A low cost sulphur containing non-quenched steel and a method of manufacturing the same

Abstract

The present disclosure relates to a sulfur-containing non-quenched and tempered steel and a manufacturing method thereof. The non-quenched and tempered steel of the present disclosure contains the following chemical elements in mass percentage: C: 0.35-0.55%; Si: 0.45-0.70%; Mn: 1.32-1.57%; P: 0.005-0.025%; S: 0.032-0.065%; Cr: 0.12-0.25%; Ni: 0.005-0.10%; Cu: 0.005-0.10%; Mo: 0.005-0.06%; V: 0.001-0.03%; Nb: 0.001-0.01%; N: 0.012-0.018%; Al: 0.005-0.025%; Ca: 0.0008-0.0025%, the balance being Fe and other inevitable impurities; the content of Ca and S satisfies Ca / S=2-5%; the F value represented by the following formula (1) is 0.78-0.88, F=[C]+([Si]+[Mn]) / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 10 (1). The non-quenched and tempered steel of the present disclosure has good strength and toughness and cutting performance.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology and relates to a sulfur-containing non-quenched and tempered steel and its manufacturing method. Background Technology

[0002] Non-quenched and tempered steel originated from the Second Energy Crisis and subsequently experienced rapid development in Europe and globally. In applications, over 70% of forgings such as crankshafts, connecting rods, front axles, and half-shafts in the German automotive industry are made of non-quenched and tempered steel. In the Japanese automotive industry, Mitsubishi Motors has almost entirely replaced hot-forged and quenched-tempered parts used in its steering and transmission systems with non-quenched and tempered steel; Toyota, Sumitomo, and other companies use non-quenched and tempered steel for 90% of their crankshafts and 75% of their connecting rods.

[0003] With the rapid development of the automotive industry, the requirements for vehicle safety, stability, and energy consumption are becoming increasingly stringent. Correspondingly, higher demands are being placed on automotive components, including high strength and toughness, stable performance, low cost, and ease of production and processing.

[0004] Non-quenched and tempered steels typically employ precipitation strengthening with microalloying elements such as V and Nb to enhance their strength. However, the prices of raw materials like V and Nb have been rising annually, significantly impacting the production cost of non-quenched and tempered steels. While non-quenched and tempered steels eliminate the need for the quenching and tempering process, offering advantages such as low carbon emissions and environmental friendliness, the addition of microalloying strengthening elements diminishes their cost advantage. Therefore, to further leverage the carbon reduction characteristics of non-quenched and tempered steels and improve their economic viability, it is necessary to develop low-cost non-quenched and tempered steels that simultaneously meet the requirements of strength, toughness, and easy machinability.

[0005] Chinese patent application CN101275199A, entitled "High-performance low-cost non-quenched and tempered steel", discloses a non-quenched and tempered steel with the following chemical composition by weight percentage: C: 0.30-0.55%, Si: 0.2-0.8%, Mn: 0.8-1.7%, P: 0.0001-0.02%, S: 0.0001-0.15%, N / Al: 0.2-1.8, wherein Al: 0.0001-0.05%, Ti: 0.0001-0.15%, and the remainder is Fe.

[0006] Chinese patent application CN102071368A, entitled "Low-cost medium-carbon non-quenched and tempered steel for forging", discloses a medium-carbon non-quenched and tempered steel with the following composition by weight percentage: C: 0.2-0.6%, Si: 0.10-1.20%, Mn: 0.9-1.8%, P: 0.0001-0.035%, S: 0.01-0.07%, Ti: 0.01-0.10%, V: 0-0.05%, with the balance being Fe.

[0007] Although the above patent applications reduced the addition of microalloying elements V and Nb, they all added Ti to the steel, which easily forms liquid-precipitated TiN, which is detrimental to the fatigue performance of non-quenched and tempered steel.

[0008] Chinese patent application CN110894584A, entitled "A Non-Quenched and Tempered Steel and its Manufacturing Method," discloses a non-quenched and tempered steel with the following composition by weight percentage: C: 0.37-0.39%, Si: 0.56-0.62%, Mn: 1.45-1.50%, P≤0.020%, S: 0.030-0.045%, Cr: 0.15-0.20%, Ni≤0.15%. The composition is as follows: Cu ≤ 0.20%, Al ≤ 0.010%, Mo ≤ 0.05%, N: 0.013-0.017%, V: 0.010-0.020%, Ti: 0.010-0.020%, Nb: 0.010-0.020%, with the balance being Fe; carbon equivalent Ceq = C + 1 / 6Mn + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, Ceq = 0.65-0.69. This non-quenched and tempered steel contains Nb, V, and Ti elements, each added at levels exceeding 0.01%, making it uneconomical in terms of cost. Summary of the Invention

[0009] In view of the above, the inventors have obtained a low-cost non-quenched and tempered steel by optimizing the alloy composition, which has good strength, toughness and machinability.

[0010] In a first aspect, this disclosure provides a non-quenched and tempered steel, which, in addition to containing more than 90% Fe and unavoidable impurities, also contains the following chemical elements in mass percentages: C: 0.35-0.55%; Si: 0.45-0.70%; Mn: 1.32-1.57%; P: 0.005-0.025%; S: 0.032-0.065%; Cr: 0.12-0.25%; Ni: 0.005-0.10%; Cu: 0.005-0.10%; Mo: 0.005-0.06%; V: 0.001-0. 0.3%; Nb: 0.001-0.01%; N: 0.012-0.018%; Al: 0.005-0.025%; Ca: 0.0008-0.0025%; where the contents of Ca and S satisfy Ca / S = 2-5%; and the F value expressed by the following formula (1) is 0.78-0.88, F = [C] + ([Si] + [Mn]) / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 10 (1), where each chemical element symbol represents the value before the percentage sign of the corresponding chemical element mass percentage content.

[0011] In a second aspect, this disclosure provides a non-quenched and tempered steel comprising the following chemical elements in weight percentages: C: 0.35-0.55%; Si: 0.45-0.70%; Mn: 1.32-1.57%; P: 0.005-0.025%; S: 0.032-0.065%; Cr: 0.12-0.25%; Ni: 0.005-0.10%; Cu: 0.005-0.10%; Mo: 0.005-0.06%; V: 0.001-0.03%. ;Nb: 0.001-0.01%;N: 0.012-0.018%;Al: 0.005-0.025%;Ca: 0.0008-0.0025%, with the balance being Fe and other unavoidable impurities; wherein the contents of Ca and S satisfy Ca / S=2-5%;and the F value expressed by the following formula (1) is 0.78-0.88, F=[C]+([Si]+[Mn]) / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 10 (1), where each chemical element symbol represents the value before the percentage sign of the corresponding chemical element mass percentage content.

[0012] In a preferred embodiment, the Ca and S content in the non-quenched and tempered steel of this disclosure satisfies Ca / S = 2-4%. By controlling the Ca and S content to meet the above ratio, the production efficiency and machinability of the non-quenched and tempered steel can be further improved.

[0013] In a preferred embodiment, the F value in the non-quenched and tempered steel of this disclosure is 0.78-0.81. By controlling the F value within this range, the strength and toughness of the non-quenched and tempered steel can be further improved.

[0014] In a preferred embodiment, the Ni content in the non-quenched and tempered steel of this disclosure is 0.01-0.06%.

[0015] In a preferred embodiment, the Cu content in the non-quenched and tempered steel of this disclosure is 0.01-0.05%.

[0016] In a preferred embodiment, the Mo content in the non-quenched and tempered steel of this disclosure is 0.005-0.035%.

[0017] In a preferred embodiment, the V content in the non-quenched and tempered steel of this disclosure is 0.001-0.02%.

[0018] In a preferred embodiment, the Nb content in the non-quenched and tempered steel of this disclosure is 0.001-0.005%.

[0019] In a preferred embodiment, one or more (preferably all) of the elements Ni, Cu, Mo, V and Nb are provided by scrap steel.

[0020] In one embodiment, more than 90% (e.g., more than 92%, more than 94%, more than 96%, more than 98%, preferably 100%) of the Ni, Cu, Mo, V and / or Nb content in the non-quenched and tempered steel of this disclosure is provided by scrap steel.

[0021] In a preferred embodiment, the Ni, Cu, Mo, V and Nb elements in the non-quenched and tempered steel of this disclosure are all provided by scrap steel.

[0022] In a preferred embodiment, the non-quenched and tempered steel of this disclosure does not contain Ti or Mg elements.

[0023] In one implementation scheme, the metallographic structure of the non-quenched and tempered steel is ferrite + pearlite.

[0024] In a preferred embodiment, the area percentage content of ferrite in the non-quenched and tempered steel is 16-22%.

[0025] In a preferred embodiment, the pearlite lamellar spacing of the non-quenched and tempered steel is 190-230 nm.

[0026] In one embodiment, the non-quenched and tempered steel of this disclosure contains MnS inclusions.

[0027] In a preferred embodiment, the size of a single MnS inclusion is ≤150 μm, and the density of MnS inclusions is 80-100 inclusions / mm². 2 By controlling the size and density of MnS inclusions within the aforementioned range, the machinability of non-quenched and tempered steel can be further improved.

[0028] This disclosure achieves a non-quenched and tempered steel with good strength, toughness, and machinability at low cost by primarily using inexpensive alloying elements and controlling the mass percentage content of each element, while controlling the Ca / S ratio at Ca / S = 2-5%, and controlling the strength-toughness equivalent F = 0.78-0.88. The mechanical properties of the non-quenched and tempered steel disclosed in this disclosure meet the following requirements: tensile strength Rm ≥ 770 MPa, specified non-proportional elongation strength Rp0.2 ≥ 480 MPa, elongation after fracture A ≥ 15%, reduction of area Z ≥ 40%, and impact energy AKU2 ≥ 45 J.

[0029] In a third aspect, this disclosure provides a method for manufacturing non-quenched and tempered steel according to the first or second aspect, comprising the following steps: sequentially smelting, refining, vacuum degassing and continuous casting of molten steel to obtain a continuously cast billet; and hot rolling the continuously cast billet to obtain non-quenched and tempered steel.

[0030] In the manufacturing method disclosed herein, smelting, refining, and vacuum degassing can all be carried out using methods commonly used in the art, such as electric furnace smelting, LF refining, or VD vacuum degassing.

[0031] In one implementation, the molten steel consists of 62-68 wt% scrap steel and 32-38 wt% molten iron. By using 62-68 wt% scrap steel and 32-38 wt% molten iron as raw materials for smelting, production costs can be reduced while ensuring good strength and toughness of the steel.

[0032] In one implementation, after refining and before vacuum degassing, aluminum wire is fed to the target content, sulfur wire is fed to the upper limit of sulfur content, and / or calcium wire is fed 40-60m / 100 tons of molten steel after vacuum breaking.

[0033] In one implementation scheme, the tapping temperature during the smelting process is 1630-1650℃.

[0034] In one implementation, the casting start temperature is 1520-1540°C.

[0035] In one implementation, the continuously cast billet obtained from continuous casting is slowly cooled to below 500°C and then put into a heating furnace for hot rolling. The rolling heating and holding temperature is 1180-1200°C, and the final rolling temperature is 900-950°C.

[0036] In one embodiment, the method further includes cooling after hot rolling. Preferably, after hot rolling, the non-quenched and tempered steel is air-cooled to 330-380°C and then slowly cooled in a slow cooling pit. Attached Figure Description

[0037] Figure 1 The morphology of MnS inclusions in Example 1 is shown.

[0038] Figure 2 The metallographic structure of Example 1 is shown.

[0039] Figure 3 The pearlite sheets of Example 1 are shown. Detailed Implementation

[0040] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains.

[0041] As used herein, the term “and / or” refers to and covers any and all possible combinations of more than one of the listed items.

[0042] In this paper, tensile strength Rm is the critical value for the transition of a metal from uniform plastic deformation to localized concentrated plastic deformation, and it is also the maximum load-bearing capacity of a metal under static tensile conditions. Tensile strength is the resistance to the maximum uniform plastic deformation of a material.

[0043] In this paper, the nonproportional elongation strength Rp is defined as the stress when the nonproportional elongation is equal to the specified extensometer gauge length percentage, and Rp0.2 is the stress when the specified extensometer gauge length is 0.2%.

[0044] In this paper, elongation after fracture (A) refers to the percentage of the length the test bar elongates relative to its original length when the metal material fractures under external force (tensile force). Elongation after fracture is an important indicator reflecting the plasticity of metallic materials; the higher the value, the better the plasticity of the metallic material.

[0045] In this paper, the reduction of area Z is the percentage of the maximum reduction in cross-sectional area at the neck after the specimen breaks, relative to the original cross-sectional area.

[0046] In this paper, AKU2 is the impact energy of a 2mm U-shaped notch, in J.

[0047] In this paper, tensile strength Rm, specified non-proportional extension strength Rp0.2, elongation after fracture A, and reduction of area Z were determined according to GB / T228.1-2010 "Metallic materials, tensile testing - Part 1: Test at room temperature". Impact energy AKU2 was determined according to GB / T229-2020 "Metallic materials, Charpy impact test method".

[0048] In this paper, the ferrite content was determined by statistically calculating the ferrite area ratio in metallographic images using the image analysis software built into the metallographic microscope, and the unit is %.

[0049] In this paper, the average interlamellar spacing of pearlite was measured as follows: The average interlamellar spacing was calculated using the intercept method on pearlite lamellar images taken with a scanning electron microscope, based on the number of intercept points where line segments of known length intersect with the lamellars. For example, along... Figure 3 Draw a line along the diagonal, count the number of intersections between the line and the layers, and calculate the average interlayer spacing by dividing the line length by (number of intersections - 1).

[0050] The design principles of each chemical element in the non-quenched and tempered steel disclosed herein are as follows:

[0051] Carbon (C): Carbon has a significant impact on the strength and toughness of steel. The ductility and toughness of steel decrease with increasing carbon content. Therefore, the lower the carbon content in steel, the better its ductility and toughness. However, carbon is also crucial for ensuring strength; low carbon content results in insufficient strength. Therefore, to ensure that non-quenched and tempered steel has both high strength and good ductility and toughness, the carbon content in the non-quenched and tempered steel disclosed in this invention is controlled at 0.35-0.55 wt%.

[0052] Si (silicon): Si can improve the strength of steel. Increasing the Si content can improve the strength of steel to a certain extent. However, with further increases in Si content, bainite or martensite structures are easily formed in the steel. Therefore, in the non-quenched and tempered steel of this disclosure, the Si content is controlled at 0.45-0.70 wt%.

[0053] Manganese (Mn): As an alloying element, Mn can improve strength. Furthermore, within a certain range, the Mn content also contributes to toughness. When the Mn content is low, the strength is insufficient. However, when the Mn content is too high, it is detrimental to both toughness and strength. Therefore, in the non-quenched and tempered steel disclosed in this invention, the Mn content is controlled at 1.32-1.57 wt%.

[0054] Phosphorus (P): As a harmful element in steel, theoretically, the lower the P content, the better. When the P content exceeds 0.025 wt%, it tends to accumulate at grain boundaries, which is detrimental to the material's performance. However, considering factors such as smelting conditions and cost control, further dephosphorization is not performed when the P content is reduced to 0.005 wt%. Therefore, in the non-quenched and tempered steel disclosed in this invention, the P content is controlled between 0.005 and 0.025 wt%.

[0055] Sulfur (S): S is an element that contributes to hot brittleness and machinability. The machinability of steel improves with increasing S content. However, the hot workability of steel deteriorates with increasing sulfur content. Therefore, for non-quenched and tempered steels for hot forging, the S content should not be excessive, with an upper limit controlled below 0.065 wt%. However, when the S content in steel falls below 0.032 wt%, its machinability begins to deteriorate. Therefore, in the non-quenched and tempered steels disclosed in this invention, the S content is controlled between 0.032 and 0.065 wt%.

[0056] Cr (chromium): Cr can improve the strength and hardenability of steel. Since non-quenched and tempered steel parts require surface induction hardening, adding a certain amount of Cr can ensure the depth and hardness of the hardened layer. When the Cr content is below 0.12 wt%, its hardenability is insufficient. However, when the Cr content exceeds 0.25 wt%, more alloy carbides will form. Therefore, in the non-quenched and tempered steel of this disclosure, the Cr content is controlled at 0.12-0.25 wt%.

[0057] Ni (nickel), Cu (copper), and Mo (molybdenum): Ni, Cu, and Mo elements can improve the strength and hardenability of steel. However, high concentrations of these alloying elements tend to form bainitic structures. Therefore, in the non-quenched and tempered steel of this disclosure, the Ni content is controlled at 0.005-0.10 wt%, the Cu content at 0.005-0.10 wt%, and the Mo content at 0.005-0.06 wt%. Furthermore, the addition of these alloying elements can easily increase costs. Therefore, in the preferred embodiment of this disclosure, the residual element content in sorted scrap steel is used to provide the required Ni, Cu, and Mo in the non-quenched and tempered steel of this disclosure, thus ensuring the strength of the steel while reducing costs.

[0058] Vanadium (V) and Niobium (Nb): V and Nb are precipitation strengthening and grain refining elements in steel. Adding appropriate amounts of V and Nb to steel can effectively improve the strength of the material. However, the raw materials for these elements are expensive, and adding raw materials containing these elements will significantly increase costs. Therefore, in the non-quenched and tempered steel of this disclosure, the content of V is controlled at 0.001-0.03 wt%, and the content of Nb is controlled at 0.001-0.01 wt%. In the preferred embodiment of this disclosure, the residual element content in sorted scrap steel is used to provide the required V and Nb in the non-quenched and tempered steel of this disclosure, thus ensuring the strength of the steel while reducing costs.

[0059] Nitrogen (N): Nitrogen can form nitrides or nitrides with alloying elements such as V, Nb, and Al in steel. The precipitation of these nitrides or nitrides can refine the grains and improve the strength and toughness of the steel. This effect is not significant when the N content in the steel is below 0.012%. However, when the N content is too high, the solubility of nitrogen in molten steel reduces the economic viability of adding nitrogen. Therefore, in the non-quenched and tempered steel of this disclosure, the N content is controlled at 0.012-0.018 wt%.

[0060] Al (aluminum): Al plays a deoxidizing role in steelmaking. The resulting composite deoxidation products act as nuclei for MnS inclusions, effectively improving machinability. Furthermore, AlN particles formed by Al and N effectively refine grain size. To achieve these effects, the Al content should be no less than 0.005 wt%. However, when the Al content exceeds 0.025 wt%, secondary oxidation easily occurs during casting, generating Al2O3 that forms nodules at the nozzle. Therefore, in the non-quenched and tempered steel of this disclosure, the Al content is controlled between 0.005 and 0.025 wt%.

[0061] Ca (calcium): For sulfur-containing steel, Ca can modify MnS inclusions, transforming elongated MnS into spherical or ellipsoidal sulfides. However, Ca combines with S in the steel to form high-melting-point CaS. This high-melting-point CaS tends to accumulate at the nozzle during casting, causing nozzle blockage. Therefore, to reduce CaS while modifying MnS inclusions, the Ca content in the non-quenched and tempered steel disclosed in this invention is controlled at 0.0008-0.0025 wt%.

[0062] Through extensive research, the inventors designed a formula to express the strength-toughness equivalent (F): F = [C] + ([Si] + [Mn]) / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 10. In this formula, [X] represents the percentage value before the percent sign of element X by mass. For example, when the content of element C is 0.55 wt%, [C] = 0.55. The inventors found that when the strength-toughness equivalent F < 0.78, the yield strength of the non-quenched and tempered steel is low and cannot meet the strength requirements; when the strength-toughness equivalent F > 0.88, the impact energy of the non-quenched and tempered steel after hot forging decreases and cannot meet the toughness requirements. Therefore, to ensure that the non-quenched and tempered steel has sufficient strength and toughness, the F value is controlled between 0.78 and 0.88 in the non-quenched and tempered steel disclosed in this invention.

[0063] In the non-quenched and tempered steel disclosed herein, in order to modify the morphology of MnS inclusions, form MnS and CaS multiphase inclusions, reduce the aggregation and growth of MnS inclusions, increase the number of MnS inclusions, and avoid nozzle nodules, while controlling the content of Ca and S elements within a specific range, it is also necessary to control the Ca / S element content ratio between 2-5%. The inventors have found that when Ca / S < 2%, the modification effect on MnS inclusions is not significant, and MnS in the steel tends to appear as elongated strips and aggregate, with a small quantity, which is detrimental to machinability; when Ca / S > 5%, excess Ca easily combines with S to form a single high-melting-point CaS, which easily aggregates at the casting nozzle, causing nozzle blockage and affecting production efficiency.

[0064] In addition, Table 1 below compares the compositional differences between the non-quenched and tempered steel of this disclosure and the steels disclosed in CN101275199A, CN102071368A, CN110894584A and CN111455291A.

[0065] Table 1. Chemical composition (wt%) of relevant patents

[0066]

[0067] It can be seen that the non-quenched and tempered steel disclosed in this paper is different from the steel disclosed in CN101275199A, CN102071368A, CN110894584A and CN111455291A in terms of element types and element contents.

[0068] In one embodiment, the non-quenched and tempered steel of this disclosure is manufactured by a method comprising the following steps: electric furnace smelting, LF refining, vacuum degassing (VD), and continuous casting of molten steel to obtain a continuously cast billet; hot rolling of the continuously cast billet to obtain the non-quenched and tempered steel; wherein the non-quenched and tempered steel comprises the following chemical elements in mass percentage: C: 0.35-0.55%; Si: 0.45-0.70%; Mn: 1.32-1.57%; P: 0.005-0.025%; S: 0. 0.32-0.065%; Cr: 0.12-0.25%; Ni: 0.005-0.10%; Cu: 0.005-0.10%; Mo: 0.005-0.06%; V: 0.001-0.03%; Nb: 0.001-0.01%; N: 0.012-0.018%; Al: 0.005-0.025%; Ca: 0.0008-0.0025%, with the balance being Fe and other unavoidable impurities.

[0069] In one embodiment, the molten steel consists of 62-68 wt% scrap steel and 32-38 wt% molten iron. In one embodiment, the scrap steel contains one or more of the elements Ni, Cu, Mo, V, and Nb. In one embodiment, no additional Ni, Cu, Mo, V, and / or Nb elements are added in the manufacturing method disclosed herein.

[0070] In one embodiment, the non-quenched and tempered steel of this disclosure is manufactured by a method comprising the following steps:

[0071] The molten steel undergoes sequential processes including electric arc furnace (EAF) smelting, LF refining, vacuum degassing (VD), and continuous casting. During EAF smelting, scrap steel with a weight ratio of 62-68% and molten iron with 32-38% are used. During LF refining, the composition is adjusted to meet the F-value requirements. At the end of LF refining, Al wire is fed to achieve the target composition. Before VD vacuum treatment, sulfur wire is fed to the upper limit of sulfur content. The VD vacuum degree is ≤2.4 mbar, and the high vacuum time is 13-18 min to fully remove hydrogen and oxygen from the molten steel. After vacuum breaking, 40-60 m of calcium wire is fed per 100 tons of molten steel to achieve a Ca / S ratio of 2-5%. The continuously cast billet is slowly cooled to below 500℃ before entering the heating furnace. The rolling heating and holding temperature is 1180-1200℃, and the final rolling temperature is 900-950℃. After hot rolling, the non-quenched and tempered steel is air-cooled on a cooling bed to 330-380℃ before being slowly cooled in a slow cooling pit.

[0072] The non-quenched and tempered steel and its manufacturing method disclosed herein have the following advantages and beneficial effects:

[0073] 1) The non-quenched and tempered steel disclosed herein is strengthened by inexpensive alloying elements C, Si, and Mn, which is cost-effective.

[0074] 2) This disclosure proposes a formula for expressing the strength-toughness equivalent (F): F=[C]+([Si]+[Mn]) / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 10, and finds that when the strength-toughness equivalent F value is in the range of 0.78-0.88, it can ensure that the steel has high strength while also having stable impact energy.

[0075] 3) The non-quenched and tempered steel disclosed herein has superior strength and toughness compared to the national standard grade F45VS with added vanadium, and also has excellent machinability, making it particularly suitable for parts that are hot-forged or directly machined.

[0076] 4) Conventional non-quenched and tempered steels mostly improve strength or microstructure by adding microalloying elements such as V, Nb, and Ti. However, the price of raw materials containing these microalloying elements is rising year by year, leading to increased costs and hindering the further promotion and application of non-quenched and tempered steels. The preferred technical solution disclosed herein uses residual strengthening alloying elements from electric arc furnace smelting scrap steel to replace the addition of microalloying elements such as V and Nb to improve the material's strength.

[0077] 5) In the manufacturing method disclosed herein, micro-calcium treatment and Ca / S ratio are used to increase the number of MnS inclusions and limit their aggregation and growth, while avoiding nozzle blockage.

[0078] The present disclosure will now be described in further detail with reference to the accompanying drawings and embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the disclosure.

[0079] Example

[0080] Examples 1-7 and Comparative Examples 1-4

[0081] The non-quenched and tempered steels of Examples 1-7 and Comparative Examples 1-4 were prepared by the following steps:

[0082] According to the formula shown in Table 2 below, molten steel is sequentially subjected to electric arc furnace smelting, LF refining, vacuum degassing (VD), and continuous casting to obtain a continuously cast billet. The billet is then hot-rolled and cooled to produce non-quenched and tempered steel. In the electric arc furnace smelting process, scrap steel with a weight ratio of 62-68% and molten iron with a weight ratio of 32-38% are used. The tapping temperature during smelting is approximately 1630-1650℃. The VD vacuum degree is ≤2.4mbar, the high vacuum time is 13-18min, and after vacuum breaking, 40-60m of calcium wire is fed per 100 tons of molten steel to achieve a Ca / S ratio of 2-5%. During continuous casting, the initial casting temperature is approximately 1520-1540℃. The continuously cast billet is slowly cooled to below 500℃ before entering the heating furnace. The rolling heating and holding temperature is 1180-1200℃, and the final rolling temperature during hot rolling is 900-950℃. After hot rolling, the non-quenched and tempered steel is air-cooled to 330-380℃ on a cooling bed and then slowly cooled in a slow cooling pit.

[0083] The composition and specific process parameters of the steels used in Examples 1-7 and Comparative Examples 1-4 are shown in Tables 2 and 3. The steel used in Comparative Example 1 was selected from F45VS in GB / T 15712-2016.

[0084]

[0085] Table 3. Specific process parameters for Examples 1-7 and Comparative Examples 1-4

[0086]

[0087] Samples of the steels from Examples 1-7 and Comparative Examples 1-4 were taken and subjected to various mechanical property tests. The test results are listed in Table 4. The relevant mechanical property test methods are as follows: tensile properties were determined according to GB / T228.1-2010 "Metallic materials, tensile testing - Part 1: Test at room temperature"; impact properties were determined according to GB / T229-2020 "Metallic materials, Charpy impact test method".

[0088] In addition, samples of the steels from Examples 1-7 and Comparative Examples 1-4 were taken and their metallographic structures were analyzed. The ferrite content of the steel was statistically analyzed using the image analysis software built into the metallographic microscope. The pearlite lamellar spacing was measured and calculated using the truncation method on a scanning electron microscope. The number and maximum size of MnS inclusions were statistically analyzed using an ASPEX Explorer automated scanning electron microscope. The results of the ferrite content, pearlite lamellar spacing, and the number and size of MnS inclusions are listed in Table 4. The morphology of the MnS inclusions in Example 1 is as follows: Figure 1 As shown. The metallographic structure of Example 1 is as follows. Figure 2 As shown. The pearlite lamellars of Example 1 are as follows. Figure 3 As shown.

[0089] Table 4. Microstructure and mechanical properties of non-quenched and tempered steel (round bars) from Examples 1-7 and Comparative Examples 1-4

[0090]

[0091]

[0092] *In addition to MnS inclusions, macroscopic inclusions composed of elements such as Ca, Al, Mn, O, and S were also found, which were analyzed to be debris from the sprue.

[0093] As can be seen from the table above, the non-quenched and tempered steels of Examples 1-7, with compositional contents within the scope of this disclosure and meeting the requirements of strength and toughness equivalent F = 0.78-0.88 and Ca / S = 2-5%, exhibit excellent mechanical properties, with Rm ≥ 770 MPa, RP0.2 ≥ 480 MPa, A ≥ 15%, Z ≥ 40%, and AKU2 ≥ 45 J. The metallographic structure consists of ferrite and pearlite, with a ferrite content of 16-22% and a pearlite lamellar spacing of 190-230 nm. The size of the MnS inclusions is ≤ 150 μm, and the number of MnS inclusions is 80-100 inclusions / mm. 2 This provides a good morphology and distribution of MnS for cutting.

[0094] All publications, patent applications, patents and other references mentioned in this disclosure are incorporated herein by reference in their entirety.

[0095] While this disclosure has been illustrated and described with reference to certain preferred embodiments, those skilled in the art should understand that the above description is a further detailed explanation of the disclosure in conjunction with specific embodiments, and should not be construed as limiting the specific implementation of this disclosure to these descriptions. Various changes in form and detail can be made by those skilled in the art, including some simple deductions or substitutions, without departing from the spirit and scope of this disclosure.

Claims

1. A non-quenched and tempered steel, characterized in that, The non-quenched and tempered steel contains the following chemical elements by mass percentage: C: 0.35-0.55%; Si: 0.45-0.70%; Mn: 1.32-1.57%; P: 0.005-0.025%; S: 0.032-0.065%; Cr: 0.12-0.25%; Ni: 0.005-0.10%; Cu: 0.005-0.10%; Mo: 0.005-0.06%; V: 0.001-0.03%; Nb: 0.001-0.01%; N：0.012-0.018%； Al：0.005-0.025%； Ca: 0.0008-0.0025%, balance being Fe and other unavoidable impurities; The contents of Ca and S satisfy Ca / S = 2-5%; and, The F value expressed by equation (1) is 0.78-0.

88. F = [C] + ([Si] + [Mn]) / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Ni] + [Cu]) / 10 (1), where each chemical element symbol represents the value before the percentage sign of the corresponding chemical element's mass percentage content; The metallographic structure of the non-quenched and tempered steel is ferrite + pearlite, with the ferrite area percentage content being 16-22% and the pearlite lamellar spacing being 190-230nm.

2. The non-quenched and tempered steel according to claim 1, characterized in that, The contents of Ca and S satisfy Ca / S = 2-4%.

3. The non-quenched and tempered steel according to claim 1, characterized in that, The F value is 0.78-0.

81.

4. The non-quenched and tempered steel according to claim 1, characterized in that, The content of Ni is 0.01-0.06%; the content of Cu is 0.01-0.05%; the content of Mo is 0.005-0.035%; the content of V is 0.001-0.02%; and / or the content of Nb is 0.001-0.005%.

5. The non-quenched and tempered steel according to claim 1, characterized in that, The non-quenched and tempered steel does not contain Ti or Mg elements.

6. The non-quenched and tempered steel according to claim 1, characterized in that, The non-quenched and tempered steel contains MnS inclusions.

7. The non-quenched and tempered steel according to claim 6, characterized in that, The size of the MnS inclusions is ≤150μm, and the density of MnS inclusions is 80-100 inclusions / mm². 2 .

8. The non-quenched and tempered steel according to any one of claims 1 to 7, characterized in that, The non-quenched and tempered steel has one or more of the following properties: tensile strength Rm≥770 MPa, specified non-proportional elongation strength Rp0.2≥480 MPa, elongation after fracture A≥15%, reduction of area Z≥40%, and impact energy AKU2≥45J.

9. The method for manufacturing non-quenched and tempered steel according to any one of claims 1 to 8, characterized in that, The manufacturing method includes the following steps: smelting, refining, vacuum degassing and continuous casting of molten steel to obtain a continuously cast billet; hot rolling of the continuously cast billet to obtain non-quenched and tempered steel.

10. The manufacturing method according to claim 9, characterized in that, Molten steel consists of 62-68 wt% scrap steel and 32-38 wt% molten iron.

11. The manufacturing method according to claim 9, characterized in that, After refining and before vacuum degassing, aluminum wire is fed to the target content, sulfur wire is fed to the upper limit of sulfur content, and / or calcium wire is fed 40-60m / 100 tons of molten steel after vacuum breaking.

12. The manufacturing method according to claim 9, characterized in that, The method also includes cooling after hot rolling.

13. The manufacturing method according to claim 12, characterized in that, After hot rolling, the non-quenched and tempered steel is air-cooled to 330-380℃, and then slowly cooled in a slow cooling pit.

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