for carbon (C) and nitrogen (N), thereby altering the microstructure and properties of the steel. Nb's role is manifested in several ways: Nb forms Nb(C,N) precipitates, which hinder austenite grain growth during high-temperature homogenization, refining the pre-rolling austenite grains and promoting austenite and ferrite grain refinement. Furthermore, dissolved Nb atoms drag grain boundaries, and deformation-induced Nb(C,N) phases prevent austenite recrystallization, maintaining a flattened microstructure and providing favorable sites for ferrite nucleation, thus promoting its refinement. In addition, Nb significantly increases the recrystallization temperature, and its effect on the austenite recrystallization cessation temperature is far greater than that of Ti and V. Even trace amounts of Nb result in a nearly 50% higher critical pressure for austenite recrystallization compared to equivalent amounts of Ti and V. In summary, Nb plays a role in steel through both precipitation strengthening of the Nb(C,N) phase and solid solution strengthening of Nb atoms, with a more significant effect than that of Ti and V. In this invention, the Nb content is controlled at a trace level, i.e., Nb: 0.02-0.05%. Trace amounts of Nb can suppress the tendency for high-temperature grain growth, giving the steel good overall properties. Adding trace amounts of Nb can effectively refine the grains and reduce mixed-crystal defects in the microstructure; however, under certain solubility and precipitation conditions, excessively high Nb content no longer provides a significant increase in strengthening.

[0036] At the same carbon content, among Nb, Ti, and V, V has the highest solubility in austenite, resulting in the greatest strengthening effect, which remains at a high level across a wide range of heating temperatures. Therefore, V is also a major microalloying element in non-quenched and tempered steels. Compared to the addition of a single element, composite microalloying is a more effective way to improve the strength and toughness of non-quenched and tempered steels. For example, in Nb-V composite microalloying, Nb(C,N) particles are still present at forging and rolling temperatures; in Ti-V composite microalloying, the TiN phase is stable and does not easily coarsen, effectively hindering austenite grain growth at high temperatures. However, Ti will preferentially bind to C and N, weakening the precipitation strengthening effect of V. If the Ti content is too high, a coarsened cubic TiN phase will form, thus reducing the toughness of the steel. This invention adopts a design scheme to improve the strength and toughness of non-quenched and tempered steel by combining Nb and V with micro-Ti treatment, and restricts the three elements to: Nb: 0.02-0.05%, V: 0.10-0.30%, Ti: 0.010-0.040%.

[0037] Cu significantly improves the corrosion resistance of steel. When steel comes into contact with the cathode of an electrolytic cell where Cu is deposited secondary on the surface, it promotes anodizing and forms a protective rust layer. Furthermore, Cu can improve the hygroscopicity of the rust layer and increase the material's critical moisture content. However, excessive Cu content can reduce the high-temperature plasticity of the steel and increase its susceptibility to cracking during processing. This invention controls the Cu content within the range of 0.10%.

[0038] In non-quenched and tempered steel, the main functions of nitrogen (N) are twofold. First, it combines with elements such as Nb, Al, and B to form compounds, thereby refining the grain size. A suitable Al / N ratio has a significant effect on grain refinement, but excessively high N content can lead to continuous casting defects such as bubbles. Second, N provides precipitation strengthening and improves the stability of TiN in the steel. Furthermore, increasing the N content in the steel can reduce the amount of vanadium (V) to some extent. Properly controlling the N content is beneficial for improving the performance of non-quenched and tempered steel and reducing costs. In this invention, the N content is controlled within the range of 120-160 ppm.

[0039] TO forms oxide inclusions in steel; TO should be controlled to ≤10ppm. [H] forms white spots in steel, which seriously affects product performance; [H] should be controlled to ≤1.0ppm.

[0040] For F+P non-quenched and tempered steel, strength can be simplified as the superposition of the effects of ferrite, pearlite, and solid solution strengthening. The main factors influencing its strength and toughness are: ferrite volume fraction, grain size (austenite, ferrite) and pearlite cluster size, pearlite lamellar spacing, and the size and content of precipitated phases. Reducing pearlite content and increasing the ferrite proportion improves the strength and toughness of non-quenched and tempered steel. Appropriately reducing C and Si while increasing Mn and Cr, and microalloying can effectively improve the toughness of the steel. Through the microalloying element Nb as the dominant element, and the composite addition of micro-Ti and V, a large number of fine, dispersed second-phase particles are formed. Undissolved second-phase particles and homogeneous fine grains can promote an increase in ferrite nucleation sites, thereby increasing the volume fraction of proeutectoid ferrite, which is beneficial to the material's plasticity and toughness. During the rolling process, the area of ​​greatest deformation in the bar stock is the edge and the half-radius range. The difference in temperature and strain between the surface and the core is the reason why the core structure is coarse and uneven, which is difficult to eliminate. Therefore, by using a low-temperature controlled rolling and cooling process to reduce the initial and final rolling temperatures of the steel, the core temperature is lowered, deformation penetrates into the core, increasing core strain and inevitably increasing core dislocation and lattice distortion. Ultimately, this eliminates bainite and adjusts the size of ferrite grains and pearlite clusters, thus refining the grains and increasing the ferrite proportion. Furthermore, controlling intragranular ferrite (IGF) precipitation is a new approach to improving F+P non-quenched and tempered steel. Fine and dispersed precipitates result in a significant precipitation effect. However, Nb / V / Ti (C, N) in non-quenched and tempered steel is a brittle second phase; further grain refinement can reduce or even eliminate its adverse effects on toughness. This invention employs low-temperature controlled rolling and cooling, providing S and Al element contents within the designed range, promoting the formation of a large amount of blocky, divorced eutectic morphology of Type III MnS in the steel, and reducing the content of long rods and chain-like large-sized intergranular aggregates of Type II MnS. Meanwhile, individual MnS and MnS-V(C,N) composite sulfides serve as effective nucleation sites for intragranular ferrite, which is beneficial for promoting uniform and refined microstructure. Appropriately extending the controlled rolling and cooling process at low temperatures (using an insulation hood and slow cooling in the pit) causes the tips of long / chain-like sulfides in non-quenched and tempered steel to become rounded, the overall morphology to develop into a columnar shape, and some areas to radially shrink and fracture, even spheroidizing. This improves the morphology, quantity, and distribution of sulfides in the steel, thereby enhancing its machinability.

[0041] This invention provides a microalloying composition design scheme for enhancing the strength and toughness of F+P non-quenched and tempered steel. It employs a Nb / V composite + micro-Ti treatment to improve the steel's strength and toughness; and adds an appropriate amount of S to improve the material's machinability. Furthermore, this invention also provides an industrial production scheme for low-temperature controlled rolling and cooling of non-quenched and tempered steel. Specifically, by using low-temperature controlled rolling and cooling and adding an insulation hood for slow cooling in the rolling pit, the rolling temperature is reduced, the uniformity of heat dissipation during the slow cooling process is improved, and the morphology and distribution of sulfides in the steel are altered, thereby enhancing the steel's strength, toughness, and machinability.

[0042] The present invention provides a method for manufacturing high-strength and high-toughness F+P type non-quenched and tempered steel, including the design and matching of a chemical composition system dominated by Nb element and V+micro-Ti composite micro-alloying, and a design steel industrial production process based on low-temperature controlled rolling and controlled cooling.

[0043] 1) This invention employs a Nb-dominant, V+micro-Ti composite alloying design to promote the precipitation of numerous fine, dispersed second-phase particles in the steel. These particles, together with homogeneous fine grains, increase the number of ferrite nucleation sites, thereby enhancing the ferrite volume fraction. The Nb(C,N) precipitates hinder austenite grain growth, promoting austenite and ferrite grain refinement. Simultaneously, these precipitates prevent austenite recrystallization, maintaining a flattened microstructure and providing favorable sites for ferrite nucleation, thus promoting its refinement.

[0044] 2) This invention, by adding trace amounts of sulfur, alters the shape and distribution of sulfide inclusions, promoting the precipitates to become ferrite nucleation sites and accelerating precipitation, thereby increasing the ferrite content. Simultaneously, appropriate amounts of sulfur increase the quantity and shape of type III MnS inclusions and MnS-V(C,N) in the steel, improving its machinability.

[0045] 3) This invention provides a low-temperature controlled rolling and cooling process for designed steel. By reducing the rolling temperature (soaking temperature, initial rolling temperature, and final rolling temperature), the core temperature of the steel is lowered, causing rolling deformation to penetrate towards the core, increasing core strain, and thus increasing core dislocation and lattice distortion. Through low-temperature controlled rolling and cooling, and increasing the duration of the slow cooling process, the size of ferrite grains and pearlite clusters in the designed steel is adjusted, thereby refining the grains and increasing the ferrite volume fraction. Furthermore, low-temperature rolling alters the morphology and distribution of sulfides in the steel, improving its machinability.

[0046] This invention relates to a non-quenched and tempered steel based on medium-carbon manganese steel, which undergoes micro-alloying treatment, i.e., the addition of small amounts of alloying elements such as vanadium (V), niobium (Nb), titanium (Ti), and aluminum (Al), thereby improving the strength and toughness of the material through phase transformation strengthening, precipitation strengthening, and grain refinement strengthening. Furthermore, to improve the machinability of the non-quenched and tempered steel, trace amounts of sulfur (S) are added to obtain dispersed and fine sulfides (mainly MnS). In summary, this invention provides a novel design scheme for non-quenched and tempered steel and its low-temperature controlled rolling and cooling industrial production process. By increasing the volume fraction of ferrite in the steel, refining the ferrite grains, and changing the morphology and distribution of sulfides in the steel, the design achieves improved strength and toughness, as well as improved machinability, in the non-quenched and tempered steel.

[0047] Compared with existing technologies, the niobium-dominated composite microalloying design provided by this invention enhances the strength and toughness of non-quenched and tempered steel. By adding an appropriate amount of sulfur and combining it with temperature-controlled rolling and cooling processes, and through a complete production process from electric arc furnace smelting to LF refining, RH degassing, round billet continuous casting, low-temperature controlled rolling, slow cooling (with added insulation), grinding, testing, and finished product, its machinability is improved. The purpose of this invention is to optimize the composition system of non-quenched and tempered steel through microalloying design, delaying the pearlite transformation process in the steel through solid solution strengthening and grain refinement strengthening, thereby increasing the volume fraction of ferrite in the steel. This fundamentally improves its microstructure and toughness. Simultaneously, the addition of an appropriate amount of sulfur, combined with low-temperature rolling, improves the material's machinability and controls the sulfide aspect ratio in the steel to ≤9.0. The designed formulation and the entire production process of the non-quenched and tempered steel composition, through low-temperature controlled rolling and cooling, and the addition of insulation to control slow cooling, promote grain refinement and ferrite formation in the steel. Ultimately, the mechanical properties of the steel designed in this invention are achieved as follows: Rm ≥ 920 MPa, R p0.2 ≥550MPa, A≥15%, Z≥40%; impact energy measured at room temperature (KU2)≥30J. Attached Figure Description

[0048] Figure 1 The microstructure of the F+P non-quenched and tempered steel in Example 2 has the following grain size: grain size grade 8.5, ferrite volume fraction 11.17%; fine and uniform grains, and uniform distribution of precipitates;

[0049] Figure 2 The distribution of inclusion morphology in the non-quenched and tempered steel of Example 1;

[0050] Figure 3 The distribution of inclusion morphology in non-quenched and tempered steel in Example 2;

[0051] Figure 4 The distribution of inclusion morphology in non-quenched and tempered steel is shown in Comparative Example 1.

[0052] Figure 5 The distribution of inclusion morphology in non-quenched and tempered steel is shown in Comparative Example 2. Detailed Implementation

[0053] Examples 1-5

[0054] A high-strength and high-toughness ferritic pearlitic non-quenched and tempered steel, comprising the following components by weight percentage:

[0055] C: 0.35-0.42%, Si: 0.35-0.45%, Mn: 1.25-1.65%, P≤0.010%, S: 0.025-0.050%, Cr: 0.10-0.30%, Nb: 0.02-0.05%, Mo: 0.010-0.050%, Ni: 0.10-0.25%, Al: 0.025-0.045%, V: 0.10-0.30%, Cu≤0.10%, Ti: 0.010-0.040%, [N]: 120-160ppm, [H]≤1.0ppm, TO≤10ppm, 0.13%≤Nb+V+Ti≤0.39%; the remainder is Fe and unavoidable impurity elements; the steel composition of each embodiment is shown in Table 1, and the balance not shown in Table 1 is Fe and unavoidable impurities.

[0056] Comparative Examples 1-3

[0057] A non-quenched and tempered steel comprising the following mass percentage composition as shown in Table 1, with the balance not shown in Table 1 being Fe and unavoidable impurities.

[0058] Table 1 Chemical composition of each example and comparative example (unit: [N], [H], TO are ppm, others are wt.%)

[0059]

[0060]

[0061] The manufacturing method of high-strength and high-toughness ferritic pearlitic non-quenched and tempered steel in the above embodiments has the following full-process production steps: batching - electric arc furnace smelting (EAF) - LF refining - RH vacuum treatment - round billet continuous casting (CCM) - low temperature controlled rolling - slow cooling - flaw detection / grinding - finished round steel.

[0062] The low-temperature controlled rolling specifically refers to:

[0063] The continuously cast billet is heated to a soaking temperature of 1180-1220℃ and held at that temperature. The total time for preheating, heating and soaking is ≥10h. Then it is rolled with an initial rolling temperature of 950-1000℃ and a final rolling temperature of 690-740℃. After rolling, it is cooled to 550-600℃ on a cooling bed and then placed in a slow cooling pit. An insulation cover is added around the slow cooling pit to achieve slow and uniform cooling of the rolled material. The cooling time shall not be less than 4 hours, and the material is cooled to 150-200℃.

[0064] In addition, this invention uses ordinary non-quenched and tempered steel with the same carbon content as a control steel. Comparative Examples 1 and 2 above employ an electric arc furnace (EAF) smelting process followed by LF refining, RH vacuum treatment, continuous casting, rolling (finishing), and slow cooling. The continuously cast billet is heated to 1200-1260℃ and held for 10 hours before rolling. The initial rolling temperature is 1200-1260℃, and the final rolling temperature is 820-860℃. After rolling, the billet is cooled to ≥550℃ on a cooling bed and then slowly cooled in a pit. Comparative Example 3 is produced according to the process of this invention.

[0065] The specific production process parameters for each embodiment and comparative example are shown in Table 2.

[0066] Table 2 Process parameters for each embodiment and comparative example

[0067] Table 2 Steel Rolling Production Process Parameters

[0068]

[0069] Table 3 shows the grain size and volume fraction of ferrite and pearlite phases of the steel produced in each embodiment of the present invention.

[0070] Table 3 Material microstructure distribution information in the embodiments of the present invention

[0071]

[0072]

[0073] As shown in Table 3, compared with the comparative examples, the grain size of the microstructure of the embodiments of the present invention is significantly improved by 2-3 levels, and the proportion of ferrite phase in the microstructure is also increased to a certain extent. With the further improvement of the degree of composite microalloying, the volume fraction of ferrite phase increases accordingly.

[0074] Furthermore, this invention provides a method for improving the machinability of designed steel: adding an appropriate amount of sulfur (S) and employing low-temperature controlled rolling and cooling. This method softens the sulfides (MnS) in the steel through S and low-temperature rolling, causing their microstructure to change from long rods and chains to discontinuous short lines and near-ellipsoidal shapes; and transforming their clustered, large-size distribution into a scattered, discrete, mosaic distribution. Compared to the destructive long strips and bands of other types of sulfide microstructures, this type of sulfide morphology and distribution significantly improves the machinability of the material, resulting in a significant reduction in the aspect ratio of the sulfides in the designed steel (see Table 3).

[0075] like Figures 2-5As shown, the present invention incorporates an appropriate amount of sulfur (S) in its composition design, combined with V, Al, and other elements specified in the invention. This, combined with a low-temperature controlled rolling and cooling process to reduce the cooling rate, generates uniform, fine-grained, fragmented third-type sulfides such as MnS and MnS-V(C, N). These sulfides act as nucleation sites for intragranular ferrite during phase transformation, promoting ferrite formation and improving the mechanical and machinability properties of non-quenched and tempered steel. The results of the examples and comparative examples clearly demonstrate that the morphology, quantity, and distribution of the sulfides in this invention have achieved excellent results.

[0076] In addition, the non-quenched and tempered steels of the embodiments and comparative examples of the present invention were subjected to the same heat treatment process and forging. The heat treatment process was: normalizing at 930℃ for 6 hours → slow cooling in the furnace → forging at 1200℃ for 200 minutes, with a final forging temperature of 860℃ and air cooling. Their mechanical properties (ISO 6892-1: Metallic materials, tensile testing at room temperature) were tested, as shown in Table 4.

[0077] Table 4 Mechanical properties of embodiments and comparative examples of the present invention

[0078]

[0079] The underlined data above are data that do not meet the requirements of this invention.

[0080] After normalizing and forging, the material strength, impact toughness, and reduction of area of ​​this invention are increased.

[0081] The high-strength and high-toughness F+P type non-quenched and tempered steel produced by this invention uses a specific composition and preparation method. The product is tested according to ASEM E112 standards, with a grain size grade ≥8.0, consisting of ferrite and pearlite. The ferrite volume fraction is 10%-13%, distributed at grain boundaries, with a small amount of intragranular ferrite. Compared to traditional non-quenched and tempered steel, the size and volume fraction of pearlite and ferrite are correspondingly controlled, thereby improving the material's structural strength and toughness. While ensuring strength, the steel's ductility and toughness are improved, and the mechanical properties of the steel meet the following requirements: Rm ≥ 920 MPa, R... p0.2 The steel exhibits a strength of ≥550 MPa, an A content of ≥15%, and a Z content of ≥40%. Simultaneously, it improves the machinability of the steel, achieving a sulfide aspect ratio ≤9.0. This invention, through composition and process design, ensures a sulfide aspect ratio within 9.0. If the sulfide aspect ratio exceeds this value, the material will not meet user requirements. In particular, the design of adding sulfur content to the steel improves its machinability.

Claims

1. A high-toughness ferrite-pearlite type non-quenched and tempered steel, characterized in that, The high-toughness ferrite-pearlite type non-quenched and tempered steel comprises the following mass percentage components: C: 0.35-0.42%, Si: 0.35-0.45%, Mn: 1.25-1.65%, P≤0.010%, S: 0.025-0.050%, Cr: 0.10-0.30%, Nb: 0.02-0.05%, Mo: 0.010-0.050%, Ni: 0.10-0.25%, Al: 0.025-0.045%, V: 0.10-0.30%, Cu≤0.10%, Ti: 0.010-0.040%, [N]: 120-160ppm, [H]≤1.0ppm, T.O≤10ppm, 0.13%≤Nb+V+Ti≤0.39%; the rest is Fe and inevitable impurity elements; The components of the high-toughness ferrite-pearlite type non-quenched and tempered steel further satisfy 2.98≤Mn / C≤4.

71. The components of the high-toughness ferrite-pearlite type non-quenched and tempered steel further satisfy 2.0≤Al / [N]≤2.

8. The structure of the high-toughness ferrite-pearlite type non-quenched and tempered steel is ferrite, pearlite and a small amount of carbide, the volume fraction of ferrite is 10%-13%, the structure grain size is 8.0 grade and above, the volume content of carbide is ≤4%, the intracrystalline ferrite content accounts for more than 4% of the total content, and the aspect ratio of sulfide is ≤9.

0.

2. The high tough ferrite-pearlite type non-quenched and tempered steel according to claim 1, characterized in that, The high-toughness ferrite-pearlite non-quenched and tempered steel after heat treatment has the mechanical properties of Rm≥920 MPa, R p0.2 ≥550 MPa, A≥15%, Z≥40%; and the impact energy is KU2≥30 J at room temperature.

3. A method of producing the high-toughness ferrite-pearlite type non-quenched and tempered steel according to claim 1 or 2, characterized by, The manufacturing method comprises rolling, and the rolling process comprises: controlling the temperature of the soaking section of the heating furnace to be 1180-1220°C, and the total time of preheating, heating and soaking being ≥10h.

4. The production method according to claim 3, characterized by The rolling is performed at an opening rolling temperature of 950-1000°C.

5. The production method according to claim 3 or 4, characterized by, The rolling is performed at a final rolling temperature of 690-740°C.

6. The production method according to claim 3 or 4, characterized by The manufacturing method comprises slow cooling, specifically: after rolling, the workpiece is cooled to 550-600°C on a cooling bed and then slow cooled in a pit, a heat preservation cover is added around the slow cooling pit, and the cooling time is not less than 4h.

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